Effects of nitrogen and water addition on ecosystem carbon fluxes in a heavily degraded desert steppe

Changes in global precipitation patterns and increased nitrogen (N) deposition have important effects on the dynamics of the carbon (C) cycle in grassland ecosystems. Long-term overgrazing caused heavy degradation of grasslands and more than 60% of China’s natural grasslands experienced moderate to severe degradation. To elucidate the effects of increasing precipitation and N deposition on C fluxes in a desert steppe ecosystem, experiments with nitrogen and water addition were conducted with four treatments, i.e., control (CK), N addition (N), water addition (W), combined addition of N and water (NW), in a Stipa breviflora desert steppe that had been heavily degraded. The ecosystem CO2 fluxes, including gross ecosystem productivity (GEP), net ecosystem CO2 exchange (NEE), and ecosystem respiration (ER), were determined using the closed chamber method during the growing seasons (May to October) of 2019 and 2020. Our results showed that water and nitrogen addition did not change the patterns of the seasonal dynamics of the ecosystem CO2 fluxes, indicating that water is the main limiting factor of CO2 fluxes in the studied desert steppe ecosystem. Precipitation increase (both W and NW treatments) stimulated all three components of the ecosystem CO2 fluxes, but N addition alone enhanced NEE and GEP while had no significant effects on ER, indicating water-induced increase of GEP was caused by both NEE and ER while N-induced increase of GEP was mainly caused by increases of NEE. Water addition can increase the effectiveness of N addition, but there was little interaction between the two factors. Soil moisture was more significantly correlated than soil temperature with ecosystem CO2 fluxes, and water addition could further enhance the correlation of ecosystem CO2 fluxes with soil temperature and moisture. Overall, ecosystem CO2 fluxes are more sensitive to the precipitation increase than to the N addition. These findings indicate that water supply will play more important role in ecosystem productivity of the desert steppe under the predicted scenarios of precipitation increase and increasing N deposition, which also provides scientific evidence for the restoration of those degraded desert steppe induced by overgrazing can not be achieved simply by increasing soil N availability.