Alteration of Nitrogen Retention and Soil Carbon with the Establishment of Bioenergy Cropping Systems in the Lower Mississippi Alluvial Valley.

It is currently estimated that only 25% of the original forested area remains in the Lower Mississippi Alluvial Valley (LMAV).  Most of the lands that supported these forests were converted to row crop agriculture.  Although much of the LMAV is highly productive and are well suited to row crop production, a significant portion of the soils are considered only marginally suited for agricultural crop production.  These types of soils are usually cropped only when agricultural product values are high or when climatic conditions favor successful crop establishment and production.  Due to the potential low value of cellulosic bioenergy crops and their adaption to poor soil conditions, these marginal soils are thought to be likely sites for cellulosic feedstock production.  We were interested in whether the establishment of eastern cottonwood (Populus deltoids) or switchgrass (Panicum virgatum), two potential bioenergy feedstocks, would alter the ecosystems services provided by these marginal soils.  We established the two bioenergy crops and a typical row cropping system (soybean-grain sorghum rotation) at three study sites located in Arkansas and Louisiana.  Two of the study sites had been continuously row cropped prior to the initiation of the study while the other during the past 20 years had been row cropped, planted to pasture, and then left fallow.  Three years after crop establishment, soil C content appeared to have increased (8-19%) with the bioenergy crop production at the two studies sites that had been continuously row cropped but not the study site left fallow.  The average soil C contents for the three cropping systems at the previously fallow study site decreased by 6% during this same three-year period.  Soil microbial C also was greater in the areas planted to cottonwood than the soybean-grain sorghum rotation.  Loss of nitrogen by leaching was 5-8 times greater in the soybean-grain sorghum rotation than the switchgrass or cottonwood crops.  The greatest losses of nitrogen occurred at the study site that was previously left fallow.  Results indicated that improved N retention or soil C sequestration with cellulosic bioenergy crop production is more likely in marginal soils that have been intensively row cropped.  Conversion of soils with more extensive management or have not supported row cropping for some time likely will have a different response to the establishment of cellulosic bioenergy crops.