Grassland Management and Diversity Effects on Soil Nitrogen Dynamics and Losses

Grasslands play an important role in land use, covering one third of the agriculturally utilized landscape in Europe. As a consequence of management intensification in the last 60 years, the productivity of grasslands increased and the diversity of these systems decreased. Nitrogen (N) plays a key role in these ecosystems – it limits primary productivity and affects biodiversity. Increasing N supply may alter nitrous oxide (N2O) and nitric oxide (NO) emissions, which play a major role in atmospheric chemistry and contribute to global warming. Nitrate (NO3-) leaching from soils can be a threat for groundwater quality. N losses of N2O and NO emissions and NO3- leaching from soils result from microbial processes denitrification and nitrification. In this study, we analyzed the effect of different management intensities and plant functional diversity on N losses and ecosystem functioning. The study was carried out as part of the Cluster of Excellence “Functional Biodiversity Research” at the Georg-August-University of Gottingen and was funded by the State of Lower Saxony (Ministry of Science and Culture). The study was conducted in the framework of two interdisciplinary projects (BIOMIX & GRASSMAN) from 2008 – 2010 in the Solling, in Lower Saxony, Germany. We analyzed a grassland grazed by cattle or sheep (BIOMIX) and a cut grassland with different management intensities (GRASSMAN). In both projects, functional plant diversity was influenced by the use of herbicides. Our particular focus was on N losses (N2O and NO emissions, N leaching) and N dynamics (net and gross mineralization). Within GRASSMAN, we calculated N response efficiency and N retention efficiency on ecosystem-level. N response efficiency is defined as the product of uptake efficiency (defined as plant N uptake per unit of available N) and N use efficiency (defined as productivity per unit of plant N uptake). In addition, we defined N retention efficiency as the soil available N retained in an ecosystem. In BIOMIX, we investigated the response of N2O and NO emissions to different grazers and sward composition. Paddocks partly pretreated with a herbicide against dicots were grazed rotationally by either cattle or sheep. Mean N2O emissions were 38.7 µg N2O-N m-2 h-1, mean NO emissions 2.4 µg NO-N m-2 h-1. Cumulative NO-N emissions were larger for sheep- than for cattle-grazed paddocks. In a controlled application experiment, plots with cattle excreta showed larger N2O emissions than plots with sheep excreta, reaching peak emissions of 1921 µg N2O-N m-2 h-1 on cattle urine patches compared to 556 µg N2O-N m-2 h-1 on sheep urine patches, related to different N-inputs per excretion. Peak emissions of dung-treated plots were much smaller. The N2O emission factors were 0.4% for cattle urine, 0.5% for sheep urine, 0.05% for cattle dung and 0.09% for sheep dung. In the main and the controlled application experiment, plant species composition was insignificant compared to the effect of grazers on N oxide emissions. N oxide emissions on the paddock scale were larger for sheep- compared to cattle-grazing, despite larger emissions per cattle excretion. We attributed this to the more even spread of sheep excreta compared to cattle excreta. In GRASSMAN, we investigated the effects of management practices (fertilizer application and mowing frequency) and sward composition on N losses (N2O emissions, N leaching) and N dynamics (net and gross mineralization), and calculated N response efficiency and N retention efficiency. A three-factorial design was conducted, including two fertilization treatments (180 – 30 – 100 kg NPK ha-1 yr-1 and no fertilization), two mowing frequencies (cut once and thrice per year) and three sward compositions (control, dicot-enhanced and monocot-enhanced swards) over a two-year period. In 2009, N2O emissions were significantly influenced by both management practices (fertilization and mowing frequency). In 2010, only fertilization affected N2O emissions. In 2009, NO3 leaching losses were influenced by fertilization and in 2010 by both management practices (fertilization and mowing frequency) influenced NO3 leaching. Net N mineralization rates were affected only by fertilizer application in 2009. In 2010, not only fertilization but also mowing frequency influenced net N mineralization rates. Neither management practices nor sward composition affected gross N mineralization. The N response efficiency was largely influenced by fertilization, followed by mowing frequency, explaining 41% and 3% of the variance, respectively, in 2009. In 2010, the effect of fertilization was lessened whereas the effects of mowing frequency and sward composition were more pronounced, accounting for 24%, 12% and 6% of the variance, respectively. The N response efficiency was larger in unfertilized than in fertilized plots, in plots cut thrice than once per year, and in control swards than in monocot- or dicot-enhanced swards. Fertilization decreased N response efficiency through decreases in both N uptake efficiency and N use efficiency whereas mowing frecuency and sward composition affected N response efficiency through N uptake efficiency rather than N use efficiency. N retention efficiency was calculated only for 2010 and was largely influenced by fertilization followed by sward composition explaining 22% und 17% of the variance, respectively. Unfertilized plots showed larger N retention efficiency than fertilized plots and N retention efficiency decreased in the order control > dicot-enhanced > monocot-enhanced swards with a significant difference between control and monocot enhanced swards. N retention efficiency was highly correlated with microbial ammonium (NH4+) immobilization and microbial biomass and only marginally correlated with plant N uptake, underlining the importance of microbial N retention in the soil-plant system. Our results indicated that management practices are the most important factor determining ecosystem functioning in grassland. Fertilization, mowing frequency and livestock grazing largely influenced N response efficiency, N retention efficiency and N losses. The sward botanical composition had a minor effect on N cycling or efficiency. Control sward composition of ~70% monocots and ~30% dicots, resulting from extensive long-term management showed the largest efficiencies. Deviations from these proportions - even to more balanced proportions of monocots and dicots - decreased efficiencies. N response efficiency and N retention efficiency are appropriate tools to evaluate environmental sustainability of sward compositions and management practices of grasslands.