Size and activity of the microbial biomass and N, S and P availability in earthworm casts derived from arable and pastoral soil and arable soil amended with plant residues

The interaction between the size and activity of the soil microbial biomass and the availability of N, P and S in earthworm casts was investigated in a laboratory study. Earthworms Aporrectodea calignosa (Savigny), were fed p asture or arable soil or arable soil amended with either ground lucerne hay or wheal straw. Uningested. soil and casts were incubated for 42 days and sampled periodically for analyses. Transit of soil through the earthworm gu t effected all measured parameters. Microbial biomass C was smaller for cas t than for soil material for the pasture treatment, but the reverse was the case? For the arable treatment. The microbial respiratory quotient was,how ever, greater for cast than soil material in both treatments. The smaller, but more metabolically active microbial biomass in pasture casts was sugges ted to have been caused by a change in the composition of the microbial com munity. Concentrations of mineral N were markedly higher in casts than unin gested soil and this was accompanied by higher protease activity in casts. Concentrations of SO42-S and resin-extractable P were similar in casts and soils during the initial stages of incubation, but were larger in casts aft er 28 and 42 days. Surprisingly, activities of arylsulphatase and acid phos phatase were lower in casts than uningested soil. An increase in concentrat ion of labile, readily mineralizable organic P and S during,out transit was suggested as a possible explanation for their increased availability in ca sts during incubation. Amendment of the arable soil with organic residues r esulted in casts with a larger microbial biomass and respiratory quotient a nd generally higher enzyme activities. Microbial immobilization of N and S in the wheat-amended casts was evident.