# Nitrogen Budget of Short Rotation Forests Amended with Digestate in Highly Permeable Soils
## 1 Introduction  
Resources other than fossil energy are required to face increasing problems such as the competition for energy resources and global climate change. One of the challenges of the bio-economy is to move from the use of fossil raw materials to sustainable production and utilization of renewable biomass. For instance, production of biogas using organic waste, as manure, represents a valuable resource, and, compared to fossil fuel utilization, the use of biogas yields a reduced carbon footprint [1, 2]. Another good example is the use of woody biomass as renewable energy source to satisfy energy demands [3]. The European Commission (EC) prescribes an increase in renewable energy generation (at least 27% for 2030), of which biomass currently contributes to 63%. Among woody biomasses, Short Rotation Forestry (SRF) crops have good opportunities to be considered by the EC as a conventional agricultural product useful for agricultural diversification [4].  
Biogas plant and woody biomass, besides replacing fossil fuels, can provide many environmental advantages. Biogas production through anaerobic digestion generates also digested slurry as organicwaste, which is rich in nutrients [1]. The SRF crops are multifunctional systems that reduce wind and soil erosion, and favor carbon sequestration in the soil due to less intensive soil cultivation and accumulation of organic matter [5; 6]. These benefits, in addition to energy production, are of great interest for cattle farming. However, if digestate can be considered a valuable and inexpensive fertilizer for SRF or meadow, the high concentration of livestock in areas with low crop coverage, has led to manure/digestate management concerns [7; 8]--in particular, when it must be spread on highly permeable soil in NVZ (nitrate vulnerable zones). These areas have been designated at the risk of agricultural nitrate (NO3-) pollution by EC, which identified loading limits (170 kg-N ha-1 y-1) for N fertilizers. Consequently, farmers have to follow mandatory rules to tackle NO3- loss from agriculture. In fact, the impact of organic and chemical fertilizers in soils and fresh and marine waters [9; 10] is well known. Another key issue is the atmospheric pollution, mainly due to emissions of both ammonia (NH3), which can lead to soil acidification [11] and nitrous oxide (N2O), considered a major GHG (green-house gas) [7].  
Although the economic impact of SRF crops has been investigated [12], comprehensive assessments of N dynamics when SRF crops are fertilized with digestate are rare, especially in highly permeable soils and during the first stage of SRF [13]. In fact, these environmental conditions could lead to high NO3- leaching rates due to the fast dynamic of soil hydrology and/or to high NH3 volatilization rates due to a scarce soil cover. To support cattle and forage in permeable soils, there is a need for research to identify specific measures to efficiently recycle N and to assess plant responses to organic fertilization in a range of site conditions [14; 15].  
In the upper part of the Brenta River basin, a Forested Infiltration Area (FIA) [16], characterized by highly permeable soils (being in an upland alluvial fan), was cultivated with SRF and fertilized using digested slurry from the same farm.  
In order to improve our knowledge on farms' sustainability in vulnerable areas, the main aim of this study is to assess if, and to what extent, SRF crops during their first stage can reduce the risk of N pollution caused by organic fertilization with digestate on permeable soils. In addition, the N balance was performed also in an adjacent traditional permanent meadow, in order to assess the sustainability of organic fertilization with digestate on SRF plantations with respect to grasslands. For this purpose, NH3 volatilization, N2O emissions and NO3- leaching, vegetation uptake and soil N content were investigated to gain a comprehensive N balance.