Season and soil management affect soil microbial communities estimated using phospholipid fatty acid analysis in a continuous cabbage (Brassica oleracea var. capitata) cropping system

The effects of soil management on seasonal changes in soil microbial communities were examined in a continuously cropped field of cabbage (Brassica oleracea var. capitata) using phospholipid fatty acid (PLFA) analysis. Soil samples from each plot treated with cattle manure compost (CMC), grass compost (GRC) and chemical fertilizers (CHF) were taken at monthly intervals from April 2003 to March 2004. The results showed that microbial biomass was smaller in the CHF-treated plot than in the other plots. The mole-percentage contributions of biomarkers for Gram-negative bacteria slightly decreased from April to September and increased from September to March in all plots. The mole-percentage contributions of biomarkers for fungi and actinomyces were greater in the CHF-treated plot compared with the other plots. The mole-percentage contributions of biomarkers for Gram-positive bacteria increased in the order of CMC > GRC > CHF. Higher ratios of cyclopropyl fatty acids to their precursors and lower ratios of monounsaturated fatty acids to saturated fatty acids suggested greater stress to bacteria in the CHF-treated plot than in the other plots. Principal component analysis of the PLFA profiles indicated that soil microbial communities exhibited similar shifts in response to seasonal changes irrespective of the treatment.