Soil respiration and its autotrophic and heterotrophic components in response to nitrogen addition among different degraded temperate grasslands

Atmospheric nitrogen (N) deposition greatly affects grassland soil respiration (Rs) and its components. However, whether there exists a similar pattern in the response of Rs and its components to N deposition among the grasslands with varying degradation status remains unclear. We established a 3-year field experiment with six N addition levels on three grassland sites (non-degraded, moderately degraded and severely degraded) and measured autotrophic respiration (Ra), heterotrophic respiration (Rh), Rs and their influencing factors, including abiotic and biotic variables. Our results showed that Ra had an increasing tendency, whereas Rh and Rs had decreasing tendencies, in response to increasing N addition in the non-degraded and moderately degraded grasslands. In the severely degraded grassland, Ra, Rh and Rs exhibited increasing tendencies under lower N addition levels and decreasing tendencies under higher N addition levels. In the non-degraded grassland, N addition mainly affected Rs by directly increasing Ra and decreasing Rh. Conversely, in the moderately degraded grassland, N addition tended to indirectly affect Ra through increased aboveground biomass. In the severely degraded grassland, Rh was influenced indirectly by an increase in aboveground biomass. The findings of this research highlight the importance of considering the degradation level of grasslands when assessing grassland soil carbon emissions under N deposition scenarios.