DETERMINANTS OF SOIL MICROBIAL COMMUNITIES - EFFECTS OF AGRICULTURAL MANAGEMENT, SEASON, AND SOIL TYPE ON PHOSPHOLIPID FATTY-ACID PROFILES

Phospholipid fatty acid (PLFA) profiles were measured in soils from or ganic, low-input, and conventional farming systems that are part of th e long term Sustainable Agriculture Farming Systems (SAFS) Project. Th e farming systems differ in whether their source of fertilizer is mine ral or organic, and in whether a winter cover crop is grown. Sustained increases in microbial biomass resulting from high organic matter inp uts have been observed in the organic and low-input systems. PLFA prof iles were compared to ascertain whether previously observed changes in biomass were accompanied by a change in the composition of the microb ial community. In addition, the relative importance of environmental v ariables on PLFA profiles was determined. Redundancy analysis ordinati on showed that PLFA profiles from organic and conventional systems wer e significantly different from April to July. On ordination plots, PLF A profiles from the low-input system fell between organic and conventi onal systems on most sample dales. A group of fatty acids (i14:0, a15: 0, 16:1 omega 7c, 16:1 omega 5c, 14:0, and 18:2 omega 6c) was enriched in the organic plots throughout the sampling period, and another grou p (10Me16:0, 20H 16:1 and 10Me17:0) was consistently lower in relative abundance in the organic system. In addition, another group (15:0, a1 7:0, i16:0, 17:0, and 10Me18:0) was enriched over the short term in th e organic plots after compost incorporation. The relative importance o f various environmental variables in governing the composition of micr obial communities could be ranked in the order: soil type > time > spe cific farming operation (e.g., cover crop incorporation or sidedressin g with mineral fertilizer) > management system > spatial variation in the field. Measures of the microbial community and soil properties (in cluding microbial biomass carbon and nitrogen, substrate induced respi ration, basal respiration, potentially mineralizable nitrogen, soil ni trate and ammonium, and soil moisture) were seldom associated with the variation in the PLFA profiles.