SIZE OF THE SOIL MICROBIAL BIOMASS IN A LONG-TERM FIELD EXPERIMENT ASAFFECTED BY DIFFERENT N-FERTILIZERS AND ORGANIC MANURES

The size of the soil microbial biomass was measured in a more than 30 yr old field experiment, whose treatments included different N fertili zers and organic manures. The size of the microbial biomass was measur ed as biomass C and N by the chloroform fumigation-incubation techniqu e, as K2SO4 extractable ninhydrin-reactive N released upon fumigation and as the soil's ATP content. There was a high degree of correlation (r > 0.88) between the fumigation-based methods and the ATP determinat ions. Compared with the biomass estimate by ATP, biomass C was underes timated in the ammonium sulphate fertilized soil (pH 4.4), the peat-am ended soils, and the sewage sludge amended soil. Biomass N was only un der-estimated in the ammonium sulphate and peat-amended soil, whereas there was a good correlation between the ninhydrin assay and the ATP a ssay for all soils. Between three successive years biomass C showed la rger, statistically significant, variations than the size of the bioma ss measured by the ninhydrin assay. There was a high degree of correla tion (r > 0.90) between both the rate of base respiration and the size of the microbial biomass and the soil's carbon content. These relatio nships generally held independent of whether carbon was derived from s tabilized soil organic matter (in the fallow soil), from crop residues , or from organic manures such as straw, green manure, farmyard manure , or sawdust. Relative to the soil's carbon content the microbial biom ass was smaller than expected in the peat amended-soils, the ammonium sulphate fertilized, and the sewage sludge-amended soil. The rate of b ase respiration was only lower than expected in the sewage sludge trea ted soil. The size of the biomass was negatively affected by a low soi l pH, but the rate of base respiration was not. Liming some of the soi ls indicated that other factors than low pH restricted the size of the biomass in the peat and sewage-sludge amended soils, but not in the a mmonium sulphate fertilized soils.