Energy sorghum biomass harvest thresholds and tillage effects on soil organic carbon and bulk density

Bioenergy feedstock production systems face many challenges, among which is the lack of guidelines on sustainable biomass harvest thresholds and tillage cropping systems that maintain soil quality and productivity. We used the ALMANAC crop model to evaluate four biomass removal rates, 0%, 50%, 75% and 100%, and four tillage cropping systems, continuous No Till (NT), and Conventional Till (CT), and periodically plowed or subsoiled NT lands at Shorter, AL, for a Lynchburg loamy sand soil, over 51 yr of actual weather data: 1960–2010. Farmers periodically plow or subsoil NT lands to alleviate problems of drainage, pests, and soil compaction. 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. As expected, NT had the highest SOC and lowest bulk density (BD) across the four biomass removal rates and gained the highest percent SOC over the 51-yr simulation period. For this study, the 75% biomass removal rate was applied sustainably on NT energy sorghum production systems, giving an annual harvestable biomass yield of 18.0±0.9, residue biomass, 6.2±0.3, and a root biomass of 7.2±0.4Mgha−1. However, the 75% removal rate also significantly increased soil bulk density, a critical indicator of soil compaction, by 30%. Compared to conventional tillage, subsoil tillage maintained SOC and better alleviated soil compaction in NT systems, but at the reduced biomass removal threshold of 50%. Long-term biomass removal resulted in reduced total biomass yields over time due to nutrient depletion as reflected by increased N stress days on subsequent crops. We attributed the N stress to N immobilization by the decomposing residues, reduced mineralization and N losses. Additional inputs will be needed to avoid increased N uptake from the soil which could result in soil mining.