Nitrogen (N) released from soil organic matter (SOM) is quantitatively important for crop uptake, even when adequate fertiliser N is supplied. Understanding of SOM has shifted to recognise distinct fractions that correlate with properties such as turnover time, carbon (C) and N content, and chemical composition. Yet, how these fractions relate to N supply from SOM is poorly understood. This study aimed to link N mobilisation and C stability in coarse (≥50 μm) and fine (≤50 μm) fractions, and evaluate the roles of these fractions in supplying N in cropping soil. Soils from long‐term continuous cotton, cotton‐vetch, and cotton‐wheat rotations and a nearby uncleared site (NV) were separated into coarse and fine fractions, left as whole soil, or dispersed and freeze‐dried as a fractionation control. Initial C chemistry in fractions and whole soils was measured by solid state 13C NMR spectroscopy. N mobilisation and CO2 loss were measured over a 14‐day incubation experiment. In the cropping soils, net immobilisation of N was measured in the separate fractions, while net mobilisation was found in the whole soils. In the NV soil, N mobilisation was greater in the fine fraction. C mineralisation followed the order fine fraction > fractionation control = coarse fraction > whole soil. C stability was best explained by physical protection within whole soil structure rather than chemical recalcitrance or mineral stabilisation. The results revealed an unexpected contrast between C and N mineralisation from SOM fractions and demonstrated the importance of soil aggregates for SOM stability. We show a cautionary impact of fractionation on C and N dynamic, highlighting the need for further research to understand the synergistic behaviour of SOM fractions in whole soils.

中文翻译：

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与整个土壤相比，土壤有机质组分中碳和氮稳定性的相反模式


即使提供充足的氮肥，从土壤有机质 (SOM) 中释放的氮 (N) 对于作物的吸收也具有重要的数量意义。对 SOM 的理解已转变为识别与周转时间、碳 (C) 和 N 含量以及化学成分等特性相关的不同部分。然而，人们对这些分数与 SOM 氮供应的关系知之甚少。本研究旨在将粗颗粒（≥50 μm）和细颗粒（≤50 μm）中的氮动员和碳稳定性联系起来，并评估这些颗粒在种植土壤中提供氮的作用。将来自长期连续棉花、棉-野豌豆和棉-麦轮作以及附近未清理地点（NV）的土壤分为粗粒和细粒部分，保留为整个土壤，或分散并冷冻干燥作为分馏控制。通过固态 13C NMR 光谱测量部分和整个土壤中的初始 C 化学成分。在 14 天的孵化实验中测量了氮动员和二氧化碳损失。在耕种土壤中，在单独的部分中测量了氮的净固定化，而在整个土壤中发现了净移动化。在 NV 土壤中，细粒部分的氮动员更大。 C矿化遵循的顺序是细粒>分馏控制=粗粒>全土。 C 稳定性最好的解释是整个土壤结构内的物理保护，而不是化学顽抗或矿物稳定性。结果揭示了 SOM 组分中的 C 和 N 矿化之间出乎意料的对比，并证明了土壤团聚体对 SOM 稳定性的重要性。我们展示了分馏对碳和氮动态的警示性影响，强调需要进一步研究以了解整个土壤中 SOM 分数的协同行为。