Short- and long-term labile soil carbon and nitrogen dynamics reflect management and predict corn agronomic performance

Published in Agron. J. 105:493–502 (2013) doi:10.2134/agronj2012.0382 Copyright © 2013 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. C system productivity and sustainability are highly reliant on soil organic matter dynamics, including the turnover of labile C and N, and the renewal of stabilized pools (Wander, 2004; Weil and Magdoff , 2004). Th ese dynamics operate on short (seasonal) and long (years to decades) time scales, and understanding these dynamics is essential in moving toward more biologically-based cropping systems. Although soil organic matter is an extremely important indicator of overall soil quality, it can be insensitive to new management practices, as changes in total organic matter can take years to detect (Wander and Drinkwater, 2000; Wander, 2004). Th e limitations of total organic matter as an indicator have led many researchers to focus on the labile pool of organic matter. Th is pool is small (typically <20% of the total), but pivotal to the rapid cycling of nutrients, soil aggregation, and C sequestration (Wander, 2004; Weil and Magdoff , 2004; Schmidt et al., 2011). Many measures of labile organic matter are sensitive and robust indicators of soil ecosystem change, but most are expensive to measure, and accordingly, are not off ered by standard commercial soil testing laboratories (Phillips, 2010). Th ere is a clear need for more inexpensive alternative measures of labile organic matter that enable farmers, extension educators, agronomists, and soil scientists to track and predict soil C and N dynamics in their fi elds. Predicting soil N availability in cropping systems has been an ongoing challenge due to the complexities and interacting forces of weather, soil biology and physical properties, residue quality, and management practices (Cabrera et al., 2005; Schomberg et al., 2009). In a research setting, soil N availability is oft en predicted with laboratory incubations of soil, that is, N mineralization potential (Stanford and Smith 1972). Th e time and costs associated with this analysis has limited the adoption into most standard commercial soil testing laboratories, although some specialized labs do off er N mineralization tests (e.g., Idowu et al., 2008). Instead of incubations, growers currently use two primary tools to predict soil N availability: the pre-sidress nitrate test (PSNT; Magdoff et al., 1984) and leaf chlorophyll content (Piekkielek and Fox, 1992; Scharf et al., 2006). Th e PSNT measures soil nitrate in corn at V4–V6 (Fox et al., 1989), while leaf chlorophyll content measures the greenness of early stage crop leaves relative to a highly-fertilized reference strip. Although both of these tests provide a prescriptive fertilizer recommendation, many growers do not use them for a variety of reasons (Markwell et al., 1995; Andraski and Bundy, 2002; Schmidt et al., 2009). Another approach has been to approximate N mineralization potential by measuring short-term C mineralization (Franzluebbers et al., 2000). Carbon mineralization is a less expensive measurement than N mineralization potential, ABSTRACT