Soil Budget, Net Export, and Potential Sinks of Nitrogen in the Lower Oglio River Watershed (Northern Italy)

Abstract

AbstractA nitrogen mass balance, realized for the lower Oglio River basin (Po River Plain, northern Italy), suggested an elevated impact of agricultural activities in this watershed. Livestock manure, synthetic fertilizers, biological fixation, atmospheric deposition, and wastewater sludge contributed 51, 34, 12, 2, and 1% of total N (TN) input, respectively (basin average 450 kg N ha−1 arable land (AL) year−1, overall input 100 115 t N year−1). Crop uptake, ammonia volatilization and denitrification in soils contributed 65, 21, and 14%, respectively, of TN output (basin average 270 kg N ha−1 AL year−1, overall output 60 060 t N year−1). N inputs exceeded outputs by 40 056 t N year−1, resulting in a basin average surplus of about 180 kg N ha−1 AL year−1. About 34% of the N surplus was exported annually from the basin while the remaining amount (about 26 800 t N year−1) underwent other unaccounted for processes within the watershed.The relevance of nitrogen removal via denitrification in aquatic compartments within the watershed was evaluated. Denitrification in the secondary drainage network can represent a relevant nitrogen sink due to great linear extension (over 12 500 km), with estimated nitrogen loss up to 8500 t N year−1. Denitrification in the riverbed and in perifluvial wetlands have the potential to remove only a small fraction of the nitrogen surplus (<3%). Evidence suggests the relevance of groundwater as a site of nitrogen accumulation.