Soil nematode trophic structure changes with water availability. Root herbivores and microbivores are at the base of this trophic chain and are consumed by predators forming a complex trophic structure. However, not all levels of the trophic structure are affected by water availability in the same way. Large differences have been found in the response of different trophic groups along a drought event. In this study we hypothesized that soil nematodes located at the top of the trophic structure are more drought sensitive than organisms located at the base of the trophic structure. Therefore, as water availability decreases predators would be constrained in their function first while herbivores and microbivores would remain quite active. We tested these hypotheses by analyzing the trophic structure of soil nematode communities under manipulated growing-season precipitation regimes during three consecutive years in climate contrasting Long Term Ecological Research grassland sites: (i) arid, Jornada basin, New Mexico, (ii) semiarid, Semiarid Grassland Research Center, Colorado, and (iii) mesic, Konza prairie, Kansas. At each site we used rainout shelters that intercept and transfer the water to water-addition plots to impose five levels of precipitation manipulation: extreme and intermediate reduction, extreme and intermediate addition, and ambient control.
Results/Conclusions
Results of our cross-site experiment showed that the magnitude of nematode community responses to water availability increased across the regional climatic gradient. While in the mesic grassland variations in soil moisture content caused by the precipitation treatments resulted in changes in total abundances and trophic structure of the nematode community, communities in both the arid and semiarid grasslands remained generally unaffected. At the mesic site, total abundance of nematodes significantly increased with decreases in soil moisture caused by drought treatments. This pattern was also observed for all trophic groups at that site, excepted for predators, which showed significant negative responses to reductions in soil moisture. We found further support to these findings by using data from our ambient control plots to compare inter-annual differences among dry to wet years in relation to long-term precipitation averages. At the mesic site, a dry year showed 40% more nematodes than a wet year due to large increases in the population of both root herbivores and microbivores, while predators were 2.3 times less numerous in the same dry year. These results show that, in contrast to arid and semiarid ecosystems, drought suppresses soil predators and promotes root herbivores and microbivores in mesic grasslands.