Elevated temperature has stronger effects on the soil food web of a flooded paddy than does CO2

Paddy fields are valuable wetlands for both food production and conserving biodiversity. However, while global warming poses a major threat to biodiversity, evaluations on its impacts within flooded paddy fields have been very limited. Soil nematodes present an ideal model organism for this issue because they are dominant and diverse native animals within this system; moreover, they support aboveground biodiversity by serving as a food source for larger animals, and they are influential rice pests that affect our food output from paddy fields. We examined the effects of a predicted elevation in CO2 and temperature over the next 50 years on the dynamics of dominant nematode taxa and their food source, i.e., rice roots and microbes in a flooded paddy field equipped with a FACE + heater system. We hypothesized that, 1) the impact of an elevation in CO2 concentration (+200 ppm) would be dependent on the trophic level of the organisms (it would be greater on rice roots compared to microbes or nematodes), although the impact can be consistently detected on each of rice roots, microbes and nematodes, 2) the impact of an elevation in temperature (+2 °C) could affect each organism simultaneously but would be different among organisms due to possible differences in thermal optima. Within our 2-year study, only our temperature hypothesis was supported, i.e., that an elevation in temperature increased rice root biomass, if any, but consistently decreased microbial biomass and retarded reproduction in the nematode taxa, which are potential rice pests. Elevated CO2 increased rice root biomass, but did not consistently affect microbial biomass or reproduction of any type of nematode taxon. We infer that the small and inconsistent impact of elevated CO2 on both microbes and nematodes could be attributable to an insensitive growth response to an elevation in CO2 concentration by the rice variety we examined.