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).