# Nitrogen Budget of Short Rotation Forests Amended with Digestate in Highly Permeable Soils
## Materials and Methods  
### Experimental Site  
The study area was located in northeastern Italy within a Quaternary unconfined aquifer composed by alluvial deposits of the Brenta River. The aquifer constitutes a large portion of the recharge area of the Venice lagoon drainage basin. The SRF experimental site, due to the high soil permeability, is part of an experimental FIA [16]. The whole system was set during the spring of 2009 in a field formerly cultivated with maize and wheat rotation. The experimental site consists of a monoculture plot (\(60\times 50\) m) of _Platanus hispanica_ (Mill ex Muench, a hybrid of _Platanus orientalis_ x _Platanus occidentalis_), which was planted with a distance of 2.5 m between rows and a distance of 2.0 m between columns (the ratio between the available distribution area and the total area is 2,5/7, thus 36%). Infiltration in the unsaturated zone is promoted by a system of ditches with a width of 1.0 m and spaced every 7.0 m (Figure 1). An adjacent permanent meadow field, with the same soil characteristics, was chosen for the comparison with _Platanus hispanica_. In the upper 0.90 m, the soil is composed of a mixture of coarse and fine sediments (loamy gravel soil) characterized by fast infiltration rates. The soil is classified as Typic Hapludalfs loamy-skeletal, mixed, mesic soil according to the USDA classification. In the entire area, the presence of a significant content of fine sediments (silt and clay) in the top 45 cm decreases the soil's permeability and increases its water retention capacity. The groundwater level fluctuatesapproximately between 15 and 19 m below ground level (b.g.l. ), thus the vadose zone is very thick and soil water logging conditions are not an issue at this site. The climate is subcontinental with about 1100 mm rainfall per year and mean temperature of 12.8 \({}^{\circ}\)C, a relative humidity of 76% and low average wind speed (6.5 km h\({}^{-1}\)). Measurements on N losses were performed over three experimental plots in the FIA (A\({}_{0}\)-A\({}_{2}\) in Figure 1) and in the permanent meadow (P\({}_{0}\)-P\({}_{2}\) in Figure 1), representing respectively \"no distribution, \"170 Kg-N ha\({}^{-1}\) y\({}^{-1}\)\" and \"340 Kg-N ha\({}^{-1}\) y\({}^{-1}\)\", as shown in Figure 1. Unless otherwise specified, the experimental period was from January 2010 to September 2011, while the annual N balance was performed in the period between 10 May 2010 and 9 May 2011. During the experiment, the two different amounts (170 and 340 kg-N ha\({}^{-1}\) y\({}^{-1}\)) of digested slurry were spread in three separate events, as reported in Table 1. The digestate was collected from a biogas plant treating dairy cow manure and ensiled energy crops (corn and sorghum silage). The digestate was applied with a 10 m\({}^{3}\) tank attached to a precision agriculture tractor delivering the planned amount of digestate (one third of the total in three times) using a flow meter (see Table 1). The spreading method employed a horizontal bar with 10 plastic tubes (inner diameter 5 cm) to allow an even distribution of digestate along the inter-rows in the FIA and in the permanent meadow, adopting the trailing shoe technique (low pressure distribution in a small groove 2 cm deep). Due to the presence of cobbles and to the high soil permeability, instead of burying the slurry in the soil, it was sprinkled in the surface, through a trailing shoe spreading system, slightly disturbing the superficial soil layer. In this way, the permanence on the soil's surface was short anyhow, but, at the same time, the first layer, rich in fine sediments, was fully exploited. In order to have comparable quantities of N for units of agricultural soil (kg ha\({}^{-1}\)), in the SRF plots, the total amount of N (Table 1) was spread in the soil of the distribution area, which represents only the 36% of the total area (excluding the surface occupied by tree and ditches). The chemical and physical characteristics of the digestate were analyzed before each event (Table 2).