# The Importance of Incorporating Denitrification in the Assessment of Groundwater Vulnerability
## 3 Results and Discussion
### Italian Application  
For the CP study area, all the necessary information has been collected and further digitalized for the application of LOSN-P. In the LOSN-P formulation, the values of OM in the soil layer surely represent the most critical parameter for the assessment of N losses, as the organic carbon is the main electron donor required by denitrifying bacteria. The data concerning the OM content in the first 30 cm of soil have been obtained from the SIAS project (Sviluppo di Indicatori Ambientali sul Suolo in Italia--Development of Soil Indicators in Italy). The spatial distribution of OM in the Campania plain is highly variable, ranging from 0.03 % in the soils of the coastal area to 10% in the soils near the Volturno River, where thin peat layers occur. The mean annual precipitation, temperature and evapotranspiration have been calculated using an available record of 15 years from 2000 to 2015 on 4 meteorological stations scattered upon the study area; they are 900 mm/y, 22 \({}^{\circ}\)C and 750 mm/y, respectively. The Ks has been obtained analyzing and interpreting the soil classification available in \"I Sistemi di terre della Campania\" [41]. Here a shapefile of soil classification is presented together with some soil characteristics like texture and bulk density that have been utilized for state the values of the soil hydraulic conductivity (Ks). Finally, the slope has been calculated with a GIS function from the Digital Terrain Model (DTM). The LOSN-P was calculated according to Equation (2) using the raster calculator toolbox and it is represented in Figure 2a. The total amount of N loss for the CP ranges from a minimum of 10 kg/ha/y to a maximum of 65 kg/ha/y. Among all the seven parameters, soil texture, OM and slope strongly affected this index. Generally, the areas with the lower slope values are characterized by the higher N loss rate. In fact, the entire coastal zone in the West and the South part of the plain are characterized by high N loss rate through percolation, promoted by the flat topography and the high Ks, despite the low percentage of OM in the soil. Figure 2: LOSN-P application for (**a**) the Campania plain and (**b**) the Garigliano plain. Low N loss rates, instead, are located along the carbonate relief in the Northern and Eastern part of the plain. Looking at LOSN-P spatial distribution (Figure 1a), the areas where a low LOSN-P index is registered, correspond to the sites with a high denitrification potential. This is confirmed by the assumption that those areas are the ones with low Ks values and strong reducing conditions due to the presence of peat layers. In fact, it has been highlighted that denitrification processes are often negatively correlated with Ks, because even though a large pool of organic carbon and nitrate are available in high Ks porous matrices, the microbial community grows better in low Ks sediments where low flow conditions allow the simultaneous presence of substrates, nutrients and favorable redox conditions [42]. Moreover, strong reducing conditions positively affect denitrification where other elements like Fe\({}^{2+}\), Mn\({}^{2+}\), could also be used as electron donors instead of organic carbon [43]. The same procedure adopted for the CP has been followed for the LOSN-P application in the Garigliano river plain. Since the plain is located just North to CP, all the input parameters were collected using the same sources used for the previous study area. The meteorological conditions are the same found in CP, as well as the soil's characteristics; the main difference concerns the percentage of OM, which is almost half respect to the CP, with values going from 0.1% to 4.0%. The highest values of OM are located in the Northern and central part of the plain, while the lowest values characterize the coastal soils. In addition, in this case, the highest rates of N loss occur in the coastal area with values around 30 kg/ha/y, with a general decrease moving inland, with values around 18 kg/ha/y.