Nitrogen (N) fertilization has been found to alter soil carbon storage in agroecosystem as various soil organic matter (OM) components respond differently to environmental change. However, how the direction and magnitude of soil carbon storage changes in response to N fertilization remains unclear, and a lack of deep understanding on soil OM composition particularly under various fertilization rates constrains our ability to reveal soil OM dynamics. In this study, we investigated soil OM composition and degradation at the molecular-level in response to various rates of N fertilization, and the potential mechanisms that govern these alterations. Soil samples (0−10 cm) were collected from a 10-year N fertilization experiment in Southern Ontario, Canada with increasing rates (non-fertilized control, 30, 87, 145 and 260 kg ha⁻¹ yr⁻¹). The samples were analyzed for soil organic carbon and total N concentrations, soil OM composition (i.e. plant-derived steroids, cutin-, suberin-, lignin-derived compounds), and microbial biomass and community structure. Despite similar soil organic carbon (21.7–23.1 g/kg) and total N contents (2.0–2.2 g/kg) across all treatments, decreased concentrations of plant-derived steroids, cutin- and suberin-derived compounds under some N addition treatments were observed. This was consistent with lower alkyl carbon contents from solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy analysis. The lower plant-derived steroids, cutin- and suberin-derived lipids may be explained by the enhanced degradation of steroids and cutin likely associated with higher microbial biomass under N fertilization. Lignin-derived phenols were elevated under N fertilization, which is consistent with the aromatic/phenolic carbon contents (mainly from lignin) from NMR analysis. The enhanced lignin degradation, which is consistent with higher fungal biomass (lignin degraders), did not result in lower lignin-derived compound or aromatic/phenolic carbon content under N addition. As other studies reported higher lignin contents in corn residues under N fertilization, we suggest that the turnover of lignin-derived phenols may be governed not only by degradation processes but also by the quantity and quality of crop residues. Furthermore, we observed that different N rates exert various controls on soil OM cycling. For example, the increase in degradation of steroids, cutin and lignin was smaller above the threshold N rates 145 kg ha⁻¹ yr⁻¹, implying that the degradation of these compounds was fertilization rate-dependent. These results highlight that various N fertilization levels alter the allocation of carbon and soil OM dynamics likely through changing biodegradation and the quantity and quality of crop residues.

由于各种土壤有机质（OM）组分对环境变化的反应不同，因此氮（N）施肥会改变农业生态系统中的土壤碳储量。然而，土壤碳储藏的方向和大小如何随氮肥的变化而变化尚不清楚，而且对土壤有机质组成缺乏深刻的了解，尤其是在不同的施肥速率下，这不足以限制我们揭示土壤有机质动态的能力。在这项研究中，我们调查了土壤OM的组成和分子水平对各种氮肥施用量的响应，以及控制这些变化的潜在机理。土壤样品（0-10厘米）是从加拿大安大略省南部的一个为期10年的N施肥实验中收集的，其比例不断增加（非施肥对照，30、87、145和260 kg公顷^-1 yr ^-1）。分析样品的土壤有机碳和总氮浓度，土壤有机物成分（即植物衍生的类固醇，角质，木栓质，木质素衍生的化合物）以及微生物生物量和群落结构。尽管在所有处理中土壤有机碳（21.7–23.1 g / kg）和总氮含量（2.0–2.2 g / kg）相似，但在某些氮添加处理下，植物类固醇，角质和木栓质衍生的化合物的浓度仍降低观测到的。这与固态^13中较低的烷基碳含量相符C核磁共振（NMR）光谱分析。较低的植物来源的类固醇，角质和木栓质衍生的脂质可能是由于氮肥下类固醇和角质的降解增强，可能与较高的微生物量相关。木质素衍生的酚在氮肥下升高，这与NMR分析得出的芳香族/酚类碳含量（主要来自木质素）一致。增加的木质素降解与较高的真菌生物量（木质素降解剂）一致，在添加氮的情况下，不会导致木质素衍生的化合物或芳族/酚碳含量降低。正如其他研究报道，氮肥下玉米残渣中木质素含量更高，我们建议，木质素衍生的酚的转化不仅受降解过程的控制，而且还受农作物残渣的数量和质量的控制。此外，我们观察到不同的氮肥施用量对土壤有机质循环具有多种控制作用。例如，高于氮临界浓度145 kg ha时，类固醇，角质和木质素降解的增加较小。^-1 yr ^-1，这意味着这些化合物的降解与施肥速度有关。这些结果表明，不同的氮肥水平可能通过改变生物降解能力以及作物残留物的数量和质量来改变碳和土壤有机质动态的分配。