Long-Term Film Mulching with Manure Amendment Drives Trade-Offs Between Spring Maize Nutrient Uptake and Topsoil Carbon Stability: Labile Carbon Fractions as Key Predictors of Agroecosystem Functioning on the Loess Plateau

Film mulching and gravel mulching are effective methods for increasing crop yields in Northwest China but exacerbate soil organic carbon (SOC) mineralization, while manure amendment is a viable method for offsetting carbon (C) losses from mulching. SOC stability is a key factor in determining the nutrient supply capacity of soil, as it affects the C sources available to microorganisms. However, the synergistic effects of film mulching with manure amendment on SOC stability and crop nutrient uptake are still unclear. Thus, four treatments of no mulching (CK), gravel mulching (GM), film mulching (FM) and film mulching with manure amendment (FCM) were established on the Loess Plateau. The experiments investigated nitrogen (N), phosphorus (P), potassium (K) uptake in the plant and grain, SOC, labile organic C fractions (LOCFs), stability-based organic C fractions (SOCFs) and C management index (CMI) in 2019 and 2020. The results indicated that FM and FCM treatments significantly improved crop dry matter accumulation in both years compared with that of the control, and the FCM treatment significantly increased the two-year NPK average of plants to 44.9%, 50.7%, and 54.5%, and significantly increased the two-year NPK average of grain to 46.7%, 58.2%, and 30.4%. The FCM treatment significantly increased all LOCFs, namely water solution C (WSC), hot-water extractable C (HWC), permanganate oxidizable C (POXC) and particulate organic C (POC) in the topsoil (0-20 cm) in both years, and the fractions of active C pool (AP) in SOCFs, namely very labile C (CVL) and labile C (CL), were significantly increased, which suggested the FCM treatment significantly decreased the C stability of topsoil. The results of CMI confirmed this. The sensitivity index indicated that among all SOC fractions, POC (21.5-72.9%) and less labile C (CLL) (20.8-483.8%) were the most sensitive fractions of LOCFs and SOCFs compared with SOC (1.93-35.8%). The random forest analysis showed that most labile C fractions and CMI had significant contributions to crop N, P and K uptake, especially POXC to crop N uptake, CMI to crop P uptake, and AP to crop K uptake. And the quantitative relationships were determined by linear regression analysis. The study concluded the FCM treatment synergistically enhanced SOC lability, crop NPK uptake, and labile C fractions (especially POXC, AP) and CMI serve as robust indicators for guiding precision nutrient management in semi-arid croplands.