Effect of different application rates of organic fertilizer on soil enzyme activity and microbial population

After cultivating 24 crops of vegetables for three consecutive years in a greenhouse, the effects of different application rates of compost (Rate 1, 270 kg N ha(-1) y(-1); Rate 2, 540 kg N ha(-1) y(-1); Rate 3, 810 kg N ha(-1) y(-1); Rate 4, 1,080 kg N ha(-1) y(-1)) were compared with the effects of chemical fertilizer (CF) and no application of fertilizer treatments (CK) for some selected soil chemical properties, microbial populations and soil enzyme activities (dehydrogenase, cellulase, beta-glucosidase, protease, urease, arysulphatase, and acid and alkaline phosphatases). The results show that the pH, electrical conductivity, concentrations of total nitrogen (N) and the organic matter received from compost treatment were generally higher than those received through CF treatment. The soil microbial biomass, populations of bacteria, fungi and actinomycetes, as well as soil enzyme activities increased significantly in the compost-treated soils compared with the CF-treated soil. In most instances, no significant increase was observed in the enzymatic activities studied for compost applications higher than a Rate 2 treatment. However, all enzymatic activities examined showed significant linear correlations with the organic matter contents of the soils. The vegetable yield reached its highest level at the Rate 2 treatment and declined or leveled off in the higher treatments, implying that a high application rate of compost cannot further increase the crop yield after the soil fertility has been established. High organic matter content in the soil was found to alleviate the adverse effect of soluble salts on vegetable growth. In conclusion, an application rate of compost at Rate 2, 540 kg N ha(-1) y(-1), is adequate on the basis of vegetable yields and soil chemical, biochemical and enzymatic properties in greenhouse cultivation under subtropical climatic conditions.