Soil and Plant Responses to Lipid-Extracted Algae
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Although algae is much more productive per area of cultivation compared to first-generation biofuel feedstocks, algae production may not be economically sustainable without high value coproducts. One of many possible coproducts may be algae residue following lipid extraction that might be used as a soil amendment for agricultural production. The overall objective of this series of experiments was to determine the feasibility and management strategies required to best utilize lipid-extracted algae (LEA) as an organic fertilizer and soil conditioner. Effects of LEA on nutrient availability, soil C storage, aggregate stability, soil acidity and salinity, greenhouse gas (GHG) loss, changes in soil microbial activity and community composition, and forage growth were assessed. Soil organic C (SOC) measured 392-d after amending soil with 1.5 and 3.0% LEA for a microcosm incubation was increased by approximately 0.2 and 0.3% OC, respectively, compared to the control. Approximately 10% of added LEA-C was mineralized and lost as CO2 compared with 15% of added wheat straw-C. Lipid-extracted algae enhanced aggregate formation and soil SOC storage in microaggregates at 0-15 cm depth over a 12-month field incubation with greater mean weight diameter by 12 months and approximately 42 and 66% of total SOC from 1.5 and 3.0% LEA treatments, respectively. With glass house and field studies, nutrient availability was enhanced with LEA amendments; however, LEA applied at a 30% rate decreased seedling emergence of foxtail millet (Setariaitalica) and salt-tolerant ryegrass (Lolium multiflorum), and thus, herbage mass (HM) and nutrient uptake were also decreased. Soil amended with 1.5% a LEA, however, increased HM of peal millet (Pennisetum glaucum), salt-tolerant ryegrass, and a sorghum-sudangrass hydrid [(Sorghum bicolor (L.) Moench ? Sorghum Sudanese (P.)]. Soil LEA-application should be a significant source of organic nutrients for microbial transformation and usage and plant uptake, and thus, may reduce inputs of inorganic fertilizer. The potential for LEA amendments enhancing aggregate formation, and consequently soil C storage, was indicated by mean weight diameter and SOC within macro- and microaggregates increasing over time. Lipid-extracted algae application may be a means of mitigating SOC losses due to agricultural production, and also, maintaining or improving soil structure and quality. However, problems with excess soil salinity, sodicity, and nutrients may occur at high LEA addition rates.