Abandonment is a strategy applied to increase soil organic C (SOC) in degraded cropland, but such efforts may fail because of microbial N limitation after abandonment in the absence of fertilization. In this study, we investigated the associations between SOC and microbial necromass C (MNC) dynamics in bulk soil and particle-size pools with N availability in a cropland abandonment chronosequence on the Loess Plateau. The total SOC, total MNC, and their particulate fractions (> 0.05 mm) in soil declined in the first eight years after cropland abandonment, but increased thereafter. By the 23rd year, the SOC content in abandoned soils increased towards the levels of cropland (16.5 g kg–1) but were still far lower than those of natural vegetation (21.5 g kg–1). The mineral–associated SOC (< 0.05 mm) content maintained after abandonment; but by contrast, the mineral-associated MNC profoundly decreased. This indicated that the reduction in MNC in this fraction was compensated for by plant-derived substances from the particulate fraction. Enzymatic stoichiometry analysis identified microbial N limitations in abandoned soils compared with cropland soils. As such, microbial N limitation led to increases in mineralization and/or decreases in synthesis of MNC in both particulate and mineral-associated fractions after abandonment, attributable to the decreased total SOC. Across the abandonment chronosequence, up to 20 % of particulate SOC was derived from microbes, whereas more than half of mineral-associated SOC came from plants. These findings challenge the general consensus that particulate SOC is dominated by plant residues whereas the mineral-associated fraction contains mainly microbially derived substances. The MNC contained a smaller proportion of fungal substances in mineral-associated fractions compared to particulate fractions, reflecting microbial ecological niche differentiation in the SOC formation between particle-size fractions. In conclusion, cropland abandonment decreased MNC accumulation because of microbial N limitation, and the mineral-associated SOC was stable in quantity but not in its source composition.

中文翻译：

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耕地撂荒后土壤矿物相关有机碳组分在定量上保持不变，而在生化上不生化维持


弃置是一种用于增加退化农田土壤有机碳 （SOC） 的策略，但由于在没有施肥的情况下弃置后微生物氮的限制，这种努力可能会失败。在这项研究中，我们研究了黄土高原农田撂荒时间序列中块状土壤和具有 N 可用性的颗粒大小池中 SOC 与微生物坏死量 C （MNC） 动态之间的关联。土壤中的总 SOC、总 MNC 及其颗粒分数 （> 0.05 mm） 在耕地撂荒后的前 8 年呈下降趋势，此后呈上升趋势。到第 23 年，撂荒土壤中的 SOC 含量增加到农田水平 （16.5 g kg–1），但仍远低于自然植被的水平 （21.5 g kg–1）。废弃后保持的矿物相关 SOC 含量（< 0.05 mm）;但相比之下，矿物相关的 MNC 大幅下降。这表明该馏分中 MNC 的减少被颗粒部分的植物衍生物质所补偿。酶化学计量分析确定了与农田土壤相比，废弃土壤中的微生物氮限制。因此，微生物氮的限制导致废弃后颗粒和矿物相关组分中 MNC 的矿化增加和/或合成减少，这归因于总 SOC 的减少。在整个废弃时序中，高达 20% 的颗粒 SOC 来自微生物，而超过一半的矿物相关 SOC 来自植物。这些发现挑战了颗粒 SOC 以植物残留物为主，而矿物相关部分主要包含微生物衍生物质的普遍共识。 与颗粒级分相比，MNC 在矿物相关级分中含有较小比例的真菌物质，反映了颗粒级分之间 SOC 形成中的微生物生态位分化。综上所述，由于微生物氮的限制，耕地撂荒减少了 MNC 积累，矿物伴生 SOC 数量稳定，但其来源组成不稳定。