Drivers of nematode diversity in forest soils across climatic zones

Nematodes are the most abundant multi-cellular animals in soil, influencing key processes and functions in terrestrial ecosystems. Yet, little is known about the drivers of nematode abundance and diversity in forest soils across climatic zones. This is despite forests cover approximately 30% of the earths land surface, provide many crucial ecosystem services but strongly vary in hydrothermal conditions and associated ecosystem properties across climatic zones. Here, we collected nematode samples from 13 forests across a latitudinal gradient. We divided this gradient in temperate, warm-temperate, and tropical climatic zones. Using boosted regression trees, we showed that across the gradient, nematode abundance and diversity were mainly influenced by soil organic carbon. However, within climatic zones, other factors were more important in driving nematode alpha-diversity, nematode biomass and the abundance of different trophic groups: mean annual temperature and total soil phosphorus in temperate zones, soil pH in warm-temperate zones, and mean annual precipitation in tropical zones. Additionally, nematode beta-diversity was higher in temperate than in warm-temperate and tropical zones, and we did not find significant differences among climatic zones in nematode gamma diversity. Together, our findings indicate a latitudinal shift in the main climatic variables controlling soil nematode communities and demonstrate that the drivers of soil nematode diversity in forested ecosystems are affected by the spatial scale and climatic conditions considered. This implies that high resolution studies are needed to accurately predict how soil functions respond if climate conditions move beyond the coping range of soil organisms. Our results also emphasize the importance of studying the area-diversity relationship of soil organisms under different climatic conditions.