The structural complexity of soil dissolved organic matter (DOM) may reflect soil biogeochemical processes due to its spectral characteristics. However, the features of DOM structural complexity in paddy soil amended with long-term chemical P fertilization are still unclear, which may limit understanding of nutrient-related soil C cycle. We collected soil samples from field experiments receiving application of 0, 30, 60, and 90 kg P ha⁻¹ yr⁻¹ to assess the effect of exogenous P on the complexity of soil DOM structure. Three-dimensional excitation-emission matrix fluorescence analysis and enzymatic activity assay were used to determine the features of soil DOM molecular structure and the associated microbial reactions. The results showed that P input increased the biodegradability of DOM, indicating by the increased lower molecular weight components and decreased humic degree in the DOM. P input also reduced the structural complexity of DOM with blue shifts of fluorescent signals. The fluorescence index and β/α index of DOM increased with increasing P application by 4–5% and 3–11%, respectively, while humification index decreased by 8–13%. The P input increased the abundance of bacteria and fungi by 34–167% and 159–964%, respectively, while 29–54% increments were found for the β-1,4-glucosidase activities. These results implicated that P fertilization accelerated the soil DOM cycle, although the structural complexity of DOM declined, which potentially benefits soil C sequestration in paddy fields and may be a C sequestration mechanism in the P-dependent paddy.

土壤溶解有机物（DOM）的结构复杂性可能由于其光谱特征而反映了土壤生物地球化学过程。然而，通过长期化学磷肥改良的水稻土中DOM结构复杂性的特征仍不清楚，这可能会限制对与养分有关的土壤碳循环的理解。我们从接受0、30、60和90 kg P ha ^-1 yr ^-1施用的田间试验中收集了土壤样品评估外源磷对土壤DOM结构复杂性的影响。利用三维激发-发射矩阵荧光分析和酶活性测定方法，确定了土壤DOM分子结构特征和相关的微生物反应。结果表明，P输入增加了DOM的生物降解性，表明DOM中较低的分子量成分增加，腐殖度降低。P输入还通过荧光信号的蓝移降低了DOM的结构复杂性。荧光指数和β/α随着磷肥施用量的增加，DOM指数分别增加4–5％和3–11％，而腐殖化指数下降8–13％。磷的输入分别使细菌和真菌的丰度增加了34–167％和159–964％，而β-1,4-葡萄糖苷酶的活性增加了29–54％。这些结果表明，尽管DOM的结构复杂性下降了，但施肥加速了土壤DOM循环，这可能有利于稻田固碳，并且可能是依赖P的稻田固碳的机制。