Abstract
Denitrification is a key process buffering the environmental impacts of agricultural nitrate loads but, at present, remains the least understood and poorly quantified sink in nitrogen budgets at the watershed scale. The present work deals with a comprehensive and detailed analysis of nitrogen sources and sinks in the Burana–Volano–Navigabile basin, the southernmost portion of the Po River valley (Northern Italy), an intensively cultivated (> 85% of basin surface) low-lying landscape. Agricultural census data, extensive monitoring of surface–groundwater interactions, and laboratory experiments targeting N fluxes and pools were combined to provide reliable estimates of soil denitrification at the basin scale. In the agricultural soils of the basin, nitrogen inputs exceeded outputs by nearly 40% (~ 80 kg N ha−1 year−1), but this condition of potential N excess did not translate into widespread nitrate pollution. The general scarcity of inorganic nitrogen species in groundwater and soils indicated limited leakage and storage. Multiple pieces of evidence supported that soil denitrification was the process that needed to be introduced in the budget to explain the fate of the missing nitrogen. Denitrification was likely boosted in the soils of the studied basin, prone to waterlogged conditions and consequently oxygen-limited, owing to peculiar features such as fine texture, low hydraulic conductivity, and shallow water table. The present study highlighted the substantial contribution of soil denitrification to balancing nitrogen inputs and outputs in agricultural lowland basins, a paramount ecosystem function preventing eutrophication phenomena.
Highlights
In the last century, the intensification of agricultural activities has deeply altered the nitrogen (N) budgets in the cropping systems of agriculturally dominated watersheds around the world
The present study aims at a better understanding of the watershed-scale importance of soil denitrification as a NO3- buffering mechanism in agricultural settings
Within the 5-year period 2006–2010, the N budget in the agroecosystems of the Burana-Volano-Navigabile watershed evidenced that synthetic fertilizers were the main contribution to all the N imported to agricultural soils in the basin, averaging at 68% of total N inputs, followed by biological fixation (23%), livestock manure (6%), and atmospheric deposition (3%) (Table 1)
Summary
The intensification of agricultural activities has deeply altered the nitrogen (N) budgets in the cropping systems of agriculturally dominated watersheds around the world. About only 25%, on average, of the N load generated within worldwide watersheds is delivered to terminal waterbodies and coastal zones via river export, but still little is known about the relative importance of different N retention mechanisms (Howarth and others 2012; Goyette and others 2016; Romero and others 2016). On this basis, questions on where and for how long N may accumulate in soils and aquifers and what transformations and removal processes it may undergo, preventing its export, are not yet properly addressed
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