1. Plant diversity is an important driver of below‐ground ecosystem functions, such as root growth, soil organic matter (SOM) storage and microbial metabolism, mainly by influencing the interactions between plant roots and soil. Dissolved organic matter (DOM), as the most mobile form of SOM, plays a crucial role for a multitude of soil processes that are central for ecosystem functioning. Thus, DOM is likely to be an important mediator of plant diversity effects on soil processes. However, the relationships between plant diversity and DOM have not been studied so far.
2. We investigated the mechanisms underlying plant diversity effects on concentrations of DOM using continuous soil water sampling across 6 years and 62 plant communities in a long‐term grassland biodiversity experiment in Jena, Germany. Furthermore, we investigated plant diversity effects on the molecular properties of DOM in a subset of the samples.
3. Although DOM concentrations were highly variable over the course of the year with highest concentrations in summer and autumn, we found that DOM concentrations consistently increased with plant diversity across seasons. The positive plant diversity effect on DOM concentrations was mainly mediated by increased microbial activity and newly sequestered carbon in topsoil. However, the effect of soil microbial activity on DOM concentrations differed between seasons, indicating DOM consumption in winter and spring, and DOM production in summer and autumn. Furthermore, we found increased contents of small and easily decomposable DOM molecules reaching deeper soil layers with high plant diversity.
4. Synthesis. Our findings suggest that plant diversity enhances the continuous downward transport of DOM in multiple ways. On the one hand, higher plant diversity results in higher DOM concentrations, on the other hand, this DOM is less degraded. This study indicates, for the first time, that higher plant diversity enhances the downward transport of dissolved molecules that likely stimulate soil development in deeper layers and therefore increase soil fertility.

中文翻译：

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植物多样性提高了可生物降解的可溶性有机物的产量和向下运输的能力

1. 植物多样性是地下生态系统功能的重要驱动力，例如根系生长，土壤有机质（SOM）储存和微生物代谢，主要是通过影响植物根系与土壤之间的相互作用来实现的。溶解有机物（DOM）作为SOM的最易移动形式，对于生态系统功能至关重要的多种土壤过程起着至关重要的作用。因此，DOM可能是植物多样性对土壤过程影响的重要媒介。但是，到目前为止尚未研究植物多样性与DOM之间的关系。
2. 在德国耶拿的一项长期草地生物多样性实验中，我们通过连续6年和62个植物群落的土壤水连续采样，研究了植物多样性对DOM浓度的潜在影响机制。此外，我们调查了植物多样性对子集样本中DOM分子特性的影响。
3. 尽管DOM的浓度在一年中变化很大，夏季和秋季的浓度最高，但我们发现DOM的浓度随着季节的变化而不断增加。植物对DOM浓度的正向影响主要是由于微生物活性的增加和表土中新近被隔离的碳所介导的。但是，土壤微生物活性对DOM浓度的影响在不同季节之间有所不同，表明冬季和春季食用DOM，夏季和秋季食用DOM。此外，我们发现较小且易于分解的DOM分子的含量增加，到达具有较高植物多样性的更深的土壤层。
4. 综合。我们的发现表明植物多样性以多种方式增强了DOM的持续向下运输。一方面，较高的植物多样性导致较高的DOM浓度，另一方面，该DOM降解程度较小。这项研究首次表明，更高的植物多样性增强了溶解分子的向下运输，这可能刺激了更深层的土壤发育，从而提高了土壤肥力。