# Nitrogen dynamics in cropping systems under Mediterranean climate: a systemic analysis

Luis Lassaletta, Alberto Sanz-Cobena, Eduardo Aguilera, Miguel Quemada, Gilles Billen, Alberte Bondeau, Maria Luz Cayuela, Wolfgang Cramer, Joris P.C. Eekhout, Josette Garnier, et al.

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Luis Lassaletta, Alberto Sanz-Cobena, Eduardo Aguilera, Miguel Quemada, Gilles Billen, et al. Nitrogen dynamics in cropping systems under Mediterranean climate: a systemic analysis. Environmental Research Letters, 2021, 16 (7), pp.73002. 10.1088/1748-9326/ac002c hal-03265203

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# Nitrogen dynamics in cropping systems under Mediterranean climate: a systemic analysis

To cite this article before publication: Luis Lassaletta et al 2021 *Environ. Res. Lett.* in press https://doi.org/10.1088/1748-9326/ac002c

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1 Nitrogen dynamics in cropping systems under Mediterranean climate: a systemic analysis

3
Luis Lassaletta¹*, Alberto Sanz-Cobena¹, Eduardo Aguilera¹, Miguel Quemada¹, Gilles Billen²,
4
Alberte Bondeau³, Maria Luz Cayuela⁴, Wolfgang Cramer³, Joris P.C. Eekhout⁴, Josette Garnier⁵,
5
Bruna Grizzetti⁶, Diego S. Intrigliolo⁴, Margarita Ruiz Ramos¹, Estela Romero⁷, Antonio Vallejo¹,
6
Benjamín S. Gimeno⁸

8 * luis.lassaletta@upm.es

1 ETSI Agronómica, Alimentaria y de Biosistemas, CEIGRAM, Universidad Politécnica de Madrid, Spain

2 UMR 7619 Metis, Sorbonne Université, Paris, Île-de-France, France

3 IMBE, Aix Marseille University, CNRS, IRD, Avignon University, Aix-en-Provence, France

4 Applied Biology and Soil Sciences Center (CEBAS), Spanish National Research Council (CSIC), Murcia, Spain

5 UMR 7619 Metis, CNRS, Paris, Île-de-France, FRANCE

6 European Commission Joint Research Centre (JRC), Ispra (VA), Italy

9 Dept. de Medio ambiente, Instituto Nacional de Investigación y Tecnología Agraria y Alimentaria (INIA-INAGEA), Madrid, Spain

Abstract

Worldwide, Mediterranean cropping systems face the complex challenge of producing enough high-quality food while preserving the quantity and quality of scarce water for people and agriculture in the context of climate change. While good management of nitrogen (N) is paramount to achieving this objective, the efficient strategies developed for temperate systems are often not adapted to the specificities of Mediterranean systems. In this work we combined original data with a thorough literature review to highlight the most relevant drivers of N dynamics in these semiarid systems. To do so, we provide an analysis at nested scales combining a bottom-up approach from the field scale with a top-down approach considering the agro-food system where cropping systems are inserted. We analyze the structural changes in the agro-food systems affecting total N entering the territory; the contrasting response of yields to N availability under rainfed and irrigated conditions in a precipitation gradient; the interaction between N management and climate change adaptation; the main drivers affecting the release of Nr compounds (NO₃⁻, NH₃, NO, N₂O) as compared with temperate systems; and finally, the behavior of N once exported to highly regulated river networks. We conclude that a sustainable N management in Mediterranean cropping systems requires the specific adaptation of practices to the particular local agroenvironmental characteristics with special emphasis on water availability for rainfed and irrigated systems. This approach should also include a systemic analysis of N inputs into the territory that are driven by the configuration of the agro-food system.
# 1. Background

The Mediterranean biome, characterized by its climate with warm, dry summers and mild, wet winters, is located between 30° and 45° latitude and is present in five areas of the world, namely, the Mediterranean Basin, California (USA), Chile, South Africa, and Australia (Fig. 1) (Aschmann, 1973). Mediterranean-type ecosystems are biodiversity hotspots concentrating a disproportionately high level of endemism (Myers et al., 2000; Vogiatzakis et al., 2006), making this biome a global conservation priority (Olson and Dinerstein, 2003). The total area covers 2.6 million km² and in 2010 had a population density of 95 inh/km² (i.e., 310 M inhabitants).

The land area of the Mediterranean Basin accounts for 66% of the world Mediterranean biome. The sustainability of this region for people and ecosystems is at stake due to: (1) scarcity of water (García-Ruiz et al., 2011), (2) significant land-use changes, notably urbanization and abandonment of pastoralism (Malek et al., 2018; Aguilera et al., 2020), (3) increasing air and water pollution (Sanz-Cobena et al., 2017), (4) biodiversity loss (Sala et al., 2000), and (5) accelerating impacts of climate change, as well as other drivers of change (Cramer et al., 2018; Zampieri et al., 2020), making this region a "climate change hotspot" (Giorgi, 2006).

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Global distribution of the Mediterranean biome (Olson et al. 2001) including potential areas of expansion (red arrows, "Mediterranization") and contraction (blue lines). Adapted from Klausmeyer and Shaw (2009).</figcaption></figure>

Due to global warming, some Mediterranean regions are expected to expand ("Mediterranization") into currently temperate areas in South America and Europe, while contracting in others such as Australia and North Africa, giving way to a more arid climate (Klausmeyer and Shaw, 2009; Hernández, 2017). The key characteristic of the Mediterranean Climate is the asynchronicity between the maximum water availability (from autumn to spring) and the maximum irradiance and temperature, with important consequences for agricultural
60 production. It is also characterized by flash floods and droughts, which are projected to be exacerbated (Michaelides et al., 2018).

62 During thousands of years of evolution, agriculture in the Mediterranean Basin has been adapted to environmental conditions of relative instability, rainfall variability, and summer water stress through the selection of crops, varieties and rotations, as well as hydraulic infrastructures and management practices, resulting in diverse and unique agricultural systems (Blondel, 2006). This diversification permits the cultivation of rainfed crops such as cereals, legumes, oilseeds, and some vegetables in winter, as well as of drought-resistant permanent crops such as almonds, grapes, and olives. Efficient irrigation can increase crop yields from 10% to 90%, depending on the annual water stress and the technology (Wriedt et al., 2009). Irrigation also allows for the cultivation and expansion of horticultural crops, maize, tobacco, and citrus fruits in these semiarid areas. As a result, irrigation is a common practice in traditional Mediterranean cropping systems, and it has expanded more than in temperate areas in the past 20 years (Fig. 2).

73 Climate change is affecting the entire planet, and Mediterranean climate zones are no exception. As occurring in the entire planet (Klausmeyer and Shaw, 2009) climate change impacts Mediterranean biomes (Famiglietti et al., 2011), where it is expected to increase water scarcity via sustained warming and decreasing precipitation, consequently leading to an increase in evaporative demand and in the intensity, duration, and frequency of droughts (García-Ruiz et al., 2011; Spinoni et al., 2018). Without adaptation crop yields will likely be impacted (Iglesias et al., 2011), (Zampieri et al., 2020). Irrigation requirements may increase substantially with climate change (Fader et al., 2016), and therefore irrigation techniques will have to be adapted as water resources may turn out to be insufficient (Malek and Verburg, 2018).
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Evolution of cropland fraction equipped for irrigation in several Mediterranean countries (all of them with than 50% of their agriculture under Mediterranean conditions) and a sample of temperate countries in Europe (based on FAOSTAT, 2020).</figcaption></figure>

Together with water, nitrogen (N) is the main limiting factor in agricultural production and must be properly managed to sustain crop yields (Mueller *et al.*, 2012; Quemada *et al.*, 2020a). Lassaletta et al (2014a) showed that a robust hyperbolic relationship relates harvested yield, expressed as N content and integrated over a whole crop rotation cycle, to average total annual N inputs to soil, with a single parameter $Y_{max}$ characterizing the pedo-climatic and technical and agronomic context. This relationship implies that not all N applied to soil is recovered with the harvested crop. Overall, N use efficiency (NUE) of the world cropping systems is less than 50% (Bouwman *et al.*, 2013; Lassaletta *et al.*, 2014a; Bouwman *et al.*, 2017). Most of the N surplus, i.e., the amount of N input to soils not exported by harvested crops, is released to the environment), where it undergoes a series of transformations, impacting different ecosystem compartments through the so-called N cascade (Galloway *et al.*, 2003; Billen *et al.*, 2013). In agricultural systems, reactive N (Nr, all nitrogen species except $N_2$) (Leach *et al.*, 2012) is preferentially emitted in the form of: volatilized ammonia ($NH_3$), an aerosol precursor and soil-acidifying compound (Van Damme *et al.*, 2018) that causes biodiversity losses when deposited (Ochoa-Hueso *et al.*, 2011); nitrous oxide ($N_2O$), a potent greenhouse gas (Thompson *et al.*, 2019); and nitric oxide (NO), which promotes the formation of tropospheric ozone ($O_3$) (Guardia *et al.*, 2017b). N can be also leached as nitrate ($NO_3^-$), which is a threat to drinking water resources and freshwater biodiversity (van Grinsven *et al.*, 2010) and promotes eutrophication of coastal waters (Garnier *et al.*, 2010). The emission of these compounds imposes a high economic cost for society (Sutton *et al.*, 2013; Van
105 Grinsven *et al.*, 2013) and their adequate management has been linked to nine of the United Nations Sustainable Development Goals (Morseletto, 2019).

107 The total N inputs to the world's Mediterranean cropping systems in 2010 were 5.9 Tg N, most of them in the form of synthetic fertilizers. 75% of this N is applied in the crops of the Mediterranean Basin under Mediterranean climate (see the Supplements for a complete N budget of Mediterranean cropping systems). Adequate crop management in the Mediterranean biome involves the simultaneous control of water and N inputs (Sadras, 2005; Sadras *et al.*, 2016). Two contrasting situations are experienced: rainfed systems subject to important spatiotemporal variability, which complicates the adjustment of optimal fertilization doses (Ryan, 2008; Ryan *et al.*, 2009; Quemada and Gabriel, 2016); and irrigated systems dealing in many cases with water supply limitations and high risk of nitrate leaching (Causape *et al.*, 2006; Barros *et al.*, 2012; Malik and Dechmi, 2019). Other intrinsic characteristics of the Mediterranean systems such as the high temperatures and the low carbon content of soil (Aguilera *et al.*, 2013a; Rodríguez Martín *et al.*, 2016; Aguilera *et al.*, 2018) have a strong impact on the microbial activity that drives the form in which N is emitted and the magnitude of the flow from croplands to other compartments (Sanz-Cobena *et al.*, 2017). N₂O emission factors, for example, are lower than those in temperate systems (Cayuela *et al.*, 2017).

122 Sustainable N management, adapted to the particularities of the Mediterranean biome, depends on knowledge of specific processes at the plot and farm scale (bottom-up approach), but also on the understanding of the agro-food system that controls the new N entering a territory (top-down approach). Organic manure may be efficiently recirculated at the regional scale, reducing the demand for new fertilizers, while different agricultural commodities can be locally produced or imported, all of which configure the structure of the N cycle (Billen *et al.*, 2014; Billen *et al.*, 2015; Le Noë *et al.*, 2017; Mueller and Lassaletta, 2020). For example, in Spain, the input of new N into the national territory (NANI, net anthropogenic N input) (Howarth *et al.*, 1996) has increased fourfold during the past 60 years, contributing to important yield improvements but also to significantly increased N pollution. Once N reaches the waterbodies, the high regulation of freshwater resources for crop irrigation and drinking water supply can affect N dynamics in Mediterranean watersheds. The fraction of runoff exported to coastal waters is lower than in temperate watersheds, increasing the risk for pollution of aquifers, rivers, and lakes (Bartoli *et al.*, 2012; Romero *et al.*, 2016).

136 Worldwide, Mediterranean cropping systems face the complex challenge under the climate change context of producing enough high-quality food for an often increasing population,
138 together with the need to preserve the quantity and quality of scarce water resources for people and agriculture, while also reducing their impacts on air pollution. We present this review with the overall aim of analyzing, from a systemic perspective, the particularities of N dynamics in Mediterranean cropping systems associated with the emission of Nr compounds. Our approach aims to extend from the plot or farm-scale to the agro-food systems that these cropping systems are embedded in.

144 This study combines primary data with a literature review in order to provide an analysis at nested scales of (1) the evolution of the N cycle in agro-food systems at the regional scale including crop N budgets, (2) water and N interactions in contrasted irrigated and rainfed systems along a precipitation gradient, (3) the implications of climate change adaptation for N management, (4) the main drivers affecting the release of Nr compounds as compared with temperate systems, (5) the behavior of N in Mediterranean watersheds, (6) the resulting policy recommendations for the design of agro-environmental strategies tailored to Mediterranean conditions. The work mainly focuses on the Mediterranean Basin, which forms a unique, conterminous and coherent territory, suitable for developing a systemic analysis covering from the agro-food system to the river basin.
153 We postulate that the main lessons learnt for these semiarid systems managed under rainfed and irrigated conditions are also relevant, not only for the rest of the world's Mediterranean areas but also for the new areas undergoing "Mediterranization" due to climate change.

## 2. Methods

157 We analyzed the evolution of the N cycle in the agro-food system at the regional scale. Crop production and pasture production are connected with livestock farming to supply food to the local population, and also with the export or import of agricultural products to or from other regions. Using N embedded in proteins or in N fertilizers as a marker, the structure of the agro-food system is presented in a diagram showing N flows through the main transformation compartments involved in the system, namely, crop and grass production, livestock breeding, food processing, international trade, and final human consumption (General Representation of Agro-Food Systems, GRAFS) (Billen et al., 2014; Le Noë et al., 2017; Billen et al., 2018).

165 To construct the GRAFS diagrams, and since some statistics were available at the country scale only, we considered 11 out of the 22 countries in the Mediterranean Basin. In these countries, more than 50% of the crop and grassland area is under Mediterranean conditions: Portugal, Spain, Italy, Albania, Greece, Turkey, Cyprus, Israel, Tunisia, Algeria, and Morocco (Supplements Table S1). Countries presenting a marked temperate-to-Mediterranean gradient (France) or with incomplete observations (Palestine), or those with relevant data missing for other reasons (Syria
171 and Libya), were not included. In total, the selected countries comprise 91% of crops and grasslands under Mediterranean climate in the Mediterranean Basin and 61% of crops and grasslands under Mediterranean climate worldwide. The crop mix at country level was estimated by clustering crop data from FAOSTAT into generic groups, namely, cereals, legumes, roots, oilseed, horticulture, permanent crops, and others. N budgets in cropping systems of the Mediterranean biome were calculated by combining spatially-explicit, crop-specific datasets on crop and livestock distribution, yields and N inputs with country-level data from FAOSTAT (see a full description of the procedure in the Supplements).

179 Water-N interactions were analyzed in Spain as a putatively representative Mediterranean country with a strong precipitation gradient with cereal crops under rainfed and irrigated conditions. Data on yearly crop-specific yield and N fertilization rates (synthetic and organic) were provided by the Spanish Ministry of Agriculture for wheat cultivation, split into rainfed and irrigated systems at the province level (40 Spanish provinces with Mediterranean climate) for 26 years (1990-2015). Yield instability during that particular period was calculated separately for rainfed and irrigated conditions at the province level by means of the coefficient of variation (CV) of crop yield in each province. Yearly precipitation data were obtained from the CRU global dataset (Seager et al., 2019). Irrigation water application was estimated for each crop in each province at monthly time steps based on crop evapotranspiration, which was calculated as described in (Vila-Traver et al., 2021). Crop water requirements were calculated by subtracting effective precipitation from crop evapotranspiration. Irrigation water application was calculated by dividing crop water requirement by the water efficiency of each irrigation technology (Wriedt et al., 2009).

193 Regional crop nutrient budgets at the Spanish province level include synthetic and organic N fertilizers, and atmospheric deposition, as inputs (Nin), and N extracted in the main harvest as outputs (NY). Nitrogen use efficiency (NUE) corresponds to NY/Nin and N surplus to Nin -NY (Zhang et al., 2020). Averaged NY vs. Nin response curves were estimated for each province and fitted by the one-parameter hyperbolic curve :

198 $NY = Y_{max} Nin / (Nin+Y_{max})$

199 Where the parameter $Y_{max}$ = (Nin * NY) / (Nin s - NY) represents the maximum yield at N saturation conditions. This approach is commonly used in regional and large-scale approaches (Lassaletta et al., 2016; Mueller et al., 2017; Mogollón et al., 2018).
202 3. Trajectories of N use in the whole agro-food system in the Mediterranean Basin

203 The structure of the agro-food system for the periods 1961–1965 and 2009–2013 in the selected Mediterranean countries is represented in diagrams showing N fluxes through the main compartments. The components involved are crop and grass production, livestock breeding, food transformation, international trade, and human consumption (Fig. 3). In the early 1960s, Mediterranean countries as a whole (as defined in the Methods section) were close to self-sufficiency for feeding their population of 166 million inhabitants, with a diet composed of 3.1 kg N/cap/yr of proteins from vegetal origin, and 1.5 kg N/cap/yr of animal proteins, including 0.2 kg N/cap/yr from fish. Livestock manure, together with symbiotic fixation of atmospheric N₂ and atmospheric deposition of Nr, was the dominant form of N fertilizing input to cropland, even if synthetic fertilizers already contributed significantly at that time. Livestock, dominated by ruminants, thus played a major role by conveying N from permanent and seminatural grassland to cropland soils. Imported feed was of limited significance, and approximately 60% of crop production was dedicated to animal feeding. The remaining crop production was used for local human nutrition together with animal production, such as meat and milk. Net imports of animal and vegetable food products were minimal.

218 During the past 50 years, the population of the Mediterranean Basin countries increased by 80% and changed its diet by increasing total protein ingestion by 30% (from 4.5 to 6.1 kg N/cap/yr), while the share of animal proteins in this total rose from 32 to 43%. This transition has been more evident in European Mediterranean countries and Israel than in the rest of the basin. On the one hand, these changes made it possible that the protein consumption per capita did not fall below WHO recommendations at any time. But these changes also indicate the gradual abandonment of the so-called Mediterranean diet that was replaced by a “Western-type” diet that totally alters the recommended relative proportions of each food group and negatively affects human health (Blas et al., 2019). This dietary change has profoundly impacted the structure of the entire agro-food system, with a considerable intensification of animal breeding: livestock production increased 3.5-fold, mostly in terms of monogastric animals feeding on grains.

229 Despite a concomitant intensification of cropping systems, increasing its use of synthetic N fertilizers by a factor of 3, the production of forage and cereal was not enough to feed the livestock. The rising demand for feed occurred in parallel to a change in the crop mix of the region. The cultivation of cereals was reduced by 16% of the region's total cropland area during the past few decades. Only Morocco significantly increased cereal cropping areas. Legume crop surfaces decreased in European countries but not in south and east Mediterranean countries. On the other
235 hand, cultivation of oilseed with potential allocation to animal consumption has almost doubled,
236 although today its share is still too low to provide a sustained source of protein (5.2% of cropland).

Mediterranean countries, TgN/yr

1961-1966

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

a.

<figure><img src="image_6.png" /></figure>

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238 **Figure 3.** Generalized representation of the Agro-Food System (GRAFS) of countries in the Mediterranean Basin with more than 50% of their agricultural land under Mediterranean climate (11 selected countries, see Methods Section), in 1961–1965 (a) and 2009–2013 (b)

241 Massive imports of feed, namely, soybean and maize from the Americas, were required for livestock production (Lassaletta *et al.*, 2014b; Leip *et al.*, 2015; Billen *et al.*, 2019). Also, 242 considerable net imports of animal food and cereals were needed to feed the human population. 243 As a result, the external dependency of the agro-food system of Mediterranean countries, defined 244 as the ratio of net import of food and feed proteins to the local animal and human ingestion, rose
246 from 6% in 1960 to 30% in 2013. This trend contributes to surpassing the carrying capacity of the region by a factor of 2.5 (Galli *et al.*, 2015).

248 The trends in agro-food system components from both the demand side and the supply side indicate massive temporal change (Fig. S2 Suppl.). The gradual increase of animal and human consumption cannot be met by the increase of cropland yield and the stagnation of grassland production, requiring recourse to increased importation. Thus, the level of external dependency paralleled this gradual increment (Fig. S3 Suppl.).

253 Yield of arable land increased by a factor of two over the past 50 years. This is largely the result of increased N fertilization. The trajectory of N yield versus soil N inputs follows the same distinct hyperbolic relationship with a $Y_{max}$ of approximately 60 kg N/ha/yr until the end of the 1970s (Fig. 4). From this time onward, improvements in yield were obtained with much lower increases in fertilization than in the first decades. Consequently, the surplus, i.e., the excess of total N soil inputs over N in harvested products, increased until the early 1980s and then levelled off until the present period. Crop NUE decreased from 60% in the early 1960s to 44% in the 1980s, and then increased again up to 50%. There are several reasons to explain the changes observed in the aggregated N crop yield and NUE. On the one hand, the expansion of irrigated land (see Fig. 2) and other agronomic improvements such as better fertilization and irrigation techniques, as well as the development of improved crop varieties through breeding and selection, had a significant effect on the observed yield increases. For example, wheat and tomato yields grew between two- and threefold, and two- and fourfold, respectively. On the other hand, over-fertilization may have moved the system to the inefficient part of the yield vs. N input response curve. In addition to these agronomic factors, the aforementioned changes in the crop mix may have had an important effect on the NUE of the system (Zhang *et al.*, 2015). High N input/N output systems such as cereals were replaced by low N input/low N output systems such as permanent crops, systems that have doubled in presence. This is linked to the specialization of Mediterranean agriculture in vegetables, fruits, and olive oil for export to temperate European countries. Counterintuitively, the cultivated area with high N input/low N output systems such as horticulture has been reduced in European Mediterranean countries while their total production has increased, indicating high intensification practices (e.g., greenhouses). By contrast, both the agricultural area and the productivity of horticulture substantially increased in south and east Mediterranean countries.
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Intensification and improvement of cropping practices of countries in the Mediterranean basin over the period 1961–2013: relationship between yield (in terms of protein content of harvested products per ha) and total N soil inputs (a) and corresponding surplus (N soil input in excess over harvested N).</figcaption></figure>

Intensification of animal production, accompanied by an increase in the share of monogastric animals (representing 10% of the livestock units in 1961 and 27% in 2013), resulted in a limited improvement of aggregated protein conversion efficiency from 8% in the 1960s to 13% (at the herd level and considering edible protein only) in recent years. These NUE values aggregate many contrasting types of livestock systems. The most intensified systems have increased N efficiency significantly, gradually reducing the total demand for feed (expressed as dry matter) to produce 1 kg of meat during the past decades and subsequently reducing the N excretion/kg meat during recent years (Lassaletta *et al.*, 2019; MAPA, 2020a, b). These systems strongly depend on imported feed, causing the externalization of N impacts (Bai *et al.*, 2014; Quemada *et al.*, 2020a) and increasing the input of new N embedded in the feed via international trade.

Alongside the intensification of both livestock and crop farming, environmental N losses increased considerably, as shown by the rise in Net Anthropogenic Nitrogen Input (NANI) from 620 to 1900 kg N/km²/yr (Fig S3. Suppl.). While the input of synthetic N fertilizers was stabilized from 1985, the net import of food and feed steadily increased and today accounts for 2 Tg N/yr, i.e., 50% of synthetic fertilizers. This massive and increasing amount of feed largely transformed into manure represents a huge challenge for cropping and livestock systems. As a result of these changes, agricultural surplus and gaseous N emission from livestock breeding have also increased. Clearly, the trend toward losing self-sufficiency of the agro-food system was accompanied by the opening of nutrient cycles. The period from 1960 to the 1980s experienced the fastest increase in environmental losses, mostly from cropland. A stabilization of these losses was observed in the latest period in parallel to NUE increases.

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303 **4. The relevance of the interaction between N and water exemplified in Spanish regions**

304 The crop yield response to water and N availability was stated as a paradigm of co-limitation, the simultaneous limitation of plant productivity by multiple resources. This co-limitation has been well studied in cereal crops of Mediterranean-type environments in Australia and Europe (Cossani and Sadras, 2018). Prolonged water stress leads to yield reduction and decreased N uptake as well as NUE, whereas insufficient N may limit water response and water use efficiency (Savin et al., 2019). The implication of co-limitation is that strategies aiming to maximize crop response to both resources should ensure that water and N are equally available (Quemada and Gabriel, 2016; Hochman and Horan, 2018). Also, water is one of the main drivers of N losses at a cropping systems level, as excessive water input leads to leaching losses and favors denitrification, thereby enhancing N₂O emissions (Section 6). As a consequence, proper management of the interaction between NUE and water use efficiency (WUE) is required in many cropping systems in order to reach a compromise between productivity and environmental sustainability.

316 Water availability is thus a limiting factor in Mediterranean areas that determines the pattern of land use (López-Bellido, 1992). High solar radiation and extended frost-free periods make these regions capable of attaining high crop yields. When water is not available, agricultural systems are either adaptations of woody crops to xeric conditions (i.e., olive orchards, vineyards, oak *dehesas*) or low-productivity rainfed arable crops (i.e., barley, wheat, sunflower, leguminous grain) subject to high yield variability. N application in these rainfed systems tends to be opportunistic depending on the weather forecast, and the NUE attained is determined by the adjustment of fertilization to the actual crop growth limited by rainfall (Sadras et al., 2003). Water application to compensate for the frequent spring and summer droughts contributes not only to stabilized productivity but also to crop diversification, providing the opportunity for more complex rotations including highly profitable crops (i.e., horticultural) usually characterized by a demand for large amounts of inputs including N. However, this increase in productivity is not always associated with an enhancement of NUE because crops are frequently over-fertilized or over-watered in order to achieve high yields (Quemada et al., 2013; Salazar et al., 2019), thereby generating a high risk of N leaching (Section 6). Therefore, increasing WUE and NUE in the Mediterranean is challenging and requires the development of specific practices adapted to rainfed and irrigated systems.

332 The analysis of the N budgets for wheat cultivation under rainfed and irrigated conditions at the province scale in Spain clearly illustrates the agro-environmental implications of N-water co-limitation. This analysis includes information on N input and wheat yields for 40 Spanish provinces (administrative units ranging 5000-20,000 km²) over 26 years (1990-2015). These provinces have
336 Mediterranean climate but different levels of annual precipitation (province range 341-869 mm/yr), temperature (province annual mean temperature range 11-18 °C) and latitude (province range 36.5-42.8º latitude N). Average yield per province ranged from 0.9-2.5 t/ha/yr for rainfed systems and 2.6-5.5 t/ha/yr for irrigated systems.

340 Yield instability was significantly lower in irrigated than in rainfed systems in all of the provinces (Fig. 5a). Thus, irrigation consistently homogenized yields across Mediterranean regions. We found a strong relationship between mean total water inputs (precipitation and irrigation) and mean yields in rainfed and irrigated systems ($R^2$ = 0.79), explaining the strong effect of water on yields (Fig. 5b). In response to the higher yields expected in irrigated systems, N fertilization rates are significantly higher (Fig. 5c). Water-N interactions in Mediterranean rainfed systems were revealed through the robust relationship ($R^2$ = 0.46) between precipitation and $Y_{max}$ in rainfed systems (Fig. 5d), new water inputs through precipitation increase $Y_{max}$ and therefore the capacity of the crop to respond to new N additions. In irrigated systems the relationship is lost; there is no water limitation and other factors affect crop response to N additions.

<figure><img src="image_8.png" /><figcaption>**Figure 5** Yield instability of wheat in rainfed and irrigated systems for the 40 Spanish provinces under Mediterranean conditions (1990-2015) (a); relationship between mean water input (precipitation + irrigation) and mean yield for each of the 40 provinces for rainfed and irrigated systems (b); mean N fertilizer input (1990-2015) of the 40 provinces for rainfed and irrigated systems (c); and relationship between total water input and $Y_{max}$ (the parameter of the hyperbolic response curve that relates yield and N inputs representing maximum yield at saturating N rates), $R^2$= 0.46 for rainfed systems, and no significant (n.s.) relationship for irrigated crops (d).</figcaption></figure>
This relationship has important implications for agro-environmental management of fertilization in rainfed systems. In regions with water scarcity, crop N uptake is rapidly saturated and N surpluses accompanied by pollution (e.g., leaching during rainfall events) could occur at low fertilization rates (e.g., Albacete province, Fig. 6). However, in areas where water availability is higher, together with other drivers such as less heat that is common in more northern latitudes, rainfed systems had higher $Y_{max}$ values that were closer to those from irrigated systems. Here, the same fertilization rates may lead to higher yield responses (e.g., Burgos province, Fig. 6). Even in these wetter regions, a dry year can significantly reduce $Y_{max}$, increasing N surplus and therefore the risk for N leaching.

<figure><img src="image_9.png" /><figcaption>**Figure 6** Yield vs. total N inputs (1990–2015) in four contrasting Spanish provinces. The curve is the mean yield fertilization response curve estimated by using the average input and yield during the study period (1990–2015) for rainfed (red) and irrigated (blue) systems. Annual mean precipitation: Albacete (388 mm/yr), Toledo (426 mm/yr), Valladolid (465 mm/yr), Burgos (688 mm/yr).</figcaption></figure>

In summary, for rainfed systems, farmers have the difficult task of adjusting fertilization in order to maximize yields while reducing N surpluses in a highly variable environment. Mean fertilization rates per province ranged from 42 to 150 kg N/ha/yr in rainfed systems and from 77 to 186
375 kg N/ha/yr in irrigated systems. In general, actual fertilization rates (estimated as the base for the national inventories of greenhouse gas emissions) are lower in the rainfed systems of those provinces with lower precipitation and faster N saturation (Fig. 7), and, with some exceptions, moderate N surplus (usually lower than 50 kg N/ha/yr). Poor planning with high fertilization rates in a very dry year entails a disproportionately low NUE and significantly high emissions of N to the environment. As described in Section 3, the increasing livestock production is generating a growing amount of manure that has to be properly managed and recirculated to crops. This manure application has to consider the distinct response of cropping systems to N applications. For example, the EU Nitrates Directive limits the application of N in manure to 170 kg N/ha/yr for areas considered as Nitrate Vulnerable Zones. These rates, albeit legal, would strongly reduce NUE in the drier regions, severely impairing the environment (Fig. 7).

<figure><img src="image_10.png" /><figcaption>**Figure 7.** Yield vs. fertilization response curves, NUE and surplus under rainfed and irrigated conditions in several Spanish provinces in a precipitation gradient (period 1990-2015). Water input (mm/yr) is the mean annual water input including precipitation and irrigation. Averaged fertilization rates and yields (1990-2015 averages) are provided by the Spanish Ministry of Agriculture as well as the result of a hypothetical fertilization rate of 170 kg N/ha/yr that is the limit of application of N in manure at the Nitrate Vulnerable Zones.</figcaption></figure>

## 5. Interactions of N use with adaptation to climate change

As a summary of the climate change conditions in the Mediterranean described in Section 1, increases in maximum and minimum temperatures and in atmospheric CO₂ concentration are expected to be associated with decreased and more unpredictable precipitation. These changes combined will result in stronger fluctuations in water stress for crops (Fig. 4 Suppl.). Besides these climatic effects, “elevated atmospheric CO₂ concentration” (hereafter eCO₂) also affects plant N
400 dynamics. Therefore, under climate change scenarios with eCO₂, N dynamics with water and temperature interactions have to be comprehensively studied. These effects are in general less understood; however, in the past few decades, Free Air CO₂ Enrichment (FACE) experiments (O'Leary et al., 2015) and other laboratory or modeling studies indicate that CO₂ might enhance photosynthesis in C₃ plant species such as wheat, rice, and barley (Ainsworth and Rogers, 2007), especially under dry conditions (Bishop et al., 2014), as well as N fixation by legumes (Parvin et al., 2018). This might increase total N crop requirements to build more biomass (Ainsworth and Rogers, 2007). Under elevated CO₂ conditions, transpiration benefits from reduced stomata opening to capture the same amount of CO₂. Therefore, low stomatal conductance, more conservative water use, and higher transpiration efficiency may jointly increase WUE as well as biomass and total water use (Houshmandfar et al., 2018). On the negative side, a lower N content in wheat grain has been reported under high CO₂ conditions (Tausz et al., 2017). For legumes, however, reduced N content in grain following a heatwave might be restored with eCO₂ (Parvin et al., 2020).

414 Adaptation to climate change in Mediterranean cropping systems needs to consider these interactions and how they are modified by heat and water stress, as both of these adverse conditions for crop growth are projected to occur more frequently in the future (Trnka et al., 2014). Briefly, positive (crop growth stimulation and water saving) and negative (grain N dilution) eCO₂ effects alike may be diminished under stressful conditions (Ainsworth et al., 2008; van der Kooi et al., 2016). Also, stresses limit the crop response to N, as some part of the fertilizer cannot be used while crop growth is water- or temperature-limited (Savin et al., 2019). This implies low NUE and high N surplus and greenhouse gas emissions (Quemada and Gabriel, 2016) (Section 4). Under these conditions, adaptation of annual crops in Mediterranean environments usually implies using varieties with high transpiration efficiency traits and early sowing dates (Porter et al., 2014). The aim of such strategies is to increase water accumulation over the crop cycle and to restore the duration of the vegetation period shortened by warming, which accelerates crop development. The target then is to identify the optimum combination of crop phenology and sowing date, selecting the earliest possible sowing date ensuring that: (1) there is enough soil water for successful germination, emergence, and canopy establishment, (2) the crop will flower after the last spring frosts and before late spring-early summer heatwaves, and (3) most of grain filling happens before the dry summer period (Kirkegaard and Hunt, 2010; Christy et al., 2018; Ruiz-Ramos et al., 2018). These are already the goals of current strategies recommended in Mediterranean systems, together with management for conserving soil moisture and reducing soil and canopy temperature. For climate change adaptation, the main novelty is that the suitable
434 sowing window could become narrower in the future, as may the range of suitable varieties in
terms of phenology, potential yield, and resistance to stresses. Therefore, in these systems the
fulfilment of mitigation targets to reduce Nr emissions within the limits of effective adaptation
will require the combination of new or existing tailored varieties, optimized management for soil
and water conservation, and improved efficiencies, supported by knowledge and technological
solutions (Springmann et al., 2018).

440 Farm-level management options such as agroforestry are currently studied in detail for their potential to improve the sustainability of Mediterranean farming system, especially under climate change (Arenas-Corraliza et al., 2016; Mosquera-Losada et al., 2018). Among others, they offer access to deeper soil water tables (Cardinael et al., 2015) and they reduce evaporation due to the wind-break effect of trees (Campi et al., 2012), thereby enhancing groundwater recharge (Kay et al., 2019), which are important issues regarding the climate projections for the Mediterranean. While it has been shown that agroforestry can increase soil fertility and nutrient availability in Europe (mostly Mediterranean cases) and North Africa (Torralba et al., 2016; Zayani et al., 2020), and that tree-legume interactions maintain a sufficient level of N fixation under the shade of trees in a Mediterranean agroforestry system (Querné et al., 2017), the implications of agroforestry for nutrient acquisition under water and heat stress are still poorly understood (Guillot et al., 2019). As stated by (Isaac and Borden, 2019), extensive research is needed on how agroforestry practices stabilize key nutrient acquisition patterns in the face of environmental change.

453 Conservation agriculture (CA) – with no (or reduced) tillage, permanent organic soil cover, and
crop diversification – is another innovative agricultural system that has been suggested for climate
mitigation and adaptation in the Mediterranean due to its soil and water conservation capability
(Kassam et al., 2012). The physical, chemical, and biological properties of soil are improved,
resulting in higher yields for dry conditions due to improved soil moisture availability (Mrabet,
2002). Although authors logically hypothesize on improved nutrient availability, studies indicate
that extractable N is not significantly greater under CA (López-Garrido et al., 2011).
Measurements at all soil depths are needed to better understand the effect of CA on nutrient
distribution and its potential contribution to sustainable Mediterranean agriculture (Mrabet et
al., 2012). Moreover, in an Italian dry environment, the limited amount of wheat crop residues
during the transition phase to CA required high N fertilization to maintain yields (Galieni et al.,
2016).

465

466

467
468 6. Emission of Nr compounds: drivers and adapted solutions

469 6.1 Ammonia

470 Similar to temperate agroecosystems, the main pathways of Nr loss from fertilized crops under Mediterranean conditions are NH₃ volatilization to the atmosphere and NO₃⁻ leaching to hydrosystems (Jarvis, 2011; Quemada et al., 2013; Sanz-Cobena et al., 2014). Ammonia (NH₃) volatilization in cropping systems mainly occurs following NH₃ production through mineralization or application of NH₄⁺-containing fertilizers, urea- or manure- and other organic-based N fertilizers. The process of NH₃ volatilization is physicochemical (Walker et al., 2013). Meteorological conditions (air temperature, wind speed near the soil surface, atmospheric stability, and rainfall), soil properties (e.g., texture, cation exchange capacity, and pH), and N management options are the main drivers involved (Bouwman et al., 1997). Most of these factors are vastly determined by Mediterranean unique pedoclimatic conditions and management practices, the latter being shaped by the dominant types of crop and livestock systems present in these areas (Sanz-Cobena, 2010).

482 Atmospheric conditions greatly influence NH₃ volatilization rates (Cape et al., 2009). Typical stable atmospheric conditions with calm periods of wind (commonly associated with high pressures) under Mediterranean conditions may lead to low volatilization rates following fertilization, mainly in winter and during the night (Theobald et al., 2015). Lower emission factors (EF: the percentage of applied N that is lost as NH₃) are expected for winter crops as compared with irrigated summer crops, even if similar agricultural practices are used. The rainfall regime is also critical for the enhancement (or mitigation) of NH₃ losses. Sanz-Cobena et al. (2011) showed that more than 7-14 mm water application immediately after fertilizer application is an effective strategy for abating NH₃ volatilization due to enhanced solubilization and incorporation of the fertilizer into the upper soil. By contrast, under low rainfall (i.e., 3 mm) conditions, NH₃ volatilization increases because high concentrations of soluble N forms might be kept at the soil surface and hydrolysis rates triggered following urea application (Sanz-Cobena et al., 2011). Based on this, applying water immediately after surface fertilization would be an efficient NH₃ abatement strategy in irrigated Mediterranean agroecosystems. In rainfed crops, as the Mediterranean climate is characterized by a limited number of rainfall events, the possibility of NH₃ mitigation due to rainfall requires a good weather forecast.

498 Regarding fertilization practices, in Mediterranean cropping systems, even under conditions of high atmospheric stability, the risk of NH₃ volatilization is greater if urea or liquid manure (i.e., slurries) is surface-applied (Marchetti et al., 2004; Sanz et al., 2010; Abalos et al., 2012; Yagüe and
501 Bosch-Serra, 2013; Bosch-Serra *et al.*, 2014; Recio *et al.*, 2018; Sanz-Cobena *et al.*, 2019). The use of urea as a synthetic N fertilizer has increased in the past decades in Mediterranean cropping systems, as in the rest of the world (IFASTAT, 2020). However, the large NH₃ losses associated with its application, triggered by the hydrolysis-mediated increase in soil pH, necessitate the implementation of specific NH₃ abatement for this fertilizer (Bittman *et al.*, 2014). The most common abatement practices are the incorporation of urea immediately following application, mechanically or by means of water (Sanz-Cobena *et al.*, 2011; Rochette *et al.*, 2013), or the use of urease inhibitors (Sanz-Cobena *et al.*, 2014; Recio *et al.*, 2020). These synthesized compounds reduce the rate of urea hydrolysis thus allowing for its incorporation into the soil (Sanz-Cobena *et al.*, 2008; Turner *et al.*, 2010; Sanz-Cobena *et al.*, 2017; Fu *et al.*, 2020). Pollution swapping (such as N leaching) might also occur if these strategies are improperly managed (Sanz-Cobena *et al.*, 2017). On the contrary, due to the predominant nitrifying conditions in rainfed systems under Mediterranean conditions, the decreased NH₄⁺ pool induced by urease inhibitors can have a co-benefit due to mitigated N₂O emissions from this microbiological process (Sanz-Cobena *et al.*, 2016). The incorporation of urea by water in irrigated systems is facilitated (Sanz-Cobena *et al.*, 2011), thus avoiding excessive soil compaction as well as greenhouse gas emissions from machinery. Increasing generation of animal manures in certain Mediterranean areas (e.g. Italy and Spain) has triggered ammonia emissions due to surface application of slurries. Although less effective than injection (Sanz-Cobena *et al.*, 2019), the possibility of adapting existing on-farm equipment to mix the slurry with the soil makes this option technically suitable and cost-effective (Bittman *et al.*, 2014).

522 6.2 Nitrous oxide

523 Studies on N₂O emissions in Mediterranean cropping systems are underrepresented compared to other temperate areas. (Aguilera *et al.*, 2013b), and studies deciphering N₂O formation pathways are conspicuously low. The few published studies to date point at nitrification and nitrification-denitrification as particularly relevant mechanisms in Mediterranean soils (Zhu *et al.*, 2013). The prevalence of distinct formation pathways in Mediterranean soils, driven by the specific climatic conditions and agronomic practices, will ultimately determine the best N₂O mitigation strategies.

529 There is ample evidence that N₂O emission factors (EF: the percentage of applied N that is lost as N₂O) in Mediterranean rainfed cropping systems (EF: 0.27% according to Cayuela *et al.*, 2017) are significantly lower than in other temperate areas (0.9% according to Stehfest and Bouwman 2006). However, as irrigation is a common practice in Mediterranean cropping systems in summer, it changes the moisture pattern and substantially increases the total amount of water in
534 soil. As a consequence, EFs in irrigated crops (0.63%) are significantly higher than in rainfed crops (0.27%) (Aguilera et al., 2013b; Cayuela et al., 2017).

536 In both rainfed and irrigated systems, the adjustment of N application to crop needs is one of the best recommended strategies to mitigate N₂O emission (Guardia et al., 2018). Additionally, the use of nitrification inhibitors with both synthetic and organic N fertilizers (Sanz-Cobena et al., 2012; Abalos et al., 2014; Gilsanz et al., 2016; Guardia et al., 2017b) is also an effective mitigation strategy.

541 The type of irrigation directly determines N₂O emissions (Cayuela et al., 2017). Localized irrigation systems such as drip irrigation significantly abate N₂O losses due to a highly localized moist area. On the contrary, sprinkler-irrigated fertilized systems are an important source of N₂O emissions due to the increased soil moisture in large surface areas (Guardia et al., 2017b). Nevertheless, recent studies show how optimized sprinkler irrigation systems can considerably reduce N₂O emissions in intensive maize crops (Franco-Luesma et al., 2019; Franco-Luesma et al., 2020).

547 Precipitation following long drought periods promotes high N₂O emissions. This is very common in Mediterranean soils in summer or following the first rainfall in autumn (Cárdenas et al., 1993). A recent meta-analysis demonstrates that flooding the soil after extended periods of drought produces a larger pulse in N₂O emissions, compared with soils that were already moderately wet before the flood event (Barrat et al., 2020). These pulses are likely more common under Mediterranean conditions than in temperate agroecosystems where pulses are usually due to freeze-thaw events (Sanchez-Martin et al., 2010).

554 Another important factor modulating N₂O emissions in Mediterranean cropping systems is soil pH. Mediterranean soils are predominantly neutral or alkaline (Slessarev et al., 2016) and this has important implications for the final ratio of denitrification products. (Cayuela et al., 2013) found an average N₂O/(N₂+ N₂O) ratio of 0.4 in five Mediterranean high-pH soils, whereas the average value for other temperate soils was 0.8. This implies that the last step of denitrification (the transformation of N₂O to N₂) was enhanced in Mediterranean soils. A recent global meta-analysis of 1104 field measurements found soil pH to be the most relevant parameter defining regional N₂O emissions, with significantly lower EFs for soils with increasing pH (Wang et al., 2018).

562 6.3 Nitric oxide

563 Together with N₂O, nitric oxide (NO) is a by-product of nitrification (Medinets et al., 2015), thus particularly common in Mediterranean rainfed systems (Vallejo et al., 2005; Meijide et al., 2007;
565 Meijide et al., 2009; Sanchez-Martín et al., 2010; Sanz-Cobena et al., 2012; Guardia et al., 2017a; Guardia et al., 2017b; Recio et al., 2019). Significant emissions of NO, a precursor of tropospheric ozone ($O_3$), have been reported after N fertilization with either organic or synthetic N fertilizers and in both rainfed and irrigated systems, particularly if drip-irrigation is performed (Guardia et al., 2017a). These authors associated increased NO fluxes with enhanced nitrification in the proximity of the wet front that surrounds the irrigation bulb. There is evidence that $NO_x$ (both NO and $NO_2$) emission exponentially increases with temperature and that, after soil rewetting, strong pulses of $NO_x$ emission are produced in high-temperature regions (Garrido et al., 2002). In the Mediterranean region, as mentioned earlier, the pulses that occur after rewetting result in high contributions to the total NO budget. Analyzing the different agroecosystems, irrigated crops are also important contributors of NO, because the importance of nitrification and the temporal pattern of soil moisture occurred when the soil temperature is high, with frequent irrigation-drying cycles. Abating soil $NO_x$ emissions should be considered in future efforts to improve regional air quality in the surrounding of irrigated crops, because these emissions contribute to the high concentration of $O_3$ found in many rural areas in the Mediterranean region (Calvete-Sogo et al., 2014).

581 6.4 Nitrate and losses to the hydrosphere

582 In Mediterranean irrigated agriculture, nitrate leaching is strongly dependent on water application. Intensive irrigation together with over-fertilization is the main cause of nitrate leaching (Isidoro et al., 2006; Cavero et al., 2012; Laspidou and Samantzi, 2014; Lasagna and De Luca, 2019). Excessive water is often applied either to avoid salt accumulation in soil or to compensate for the heterogeneity of the water-delivery system, enhancing nitrate leaching during crop growth (Díez et al., 2000). Low crop N availability is then compensated by increasing fertilizer rates leading to a vicious circle characterized by low WUE and NUE. Therefore, the two main drivers of nitrate leaching are water percolation and soil inorganic N accumulation, and strategies to mitigate losses are oriented toward keeping these processes under sustainable thresholds.

592 Strategies to improve water management rely on adapting water application to crop requirements and are mainly based on improving irrigation schedules or technologies. Irrigation should be scheduled to avoid over-irrigation and limit water percolation (Zinkernagel et al., 2020).
593 Additionally, the water delivery technology may have a major impact on the main N processes governing N losses. Drip and sprinkle irrigation have the potential to localize water in the root zone and limit percolation and nitrate leaching (Causape et al., 2006; Vazquez et al., 2006; Cavero et al., 2017b; Recio et al., 2019).
598 et al., 2012; Garcia-Garizabal et al., 2014; Malik et al., 2019), whereas with flood irrigation it is
difficult to achieve a high water efficiency and control leaching losses (Garcia-Garizabal et al.,
599
600 2012).

601 Adjusting N fertilization, both synthetic and organic, to crop requirements remains crucial in order to avoid soil N accumulation and mitigate nitrate leaching (Quemada et al., 2013). Intensive livestock associated with irrigated land is common in Mediterranean areas and avoiding excessive application of manure to cropland is highly recommended (Mantovi et al., 2006; Nikolaidis et al., 2009; Yague and Quilez, 2010). In the Po valley, establishing a N balance to minimize N surplus was identified as the first step toward mitigating the effect of high livestock concentration that leads to poor water quality (Perego et al., 2012).

608 Replacing the traditional fall-winter fallow by cover crops in irrigated areas reduced nitrate leaching by 35% on average in a meta-analysis of irrigated land (Quemada et al., 2013). Compare to fallow soils, cover crops minimize the risk of N leaching by reducing water percolation due to an increase in evaporation, assimilating the soil residual N after harvesting the summer crop thus reducing nitrate concentration in the leachate, and enhancing microbial N immobilization associated to C inputs from cover crops roots (Gabriel et al., 2012; Benoit et al., 2016; Thapa et al., 2018). Replacing the fallow with a non-legume is more efficient than with a legume in reducing leaching, and mixes of grass/legume greatly reduce leaching with respect to the fallow (Tosti et al., 2014). Additionally, the combination of N recycling and the boost of soil N mineralization in the short and long term after cover crops may allow to reduce fertilization rates without affecting yields. Indeed, the increase in N₂O emissions caused by legume cover crops could be offset by the N fertilizer savings and its associated indirect CO₂ emissions (Guardia et al., 2019; Quemada et al., 2020b). In general, Mediterranean irrigated soils are prone to high mineralization rates, and strategies that enhance soil stable organic compounds, allow for the recovery of soil physical properties and fertility in the long term and contribute toward mitigating nitrate leaching losses (Diacono and Montemurro, 2010; García-González et al., 2018).

624 An additional pathway of N losses that is influenced by practices such as irrigation and cover cropping is leaching of dissolved organic N (DON). Soil water content regulates the ratio of DON and inorganic N forms in Mediterranean soils, being DON dominant under various plant communities during wet periods (Delgado-Baquerizo et al., 2011). In an irrigated maize field of central Chile, DON leaching under a grass cover crop was twice the rate of nitrate leaching, even if the total N losses were reduced compared with the fallow (Salazar et al., 2019). Although the full ecological significance of DON loss remains unclear, it may represent a large part of the total
631 dissolved N, and monitoring DON leaching may contribute toward mitigating diffuse pollution
632 from agricultural systems.

633 Losses to hydrosystems from rainfed agriculture in the Mediterranean basin are mainly associated
634 with dissolved N in runoff or attached to soil-eroded particles (Section 7). In rainfed cropland,
635 nitrate leaching showed to be relevant after intense rainfall events or wet periods that are
636 common in semi-arid (Ruiz-Ramos et al., 2011) or humid (Arregui and Quemada, 2006)
637 Mediterranean areas. High winter nitrate flows from upland cereals are very common when
638 winter fertilization is applied (Lassaletta et al., 2009, 2010). Finally, transformation of rainfed into
639 irrigated fields may greatly enhance nitrate leaching and become a major risk for water pollution
640 if it concerns large areas (Merchan et al., 2015). During the transition period, N mineralization
641 may release a large amount of nitrate until the soil organic matter content reaches a new
642 equilibrium (Quemada and Gabriel, 2016). Promoting temporary soil mining by minimizing N
643 fertilizer application may help mitigate the risk of nitrate pollution (Vazquez et al., 2006). The
644 main pathways of environmental N losses from rainfed and irrigated cropland are summarized in
645 Figure 8.
<figure><img src="image_11.png" /><figcaption>**Figure 8.** Schematic representation of the main pathways of reactive N losses in Mediterranean agroecosystems, both rainfed (a, c) and irrigated (b, d) under broadcast N fertilization. The size of the arrow is proportional to the net amount of N being lost to both the atmosphere and water bodies. Bottom figure (c, d) shows the impact of the most recognized Nr abatement strategies in the area focusing on N fertilizer management, irrigation (e.g. drop or well-planned sprinkler irrigation), and cover cropping. Red arrows indicate the potential pathways of pollution swapping. * Similar to temperate areas, high nitrate leaching can occur in upland cereal croplands during intensive precipitation in winter. Our own elaboration based on the scientific literature referred to in the text.</figcaption></figure>

## 7. From the croplands to the aquatic continuum

### 7.1 Soil erosion and N flows

Mediterranean soils are particularly vulnerable to water erosion, due to frequent intense rainfall events (Cortesi et al., 2012; Merheb et al., 2016), the typical steep slopes (Tarolli et al., 2012) and the climatic conditions that make Mediterranean regions prone to wildfire, often associated with
661 soil erosion (Shakesby, 2011). While annual precipitation in many Mediterranean regions is expected to decrease under climate change (Cramer et al., 2018; Brogli et al., 2019), several studies highlight an expected increase in extreme precipitation events (Hagos et al., 2016; Polade et al., 2017; Tramblay and Somot, 2018) which could lead to increased soil erosion (Morán-Ordóñez et al., 2020).

666 It has been estimated that globally 23-42 Tg N is moved by soil erosion each year, which is in the same order of magnitude as N applied as chemical fertilizer and N removed by harvested crops (Quinton et al., 2010). In spite of this, the transport of N has been largely neglected in soil erosion studies, which mostly focus on the impact of erosion on the redistribution of soil organic matter (SOM) and associated phosphorus (Garnier et al., 2015). N is a structural component of SOM (Tipping et al., 2016), hence, the transport of SOM after extreme rainfall events affects soil N stocks. The redistribution of SOM is associated with the dynamics of sediment transport and deposition (Boix-Fayos et al., 2015). As such, SOM may be deposited in common depositional landscape units, including river bars, floodplains, and reservoirs (Fissore et al., 2017) (see also Section 7.2). The impact of soil erosion on SOM dynamics is still open to debate. Eroded SOM may be subject to mineralization during transport, potentially leading to increased SOM decomposition (Lal, 2004), depending on the travel time and distance (Quinton et al., 2010). On the contrary, the deep burial of SOM-rich sediments may lead to reduced SOM decomposition rates (Berhe et al., 2007). Due to heterogeneous land use configurations, the transport of sediment and SOM in Mediterranean catchments is highly complex (Boix-Fayos et al., 2015), making it more difficult to assess how SOM, and consequently N, is redistributed in Mediterranean environments.

683 7.2 N retention in Mediterranean river basins

684 Nitrogen losses from agricultural land reach water bodies by surface runoff and leaching to groundwater, as well as by soil erosion and atmospheric deposition. Once in the aquatic system, N undergoes different processes of transport, storage, transformation and elimination to gaseous forms, and part of it is finally exported to coastal areas.

688 Mediterranean climatic features, including high temperatures and low rainfall, often concentrated in a few large precipitation events, favor N retention in the watersheds -i.e. considering retention as the sum of storage, transformation and elimination processes-. High temperatures are accompanied by high evapotranspiration rates, so runoff mainly occurs after intense events. Moreover, the irregular precipitation patterns and the recurrence of summer
693 droughts have driven intensive hydraulic management to secure water availability (Castaldelli et al., 2013; Soana et al., 2017; Soana et al., 2019; Romero et al., 2021), and most rivers comprise a large number of dams, reservoirs and irrigation channels (Sadaoui et al., 2018). Reservoirs and channels increase land-water contact surface and residence time in the watersheds, and allows for enhanced nutrient processing via denitrification, uptake by riparian vegetation and freshwater macrophytes, infiltration and leaching to groundwater (Bartoli et al., 2012; Lassaletta et al., 2012; Romero et al., 2016). Besides, reservoirs are built to contend with water scarcity and buffer great episodes of rain and flooding, which means that they also decrease the transport of N to coastal areas during flashy rainfall events.

702 All these waterwork developments contribute to increasing N retention in the river basin with potentially higher pressure on inland waters (including groundwater) and less export to coastal systems. Higher retention percentages in the Mediterranean with respect to other climatic conditions –and especially in arid and semiarid Mediterranean basins– have been reported in several recent studies that include different geographical scales (the Iberian Peninsula, European Mediterranean basins, and the entire Mediterranean basin) and different modeling approaches (Romero et al., 2016; Grizzetti et al., 2021; Romero et al., 2021).

709 Despite high retention and hence high elimination rates, N concentration in water bodies might be increased in regions affected by water scarcity due to the low dilution capacity by water flow, as has been observed in southern Europe, mainly in Spain, Italy and Greece (Grizzetti et al., 2017).
712 In addition, as N retention includes permanent and temporal components, high values of retention might hide an accumulation of N in soils and groundwater in the river basin, and a delay between sources and impacts in aquatic ecosystems (Grizzetti et al., 2015).

715

## 8. Lessons learned and policy dimensions

717 The climatic and agronomic specificities of the Mediterranean region worldwide demand N management techniques specifically suited for this region. Agronomic practices (rainfed vs. irrigated crops; annual crops vs. permanent crops) or soil characteristics (acidic vs. calcareous; soil carbon content) could limit the beneficial effects of sustainable practices successfully developed and implemented in other regions, and even prevent their implementation. In any case, they should be adapted considering the climate change context and potential adverse side effects, such as pollution swapping. The definition of the most suited management practices at the local level
724 should be defined under a co-creation process considering the end-users' needs and constraints
725 with the support of researchers and the various implicated agents.

The systemic approach considering the overall N budget within the agro-food system in the Mediterranean Basin has highlighted those processes demanding urgent action. Anyhow, accounting, surveillance, and control of the different elements of the agro-food system need to be implemented. Although measures taken at the farm level eventually impact the overall system, multilateral, subregional, and national actions are needed to control N surpluses and their environmental impacts. This study has identified important differences in the agro-food system across the Mediterranean Basin, mostly related to climatic factors (coastal vs. in-land areas; subarid vs. arid areas). The study of these systems at the national and province level has identified where governmental actions via incentives or regulations are needed, considering the geopolitical angle in the degree of intensification or import-export of feed and food.

In the Mediterranean biome, management of water and N should be jointly appraised developing N fertilization schemes tailored not only to the potential N crop demand but also to water availability (quantity) and quality, the soil chemical and physical characteristics, as well as the fertilizer resources locally available. The case of the agro-food system transformation observed in the countries of the Mediterranean Basin is paradigmatic. The increasing manure production and the associated crop-livestock disconnection are critical in other regions of the world (Garnier et al., 2016; Jin et al., 2020; Mueller and Lassaletta, 2020), but they are of particular concern in the Mediterranean Basin. The best use of the available local manure has to consider the much lower response of rainfed systems and the high leaching potential of irrigated systems. Thus, the consideration not only of the two contrasting worlds –namely, rainfed and irrigated systems – but also of the precipitation gradient to rainfed systems is important. Analysis of potential maximum yields and yield vs. fertilization relationships, adapted to local conditions, should be made to set sustainable N application rates.

In conclusion, bottom-up and top-down approaches should go hand in hand, combining (1) local assessments of the most suited farming systems that jointly improve N and water use efficiency considering the crop specificities, water availability for irrigation, and the edapho-climatic conditions up-scaled to national and continental level, and (2) a systemic analysis of N inputs into the systems fueled by food demand. Indicators for monitoring should be derived in order to suggest policies and governance systems tailored to the local specificities. In addition, structural measures promoting the reconnection of crop and livestock farming, including the relocation of feed production, the reduction of food waste, and the consideration of the key effect of human consumption patterns (diet, reconnection of production and consumption in time and space) have
758 to be considered (Billen et al., 2019). This integrated vision is aligned with the spirit of the recent
759 Farm to Fork Strategy (and Biodiversity Strategy) of the European Union that aims to halve N
760 pollution in agricultural systems and also considers recommendations at the consumption level.

761 Markets are not currently considering N-related environmental externalities in the region, which
762 also have socio-economic implications. However, the lack of proper connection of our scientific
763 findings with socio-economic indicators prevents defining relevant guidelines integrating
764 agronomic, environmental and social aspects of the Mediterranean farming systems.
765 Nevertheless, farmers' adoption of locally suited best sustainable practices could very likely result
766 in yield losses compared to "business as usual" practices, demanding the implementation of
767 compensating schemes to account for the environmental services they provide.

768 As a whole, the implementation of these practices also demands modifications in rural planning,
769 considering the local biophysical, climatic, and socioeconomic conditions but also the
770 heterogeneity of different land uses at the landscape and catchment levels. The integration of
771 sustainable practices at the regional scale may help address coexisting environmental challenges
772 such as land degradation, biodiversity loss, food security, climate change mitigation and
773 adaptation, and water scarcity (Ruiz et al., 2020). These features should be taken into account
774 when developing future policies by integrating top-down system analysis with bottom-up
775 initiatives aiming to identify the most urgent problems and feasible solutions, based on the
776 quantification of ecosystem services as well as sustainable economic issues and the well-being of
777 farmers (van Leeuwen et al., 2019).

778

779 Acknowledgments

780 This work was partly supported by 7th Framework Programmes funded by the European Union
781 through the LUC4C project (grant agreement 603542). A. Bondeau and W. Cramer acknowledge
782 Labex OT-Med (ANR-11-LABX-0061) funded by the French Government Investissements d'Avenir
783 program of the French National Research Agency (ANR) through the A*MIDEX project (ANR-11-
784 IDEX-0001-02). We are grateful to the Spanish Ministry of Economy and Competitiveness
785 (AgroSceNA-UP, PID2019-107972RB-I00), the Comunidad de Madrid, Spain (AGRISOST-CM
786 S2018/BAA-4330 project). L. Lassaletta is supported by Spanish Ministry of Economy and
787 Competitiveness (MINECO) and European Commission ERDF Ramón y Cajal grant
788 (RYC-2016-20269), Programa Propio from UPM, and acknowledges the Comunidad de Madrid
789 (Spain) and structural funds 2014-2020 (ERDF and ESF). E. Aguilera is supported by a Juan de la
790 Cierva research contract from the Spanish Ministry of Economy and Competitivenes (FJCI-2017-
791 34077). A. Sanz-Cobena thanks to the Autonomous Community of Madrid and UPM for their
792 economic support through the research project APOYO-JOVENES-NFW8ZQ-42-XE8B5K. We are
793 very grateful to the two anonymous reviewers for their constructive and useful suggestions.
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