In a context of global change, soil has been identified as a potential carbon (C) sink, depending on land-use strategies. To detect the trends in carbon stocks after the implementation of new agricultural practices, early indicators, which can highlight changes in short timescales, are required. This study proposes the combined use of stable isotope probing and chemometrics applied to solid-state ¹³C nuclear magnetic resonance (NMR) spectra to unveil the dynamics of the storage and mineralization of soil carbon (C) pools. We focused on light organic matter fractions isolated by density fractionation of soil water stable aggregates because they respond faster to changes in land use than the total soil organic matter (SOM). Samples were collected from an agricultural field experiment with grassland, continuous maize cropping, and ley grassland under temperate climate conditions. Our results indicated contrasting aggregate dynamics depending on land-use systems. Under our experimental conditions, grassland returns larger amounts of C as belowground inputs than maize cropping, evidencing a different distribution of light C fractions between aggregate classes. Coarse aboveground inputs from maize contributed mostly to larger macroaggregates. Land-use changes with the introduction of ley grassland provoked a decoupling of the storage and/or degradation processes after the grassland phase. The newly derived maize inputs were barely degraded during the first 3 years of maize cropping, whereas grassland-derived material was depleted. As a whole, results suggest large microbial proliferation as shown by ¹³C NMR under permanent grassland, then reduced within the first years after the land-use conversion, and finally restored. The study highlighted a fractal structure of the soil, determining a scattered spatial distribution of the cycles of storage and degradation of soil organic matter related to detritusphere dynamics. As a consequence, vegetal inputs from a new land use are creating new detritusphere microenvironments that may be disconnected from the dynamics of C cycle of the previous land use. The formation of those different and unconnected microenvironments may explain the observed legacy effect of the previous land use, since each microenvironment type contributes separately to the overall soil C cycle. The effects of the new land use on the soil C cycle are delayed until the different detritusphere microenvironments remain unconnected, and the ones from the previous land use represent the predominant microenvironment type. Increasing knowledge of the soil C dynamics at a fine scale will be helpful in refining the prediction models and land-use policies.

中文翻译：

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ley草原的土地利用扰动使古代土壤有机质的降解与新获得的碳输入的存储脱钩

在全球变化的背景下，根据土地利用策略，土壤已被确定为潜在的碳汇。为了在实施新的农业实践之后发现碳储量的趋势，需要早期的指标，该指标可以突出短期内的变化。这项研究提出将稳定同位素探测与化学计量学结合应用于固态¹³C核磁共振（NMR）光谱揭示了土壤碳（C）库的存储和矿化动力学。我们专注于通过土壤水稳定聚集体的密度分级分离的轻质有机物级分，因为它们对土地利用变化的反应比总土壤有机质（SOM）更快。在温带气候条件下，从农田试验，草原连续玉米种植和ley草原的农业田间试验中收集样品。我们的结果表明，取决于土地利用系统的总体动力形成对比。在我们的实验条件下，与地下作物相比，草地作为地下投入返回的C量更大，这证明了不同类别的轻质C组分的分布不同。玉米的地上粗输入量主要贡献于较大的宏观总量。引入ley草原引起的土地利用变化引发了草原阶段之后存储和/或退化过程的脱钩。在玉米种植的前三年中，新获得的玉米投入几乎没有退化，而草地来源的材料却被消耗掉了。总体而言，结果表明微生物大量繁殖，如¹³永久草地下的13 C NMR，然后在土地利用转换后的头几年内降低，最后恢复。这项研究强调了土壤的分形结构，确定了与有机地球动力学有关的土壤有机物的存储和降解循环的分散空间分布。结果，来自新土地利用的植物投入正在创造新的破坏性微环境，这些环境可能与先前土地利用的C循环动力学无关。这些不同且不相关的微环境的形成可能解释了先前土地使用的观察到的遗留效应，因为每种微环境类型都分别对整个土壤碳循环产生影响。新土地利用对土壤碳循环的影响被延迟，直到不同的破坏层微环境保持不连通为止，先前土地用途中的土地代表了主要的微环境类型。在精细尺度上增加对土壤碳动力学的了解将有助于完善预测模型和土地利用政策。