E. Witter, SOIL C-BALANCE IN A LONG-TERM FIELD EXPERIMENT IN RELATION TO THE SIZE OF THE MICROBIAL BIOMASS, Biology and fertility of soils, 23(1), 1996, pp. 33-37
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.