# Denitrification in Intrinsic and Specific Groundwater Vulnerability Assessment: A Review
## 2 Denitrification in the Unsaturated and Saturated Zones  
NO3- attenuation is controlled by several physical and biogeochemical processes occurring in the saturated and unsaturated zones. The understanding and quantification of these processes are mandatory to protect groundwater from the risk associated with point and non-point sources of NO3- pollution [44]. Figure 1 reports a schematic representation of the processes acting in the unsaturated and saturated zones. Among all biogeochemical processes, DNT is considered the predominant factor responsible for NO3- attenuation in agricultural soil [45], representing an important pathway for Nr losses. Nevertheless, DNT also has a serious adverse environmental effect, being the principal source of N2O and NO, accounting for 70% of the N2O emitted annually from the biosphere into the atmosphere [46]. Several factors regulate DNT in the unsaturated and in the saturated zones; in fact, the DNT rate is a direct function of:  
Figure 1: Conceptual model of DNT occurring in the unsaturated and saturated zones. (i) soil water content, (ii) hypoxic and anoxic conditions, (iii) NO3\({}^{-}\) concentrations, and (iv) organic carbon substrates, such as soil organic matter and crop residues [47]. Soil texture could also directly influence DNT, especially in the topsoil, via the formation of micro-niches where anoxic condition would prevail [48]. Nr mass loading rate significantly affects DNT, especially when the Nr supplied through fertilizer is higher than the crop Nr requirement [49; 50]. Within the unsaturated profile, DNT could prevent leached NO3\({}^{-}\) from reaching the water table, if labile carbon sources are also present [51; 52]; in any case, DNT rates could be significantly limited by the vertical unsaturated geochemical and microbiological variability. DNT in the saturated zone has been less extensively studied with respect to DNT in the unsaturated zone. Despite this, DNT in groundwater systems is still relevant and mainly controlled by the same conditions as for the unsaturated zone. For instance, the presence of electron donors (organic carbon, reduced iron, and/or reduced sulfur) have been identified as the most limiting factor among with anoxic conditions, more than NO3\({}^{-}\) and nutrient availability [44]. The above-mentioned DNT controlling factors (Figure 1), both in the unsaturated and saturated zones, are usually considered in vulnerability assessment methodologies to indirectly account for DNT where direct measurements are not available. Furthermore, as will be further discussed in the following chapters, the main factor considered in most used methodologies (namely the rating and weights methods) is the soil texture, which is easier to measure and analyze compared to the other factors controlling DNT in the unsaturated or saturated zone. Based on these considerations, Table 1 shows the DNT rates for the unsaturated zone, and more specifically in the topsoil, obtained from both laboratory and field studies, depending on the soil texture and for different land covers. The average DNT value for laboratory studies is 460 Kg-N/ha/year, with values that can reach up to 3600 Kg-N/ha/year, while for field studies the average DNT rate is 60 Kg-N/ha/year, with a maximum of 550 Kg-N/ha/year. It is evident that under controlled laboratory conditions, the DNT rates tend to be much higher (approximately one order of magnitude) than in field conditions, where local limiting factors may act to reduce the effectiveness of DNT both spatially and seasonally. Concerning the DNT rates in the saturated zone, it must be stressed that the studies available in the literature are not numerous, especially in the field, given the greater difficulty of the monitoring techniques required and the duration of the studies to be carried out.