Abstract
Although representing a paramount mechanism against nitrogen excess in agricultural landscapes, soil denitrification is still a largely unknown term in nitrogen balances at the watershed scale. In the present work, a comprehensive investigation of nitrogen sources and sinks in agricultural soils and waters was performed with the aim of gaining insights into the relevance of soil denitrification in a highly farmed sub-basin of the Po River delta (Northern Italy). Agricultural statistics, water quality datasets, and results of laboratory experiments targeting nitrogen fluxes in soils were combined to set up a detailed nitrogen budget along the terrestrial–freshwater continuum. The soil nitrogen budget was not closed, with inputs exceeding outputs by 72 kg N·ha−1·year−1, highlighting a potential high risk of nitrate contamination. However, extensive monitoring showed a general scarcity of mineral nitrogen forms in both shallow aquifers and soils. The present study confirmed the importance of denitrification, representing ~37% of the total nitrogen inputs, as the leading process of nitrate removal in heavily fertilized fine-texture soils prone to waterlogged conditions.
Highlights
Nitrogen (N) pollution is a still unsolved global environmental issue and with multiple implications in terms of water quality deterioration, biodiversity loss, human health problems, global carbon-cycle alterations, and climate change [1,2,3]
The aim of the present work was to evaluate the relative importance of soil denitrification in buffering N excess at the watershed scale in a deltaic, intensively cultivated, and irrigated sub-basin of the Po River plain, the Po di Volano-Sacca di Goro
N exported via crop harvest was the main output from agricultural soils among the three terms included in the budget, accounting for 95% of the sum of all items (Table 2)
Summary
Nitrogen (N) pollution is a still unsolved global environmental issue and with multiple implications in terms of water quality deterioration, biodiversity loss, human health problems, global carbon-cycle alterations, and climate change [1,2,3]. Uncertainty remains on processes and paths, since about 75% (range 40–95%) of the N load generated within worldwide catchments is assessed not to be delivered to terminal waterbodies, via river export, but internally retained [7,8,9]. This high retention capacity may mask both permanent (i.e., denitrification in soils and aquatic environments) and temporary removal processes (i.e., storage in the vadose zone–groundwater system). Assessing the fate of missing N is a major scientific challenge for defining codes of good agricultural practices and implementing effective management strategies aimed at protecting water quality in human-impacted watersheds
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