Soil budget, net export and potential sinks of nitrogen in the lower Oglio River watershed (northern Italy)

A 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 450kgNha -1 arable land (AL)year -1, overall input 100115tNyear -1). Crop uptake, ammonia volatilization and denitrification in soils contributed 65, 21, and 14%, respectively, of TN output (basin average 270kgNha -1ALyear -1, overall output 60060tNyear -1). N inputs exceeded outputs by 40056tNyear -1, resulting in a basin average surplus of about 180kgNha -1ALyear -1. About 34% of the N surplus was exported annually from the basin while the remaining amount (about 26800tNyear -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 12500km), with estimated nitrogen loss up to 8500tNyear -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. The novelty in the proposed approach consists in the evaluation of multiple N sources and sinks in over 200 single municipalities aggregated then on the watershed level. The present study demonstrates, via the soil system budget approach, that in the lower Oglio River (northern Italy) basin N inputs to arable lands largely exceed N requirements by crops.