Soil structure and organic carbon are key factors that reflect soil quality in sustainable agricultural production. Many studies have concentrated on soil aeration under intensive anthropogenic agricultural practices, but the relationship between soil aggregate stability and soil organic carbon (SOC) functional groups is poorly understood, particularly in degraded greenhouse vegetable fields converted from paddy fields. To solve this problem, soil samples were collected from local vegetable fields and adjacent paddy fields. Conventional open-air vegetable fields and greenhouse vegetable fields, both with a 6–8-year cultivation history following conversion from paddy fields, were considered. SOC functional groups were characterised by ¹³C cross-polarisation magic-angle-spinning nuclear magnetic resonance (¹³C CPMAS NMR). The results showed that the mean weight diameter (MWD) values of water-stable soil aggregates in the open-air vegetable fields and greenhouse vegetable fields decreased by 64.6% and 66.7%, respectively, compared with those in the paddy fields. Similar results were obtained for the soil aggregate distribution, and soil aggregates in the greenhouse vegetable fields and open-air vegetable fields were mainly in the 0.25–2 mm size classes. Among all SOC functional groups, O-alkyl C exhibited the greatest decrease after the conversion of paddy fields to vegetable fields, no matter whether greenhouse vegetable fields or open-air vegetable fields. The degradation of soil structure decreased the physical protection of SOC functional groups by soil aggregates, and therefore, decreased the total SOC content and labile C functional group fractions in bulk soil. At the soil aggregate scale, the decrease of O-alkyl C mainly occurred in microaggregates (0.053–0.25 mm), while aromatic C decreased significantly in macroaggregates (>0.25 mm) in the greenhouse vegetable fields converted from paddy fields. Correlation analysis showed that O-alkyl C was significantly positively related to MWD and the proportion of large soil macroaggregate (>2 mm). This result suggested that soil aggregates become fragile in long-term vegetable cultivation after conversion from paddy fields, and a large loss of O-alkyl C might be the main reason for this phenomenon.

中文翻译：

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稻田转化的常规菜田土壤团聚度降低，土壤有机碳活性降低

土壤结构和有机碳是反映可持续农业生产中土壤质量的关键因素。许多研究都集中在集约化人为农业实践下的土壤通气上，但人们对土壤团聚体稳定性与土壤有机碳 (SOC) 官能团之间的关系知之甚少，特别是在从稻田转化而来的退化温室蔬菜田中。为了解决这个问题，从当地的菜地和邻近的稻田采集土壤样本。考虑了常规的露天菜地和温室菜地，它们在稻田转换后都有 6-8 年的种植历史。SOC 官能团通过¹³ C 交叉极化魔角旋转核磁共振来表征（¹³C CPMAS NMR)。结果表明，与水田相比，露天菜地和大棚菜地的水稳性土壤团聚体平均重量直径（MWD）值分别降低了64.6%和66.7%。土壤团聚体分布也得到了类似的结果，温室菜地和露天菜地的土壤团聚体主要在0.25-2 mm大小的范围内。在所有SOC官能团中，无论是温室菜地还是露天菜地，O-烷基C在稻田转化为菜地后的下降幅度最大。土壤结构的退化降低了土壤团聚体对 SOC 官能团的物理保护，因此，降低了块状土壤中的总 SOC 含量和不稳定的 C 官能团分数。在土壤团聚体尺度上，O-烷基碳的减少主要发生在微团聚体（0.053-0.25 mm）中，而芳烃碳在水田转化的大棚菜田的大团聚体（>0.25 mm）中显着降低。相关性分析表明，O-烷基C与MWD和大型土壤大团聚体（>2 mm）的比例呈显着正相关。这一结果表明，水田转化后的长期蔬菜种植中土壤团聚体变得脆弱，而O-烷基C的大量损失可能是造成这种现象的主要原因。而在稻田转化的温室菜田中，大团聚体（>0.25 mm）的芳香烃C显着降低。相关性分析表明，O-烷基C与MWD和大型土壤大团聚体（>2 mm）的比例呈显着正相关。这一结果表明，水田转化后的长期蔬菜种植中土壤团聚体变得脆弱，而O-烷基C的大量损失可能是造成这种现象的主要原因。而在稻田转化的温室菜田中，大团聚体（>0.25 mm）的芳香烃C显着降低。相关性分析表明，O-烷基C与MWD和大型土壤大团聚体（>2 mm）的比例呈显着正相关。这一结果表明，水田转化后的长期蔬菜种植中土壤团聚体变得脆弱，而O-烷基C的大量损失可能是造成这种现象的主要原因。