# Nitrogen Budget of Short Rotation Forests Amended with Digestate in Highly Permeable Soils
## 3 Results and Discussion
### Nitrogen Balance  
Nitrogen balance results are resumed in Table 6. In all the fertilized plots, for both SRF systems and permanent meadows, the N coming from digestate was the most important input, whereas, where fertilization was not carried out, only N from rain contributed. In SRF systems, the most important output was the N loss via leaching, estimated in 81 and 111 kg-N ha\({}^{-1}\) y\({}^{-1}\), for A1 and A2, respectively. High levels of leaching could be related to both highly permeable soils and low levels of water uptake by the roots of the young trees. In fact, N retained by woody biomass was estimated at 19 and 26 kg-N ha\({}^{-1}\) y\({}^{-1}\), respectively, for A1 and A2, whereas herbaceous weeds removed from the systems 24-36 kg-N ha\({}^{-1}\) y\({}^{-1}\). Production of herbaceous biomass from the permanent meadow allowed the highest N export, which amounted to 256 and 340 kg-N ha\({}^{-1}\) y\({}^{-1}\), respectively, for P1 and P2 thesis. Because of this, N leaching from the permanent meadow was lower than that from the SRF, amounting to 35 and 83 kg-N ha\({}^{-1}\) y\({}^{-1}\), respectively for P1 and P2. The highest levels of N\({}_{\text{res}}\) (difference between N input and N output) were found in the SRF systems. In particular, the only positive values of N\({}_{\text{res}}\) were found in A1 and A2 (64 and 163 kg-N ha\({}^{-1}\) y\({}^{-1}\), respectively). Considering these data linked to the difference of N concentration in the soil (-59 and -26 kg-N ha\({}^{-1}\) y\({}^{-1}\), Table 4), it was possible to suppose that N was not preserved into the soil, but, at least partially, was denitrified. As a matter of fact, some clues suggested that denitrification was an ongoing process: (i) the measured denitrification rates showed positive results in all the measuring campaigns, with an average rate of 1 \(\pm\) 0.1 g N-NO\({}_{3}\) ha\({}^{-1}\) d\({}^{-1}\); (ii) the N-NO\({}_{2}\) concentration in tension lysimeters was 0.42 mg L\({}^{-1}\), suggesting an active denitrification, with four isolated peaks over 1 mg L\({}^{-1}\), indicating that the limiting condition is the availability of the labile organic substrates, which usually leads to transient N-NO\({}_{2}\) accumulation [28; 29]; (iii) the average redox potential measured in the tension lysimeters was \(-112\pm 7\) mV, indicating anoxic-hypoxic conditions suitable for denitrification in soils [30]. All the above-mentioned clues can be considered as evidence of the ongoing denitrification process, although they cannot be used directly to quantify this process, which was then estimated via mass balance (Table 6). The high denitrification capacity could be linked to high amounts of distributed digestate in a smaller area, which can lead to anaerobic conditions in soils. In the permanent meadow, strongly negative values of N\({}_{\text{res}}\) (ranging \(-86\) to \(-169\) kg-N ha\({}^{-1}\) y\({}^{-1}\), passing from the highest level of fertilization to the lowest) and differences of N concentration in soils (ranging \(-89\) to \(-190\) kg-N ha\({}^{-1}\) y\({}^{-1}\)) suggested that soils became depleted of N, and that permanent meadows would benefit from higher levels of fertilization. This finding was further supported by the increase in production of herbaceous biomass when passing from P0 to P2.