ARS | Publication request: Energy sorghum biomass harvest thresholds and tillage effects on soil organic carbon and bulk density

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 16, 2012
Publication Date: N/A

Interpretive Summary: Bioenergy feedstock production systems face many challenges, among which is the lack of guidelines on sustainable biomass harvest thresholds, and tillage cropping systems that minimize the potential cumulative effects of fresh biomass harvesting equipment-induced soil compaction. We used the ALMANAC model to evaluate four biomass removal rates, 0%, 50%, 75% and 100%, and four tillage cropping systems, No Till (NT), Conventional Till (CT), and periodically plowing NT (NTCT), or subsoiling NT (NTSS) lands to alleviate compaction, at Shorter, AL. Overall, biomass removal resulted in reduced total biomass, left-over residues, roots and total soil biomass inputs. Reduced biomass inputs to the soil resulted in N nutrient pool depletion, which were reflected as increased N stress days suffered by the crop. Whole profile soil organic carbon (SOC) storage was directly proportional to the soil biomass inputs. Given the importance of SOC as a soil quality indicator, we premised sustainability upon the maintenance of SOC at or above the initial SOC levels. For the current study, the 75% biomass removal rate can be applied on continuous NT energy sorghum production systems, assuming annual biomass inputs to the soil of 13.4 Mg ha**-1. Compared to plowing, subsoiling would be more effective in offsetting the potential cumulative effects of harvesting equipment-induced soil compaction on NT systems, with less impact on SOC storage, but at the reduced biomass removal threshold of 50%. There is an urgent need for guidelines on biomass harvest thresholds and biomass feedstock production systems that sustain productivity, environmental integrity, and ecosystem services.

Technical Abstract: Bioenergy feedstock production systems face many challenges, among which is the lack of guidelines on sustainable biomass harvest thresholds, and tillage cropping systems that minimize the potential cumulative effects of fresh biomass harvesting equipment-induced soil compaction. We used the ALMANAC model to evaluate four biomass removal rates, 0%, 50%, 75% and 100%, and four tillage cropping systems, No Till (NT), Conventional Till (CT), and periodically plowing NT (NTCT), or subsoiling NT (NTSS) lands to alleviate compaction, at Shorter, AL, for a Lynchburg loamy sand soil, over 51 years of actual weather data:1960-2010. The biomass removal rates of 50%, 75% and 100% reduced overall biomass yields by 17%, 28%, 31%, and soil biomass inputs (residues+roots) by 49%, 70%, 82%, respectively. Biomass removal resulted in soil N nutrient pool depletion. On average, the 50% and 100% removal rates respectively showed higher annual N stress days of 25 and 40 days, when compared to no biomass removal. Given the importance of soil organic carbon (SOC) as a soil quality indicator, we premised sustainability upon the maintenance of SOC at or above the initial SOC levels. For the current study, the 75% biomass removal rate can be applied on continuous NT energy sorghum production systems, assuming annual biomass inputs to the soil of 13.4 Mg ha**-1. Compared to plowing, subsoiling would be more effective in offsetting the potential cumulative effects of harvesting equipment-induced soil compaction on NT systems, with less impact on SOC storage, but at the reduced biomass removal threshold of 50%.