Diversity loss has been shown to change the soil community; however, little is known about long-term consequences and underlying mechanisms. Here, we investigated how nematode communities are affected by plant species richness and whether this is driven by resource quantity or quality in 15-year-old plant communities of a long-term grassland biodiversity experiment. We extracted nematodes from 93 experimental plots differing in plant species richness, and measured above- and belowground plant biomass production and soil organic carbon concentrations (C_org) as proxies for resource quantity, as well as C/N_leaf ratio and specific root length (SRL) as proxies for resource quality. We found that nematode community composition and diversity significantly differed among plant species richness levels. This was mostly due to positive plant diversity effects on the abundance and genus richness of bacterial-feeding, omnivorous, and predatory nematodes, which benefited from higher shoot mass and soil C_org in species-rich plant communities, suggesting control via resource quantity. In contrast, plant-feeding nematodes were negatively influenced by shoot mass, probably due to higher top–down control by predators, and were positively related to SRL and C/N_leaf, indicating control via resource quality. The decrease of the grazing pressure ratio (plant feeders per root mass) with plant species richness indicated a higher accumulation of plant-feeding nematodes in species-poor plant communities. Our results, therefore, support the hypothesis that soil-borne pathogens accumulate in low-diversity communities over time, while soil mutualists (bacterial-feeding, omnivorous, predatory nematodes) increase in abundance and richness in high-diversity plant communities, which may contribute to the widely-observed positive plant diversity–productivity relationship.

多样性丧失已被证明会改变土壤群落；然而，人们对长期后果和潜在机制知之甚少。在这里，我们调查了线虫群落如何受植物物种丰富度的影响，以及这是否受资源数量或质量驱动的长期草原生物多样性实验的 15 年植物群落。我们从植物物种丰富度不同的 93 个试验地块中提取了线虫，并测量了地上和地下植物生物量产量和土壤有机碳浓度 ( C _org ) 作为资源量的代表，以及C / N_叶比率和特定根长度 (SRL) 作为资源质量的代理。我们发现线虫群落组成和多样性在植物物种丰富度水平之间存在显着差异。这主要是由于植物多样性对以细菌为食的、杂食性和捕食性线虫的丰度和属丰富度产生了积极的影响，这些线虫受益于物种丰富的植物群落中较高的枝条质量和土壤C _{org ，表明通过资源数量进行控制。}相反，以植物为食的线虫受到枝条质量的负面影响，可能是由于捕食者的自上而下控制较高，并且与 SRL 和C / N_{叶呈正相关}，表示通过资源质量进行控制。随着植物物种丰富度的降低，放牧压力比（每根质量的植物饲养者）表明在物种贫乏的植物群落中植物取食线虫的积累更高。因此，我们的研究结果支持这样的假设，即土壤传播的病原体随着时间的推移在低多样性群落中积累，而土壤共生者（以细菌为食、杂食性、捕食性线虫）在高多样性植物群落中的丰度和丰富度增加，这可能有助于与广泛观察到的正植物多样性 - 生产力关系。