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

The porous soil environment constrains grazing of microorganisms by microbivorous 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 ('exclusion hypothesis') because nematodes cannot access their microbial forage. We examined C and N mineralization, microbial biomass C (by fumigation-extraction), the metabolic quotient (C mineralization per unit biomass C), nematode abundance, and soil water content in intact soil cores from an old field as a function of soil matric potential (−3 to −50 kPa). We expected, in accordance with the exclusion hypothesis, that nematode abundance, N and C mineralization 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 kPa than at −10 kPa, but not significantly different from biomass C at −3 and −20 kPa. The metabolic quotient was greatest at −50 kPa than any of the other matric potentials. From the exclusion hypothesis we expected significantly fewer nematodes to be present at −50 and −20 kPa representing water-filled capillary pore sizes less than 6 and 15 μm, respectively, than at −3 and −10 kPa. Microbivorous (fungivorous+bacterivorous) nematode abundance per unit mass of soil was not significantly different among matric potentials. Body diameters of nematodes ranged from 9 μm to 40 μm. We discuss several alternatives to the exclusion hypothesis, such as the 'enclosure hypothesis' which states that nematodes may become trapped in large water-filled 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.