Mineral-associated carbon stability weakens the positive feedback of soil organic carbon to warming in abandoned croplands

Natural recovery can enhance soil organic carbon (SOC) accumulation in abandoned croplands. However, the response of different SOC fractions to warming in abandoned croplands remains unclear. Here, categorizing SOC into particulate and mineral-associated organic carbon (POC and MAOC) with physical fractionation, we investigated the responses of POC and MAOC content and temperature sensitivity (Q10) to warming, and explored the underlying mechanisms through a 3-year in situ warming experiment (open-top chambers, +1.6°C) in abandoned croplands across China (latitude: 22.33-46.58°N). Our results indicated that POC content remained unchanged while MAOC content significantly increased under warming. Changes in POC and MAOC content were mainly influenced by changes in belowground biomass and microbial necromass carbon, respectively. Warming affected the Q10 of POC and MAOC by influencing carbon quality and mineral protection, respectively. The Q10 of MAOC was significantly lower than that of POC regardless of the warming or control treatment, suggesting that MAOC represented the most persistent and least vulnerable carbon fraction within SOC. Therefore, the increase in MAOC content in abandoned croplands would weaken the positive feedback of SOC to climate warming.自然恢复可提升弃耕地土壤有机碳（SOC）累积量。然而，不同SOC组分对弃耕地增温的响应仍不明晰。本研究采用物理分组法将SOC划分为颗粒态有机碳（POC）与矿质结合态有机碳（MAOC），依托我国北纬22.33°-46.58°范围内弃耕地的3年原位增温试验（开顶式气室，增温1.6℃），探究了POC、MAOC含量及其温度敏感性（Q10）对增温的响应，并揭示了其内在机制。研究结果显示，增温处理下POC含量无显著变化，而MAOC含量则显著升高。POC与MAOC含量的变化分别主要受地下生物量与微生物残体碳的调控。增温分别通过影响碳质品质与矿物保护作用，改变了POC与MAOC的温度敏感性Q10。无论增温组还是对照组，MAOC的Q10均显著低于POC，表明MAOC是SOC中稳定性最强、最不易受干扰的碳组分。因此，弃耕地中MAOC含量的提升将削弱SOC对气候变暖的正反馈效应。