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 N-2 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 similar 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.

Automated summary

The nitrogen cycle in Swiss lakes is affected by various factors, with nitrate concentration, external nitrogen load, and hydraulic loading rate being key predictors of denitrification rates. Net sedimentation of nitrogen is strongly related to total phosphorus concentration. Nitrogen removal efficiency is negatively correlated with hydraulic loading rate. These findings highlight the importance of nitrogen management alongside phosphorus management in order to protect lake water quality.