Soil bulk density effects on soil microbial populations and enzyme activities during the growth of maize (Zea mays L.) planted in large pots under field exposure

Li, C. H., Ma, B. L. and Zhang, T. Q. 2002. Soil bulk density effects on soil microbial populations and enzyme activities during the growth of maize (Zea mays L.) planted in large pots under field exposure. Can. J. Plant Sci. 82: 147-154. Soil compaction associated with inappropriate maneuvering of field equipment, and/or modem cropping system negatively affect soil physical properties, and thus, may limit microbial activities and biochemical processes, which are important to nutrient bioavailability. An experiment was carried out using the pot-culture technique to determine the effect of bulk density on soil microbial populations and enzyme activities in an Eutric Cambisol sandy loam soil (United Nations' classification) planted with maize (Zea mays L.) in the Experimental Farm of Henan Agricultural University, Henan, China (34degrees49'N, 113degrees40'E). Numbers of bacteria, fungi, and actinomycetes and the enzyme activities of invertase, polyphenol oxidase, catalase, urease, protease, and phosphatase were determined at various stages during the plant growing season. Microbial numbers were negatively and linearly related to soil bulk density. With increases in soil bulk density from 1.00 to 1.60 Mg m(-3), total numbers of bacteria, fungi and actinomycetes declined by 26-39%. The strongest correlations between the soil microbial population and bulk density occurred at the plant growth stages of the 6 fully expanded leaf (V6) and anthesis (RI), with R-2 > 0.90 (P < 0.01) for all three microorganism categories. Increasing soil bulk density was related quadratically to the activities of soil invertase and polyphenol oxidase, protease and catalase. It appears that the greatest activities of most soil enzymes occurred at a bulk density of 1.0 to 1.3 Mg m(-3), which are optimum for most field crops. The plant growth stages also had an important impact on soil enzyme activities and microbial populations, with strong positive associations between soil microorganisms and enzyme activities with crop growth.