3.Results
3.1. Litter mass remaining and decomposition rate constant
The results showed that N, P and NP additions significantly contributed to the decomposition of litter matter (Fig.1). At 360 days, the mass remaining of litter was 53.62%, 62.36% and 65.16% in N1, N2 and N3 treatment; 55.56%, 53.02% and 51.35% in P1, P2 and P3 treatment; 51.05%, 47.72% and 49.23% in NP1, NP2 and NP3 treatment, respectively. One-way ANOVA showed that compared to CK, there were significant differences in mass remaining of litter in different concentrations of N, P and NP treatments, respectively (P < 0.001). Under N addition, N1, N2 and N3 treatments significantly reduced mass remaining in the early stages of decomposition, and only N1 treatment had a significant effect in the middle and late stages of decomposition; under P addition, P1, P2 and P3 significantly reduced mass remaining in each decomposition period; under NP addition, NP1, NP2 and NP3 treatments significantly reduced mass remaining in the middle and late stages of decomposition, and no significant difference was found between NP2 and NP3 treatments. The RM-ANOVA showed that the interaction of N and P additions and their synergistic effect with decomposition time had a significant effect on the mass remaining of litter (P < 0.001) (Table S1).
The litter decomposition rate constant (K ) increased significantly with increasing P addition concentration with the highest value in the P3 treatment (0.67 year-1), and showed a trend of increasing and then decreasing with increasing N and NP addition concentrations with the highest values in the N1 (0.62 year-1) and NP2 (0.73 year-1) treatments, respectively (Table 2). In the CK treatment, the time required for 50% (T0.5) and 95% (T0.95) of litter decomposition was 1.67 and 7.21 years, respectively. Under N addition, T0.5 and T0.95 were 1.11 and 4.81 years for the N1 treatment, 1.47 and 6.34 years for the N2 treatment, and 0.62 and 6.99 years for the N3 treatment, respectively. Under P addition, T0.5and T0.95 were 1.18 and 5.10 years for the P1 treatment, 1.09 and 4.72 years for the P2 treatment, and 4.49 and 1.04 years for the P3 treatment, respectively. Under NP addition, T0.5and T0.95 were 1.03 and 4.46 years for NP1 treatment, 0.94 and 4.05 years for NP2 treatment, and 0.98 and 4.12 years for NP3 treatment, respectively. The RM-ANOVA showed that the interaction of N and P additions and their synergistic effect with decomposition time had a significant effect on the decomposition rate constant (P < 0.001) (Table S1).