Relationships between habitable pore space, soil biota and mineralization rates in grassland soils

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The hypotheses that the accessible soil pore volume determines the biomass of bacteria and their grazers, and that the activity of bacteria and the mineralization rates of C and N are affected by grazing pressure on bacteria were tested. We determined the biomass of bacteria, fungi, protozoa and nematodes, the pore-size distribution, and the potential mineralization rates of C and N in grassland soils with different textures. Bacteria constituted by far the largest biomass pool. Fungi, protozoa and nematodes together comprised only 10% of the total biomass. It was found that in loams and clays, most pores had diameters <0.2 μm and between 0.2 and 1.2 μm, whereas in sandy soils most pores had diameters from 6 to 30 and 30 to 90 μm. A close positive correlation was found between the bacterial biomass and the soil volume of pores with 0.2–1.2μm dia and between the biomass of nematodes and the soil volume of pores with 30–90 μm dia. The biomass of fungi and protozoa showed no relationship with a specific pore-size class. Bacterial activity [measured as the frequency of dividing-divided cells (FDDC); the number of viable cells; and the amount of CO 2 produced per cell] were not affected by grazing intensity. The amount of N mineralized bacterium −1, however, was much higher in soils with a high grazing pressure of bacterivorous nematodes and flagellates than in soils with a low grazing pressure of these groups. This indicates that grazing of bacteria by bacterivorous nematodes and flagellates may considerably increase N mineralization. No relationship was found between the grazing pressure of amoebae and the amount of N mineralized bacterium −1.