Abstract
Quantifying N transformation processes in soil aggregates is relevant since microbial communities central to the N cycle may differ among aggregate size fractions. Our objective was to test the hypothesis that variations in microbial community composition of aggregate size fractions influence N transformation rates of soil from three long‐term (22‐yr) tillage systems (no‐till, chisel plow, and moldboard plow). Aggregate size fractions (2–4, 0.5–1, and <0.25 mm) were obtained by dry sieving. Nitrogen transformation rates were estimated by analysis of 15N pool dilution data with the FLUAZ model, and microbial community composition by phospholipid fatty acid (PLFA) profiles. Aggregate size fraction and tillage system had significant (P < 0.01) effects on total and microbial biomass C and N, gross N mineralization rate (GNMR), gross nitrification rate (GNR), and gross N immobilization rate (GIR). No‐till soils and the 0.5‐ to 1.0‐mm aggregate size fraction had the highest N transformation rates. Net N mineralization rates were greater for no‐till than for tilled soils. Multiple response permutation analysis of PLFA data revealed that microbial community composition did not differ with aggregate size fraction. Stepwise regression analysis indicated that microbial community composition (nonmetric multidimensional scaling Axis 1) accounted for 89% of the variation in GIR, soil C and N concentrations accounted for 88% of the variation in GNMR, and microbial biomass C concentration accounted for 81% of the variation in GNR. These results indicate that greater N transformation rates in no‐till than tilled soil were due primarily to increased microbial biomass (i.e., microbial population size) rather than altered microbial community composition.
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