Denitrification and Nitrogen Burial in Swiss Lakes

Earth's nitrogen (N) cycle is imbalanced because of excessive anthropogenic inputs. Freshwater lakes efficiently remove N from surface waters by transformation of NO₃^- to atmospheric N₂ and/or N₂O (denitrification; DN) and by burial of organic N in sediments (net sedimentation; NS). However, relatively little is known about the controlling environmental conditions, and few long-term measurements on individual lakes are available to quantify conversion rates. We report N-elimination rates in 21 Swiss lakes estimated from whole-lake N budgets covering up to ∼20 years of monitoring. The NO₃^- concentration in the bottom water was the main predictor of DN. Additionally, DN rates were positively correlated with external N load and the area-specific hydraulic loading rate (mean depth/water residence time; Q_s). NS of N was strongly related to total phosphorus (P) concentration. Nitrogen removal efficiency (NRE), the fraction of the load of dissolved N to a lake removed by DN and NS, was strongly negatively related to Q_s. This previously unconsidered variable improves the predictability of NRE and does not require knowledge of N and P loading rates or concentrations. We conclude that P management alone intended to oligotrophy lakes only slightly increases N export unless it is accompanied by N management.