Assessing denitrification from seasonally saturated soils in an agricultural landscape: A farm-scale mass-balance approach

• Denitrification rates were highly correlated to soil saturation frequency as described by the soil topographic index.
• Denitrification from seasonally saturated soils accounted for about half of the total nitrogen imbalance in this headwater catchment.
• Denitrification rates from riparian areas were high but only accounted for about 10% of the total nitrogen imbalance.

Riparian zones have received considerable attention as potential hotspots of denitrification at the landscape- or watershed-scale. Conceptually, the conditions that promote denitrification in these zones are also found in other parts of the landscape, namely those areas that are prone to saturate. However, spatiotemporal characterization and quantification of these potential denitrification hotspots are lacking, despite their importance to land managers tasked with mitigation of nitrogen (N) pollution, particularly in human-dominated landscapes. We quantified denitrification fluxes from the shallow saturated zone of an agricultural landscape using a topographic index-denitrification model, which facilitates scaling of in situ denitrification rates across the landscape based on frequency and duration of saturated conditions. Denitrification in the shallow saturated zone (i.e., where the water table is at or within a few meters of the soil surface) resulted in a N flux that was nearly half of the total denitrification from the landscape—in about a third of the area—as determined from a well-constrained whole-farm N balance constructed from farm records and field measures. Denitrification flux rates from saturated riparian soils were among the highest in the landscape, however the contribution of riparian areas to total landscape denitrification was less than 10%.