3.2. Litter lignin, cellulose and nutrient remaining rate
Overall, the lignin remaining rate showed a trend of decreasing and then increasing under N, P and NP addition, and the cellulose remaining rate continued to decrease with increasing decomposition time (Fig.2). Low concentrations of N, P and NP addition could promote the decomposition of lignin and cellulose, while high concentration treatment inhibited this effect, and lignin was accumulated under N3 treatment (105.61%). The one-way ANOVA showed that compared to CK, the lignin and cellulose remaining rates of N, P and NP treatments were significantly different (P < 0.001). After 360 days of decomposition, the lignin remaining rate was the lowest in N2 (68.46%), P1 (60.73%) and NP1 (57.84%) treatments, respectively, and the cellulose remaining rate in N1 (27.65%), P2 (29.72%) and NP1 (25.06%) treatments, respectively. It was noteworthy that the lignin remaining rate was significantly higher in both N3 and NP3 treatments than CK during litter decomposition (P < 0.05). The RM-ANOVA showed that the interaction of N and P addition and its synergistic effect with decomposition time had a significant effect on both lignin and cellulose remaining rates (P < 0.001) (Table S2).
The results showed that N, P and NP additions could significantly promote the release of litter carbon (C) compared to CK treatment (Fig.3). During the litter decomposition, the litter C remaining rate was not significantly different among different N addition concentrations, and the lowest in N3 treatment (38.30%) in late stage; in the late stage, the P3 (24.69%) and P2 (24.35%) treatments were significantly lower than P1 (26.63%); in the early and middle stages, NP3 treatment was significantly lower than other treatments, and in the late stage the NP2 (14.92%) treatment was significantly lower than other treatments. Compared with CK, N addition significantly inhibited the release of litter nitrogen (N) during the decomposition process, and the effect was strengthened with increasing N concentration and the highest N remaining rate (70.91%) was in N3 treatment; P addition significantly reduced the N remaining rate and the lowest value (25.06%) was in P3 treatment; NP2a and NP3 treatments significantly increased the N remaining rate in the early and middle stages, and NP1 was significantly lower than CK treatment with the lowest value of 34.29% in late stage. Compared with CK treatment, N addition significantly inhibited the release of litter phosphorus (P) in the early and late stages of decomposition, and the lowest P remaining rate (35.94%) was in N2 treatment; P3 addition significantly increased the litter P remaining rate in early and late decomposition, while P1 treatment significantly reduced the P remaining rate with the lowest value of 54.67% in the middle and late stages ; NP1 treatment significantly reduced the P remaining rate in the middle and late stages with the lowest value of 29.31%. The RM-ANOVA showed that the interaction of N and P addition and its synergistic effect with decomposition time had a significant effect on the litter C, N and P remaining rates (P < 0.001) (Table S2).