Uncoupling of carbon and nitrogen mineralization: role of microbivorous nematodes

Microfaunal grazing of soil microorganisms affects nutrient mineralization rates. However, the accessibility of microbial food resources to microfauna depends on matric potential because microfauna require water to move. Laboratory incubations of undisturbed pairs of soil cores were conducted to evaluate temporal changes in the relationships among C and N mineralization, abundance and distribution of nematode trophic groups, and matric potential. Cores were collected in May, August, and November 1997, and March 1998 from an old field. The general relationship between C and N mineralization for all data points did not hold among sampling periods. Differences in this relationship may have been a result of microbivorous grazing. Nematode abundance did not decrease as matric potential decreased, suggesting microbivorous grazers were not merely excluded from their food resources, but survived in isolated water-filled pores as soil dried. We suggest that at −50 kPa nematodes and their microbial food resources are enclosed within spatially isolated water pockets and this entrapment leads to increased microbivorous grazing and microbial activity per unit biomass ( qCO 2). Only at −50 kPa was there a strong linear relationship between qCO 2 and microbivorous nematode density. There were also negative linear relationships between qCO 2 and microbial biomass C and C:N at −50 kPa which were significantly different from the other matric potentials tested. Changes in microbial community composition appeared to affect C and N mineralization rates, but dissolved substrate availability could not directly explain differences in C and N mineralization.