In uence of Cropping Systems on Soil Aggregate and Weed Seedbank Dynamics During the Organic Transition Period Andrew

Published in Agron. J. 102:1632–1640 (2010) Published online 29 Sept 2010 doi:10.2134/agronj2010.0156 Copyright © 2010 by the American Society of Agronomy, 5585 Guilford Road, Madison, WI 53711. 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. W management has been identifi ed by organic farmers as the number one barrier to long-term success during the transition to certifi ed production systems, with soil fertility and quality a close second (Walz, 2004). Th e aggregation of soil is an essential function in soil physicochemical and biological processes, and has been shown to infl uence soil quality through the protection of existing soil organic matter (SOM), moisture holding capacity, and soil nutrient retention (Angers and Giroux, 1996; Angers and Caron, 1998; Jiao et al., 2006). Additions of farmyard manure (FYM) to organic systems have been shown to enrich SOM directly and indirectly through improved soil properties such as increased numbers and distribution of soil macroaggregates, microfauna, macroand micronutrients and improved crop yields (Edmeades, 2003; Ghoshal and Singh, 1995; Gupta et al., 1992; Jiang et al., 2006; Jiao et al., 2006; Mikha and Rice, 2004). Paré and colleagues (1999) have shown that the addition of stockpiled FYM to conventionally tilled systems signifi cantly increased the percent of water stable aggregates compared with the same addition to a no-till system. Many studies show the opposite; in general, no-till systems result in increased total water stable soil aggregation (Denef et al., 2001; Grandy and Robertson, 2006; Green et al., 2005; Mikha and Rice, 2004; Park and Smucker, 2005; Six et al., 1999, 2000a, 2000b; Taboada-Castro et al., 2006; Zotarelli et al., 2007). While no-till systems can improve soil aggregation, they oft en rely heavily on herbicides for weed control, an unaccepted weed management practice in organic production. Perennial legume rotations, such as alfalfa, have been shown to accumulate soil carbon faster than annual crop rotations. Th is is likely due to the plant residue quality and quantity as well as root biomass growth, C rhizodeposition of the legume and less reliance on tillage, all of which infl uence soil aggregation. Rates of carbon accumulation in perennial systems appear related to changes in soil aggregate size classes as these systems can modify decomposition dynamics by changing soil aeration, water dynamics, and aggregation, as well as the biochemistry and quantity of crop residues (Grandy and Robertson, 2007). Research on organic production systems, which rely on mechanical weed management and the incorporation of green manures and FYM, have oft en shown increases in the weed seedbank as a result of viable weed seed return via incorporation of manures and reduced effi cacy of mechanical over chemical weed control during the transition (Huxham et al., 2005; Riemens et al., 2007). Conversely, other studies have shown an increase in weed species richness with an overall decline in total weed populations under organic or pesticide-free systems (Liebman and Davis, 2000; van Elsen, 2000). Th e ecological relevance of weed species richness and population will vary based on landscape and ABSTRACT