Production and soil effects of sorghum biofuel cropping systems in semiarid marginal regions

In order to meet the growing demands for food, fiber, and biofuels, land management decisions will require identification of lands most suitable for each crop. Biofuel production that occurs on lands otherwise constrained for other intensive agricultural production by soil or water limitations (i.e., marginal lands) may not only meet some of these demands but, if managed properly, may also help improve soil function. For large-scale application to be sustainable, identification of crop type most efficient for feedstock production as well as impacts on soil and water resources are necessary. In this study, forage sorghum (Sorghum bicolor L. Moench) cropping systems were initiated in the semiarid Southern High Plains (SHP) of the U.S. to evaluate potential biofuel production and potential benefits on soils that are depleted of organic matter (< 0.7%) due to previous cropping history. Systems consisted of two sorghum cultivars (Sorghum Partners 1990 = SP 1990 and PaceSetter bmr = PS bmr) differing in lignin content due to brown midrib trait (bmr-12) that were tested under two different water levels (non-irrigated or deficit irrigation of 2.88 mm day-1), and biomass removal rate treatments of 50% and 100%.
Forage sorghum SP1990 (non bmr) produced significantly higher weight and volumes of biomass than PS bmr under both deficit irrigation and no irrigation in the two years of study. However, PS bmr biomass was converted into ethanol (EtOH) 54% more efficiently during both years. When below average precipitation occurred during the first year of the study, both cultivars produced similar amounts of EtOH at each irrigation level (1,600 to 3,380 L ha-1). When higher than average precipitation occurred during the second year, higher biomass production of SP 1990 resulted in more EtOH production than PS bmr (3,380 vs. 2,640 L ha-1). Irrigation resulted in 26-49% more biomass and 28-72% more EtOH production during both growing seasons, indicating that non-irrigated production resulted in deficit water conditions regardless of precipitation. Overall EtOH production ranged from 1,600 to 3,380 L ha-1 during both years of the study.
Changes in soil microbial properties (0-10 cm), known to be sensitive econsensors, were measured during the two year transition from previous long-term cotton cropping systems to the sorghum biofuel cropping systems. Increases in microbial biomass C (MBC) and N (MBN) (16-17%) and differences in fatty acid methyl ester (FAME) profiles were observed after one growing season. Additionally, soil enzyme activities (EAs) targeting C, N, P, and S increased 15-75% after two growing seasons. Increases in EAs 16-19%) and differences in FAME profiles were seen due to the irrigation treatment, which may be due to the increase in belowground biomass production even under deficit irrigation. When biomass was not fully removed (50% removal treatment), increases in MBC and MBN (11-15%), b-glucosidase (C cycling) and alkaline phosphatase (P cycling) (12-22%) occurred, which is likely attributed to the protection of the soil surface from aeolian erosion provided by the surface residue. The cultivars tested, which produced biomass with different chemical composition, had little effect on the soil microbial properties measured during the time frame of this study.
This study indicates that chemical modifications and biomass yield potential are critical factors when selecting sorghum characteristics for use as biofuel feedstocks under marginal water-deficit conditions. These cropping systems also have the potential to improve sandy, low organic matter soils in this semiarid region, as was shown by increases in microbial biomass and soil functionality indicated by EAs after only two growing seasons. Early results from this study suggest sorghum biofuel cropping systems can be a sustainable practice for marginal lands in the SHP; however, tracking of long-term changes are necessary to fully evaluate effects. It is hypothesized that soil properties will continue to improve, especially in the lower biomass removal level as more above-ground biomass will be incorporated and decomposed. It is unclear how the chemical composition of biomass from different sorghum cultivars will impact soil properties but differences in organic matter accumulation and enhanced biochemical cycling are possible. Finally, additional research on incorporating biofuel production into traditional cotton production, along with the evaluation of novel sorghum cultivars specifically bred for use as feedstock, are important focuses for the application of biofuel production in the semiarid SHP.