Reactive nitrogen losses via denitrification assessed in saturated agricultural soils

The aim of the present study was to quantify nitrate (NO3 −) removal via denitrification in an intensively cropped lowland of the Po River delta (Northern Italy). These agricultural lands are characterized by fine textured soils, generally poor of labile organic matter and amended mainly with synthetic fertilizers. Laboratory core incubations in water saturation conditions were performed on two contrasting soil types distinguished by different soil textures (silty-loam and silty-clay) either amended with NO3 − alone or a combination of NO3 − and acetate. Denitrification was evaluated by concomitant measurements of NO3 − consumption and N2 production via N2:Ar analyses by Membrane Inlet Mass Spectrometry (MIMS). The water-logged soils showed higher capacity to reduce NO3 − to N2 when supplied with acetate as an organic substrate, while, without acetate amendment, NO3 − removal was limited due to lack of labile organic substrates. Transient nitrite (NO2 −) accumulation was documented in acetate amended mesocosms, due to concurrent presence of elevated pH values and use of highly oxidized substrates (like acetate). This study suggests that agricultural practices aimed to increase the availability of labile organic matter, such as acetate, are beneficial in buffering reactive N excess in soils and to reduce NO3 − leaching towards groundwater and surface-water.