Effects of plant species diversity on nematode community composition and diversity in a long-term biodiversity experiment

Abstract

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.

Keywords: Aboveground–belowground interactions; Biodiversity loss; Plant–soil interactions; Resource quality; Resource quantity.

Fig. 1

Summary of non-metric multidimensional scaling (NMDS), abundance based on Bray–Curtis dissimilarity (a) and presence–absence based on Jaccard index (b) of 51 nematode genera identified in 93 plant communities. Circles indicate the plant communities differing in sown plant species richness, and the ellipses indicate the standard deviation of point scores for each plant species richness level. Arrow in NMDS for abundance (a) indicates which nematode genera were most important for the distribution of circles and ellipses along the sown plant species richness gradient

Fig. 2

Relationships between plant species richness and nematode diversity (a), number of bacterial feeders 100 g⁻¹ dry soil (log-transformed; b), channel ratio (= FF (FF + BF)⁻¹; c), grazing pressure ratio (= log (PF root mass⁻¹); d), number of c–p 1 and 2 nematodes 100 g⁻¹ dry soil (log-transformed; e), and number of c–p 5 nematodes 100 g⁻¹ dry soil (square root-transformed; f). Each circle represents a plant community, lines (± SE) indicate significant relationships of linear mixed-effects models (P < 0.05; Table 2)

Fig. 3

Piecewise structural equation models (SEM) exploring the effect of sown plant species richness, root mass, specific root length (SRL), shoot mass, C/N ratio of leaves (C/N_leaf), and soil organic carbon concentrations (soil C_org) on nematode diversity and composition (a), trophic group abundance (b), trophic group genus richness (c), and nematodes along the c–p scale (d). Arrows represent significant unidirectional relationships among variables (P < 0.05), while blue arrows indicate positive relationships and red arrows indicate negative relationships. Double-headed arrows show correlated errors. Standardized parameter estimates are given next to the arrows. Marginal R² (based on fixed effects only) and conditional R² (based on random and fixed effects) for component models with significant relationships are given in brackets below the respective response variable