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.