Effects of the nematofauna on microbial energy and matter transformation rates in European grassland soils

The effect of the nematofauna on the microbiology and soil nitrogen status was studied in 6 major European grassland types (Northern tundra (Abisko, S weden), Atlantic heath (Otterburn, UK), wet grassland (Wageningen, Netherla nds), semi-natural temperate grassland (Linden, Germany), East European ste ppe (Pusztaszer, Hungary) and Mediterranean garigue (Mt. Vermion, Greece). To extend the range of temperature and humidity experienced locally during the investigation period, soil microclimates were manipulated, and at each site 14 plots were established representing selected combinations of 6 temp erature and 6 moisture levels. The investigated soils divided into two grou ps: mineral grassland soils that were precipitation fed (garigue, wet grass land, seminatural grassland, steppe), and wet organic soils that were groun dwater fed (heath, tundra). Effects of the nematofauna on the microflora we re found in the mineral soils, where correlations among nematode metabolic activity as calculated from a metabolic model, and microbial activity param eters as indicated by Biolog and ergosterol measurements, were significantl y positive. Correlations with bacterial activity were stronger and more con sistent. Microbial parameters, in turn, were significantly correlated with the size of the soil nitrogen pools NH4, NO3, and Norganic. Furthermore, mo del results suggested that there were remarkable direct effects of nematode s on soil nitrogen status. Calculated monthly nematode excretion contribute d temporarily up to 27% of soluble soil nitrogen, depending on the site and the microclimate. No significant correlation among nematodes and microbial parameters, or nitrogen pools, were found in the wet organic soils. The da ta show that the nematofauna can under favourable conditions affect soil ni trogen status in mineral grassland soils both directly by excretion of N, a nd indirectly by regulating microbial activity. This suggests that the diff erences in nitrogen availability observed in such natural grasslands partly reflect differences in the activity of their indigenous nematofauna.