120 SSSAJ: Volume 74: Number 1 • January–February 2010 Soil Sci. Soc. Am. J. 74:120–129 Published online 13 Nov. 2009 doi:10.2136/sssaj2009.0006 Received 6 Jan. 2009. *Corresponding author (dan_israel@ncsu.edu). © Soil Science Society of America, 677 S. Segoe Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Nitrogen Transformations and Microbial Communities in Soil Aggregates from Three Tillage Systems Soil Biology & Biochemistry N o-till has several advantages over tilled systems, including increased water iniltration, greater sequestration of soil organic matter, and reduction of soil erosion. Additionally, a body of literature has also demonstrated that soil N reten- tion is greater in no-till than tilled systems (Alvarez et al., 1998; Beare et al., 1997). Plant-available N is not well deined for no-till systems, however, because during the transition from conventional tillage to no-till, particularly when C is accruing, increased microbial immobilization of inorganic N (Kitur et al., 1984; Rice and Smith, 1984) may temporarily reduce the plant-available N. As N immobilization and mineralization processes become more balanced, N availability to plants may become greater in no-till than in tilled soils (Rice et al., 1986). Information about the internal N cycle of no-till soils is limited. herefore, a thorough understand- ing of N cycling processes is required for formulation of the best N management practices in long-term no-till systems. Microbially mineralized NH 4 or NO 3 is also concurrently immobilized by soil microorganisms and recycled into the inorganic N pool due to the rapid turnover rate of the soil microbial biomass. hus, the N dynamics resulting from the balance between mineralization and immobilization of inorganic N (both Subathra Muruganandam Dep. of Soil Science North Carolina State Univ. Raleigh, NC 27695 Daniel W. Israel* USDA-ARS Dep. of Soil Science North Carolina State Univ. Raleigh, NC 27695 Wayne P. Robarge Dep. of Soil Science North Carolina State Univ. Raleigh, NC 27695 Quantifying N transformation processes in soil aggregates is relevant since microbial communities central to the N cycle may difer among aggregate size fractions. Our objective was to test the hypothesis that variations in microbial community composition of aggregate size fractions inluence 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 15 N pool dilution data with the FLUAZ model, and microbial community composition by phospholipid fatty acid (PLFA) proiles. Aggregate size fraction and tillage system had signiicant (P < 0.01) efects on total and microbial biomass C and N, gross N mineralization rate (GNMR), gross nitriication 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 difer 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. hese 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. Abbreviations: GIR, gross immobilization rate; GNMR, gross nitrogen mineralization rate; GNR, gross nitriication rate; MWE, mean weighted error; MRPP, multiple response permutation procedure; NMS, nonmetric multidimensional scaling; PLFA, phospholipid fatty acid.