Understanding the mechanisms that control soil organic carbon (SOC) persistence is central to soil management and climate change mitigation. In the present study, we utilised a chronosequence of Vertisols which have undergone land use change from native vegetation to cropping for up to 82 y in subtropical Australia. We examined whether the marked changes in SOC concentrations were associated with changes in the physicochemical protection of SOC in aggregate structures (occlusion) and mineral surfaces (adsorption) or with changes in chemical functional composition. Soil samples were fractionated using density and physical fractionation to isolate the free particulate organic matter (fPOM), occluded POM (oPOM) and fine mineral-associated organic matter (fine-MAOM) to assess the impact of land use change on soil organic matter (SOM) fractions with differing degrees of physicochemical protection. The impact of long-term cropping on SOC functional group composition across soil fractions was assessed using synchrotron-based near edge X-ray absorption fine structure (NEXAFS) analyses. We found that although long-term cropping caused a loss of 43 % of bulk SOC after 20 y, this marked loss over time was not associated with a change in C functional group composition. Furthermore, although the SOC retention in the various fractions differed up to 60-fold (fPOM-C decreased by 78 % after cropping for 20y, whilst fine-MAOM decreased by 25 %), there were only comparatively minor differences in SOC functional group composition between these fractions. Together, these findings suggest that the differences in C retention between fractions were less related to SOC functional group composition and more related to SOM’s physicochemical protection.

中文翻译：

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在控制长期土地利用变化后的土壤有机碳保留方面，物理化学保护比化学功能组成更重要


了解控制土壤有机碳 （SOC） 持久性的机制对于土壤管理和缓解气候变化至关重要。在本研究中，我们利用了 Vertisols 的计时序列，这些 Vertisols 在澳大利亚亚热带地区经历了长达 82 年的土地利用变化，从原生植被转变为种植。我们检查了 SOC 浓度的显着变化是否与 SOC 在聚集体结构（封闭）和矿物表面（吸附）中的物理化学保护变化或化学功能组成的变化有关。使用密度和物理分馏对土壤样品进行分馏，以分离游离颗粒有机质 （fPOM）、封闭 POM （oPOM） 和细矿物相关有机质 （fine-MAOM），以评估土地利用变化对具有不同物理化学保护程度的土壤有机质 （SOM） 组分的影响。使用基于同步加速器的近边缘 X 射线吸收精细结构 （NEXAFS） 分析评估长期种植对土壤组分 SOC 官能团组成的影响。我们发现，尽管长期种植在 20 年后导致 43% 的体积 SOC 损失，但这种随时间推移的显着损失与 C 官能团组成的变化无关。此外，尽管各种馏分中的 SOC 保留差异高达 60 倍（裁剪 20 年后 fPOM-C 下降了 78%，而精细 MAOM 下降了 25%），但这些馏分之间的 SOC 官能团组成仅存在相对较小的差异。总之，这些发现表明，组分之间 C 保留的差异与 SOC 官能团组成关系较小，而与 SOM 的物理化学保护关系更大。