Grazing in a porous environment: 1. The effect of soil pore structure on Cand N mineralization

The porous soil environment constrains grazing of microorganisms by microbi vorous nematodes. In particular, at matric potentials at which water-filled pore spaces have capillary diameters less than nematode body diameters the effect of grazing, e.g. enhanced mineralization, should be reduced ('exclu sion hypothesis') because nematodes cannot access their microbial forage. W e examined C and N mineralization, microbial biomass C (by fumigation-extra ction), the metabolic quotient (C mineralization per unit biomass C), nemat ode abundance, and soil water content in intact soil cores from an old fiel d as a function of soil matric potential (-3 to -50 kPa). We expected, in a ccordance with the exclusion hypothesis, that nematode abundance, N and C m ineralization would be reduced as matric potential decreased, i.e. as soils became drier. N mineralization was significantly greater than zero for -3 kPa but not for -10, -20 and -50 kPa. Microbial biomass C was less at -50 k Pa than at -10 kPa, but not significantly different from biomass C at -3 an d -20 kPa. The metabolic quotient was greatest at -50 kPa than any of the o ther matric potentials. From the exclusion hypothesis we expected significa ntly fewer nematodes to be present at -50 and -20 kPa representing water-fi lled capillary pore sizes less than 6 and 15 mu m, respectively, than at -3 and -10 kPa. Microbivorous (fungivorous+bacterivorous) nematode abundance per unit mass of soil was not significantly different among matric potentia ls. Body diameters of nematodes ranged from 9 mu m to 40 mu m. We discuss s everal alternatives to the exclusion hypothesis, such as the 'enclosure hyp othesis' which states that nematodes may become trapped in large water-fill ed pore spaces even when capillary pore diameters (as computed from matric potential) are smaller than body diameters. One of the expected outcomes of grazing in enclosures is the acceleration of nutrient cycling.