Selective gopher disturbance influences plant species effects on nitrogen cycling

Plant species effects on ecosystem processes are mediated by traits such as litter quality and exudation. These same traits also influence the activity and distribution of animals that play key roles in regulating ecosystem dynamics. We planted monocultures of eight plant species commonly found in California grasslands to investigate the relative importance of plant species direct effects on nitrogen cycling, versus their indirect effects mediated by plant interactions with gophers. Plant species differed in their litter C:N ratio, which closely related to species effects on rates of net mineralization and nitrification in undisturbed soil. However, the effect of selective gopher disturbance on N cycling greatly altered these species effects. Plant species differed in their effects on the type and timing of gopher disturbance. Small feeding holes were formed in late spring in plots containing species with high tissue quality. These feeding holes minimally disturbed the soil and did not alter N cycling rates over the short term. Large gopher mounds were formed in the winter and early spring, primarily in plots containing the grass, Aegilops triuncialis, and to a lesser extent in plots containing Avena barbata . These large mounds significantly disturbed the soil and greatly increased net nitrification rates, but had no consistent effects on net N mineralization. In undisturbed soil, Aegilops had the highest litter C:N ratio and one of the lowest rates of net nitrification. However, gophers preferentially built large mounds in Aegilops plots. Once the effects of gopher burrowing were considered, Aegilops had one of the highest rates of net nitrification, indicating that the indirect effects of plant species on N cycling can be more important than the direct effects alone. This experiment indicates that it is vital to consider interactions between plants and other organisms in order to predict the ecosystem effects of plant communities.