Biogeophysical factors influencing soil respiration and mineral nitrogen content in an old field soil

Microbivorous grazers are thought to enhance nutrient mineralization. The predicted effect of microbivory on nutrient cycling depends on the pore habitat model used. We evaluated CO2 evolution and mineral N content of an old field soil to test two alternative habitat hypotheses. The exclusion hypothesis predicts that nematodes are separated from their microbial food resources in water-filled pores when soils dry, resulting in slower rates of biogeochemical transformations. The enclosure hypothesis predicts that nematode densities increase relative to their forage in smaller, isolated water volumes when soils dry, accelerating rates of biogeochemical transformations. We investigated the effect of soil moisture on the relationship between microbial biomass, microbivorous and predaceous nematodes, soil respiration and mineral N concentrations in an old field five times during the course of a year. We could evaluate the validity of the two habitat hypotheses for the entire field only in August 1997 because that was the only sampling date when maximum water-filled pore diameters were smaller than microbivorous nematode body diameters in all sampled field locations. The mean microbivorous and predaceous nematode abundances for the field in August were greater than 6300 kg-1 and 80,000 kg-1, respectively. Accordingly, the exclusion hypothesis was rejected. Predaceous nematode abundance was markedly higher in August than at any other sampling date. The high abundance of predators present suggests that detrital resources were not limiting productivity and that predators and microbivores were in enclosures, allowing predators to efficiently access their prey. Spatial maps, in agreement with linear correlation analyses, suggest that under our driest sampling conditions, soil respiration and mineral N content were controlled by microbivory and predation.