The effect of free-living nematodes on nitrogen mineralisation in undisturbed and disturbed soil cores

Soil fauna, particularly nematodes, are considered to strongly contribute to nitrogen mineralisation through grazing on microflora during decomposition. Demonstration of this effect has mostly relied on calculation-based soil food web analyses or experiments involving simplified and artificially constructed food webs. We carried out an incubation experiment in which defaunated soil cores were reinoculated with entire soil nematode populations extracted from bulk soil and during which nitrogen mineralisation was measured. Both undisturbed and disturbed cores were prepared to investigate whether a representative pore structure influences the effect of entire free-living nematode populations on nitrogen mineralisation. Cores were subjected to a 5 kGy gamma irradiation dose sufficient to eliminate all soil fauna while leaving the microbial biomass largely intact. Half of the irradiated cores were reinoculated with nematodes extracted from a corresponding volume of bulk soil and incubated for 82 days. The microbial biomass was not strongly affected by gamma irradiation or nematode addition but declined strongly in all treatments during incubation. Reinoculation of nematodes was successful in establishing populations of a similar size and composition as in the control samples. Net nitrogen mineralisation from indigenous soil organic matter was observed in all treatments throughout the incubation, but was always more pronounced in irradiated cores. Total mineral nitrogen concentrations did not differ significantly between simply irradiated and irradiated then reinoculated cores. However by the end of the incubation period nematode addition resulted in 87% and 23% more NO3−–N g−1 dry soil in undisturbed and disturbed cores respectively, while NH4+–N g−1 dry soil decreased by 50% in both core types. We found no convincing evidence for a contribution of free-living nematodes on total nitrogen mineralisation, but the activity of nitrifying organisms was clearly stimulated by nematode grazing. ► Disturbed and undisturbed cores were defaunated by gamma irradiation (5 kGy). ► Defaunated soil was reinoculated with nematode communities extracted from bulk soil. ► Nematode reinoculation caused lower NH4+ and higher NO3− concentrations. ► Grazing by nematodes did not result in not greater total nitrogen mineralisation. ► Nematode and nitrogen dynamics differed between disturbed and undisturbed cores.