A comparison of soil food webs beneath C3- and C4-dominated grasslands

Soil food webs influence organic matter mineralization and plant nutrient availability, but the potential for plants to capitalize on these processes by altering soil food webs has received little attention. We compared soil food webs beneath C₃- and C₄-grass plantings by measuring bacterial and fungal biomass and protozoan and nematode abundance repeatedly over 2 years. We tested published expectations that C₃ detritus and root chemistry (low lignin/N) favor bacterial-based food webs and root-feeding nematodes, whereas C₄ detritus (high lignin/N) and greater production favor fungal decomposers and predatory nematodes. We also hypothesized that seasonal differences in plant growth between the two grassland types would generate season-specific differences in soil food webs. In contrast to our expectations, bacterial biomass and ciliate abundance were greater beneath C₄ grasses, and we found no differences in fungi, amoebae, flagellates, or nematodes. Soil food webs varied significantly among sample dates, but differences were unrelated to aboveground plant growth. Our findings, in combination with previous work, suggest that preexisting soil properties moderate the effect of plant inputs on soil food webs. We hypothesize that high levels of soil organic matter provide a stable environment and energy source for soil organisms and thus buffer soil food webs from short-term dynamics of plant communities.