Forest soil dissolved organic matter (DOM) is a major source of terrigenous dissolved organic carbon (DOC) that is an important component of biogeochemical cycles. While many studies have shown that DOM dynamics are regulated strongly by nitrogen availability, how continued or increasing deposition of reactive nitrogen to forests alters the molecular composition of soil DOM remains unexplored. We studied the storage and molecular-level composition of soil-derived DOM after 22 years of nitrogen addition in a temperate deciduous forest. Soil DOC quantity changes were small (+19.0 g/m²; ∼10% increase) but the molecular composition changed markedly. Indices based on optical spectroscopy suggested that DOM molecular size and aromaticity were elevated with nitrogen addition. Nuclear magnetic resonance analyses showed that DOM contained more carbohydrates and aromatics but less aliphatic compounds with nitrogen addition. Ultrahigh-resolution mass spectrometric analysis further supports the finding that solid-phase isolated DOM from nitrogen-added soils was larger in molecular size and aromaticity. Condensed aromatic dissolved black carbon, particularly those compounds with high molecular size/carbon number and those stored in deep soils, showed the greatest percent increase with nitrogen addition. These results countered our hypothesis that soluble carbohydrates would be selectively removed from the DOC pool compared to polyphenols and condensed aromatic components under nitrogen enrichment. These changes in DOM molecular composition may also impact microbial communities and downstream metabolism of DOC.

中文翻译：

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长期添加氮会改变土壤衍生的溶解有机物的组成

森林土壤溶解有机物（DOM）是陆源溶解有机碳（DOC）的主要来源，它是生物地球化学循环的重要组成部分。尽管许多研究表明，DOM动态受氮的有效利用的调节，但反应性氮向森林中的沉积持续或增加如何改变土壤DOM的分子组成仍待探索。我们研究了在温带落叶林中添加氮22年后土壤衍生DOM的存储和分子水平组成。土壤DOC量变化很小（+19.0 g / m ²; 约增加10％），但分子组成发生了明显变化。基于光谱学的指标表明，随着氮的加入，DOM分子的大小和芳香性增加。核磁共振分析表明，DOM中含有较多的碳水化合物和芳烃，而氮含量较低的脂族化合物。超高分辨率质谱分析进一步支持了从含氮土壤中分离出的固相DOM分子大小和芳香性更大的发现。缩合的芳香族溶解黑碳，特别是那些具有大分子尺寸/碳原子数的化合物和储存在深层土壤中的那些，显示出随着氮的添加而增加的百分比最大。这些结果与我们的假设相反，即在氮富集下，与多酚和缩合的芳族成分相比，可溶性碳水化合物将被选择性地从DOC库中去除。DOM分子组成的这些变化也可能影响微生物群落和DOC的下游代谢。