Size of the soil microbial biomass in a long-term field experiment as affected 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 fertilizers and organic manures. The size of the microbial biomass was measured as biomass C and N by the chloroform fumigation-incubation technique, as K 2SO 4 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 determinations. Compared with the biomass estimate by ATP, biomass C was underestimated in the ammonium sulphate fertilized soil (pH 4.4), the peat-amended soils, and the sewage sludge amended soil. Biomass N was only underestimated in the ammonium sulphate and peat-amended soil, whereas there was a good correlation between the ninhydrin assay and the ATP assay for all soils. Between three successive years biomass C showed larger, statistically significant, variations than the size of the biomass measured by the ninhydrin assay. There was a high degree of correlation ( r > 0.90) between both the rate of base respiration and the size of the microbial biomass and the soil's carbon content. These relationships generally held independent of whether carbon was derived from stabilized 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 biomass was smaller than expected in the peat amended-soils, the ammonium sulphate fertilized, and the sewage sludge-amended soil. The rate of base respiration was only lower than expected in the sewage sludge treated soil. The size of the biomass was negatively affected by a low soil pH, but the rate of base respiration was not. Liming some of the soils 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 ammonium sulphate fertilized soils.