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a Department of Soil and Crop Sciences, Colorado State University, Fort Collins, CO 80523
b Dep. of Crop and Soil Science, Oregon State Univ., Corvallis, OR 97331
* Corresponding author (Richard.Dick{at}orst.edu)
Microorganisms and their activities can be heterogeneously distributed among soil aggregates, and their distribution may change in response to management practices that affect aggregate formation. In this study, aggregates of a winter fallow and winter cover-cropped soil were analyzed over time for microbial biomass (MBC), respiration, and N mineralization potential. Communities were characterized across aggregate sizes according to their extractable fatty acid methyl ester (FAME) profiles and Biolog substrate utilization patterns. Size distribution of soil aggregates and total N in aggregates changed over time. The percentage of the largest aggregate size fraction (2.0–5.0 mm) declined from March to August, whereas the percentage of 0.25- to 2.0-mm aggregates increased. For aggregates <2.0 mm, total N was greatest in July compared with other sampling dates. Cover crop residues enhanced MBC, respiration, and N mineralization in whole soils and aggregates, and shifted microbial community FAME profiles. Microbial respiration, MBC, and N mineralization were greatest in the intermediate aggregate size fractions, although specific distribution patterns changed over time or in response to cover crops. Community FAME structure was correlated with total C and N, MBC, respiration, and N mineralization potential. Some FAMEs, including i17:0, 10Me16:0, 19:0 cy, and 18:2
6c, varied among soil aggregates, and communities were classified according to aggregate size class by canonical discriminant analysis. Community Biolog substrate utilization patterns changed over time but were not affected strongly by aggregate size. Lack of community differentiation may be due to the frequent mixing of soil during cultivation and tillage events, whereby microbial communities become evenly distributed among soil aggregates.
Abbreviations: CDC, canonical discriminant analysis • FAME, fatty acid methyl ester • MBC, microbial biomass C • PCA, principal components analysis • TKN, total Kjeldahl N • TOC, total organic carbon
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