Aboveground and belowground biomass
Nitrogen addition treatments, regardless of adding N levels and frequencies, significantly increased aboveground biomass (AGB) and belowground biomass (BGB) (P < 0.01). However, mowing showed no significant effect on AGB (P > 0.05) but significantly decreased BGB (P < 0.001). Besides, the levels and frequency of N addition and mowing interactively affected AGB and BGB, respectively (P < 0.01, Table 1).
Across all N addition levels, AGB increased by 30.92% on average in control (no mowing, P < 0.05) and by 40.80% in mowed plots (P < 0.05) at the lower frequency N addition (twice a year) (Fig. 1a). AGB also increased by 10.67% in control plots (P > 0.05) and by 74.81% in mowed plots (P < 0.05) at the higher frequency of N addition (monthly) (Fig 1a). At the highest N addition level (N-50), AGB tended to be higher in mowed versus control plots, and AGB at higher N addition levels (N-20, N-50) was significantly higher in mowed than un-mowed plots at the higher frequency of N addition (P < 0.05, Fig. 1a). However, no significant effect of lower-frequency N addition was found on AGB (Fig. 1a). BGB at the lower frequency of N addition was significantly lower than that at high frequency of N addition under relatively lower N addition levels (N-2, N-10) (P < 0.05, Fig. 1b) while opposite trends were observed under relatively higher N addition levels (N-20, N-50) (Fig. 1b). BGB was significantly increased by 41.46% (P < 0.05) under N addition, and it increased with increasing N addition levels at lower N addition frequency (P < 0.05).