Nonlinear responses of soil nematode community composition to increasing aridity

Aim: Increasing aridity under global change is predicted to have a profound impact on the structure and functioning of terrestrial ecosystems, yet we have poor understanding of how belowground communities respond. In order to understand the longer-term responses of different trophic levels in the soil food web to increasing aridity, we investigated the abundance, richness and community similarity of the soil nematode community along a 3200-km aridity gradient.
Location: A transect across semi-arid and arid grasslands in Northern China, where the aridity ranges from 0.43 to 0.97.
Time period: July and August 2012.
Major taxa studied: Soil-borne Nematoda.
Methods: We used Generalized Additive (Mixed) Models to analyze the abundance, richness and community similarity patterns of soil nematodes. We used Structural Equation Modelling (SEM) to disentangle the direct and indirect environmental drivers (aridity, soil and plant variables) of the nematode community.
Results: The abundance, richness and similarity of nematode communities declined non-linearly with increasing aridity. The most pronounced decline in nematode richness and community similarity occurred under arid conditions (aridity > 0.80). However, the shape of response to aridity differed among nematode feeding groups. Under arid conditions, the abundance and richness of bacterial feeders were less sensitive to changes in aridity than fungal feeders. The SEM analysis revealed that nematode community responses to aridity were not mediated via changes in plant and soil variables, but rather were directly affected by aridity.
Main conclusions: Our results show that in mesic grasslands increasing aridity primarily caused decline in nematode abundance, whereas increasing aridity in xeric grasslands led to loss of nematode diversity. The non-linear responses of nematodes to aridity could result in non-linear shifts in ecosystem functioning as well, because soil nematodes operate at various trophic levels in the soil food web, thereby influencing the performance of plants, soil biodiversity and biogeochemical cycling.