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

Microbivorous grazers are thought to enhance nutrient mineralization. The p redicted effect of microbivory on nutrient cycling depends on the pore habi tat model used. We evaluated CO2 evolution and mineral N content of an old field soil to test two alternative habitat hypotheses. The exclusion hypoth esis predicts that nematodes are separated from their microbial food resour ces in water-filled pores when soils dry, resulting in slower rates of biog eochemical transformations. The enclosure hypothesis predicts that nematode densities increase relative to their forage in smaller, isolated water vol umes when soils dry, accelerating rates of biogeochemical transformations. We investigated the effect of soil moisture on the relationship between mic robial biomass, microbivorous and predaceous nematodes, soil respiration an d 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 max imum water-filled pore diameters were smaller than microbivorous nematode b ody diameters in all sampled field locations. The mean microbivorous and pr edaceous nematode abundances for the field in August were greater than 6300 kg(-1) and 80,000 kg(-1) respectively. Accordingly, the exclusion hypothes is was rejected. Predaceous nematode abundance was markedly higher in Augus t than at any other sampling date. The high abundance of predators present suggests that detrital resources were not limiting productivity and that pr edators and microbivores were in enclosures, allowing predators to efficien tly access their prey. Spatial maps, in agreement with linear correlation a nalyses, suggest that under our driest sampling conditions, soil respiratio n and mineral N content were controlled by microbivory and predation. (C) 2 001 Elsevier Science Ltd. All rights reserved.