3.1 The effect of GE on soil pH
Most soil pH both in the grazed and GE grasslands was above 7 across China’s grasslands (Fig. 2a, b). The soil pH grazing and GE grasslands showed a significant positive correlation for all soil samples (p< 0.001) (Fig. S1). Among the 376 samples after GE, 248, 5 and 123 observations showed a decrease, neutral and increase in soil pH, respectively, along the whole soil layer relative to grazed grasslands (Fig. 2c).
On average, the changes of soil pH across the 376 grazed−GE paired samples were significant (Fig. 3a; mean pH was 8.15 and 8.02 for the grazed group and the GE group, respectively, p < 0.001). Among the 122 observations after GE, 86, 4 and 32 observations showed a decrease, neutral and increase, respectively, in soil pH in the 0–10 cm soil layer (supplementary dataset). Among the 91 observations after GE, 60 and 31 observations showed a decrease and increase in the 10–20 cm soil layer, respectively (supplementary dataset). The pH of the 0–10 and 10–20 cm layers decreased by 0.18 (p < 0.001) and 0.13 (p < 0.01) after GE, respectively (Fig. 3b). The RPC of 0–10 and 10–20 cm was −2.28% and −1.52%, respectively (Fig. 3c). Moreover, 42 of 72 and 16 of 27 observations showed a decrease in soil pH for the 20–30 and 30–100 cm soil layers, respectively (supplementary dataset). However, paired t -test showed that there were no significant differences in soil pH for the deeper soil layers (20–30 and 30–100 cm) between grazed and GE grasslands (p> 0.05) (Fig. 3b). The RPC of 20–30 and 30–100 cm was −0.50% and −0.21%, respectively (Fig. 3c).
Short- (≤5 years) and medium-term (5–15 years) GE caused a significant decrease in soil pH for GE grasslands (p < 0.001) (Fig. 4a). Long-term (≥15 years) GE had no significant influence on soil pH dynamics (Fig. 4a). The RPC of short- (≤5 years), medium- (5–15 years) and long-term (≥15 years) periods was −1.08%, −3.23% and −0.16%, respectively (Fig. 4b). The RPC of medium-term (5–15 years) GE resulted in the lowest RPC along the age gradients of GE (p < 0.05) (Fig. 4b).