The influence of microbial communities, management, and soil texture on soil organic matter chemistry

Soil organic matter dynamics following land-use change remain difficult to predict because of the complex biological, physical, and chemical mechanisms that control C turnover. We examined twelve sites, representing four broad land-use types (cultivated agriculture, pine forest, hardwood forest, and pasture), in the South Carolina Piedmont to determine whether variation in organic matter chemistry was linked to management, soil edaphic properties, microbial communities, or labile C. Organic matter chemistry was determined before and after a 232-d incubation using pyrolysis gas chromatography/mass spectroscopy and microbial community properties were determined prior to incubation by measuring extracellular enzyme activities, fungal: bacterial ratios by quantitative polymerase chain reaction (PCR), and microbial biomass. There was considerable variation in soil organic matter chemistry among the 12 sites but this could not be attributed to broad differences in land use, per se, likely because of the variation in edaphic soil properties and specific management practices within the individual land use categories. The relative abundance of N-containing compounds was correlated with the size of the labile C pool (r = 0.65). Further, the three most abundant N-containing pyrolysis products, pyridine, pyrrole, and indole, were also positively correlated with at least two enzymes and both pyridine and pyrrole were negatively correlated with fungal: bacterial ratios. In contrast, the relative abundances of lignin derivatives were often negatively correlated with enzyme activities but positively correlated with fungal: bacterial ratios. Post-incubation, silt plus clay content was negatively correlated with lignin derivatives (r = -0.68) and positively correlated with N-containing compounds (r=0.69). Our results suggest that broad land use categories are a poor predictor of soil organic matter chemistry when edaphic soil properties and specific management practices vary. However, enzyme activities, fungal: bacterial ratios, and soil texture correlate with soil organic matter chemistry across a range of ecosystems, suggesting that interactions between microbial communities and soil organic matter chemistry are important controls on soil C dynamics across landscapes. (C) 2009 Elsevier B.V. All rights reserved.