Soil Carbon, Nitrogen, Phosphorus Contents and Enzyme Activities of Sloping Cropland in Response to Robinia pseudoacacia Litter Addition in the Loess Hilly Region of China

Objective This study aimed to investigate the impact of litter decomposition from Robinia pseudoacacia plantations on soil fertility and nutrient cycling in ecologically fragile area.

Method Litter collected from 17-, 27- and 42-year-old plantations in the loess hilly region of China were combined with sloping cropland soils, incubated for 42 d, and analyzed for changes in soil carbon (C), nitrogen (N), and phosphorus (P) contents and stoichiometries, as well as shifts in soil enzyme activities.

Result Litter addition increased the contents of soil organic C (SOC), total N (TN), and total P (TP), as well as the activities of catalase (CAT), invertase (INV), urease (URE), and alkaline phosphatase (AP) by 248.5%, 80.7%, 2.6%, 419.7%, 401.9%, 13.7% and 72.3%, respectively, on average after the incubation. The responses of these indicators to litter addition exhibited a positive correlation with the stand age of R. pseudoacacia. The response ratios of SOC (RR-SOC) and TN (RR-TN) demonstrated an initial increase (7 ~ 14 days) followed by a decrease (14 ~ 42 days) during litter decomposition, while the response ratio of TP remained relatively stable. Furthermore, the response ratios of CAT and URE initially increased (7 ~ 28 days) and then decreased (28 ~ 42 days), whereas the response ratios of INV (RR-INV) and AP exhibited opposite trends. Notably, the most extreme values of response ratios for soil nutrient contents and enzyme activities were observed at 14 and 28 days, respectively. The results indicate that the delay in enzyme activity response may be attributed to fluctuations in soil nutrients resulting from litter decomposition. Correlation analysis revealed a significant positive correlation between RR-INV and RR-SOC, RR-TN, RR-C∶N, RR-C∶P, and RR-N∶P (the response ratios of C∶N∶P stoichiometries; P < 0.05). Redundancy analysis further demonstrated that RR-INV accounted for 15.4% of the variability in the response ratios of soil C, N, and P contents and their stoichiometry (P < 0.01), suggesting that INV activity could serve as a valuable biological indicator for reflecting soil nutrient levels. The results of the two-way analysis of variance indicated that litter quality played a dominant role in affecting soil nutrient contents, C:N:P stoichiometry, and enzyme activities.

Conclusion The addition of litter from high-age R. pseudoacacia plantations had the most pronounced positive effect on soil C, N and P levels, as well as enzyme activities, indicating that long-term revegetation efforts can effectively enhance soil fertility in environmentally vulnerable regions.