Apparent Red Clover Nitrogen Credit to Corn: Evaluating Cover Crop Introduction

Published in Agron. J. 105:1658–1664 (2013) doi:10.2134/agronj2013.0089 Available freely online through the author-supported open access option. 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. T value of using cover crops includes reduced soil erosion, gains in soil organic matter, capture of unused fertilizer N, decreased soil compaction, and suppression of diseases and weeds when compared with bare soil or winter fallow (Snapp et al., 2005; Tonitto et al., 2006; Cherr et al., 2006). Before industrial manufacturing of inorganic fertilizers, the most common reason for using a legume cover crop was to enhance soil fertility, especially N (Dinnes et al., 2002). Many studies have attributed the benefit of a legume cover crop before corn to an increase in soil N levels following legume incorporation (Hesterman et al., 1986; Blevins et al., 1990; Torbert et al., 1996; Vyn et al., 1999; Sanchez et al., 2001); however, other non-N rotational benefits have been identified as well (McVay et al., 1989; Raimbault and Vyn, 1991; Corak et al., 1991). In total, the direct contribution of N from legume N2 fixation coupled with factors such as improved soil physical, chemical, and biological properties may synergistically produce the overall “rotational effect” (Gaudin et al., 2013); however, legume cover crops are generally managed as a N source (Tonitto et al., 2006; Liebman et al., 2012). Red clover, when frost-seeded into winter wheat (Triticum aestivum L.), has been shown to fix and accumulate large amounts of N in the aboveground biomass and has been evaluated as a green manure prior to corn production (Hesterman et al., 1992; Tiffin and Hesterman, 1998; Schipanski and Drinkwater, 2011; Gaudin et al., 2013). Due to the low C/N ratio of red clover tissues, N turnover is generally faster than for nonleguminous cover crops (Varco et al., 1989; Wagger, 1989); however, environmental and management factors can markedly influence decomposition dynamics, and it is difficult to accurately predict the amount of N that will become available, or when it will become available, to a subsequent crop (Crews and Peoples, 2005; McSwiney et al., 2010; Ruffo and Bollero, 2003). Snapp et al. (2005) determined that uncertainty regarding N mineralization and availability was one of the chief barriers to adoption of legume cover crops among surveyed Michigan farmers. Determining the N credit value of a cover crop is critical for calculating the N fertilizer application rate that maximizes profit and protects the environment (Andraski and Bundy, 2002; Stute and Posner, 1995; Vyn et al., 1999). Knowledge is limited, however, concerning longer term, decadal interactions of producing cover crops within a given cropping system. Long-term field crop experiments greatly improve our understanding of historical management effects on soil properties, including slow processes such as soil organic matter accumulation (Sanchez et al., 2004; Robertson et al., 2008). During the first decade of a row crop experiment in southwest Michigan, the presence of cover crops in a corn–soybean [Glycine max (L.) Merr. ]–wheat rotation supported soil C increases of 10 to 40 g C m–2 annually (Robertson et al., 2000). Similarly, diversification with cover crops in a 14-yr organic corn–soybean–wheat rotation experiment in Pennsylvania enhanced soil C by 42 g C m–2 annually (Drinkwater et al., 1998). Although these studies demonstrate gains in soil C and improved soil fertility, the timing of nutrient release, particularly with regard to soil N supply, has been shown to be one of the primary yield-limiting factors for cash crop production in organic farming systems (Cavigelli et al., 2008). Soil fertility amendments (i.e., fertilizers, composted dairy manure, and green manures) are generally applied to enhance the growth and yield of the cash crop. Although rarely studied, it can be inferred that practices improving soil nutrient and organic matter status will, in turn, enhance cover crop performance. We hypothesized that agricultural systems that have ABSTRACT Corn (Zea mays L.) production systems can benefit from introducing a leguminous winter cover crop into the rotation, especially with regard to increased N availability (i.e., legume N credit); however, it is not known if the full agronomic benefit is realized in the first year of cover crop introduction or if the benefit is cumulative with time. The objective of this study was to determine the apparent red clover (Trifolium pratense L.) N credit to corn in a conventional system where red clover was introduced for the first time compared with three agricultural systems that had a 14-yr history of using cover crops. The apparent red clover N credit was calculated by the difference in unfertilized corn N accumulation between cover and no-cover split-split plots. These data suggest that corn growers can realize the full benefits of a red clover cover crop in the first year of introduction.