Soil and crop response to harvesting corn residues for biofuel production
Introduction
Producing energy from renewable resources to reduce both over-reliance on imported fossil fuels and intensity of greenhouse gas emissions is a high priority (Herrera, 2006, Graham et al., 2007). One of the potential renewable energy feedstocks is corn stover because of its high cellulose content and abundance (Pacala and Socolow, 2004). Stover is the nongrain part of a corn plant which is left on the soil surface after harvest such as stalk, husk, leaves, and cobs (Wilhelm et al., 2004). It represents the largest amount of crop biomass produced in the U.S., mostly in the Corn Belt region, estimated at 238 million tons yr− 1 (Tg yr− 1) (Sokhansanj et al., 2002). National interest for a large-scale harvesting of corn stover as biofuel feedstocks is high, technologies for stover conversion to liquid biofuel are well advanced, and the ethanol plant construction industry is booming (Somerville, 2006).
While increasing biofuel production is important to reducing dependence on foreign oil, removal of corn stover for biofuel production may be “robbing Peter to pay Paul” (Lal and Pimentel, 2007). Corn stover retention is indispensable to achieving effective soil and water conservation (Lal, 2005). An excessive removal of corn stover for biofuel production may adversely affect SOC, nutrient cycling, soil tilth, soil water reserves, biotic activity, and crop yields (Wilhelm et al., 2004, Lal et al., 2004). It may reduce soil water storage, alter soil temperatures regimes, reduce soil structural stability, increase soil compaction, decrease water infiltration, and increase soil erosion and non-point source pollution.
While little, if any, crop residues should be removed from soils on sloping lands and from those prone to wind erosion, it is hypothesized that some amount of crop residues may be removed from those with poor internal drainage, fine texture and in regions with wet springs and sub-optimal soil temperature regime. Thus, an important but unresolved question is how much stover can be removed without jeopardizing SOC sequestration, soil and environmental quality, and agronomic sustainability? Removal of excess corn stover may even be beneficial in some soils in terms of farm economy and crop yields because excessive stover mulch causes planting difficulties and, in cool and temperate regions, slows soil warming in spring and reduces plant emergence resulting in lower crop yields (Wolfe and Eckert, 1999, Mann et al., 2002).
While the negative impacts of complete stover removal are foreseeable (Karlen et al., 1994), the impact of partial stover removal on SOC, soil physical quality, and crop productivity has not been fully resolved. Yet, it is precisely this information that is needed for determining the maximum permissible rates of stover removal and developing decision support systems for a judicious management of crop residue for essential but competing uses. Some reports indicate that 30% to 50% of the total corn stover production in the U.S. Corn Belt region may be removed for biofuel production (Kim and Dale, 2005, Graham et al., 2007). These estimates are, however, based on the residue requirements for reducing soil erosion risks and do not consider the additional requirements for sequestering SOC, off-setting CO2 emissions, maintaining soil fertility, recycling nutrients, sustaining crop yields, and maintaining the overall soil quality (Lal, 2005). Studies specifically designed to assess the effects of corn stover removal from NT continuous corn, a prime candidate for stover harvesting, are needed to determine the threshold levels of stover removal. Thus, the objectives of this study were to comprehensively assess the impacts of harvesting corn stover on SOC sequestration, CO2 emissions, soil physical quality, and agronomic productivity across three soils under long-term NT systems in Ohio after 2 1/2 yr of stover management.
Section snippets
Description of the study sites
Three sites with contrasting soil textural and topographic characteristics under long-term (> 8 yr) NT management in Ohio were selected in 2004. Two sites under glaciated soils (Celina silt loam and Hoytville clay loam) and one under an unglaciated soil (Rayne silt loam) were selected for the study. Detailed information on the study sites including soil characteristics and management history is presented in Table 1. At each site, a randomized complete block design with five corn stover
Soil organic carbon and CO2 emission
Stover removal had a significant adverse impact on SOC concentration (r > 0.95; P < 0.001), which decreased with increase in rate of stover removal in the silt loam soils (RSL and CSL) but not in the clayey soil (HCL) over the study period of 2 1/2 yr (Fig. 1). On the HCL, although the SOC concentration decreased linearly with increase in stover removal rate, differences among the five stover treatments were not significant. On the RSL, removal of 25% of stover did not significantly reduce SOC
Conclusions
This study shows that rates of SOC sequestration, compaction levels, and water and temperature regimes near the soil surface are negatively affected when stover is removed from long-term NT continuous corn over a short period of 2 1/2 years. Stover removal at rates > 25% strongly reduced SOC, decreased earthworm population, increased soil strength, reduced PAW, and decreased crop yields. However; the rate and magnitude of undesirable impacts depended strongly on site-specific characteristics
Acknowledgements
We thank Martin J. Shipitalo and Lloyd B. Owens for their cooperation during the execution of the field experiments, Kimberly L. Wintringham and Evelyn Anemaet for their comments on earlier versions of the article, and Matthew H. Davis, Clarence Renk, and Joseph Davlin for managing the experimental sites.
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