The relationships between gross N turnover and soil and plant N content
Soil pH values were negatively correlated with soil GA (R2 = 0.63, P < 0.0001) and GN (R2 = 0.41; P < 0.01) (Fig. 3a). However, GA (R2 = 0.87, P< 0.0001; R2 = 0.23, P< 0.05) and GN (R2 = 0.55, P< 0.001; R2 = 0.56, P< 0.001) (Fig. 3b, 3c) were positively correlated with the soil NH4+-N and NO3--N concentrations, respectively.
Based on regression analyses, the SEM suggested that N addition directly affected plant biomass and N content and soil N and GA, while it indirectly affected GN through modifying soil N (Fig. 4). Overall, the SEM results suggested that N addition level and soil N pool were the most important factors affecting soil GA and GN.