The chemistry of soil organic matter (SOM) is critical for soil functions. However, the chemical composition and oxidation state of physical SOM fractions under long-term mineral and organic amendments in paddy soils are still unclear. Here, we investigated this issue using three particle size classes of SOM sampled from five long-term (37-years) fertilisation regimes in a subtropical paddy soil which were analysed using pyrolysis–gas chromatography–mass spectrometry (Py–GC/MS). Coarse particulate organic matter (cPOM, >250 μm), fine particulate organic matter (fPOM, 53–250 μm) and mineral-associated organic matter (MOM, <53 μm) were isolated. The five fertilisation regimes were (1) unfertilised (control), (2) nitrogen (N, urea), (3) nitrogen + phosphorus + potassium (NPK, urea + Ca(H₂PO₄)₂ + KCl), (4) NPK + straw (NPKS) and (5) NPK + Astragalus sinicus L. + pig manure (NPKM). After 37-years, the SOM content of the control, N, NPK, NPKS and NPKM treatments had increased by 16.3%, 12.3%, 31.6%, 41.7% and 59.8% compared with the initial level, respectively. The pools of soil organic carbon (SOC) and total N (TN), as well as the C/N ratio in the soil matrix, were localised by particle size, with the majority in the mineral-associated organic matter (MOM). Particle size, not fertilisation regime, primarily associated with shifts in the chemical composition of SOM, with selective preservation of lignin and polysaccharides in the particulate organic matter (POM) and enrichments of lipids and N-containing compounds in the MOM. The origin (plant- and microbial-derived) and oxidation state of SOM were also regulated by particle size (especially the coarse particles) but not fertilisation treatment. Despite long-term persistence, the MOM had a lower C oxidation state (implying higher vulnerability to microbial utilisation) than the coarse POM (cPOM) and fine POM (fPOM). In conclusion, fertilisation regime regulated the bulk SOM (SOC and TN) pool, whereas physical particle size generated a divergence trend in the chemical composition and oxidation state of SOM in the subropical paddy soil, indicating the greater role of physical arrangement in SOM chemistry than the quality of amendments.

中文翻译：

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颗粒大小主要改变稻田土壤长期施肥下有机质的化学成分

土壤有机质 (SOM) 的化学性质对土壤功能至关重要。然而，稻田土壤中长期矿物和有机改良剂下物理 SOM 组分的化学成分和氧化状态仍不清楚。在这里，我们使用从亚热带稻田土壤中的五个长期（37 年）施肥方案中采样的三个粒径等级的 SOM 研究了这个问题，并使用热解-气相色谱-质谱法 (Py-GC/MS) 进行了分析。分离出粗颗粒有机物（cPOM，>250 μm）、细颗粒有机物（fPOM，53-250 μm）和矿物相关有机物（MOM，<53 μm）。五种施肥方案是（1）未施肥（对照），（2）氮（N，尿素），（3）氮+磷+钾（NPK，尿素+Ca（H ₂ PO _{4 ）}) ₂  + KCl)、(4) NPK + 稻草 (NPKS) 和 (5) NPK + 黄芪L. + 猪粪 (NPKM)。37年后，对照、N、NPK、NPKS和NPKM处理的SOM含量分别比初始水平增加了16.3%、12.3%、31.6%、41.7%和59.8%。土壤有机碳 (SOC) 和总氮 (TN) 库以及土壤基质中的 C/N 比由颗粒大小定位，其中大部分位于矿物相关有机质 (MOM) 中。颗粒大小，而不是施肥方案，主要与 SOM 化学成分的变化有关，与颗粒有机物 (POM) 中木质素和多糖的选择性保存以及 MOM 中脂质和含氮化合物的富集有关。SOM 的来源（植物和微生物来源）和氧化状态也受颗粒大小（尤其是粗颗粒）的调节，但不受施肥处理的调节。尽管长期存在，但与粗聚甲醛 (cPOM) 和细聚甲醛 (fPOM) 相比，MOM 的 C 氧化态较低（这意味着更容易被微生物利用）。总之，施肥方案调节了 SOM（SOC 和 TN）池，而物理颗粒大小在亚热带稻田土壤中 SOM 的化学成分和氧化状态产生了分歧趋势，表明物理排列在 SOM 化学中的作用大于修改的质量。