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. Labo ratory incubations of undisturbed pairs of soil cores were conducted to eva luate temporal changes in the relationships among C and N mineralization, a bundance and distribution of nematode trophic groups, and matric potential. Cores were collected in May, August, and November 1997, and March 1998 fro m 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 dec rease as matric potential decreased, suggesting microbivorous grazers were not merely excluded from their food resources, but survived in isolated wat er-filled pores as soil dried. We suggest that at -50 kPa nematodes and the ir microbial food resources are enclosed within spatially isolated water po ckets and this entrapment leads to increased microbivorous grazing and micr obial activity per unit biomass (qCO(2)). Only at -50 kPa was there a stron g linear relationship between qCO(2) and microbivorous nematode density. Th ere were also negative linear relationships between qCO(2) and microbial bi omass C and C:N at -50 kPa which were significantly different from the othe r matric potentials tested. Changes in microbial community composition appe ared to affect C and N mineralization rates, but dissolved substrate availa bility could not directly explain differences in C and N mineralization. (C ) 2001 Elsevier Science Ltd. All rights reserved.