Mc. Savin et al., Biogeophysical factors influencing soil respiration and mineral nitrogen content in an old field soil, SOIL BIOL B, 33(4-5), 2001, pp. 429-438
Microbivorous grazers are thought to enhance nutrient mineralization. The p
redicted effect of microbivory on nutrient cycling depends on the pore habi
tat model used. We evaluated CO2 evolution and mineral N content of an old
field soil to test two alternative habitat hypotheses. The exclusion hypoth
esis predicts that nematodes are separated from their microbial food resour
ces in water-filled pores when soils dry, resulting in slower rates of biog
eochemical transformations. The enclosure hypothesis predicts that nematode
densities increase relative to their forage in smaller, isolated water vol
umes when soils dry, accelerating rates of biogeochemical transformations.
We investigated the effect of soil moisture on the relationship between mic
robial biomass, microbivorous and predaceous nematodes, soil respiration an
d mineral N concentrations in an old field five times during the course of
a year.
We could evaluate the validity of the two habitat hypotheses for the entire
field only in August 1997 because that was the only sampling date when max
imum water-filled pore diameters were smaller than microbivorous nematode b
ody diameters in all sampled field locations. The mean microbivorous and pr
edaceous nematode abundances for the field in August were greater than 6300
kg(-1) and 80,000 kg(-1) respectively. Accordingly, the exclusion hypothes
is was rejected. Predaceous nematode abundance was markedly higher in Augus
t than at any other sampling date. The high abundance of predators present
suggests that detrital resources were not limiting productivity and that pr
edators and microbivores were in enclosures, allowing predators to efficien
tly access their prey. Spatial maps, in agreement with linear correlation a
nalyses, suggest that under our driest sampling conditions, soil respiratio
n and mineral N content were controlled by microbivory and predation. (C) 2
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