Conversion of Marginal Soils in the Lower Mississippi Alluvial Valley to Bioenergy Feedstock Production: Carbon Sequestration and Nitrogen Retention.

A substantial portion of the marginal soils in the Lower Mississippi Alluvial Valley (LMAV) were cleared of native forests and planted to soybeans between 1960 and 1980.  Many of these soils are low in fertility and prone to late spring flooding.  Soybean-grain sorghum rotations are frequently grown on these soils due to the low inputs of nutrients required by these crops and/or their tolerance of diverse moisture conditions that occur in these marginal soils.  Eastern cottonwood and switchgrass are two crops that are being considered for bioenergy production in the LMAV.  To evaluate the potential for carbon sequestration and nitrogen retention by these bioenergy crops, we established eastern cottonwood, “Alamo” switchgrass, and a soybean-grain sorghum cropping system on marginal soils at three locations in the LMAV.  Carbon and nitrogen in the soils, soil water, aboveground biomass, and fine roots were monitored.  At the end of the third growing season a total of 9.0 and 11.0 Mg/ha of C (above + belowground) was respectively sequestered by the eastern cottonwood and switchgrass, compared to 4.1 Mg/ha of C by the soybean-grain sorghum cropping system.  The majority of the C was sequestered in aboveground biomass, but increases in soil C occurred for the dedicated energy crops at two of the study sites.  During this same three-year period, more than twice the amount of N (265 kg/ha) was accumulated in the biomass of the soybean-grain sorghum cropping system than in the two bioenergy crops (eastern cottonwood 125 kg/ha; switchgrass 129 kg/ha).  However, N concentrations in soil water were 6-8 times greater in the soybean-grain sorghum rotation than in the bioenergy crops.  These initial results suggested that these bioenergy crops can increase C sequestration and N retention.  Data collected through the fifth growing season will also be summarized.