SOIL C-BALANCE IN A LONG-TERM FIELD EXPERIMENT IN RELATION TO THE SIZE OF THE MICROBIAL BIOMASS

Soil C balances were calculated in a field experiment started in 1956. Treatments include a fallow and soils receiving different N fertilize rs or organic amendments. By assuming the absence of a priming effect, the degree of mineralization of crop residues and organic amendments was calculated. Crop residue mineralization was not affected by a more than 50% decrease in the size of the microbial biomass in soil fertil ized with (NH4)(2)SO4, which had caused the pH of this soil to drop fr om 6.6 to 4.4. More C had accumulated per unit C input in peat-and sew age sludge-amended soils than in any of the other soils, suggesting th at peat and sewage sludge were more resistant to microbial attack. Rec alcitrance of substrate C was an adequate explanation for the low rati o of biomass C to soil C in the peat-amended soils, but not in the sew age sludge-amended soil. There was a close linear relationship (r=0.94 ) between the content of microbial biomass C in the soil measured in 1 990 and cumulative C losses from the soil since 1956. Compared to the relationship between soil biomass C and soil organic C concentrations, the linear relationship between microbial C and cumulative C losses s uggested that the significantly reduced biomass in the sewage sludge-a mended soil was at least partially due to the presence of toxic substa nces (presumably elevated heavy metal concentrations) in this soil and was probably not affected by the somewhat low pH (5.3) in this soil.