| Summary: | Soil organic C (SOC) storage results when organic matter inputs to soil exceed losses through decomposition, and is strongly influenced by organic matter effects on soil aggregation. We evaluated the initial effects of lipid-extracted algae (LEA), a byproduct of biofuel production, on soil aggregate formation and SOC storage. In situ field incubations were conducted by amending soil with (1) 1.5% LEA, (2) 3.0% LEA, (3) 1.5% LEA + 1.5% wheat straw (WS) and (4) soil plus inorganic N (140 kg ha<sup>−1</sup> NH<sub>4</sub>NO<sub>3</sub>) and P [112 kg ha<sup>−1</sup> Ca(H<sub>2</sub>PO<sub>4</sub>)<sub>2</sub>·H<sub>2</sub>O] as the control. Soil samples were collected 0, 3, 6, 9, and 12 months after treatment application at 0–5, 5–15, and 15–30 cm. Soil was separated into macroaggregate (>250 µm), microaggregate (250–53 µm), and silt and clay (<53 µm) fractions by dry-sieving, and mean weight diameter was calculated. Soils and soil fractions were analyzed to determine C concentrations and associated δ<sup>13</sup>C values. Mean weight diameter 12 months after 3.0% LEA application was greater than the 1.5% LEA + 1.5% WS addition at the 5–15 cm depth. Soil amended with 1.5% LEA, 3.0% LEA or 1.5% LEA + 1.5% WS resulted in greater SOC after 12 months for all soil size fractions and depths. δ<sup>13</sup>C indicated that most LEA-C was initially associated with the silt and clay fraction, but later became more strongly associated with the macro- and microaggregate fractions after 12 months. Soil application of LEA enhanced initial aggregate formation and SOC storage by increasing aggregate MWD and macro- and microaggregate associated SOC over time. As the world population grows and resources become more limited, use of alternative energy sources, soil conservation, and environmental protection must be top research priorities. Our research emphasized all three and demonstrated that LEA can enhance soil structure and C storage.
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