Effect of grazing intensity on soil resperation
Desert steppe is sparsely vegetated, so soil respiration is likewise an important determinant of carbon balance in the ecosystem. The rate of SR decreased with grazing intensity (Fig. 4d, bars), and belowground biomass (Fig. S1b), ammonium N (Fig. S2e), and available P (Fig. S2g) also significantly decreased (p < 0.05), but the effect of different grazing intensities on soil organic carbon was not significant (p > 0.05, Fig. S2d). In this study, belowground biomass, available P, and soil organic carbon were all significantly correlated with SR based on redundancy analysis. We further constructed structural equations and the results showed that grazing did not directly affect SR, but indirectly reduced the rate of SR by decreasing belowground biomass and ammonium N (Fig. 7b).
Belowground biomass is highly correlated with soil respiration (Pregitzer et al. 2008; Wu et al. 2016; Diao et al.2022). Higher CO2 fluxes may be caused by higher root biomass, which can promote soil respiration by releasing more secretions at the inter-root level and providing a favorable environment for soil microbial respiration (Wu et al. 2016). In contrast, heavy grazing reduced above- and belowground biomass, thus reducing the amount of root growth, soil microbial load and soil enzyme activity, which likely led to the inhibition of microbial respiration and ultimately reduced soil respiration rate (Li et al. 2013).
In addition, based on the GLM and SEM analyses, we also found that soil ammonium N content correlated with respiration (Fig. 5d and f, Fig. 6f and h), which is consistent with the result that nitrogen addition can stimulate soil respiration in nutrient-poor soil (Smith, 2005). The affinity of dissolved oxygen and aeration tissue for NH4+ and NO3- in root respiration mainly depends on NH4+, which is enhanced when NH4+ is absorbed. The enhancement of glutamate dehydrogenase regulation and amino acid metabolic reactions increases root N use efficiency and promotes root growth (Knapp et al. 2017). Thus, the change in soil ammonium N content is one of the main factors influencing soil respiration (Onoda et al. 2004; LeBauer and Treseder, 2008; Gong et al. 2021).