Corn Yields and No-Tillage Affects Carbon Sequestration and Carbon Footprints

Published in Agron. J. 104:763–770 (2012) Posted online 20 Mar. 2012 doi:10.2134/agronj2011.0353 Available freely online through the author-supported open access option. Copyright © 2012 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. A producers increasingly are being asked to document the impacts of their products on the environment. The life cycle analysis (LCA) methodology determines the C footprint (summation of all greenhouse gases) and converts soil, animal, and plant processes data into a cradle to grave environmental accounting (Wang et al., 2007; Wang 2008; Liska et al., 2009; Plevin, 2009). The C sequestration partial C footprint for corn grain–based ethanol is calculated by dividing the g CO2 ha -1 sequestered in the soil during corn production by the energy contained in the ethanol. To be consistent with reported values, a positive value indicates that CO2 was released, whereas a negative value indicates that CO2 was sequestered. The power of LCA approach is that it provides a mechanism to compare products. For example, typical reported C footprints for coal, gasoline, and ethanol are 134, 96, and 65 g CO2 equ MJ –1 (Liska et al., 2009). For the calculated LCA to have meaning, the boundary conditions of the process must be clearly defined. In this article, the partial C footprint is restricted to corn used to produce ethanol. By defining the boundary conditions, the C sequestration partial footprint can be added to other C footprints that do not or incorrectly considers soil C storage (Wang, 2008; Liska et al., 2009; Plevin, 2009). The soil C sequestration potential is influenced by many factors including the amount of C contained in the soil, the mineralization rate constants, tillage, and the amount of nonharvested C (NHC) returned to the soil (Clay et al., 2010). Calculating the C sequestration potential requires a starting point for comparisons (benchmarks). However, obtaining accurate benchmarks may be impossible due to lack of data. In addition, it may be prohibitory expensive to collect and analyze the number of samples required to determine current benchmarks. Many studies have estimated benchmarks from the USDA-NRCS STATSGO2 and USDA-NRCS SSURGO data sets (Davidson and Lefebvre, 1993; Hunt et al., 2005; Mednick et al., 2008; Reitsma et al., 2011; Zhong and Xu, 2011; Soil Survey Staff-NRCS). A problem with STATSGO2 and SSURGO is that the SOC benchmark errors and sampling protocols are not clearly defined. An alternative source of benchmarks may be producer soil samples that were analyzed by public and private laboratories (Bauder et al., 2003; Corn Growers Association North Carolina, 2002; Janssen et al., 2008). These laboratories generally follow strict protocols where information is archived with time and location stamps. The protocols may: (i) include internal reference materials and standards; (ii) external quality control assessments; and (iii) provide protocols for collecting soil samples (Gelderman et al., 2006). If a change in the chemical analysis approach is implemented, calibration studies are conducted to compare the methods. These laboratory ABSTRACT The corn (Zea mays L.)–based ethanol carbon footprint is impacted by many factors including the soil’s C sequestration potential. The study’s objective was to determine the South Dakota corn-based ethanol surface SOC sequestration potential and associated partial C footprint. Calculated short-term C sequestration potentials were compared with long-term sequestration rates calculated from 95,214 producer soil samples collected between 1985 and 2010. National Agricultural Statistics Service (NASS) grain yields, measured root/shoot ratios and harvest indexes, soil organic C (SOC) and nonharvested C (NHC) first-order rate constants, measured SOC benchmarks [81,391 composite soil samples (0–15 cm) collected between 1985 and 1998], and 34,704 production surveys were used to calculate the short-term sequestration potentials. The SOC short-term, area weighted sequestration potential for the 2004 to 2007 time period was 181kg C (ha × yr)–1. This relatively low rate was attributed to a drought that reduced the amount of NHC returned to soil. For the 2008 to 2010 time period, the area weighted short-term sequestration rate was 341 kg (ha × yr)–1. This rate was similar to the long-term measured rate of 368 kg C (ha × yr)–1. Findings from these independent SOC sequestration assessments supports the hypothesis that many of the regions surface soils are C sinks when seeded with corn. Based on short-term C sequestration rates, corn yields, and the corn conversion rate to ethanol, the area weighted surface SOC footprints for the 2004 to 2007 and 2008 to 2010 time periods was –10.4 and –15.4 g CO2 equ MJ –1, respectively.