Salinity change induces distinct climate feedbacks of nitrogen removal in saline lakes

Current estimations of nitrogen biogeochemical cycling and N2O emissions in global lakes as well as predictions of their future changes are overrepresented by freshwater datasets, while less consideration is given to wide-spread saline lakes with different salinity (representing salinization or desalinization). Here, we show that N2O production by denitrification is the main process of reactive nitrogen (Nr, the general abbreviations of NH4+-N, NO2 -N and NO3 -N) removal in hypersaline lake sediments (e.g. Lake Chaka). The integration of our field measurements and literature data shows that in response to natural salinity decrease, potential Nr removal in-creases while N2O production decreases. Furthermore, denitrification-induced N2 production exhibits higher salinity sensitivity than denitrification-induced N2O production, suggesting that the contribution of N2O to Nr removal decreases with decreasing salinity. This field-investigation-based salinity response model of Nr removal indicates that under global climate change, saline lakes in the process of salinization or desalination may have distinct Nr removal and climate feedback effects: salinized lakes tend to generate a positive climate feedback, while desalinated lakes show a negative feedback. Therefore, salinity change should be considered as an important factor in assessing future trend of N2O emissions from lakes under climate change.

Automated summary

Current estimations of nitrogen biogeochemical cycling and N2O emissions in global lakes primarily focus on freshwater datasets, while saline lakes with salinization or desalinization have been given less consideration. This study reveals that N2O production through denitrification is the main process for reactive nitrogen removal in hypersaline lake sediments. Field measurements and literature data indicate that decreasing salinity leads to increased potential nitrogen removal and decreased N2O production. Furthermore, the salinity sensitivity of denitrification-induced N2 production is higher than that of denitrification-induced N2O production, suggesting a reduced contribution of N2O to nitrogen removal with decreasing salinity. Therefore, salinity change should be considered as an important factor in assessing future trends of N2O emissions from lakes under climate change.