Rapid response of soil microbial communities from conventional, low input, and organic farming systems to a wet/dry cycle

Soil microbial communities may be strongly influenced by agricultural practices which change the soil environment. One such practice is the use of organic amendments and cover crops which increase carbon availability to microorganisms. Another is irrigation which, in California's hot, rain-free growing season, can cause severe wet/dry cycles. We investigated (i) long-term differences in amounts of organic inputs using soils from organic, low input, and conventional farming systems, and (ii) differences in severity of soil drying following irrigation, using soil from two depths, 0-3 and 3-15 cm. All soils were air-dried and re-welted, and we measured short-term changes in microbial biomass carbon (MBC), dissolved organic carbon (DOC), respiration, and phospholipid ester-linked fatty acid (PLFA) composition before and for 27 h after re-wetting, Respiration rates were fit to a two-first-order-component model. Carbon respired from the more slowly utilized C pool of the two-component model, MBC, and DOC increased with increasing amounts of organic inputs, and PLFA composition of the organic and conventional soils clearly differed in their mole percentages of numerous fatty acids when analyzed by principal components analysis and redundancy analysis. Despite these differences, the response of microbial communities in the three farming systems to soil drying and re-wetting was similar. For example, the relative increase in MBC following soil re-wetting did not differ among the farming system soils. In contrast, the relative increase in MBC after re-wetting was greater, and the respiratory response to soil re-wetting was more rapid in the surface (0-3 cm) than deeper (3-15 cm) layer. Higher ratios of cyclopropyl fatty acids to their precursors suggested greater stress to bacteria in the deep than surface layer, and these ratios declined more rapidly after re-wetting in the deep than surface layer. This study suggested that adaptation to wet/dry cycles by surface microorganisms had occurred during the 3-month growing season, leading to changes in both microbial process rates and community composition. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.