Nitrate concentrations in groundwater under contrasting agricultural management practices in the low plains of Italy

In groundwater vulnerability assessment, it is assumed that groundwater closer to the soil surface is at greater risk of contamination by pollutants, including nitrate. However, this is not always the case, since low nitrogen (N) concentrations and losses from cultivated fields have been measured in shallow ground waters. This study aimed to evaluate the water and N balances in two low plain areas of the Veneto Region (NE Italy), characterized by a shallow water table (<5 m) and identified as Nitrate Vulnerable Zones (NVZs). In each of the two NVZs, three fields were managed with conventional (C), integrated (I) and organic (O) farming systems. Water and N fluxes through the vadose zone were monitored during the years 2004–2006. Water samples were collected from water-table wells and suction microlysimeters, whereas soil moisture and tension were measured with sets of TDR probes and dial-gauge tensiometers installed along the soil profiles. Surface runoff was also collected by gutters and diverted into containers where water volumes were quantified and sampled for chemical analysis. Water was analyzed for total N bound to sediment (only runoff) and total N and nitrate in solution. To evaluate the N in/out fluxes of the root zone (0.9 m depth), the upflux/percolation volumes were quantified by applying the water balance method. Upflux was one of the most important items of the water balance. The highest values were observed during summer (>200 mm), when upflux contributed an average of up to 65% of the evapotranspiration (ET). Percolation volumes were reduced as a consequence, with a net flux ranging from 118 to 292 mm y−1 on average. Upflux also strongly influenced the N balance, reducing potential leaching. We estimated that, on average, 59% of the leached N returned to the root zone by upward movement. Shallow groundwater conditions also affected the proportions of N forms. Nitrate fraction decreased markedly in groundwater with respect to percolation water, most probably due to the enhancing effect of denitrification processes. Following the indications in the EU 91/676 Directive, large areas of the central and low plain of the Veneto Region have been designated as NVZs. However, agricultural impact on water quality does not appear to be very high in many parts of these areas, as evidenced by the N balance of these experimental fields.

► Upflux is one of the most important items of the water balance in shallow groundwater conditions.
► Upflux and denitrification are two main processes that reduce N losses to water bodies.
► Criteria to identify Nitrate Vulnerable Zones should be revised to consider shallow groundwaters.