A general intensification of the global hydrological cycle have increased the occurrence of extreme wet and dry years over the past 2 decades in North American grasslands, where ecosystem sensitivity to these modified precipitation patterns varies widely among arid, semi-arid, and mesic sites. Ecosystem processes above- and belowground are influenced by soil nematodes in grasslands through their multiple trophic pathways, and different functional types of nematodes can be sensitive to moisture availability. In this study we tested how nematode responsiveness to moisture availability varies among three climate contrasting Long Term Ecological Research (LTER) grassland sites: (i) arid, Jornada basin, New Mexico, (ii) semi-arid, Semiarid Grassland Research Center (formerly Shortgrass Steppe LTER), Colorado, and (iii) mesic, Konza prairie, Kansas. We manipulated growing-season precipitation in 2015 and 2016 through rainout shelters that intercept and transfer the water to water-addition plots. At the end of 2016 growing season, we measured responses of nematode abundance, trophic structure, and root-feeders community composition to five levels of precipitation manipulation: extreme and intermediate reduction, extreme and intermediate addition, and ambient control.
Results of our cross-site experiment showed that the magnitude of nematode community responses to precipitation manipulations increased across the regional water-availability gradient. While nematode total abundances and community trophic structure in the mesic grassland experienced significant changes following two growing seasons of anomalous precipitation, communities in both the arid and semi-arid grasslands remained generally unaffected. At taxonomic level, genera composition of root-feeding nematode communities differed across sites but appear to be unaffected by moisture availability. At the mesic site, total abundance of nematodes significantly decreased with increases in soil moisture caused by precipitation manipulations. This pattern was also observed for all trophic groups at that site, excepted for predators, which showed non-significant positive responses to increments in soil moisture. Hence, drought plots tended towards lower abundances of predaceous nematodes and higher abundances of root-feeders and microbial-feeders compared to water-addition plots, suggesting that mesic grasslands undergoing dryness may experience increased root-herbivory and grazing of microbial decomposers with possible effects on primary production and on nutrient cycling that deserves further investigation. By unraveling divergent responsiveness of soil nematode communities to short-term precipitation anomalies across sites our study provides novel insights into the sensitivity of ecosystems structure and functioning to climate change.