Corn stover, left in fields after corn grain harvest, has been identified as a potential feedstock to help supply biofuel needed to offset a portion of the 14 million barrels of oils consumed daily by the U.S. transportation sector. It was projected to supply 256 million tons of the 1.4 billion tons of biomass (232 million Mg out of 1.3 billion Mg) estimated to be available each year.
Corn stover was identified as an important feedstock because of its abundance (~35 million ha of corn were planted in 2008 and 2009) and at least the perception that it was an unused material. It was also recognized that corn yields have nearly doubled since the first energy crisis during the 1970s, that yields are expected to continue to increase in the future, and that in Iowa, farmers currently spend $45 to $65 ha–1 ($20–$30 acre–1) to “manage” their stover.
From an engineering perspective, harvesting stover as a major feedstock appears quite favorable, but the Billion Ton Report (BTR) projections raised many concerns among soil scientists that harvesting excessive corn stover could reduce crops yields directly or indirectly by diminishing total organic carbon levels until soil’s production capacity was threatened.
This large group of researchers at several locations across the U.S. set out to present the soil quality baselines that were developed using the Soil Management Assessment Framework (SMAF) for several representative research locations established to examine the effects of harvesting corn stover across the eastern half of the United States. Initial grain and stover yields for the various locations are also presented to help illustrate the breadth of factors that need to be quantified to ensure feedstock harvest and biofuel production strategies are indeed sustainable.
The researchers established an effective and successful multi-location, multi-region bioenergy feedstock production study and soil, crop yield, and stover removal data have been collected for 2 yr to help quantify long-term effects of low and high stover harvest strategies. Based on an initial, but very limited SMAF analysis, it appears that total organic carbon (TOC) is the soil quality indicator that needs to be monitored most closely to quantify crop residue removal effects.
The group addressed using the TOC as a soil quality indicator by not only by measuring harvested and residual amounts of crop residue, but also by incorporating cover crops and using higher plant populations in twin-row planting systems to increase TOC input. Overall, grain yields averaged 9.7 and 11.7 Mg ha–1 (155 and 186 bu acre–1) in 2008 and 2009, values that are consistent with national projections and confirmation that the distribution of our research sites is representative of corn production throughout the United States.
The group found that the average amount of stover collected for the 50% treatment was 2.6 and 4.2 Mg ha–1 for 2008 and 2009, respectively, while the 90% treatment resulted in an average removal of 5.4 and 7.4 Mg ha–1, respectively. The group stated that based on current literature data, removal rates for both scenarios could result in a gradual decline in TOC, but there is a large standard error associated with those estimates, emphasizing the need for continuing this and other long-term studies for several years.