Effect of Lipid-Extracted Algae on Soil Physicochemical Dynamics.

Algae biofuel production would become more sustainable with high-revenue by-products, such as organic fertilizers and soil conditioners, derived from lipid-extracted algae (LEA). Adding LEA to soil may improve soil physical and chemical properties, such as aggregate formation and organic carbon (C) storage.  The objective of this study was to determine the effects of LEA on soil quality in a field environment by: a) investigating the influence of LEA incorporation on aggregate formation and distribution into macroaggregates (>250 µm), microaggregates (250-53 µm), and the free light fraction (<53 µm), b) determining SOC and total N storage within fractions, and c) determining the C source in aggregate fractions by utilizing the natural abundances of δ¹³C of Parrita soil (-16.3‰) and LEA (-27.6‰). A field study was conducted using the following treatments: 1.5% LEA, 3.0% LEA, and 3.0% organic material [1.5% LEA + 1.5% wheat (Triticum aestivum L.) straw (WS)] and a control (soil plus inorganic N and P). Soil from an adjacent field was collected from 0-15 cm depth, homogenized, sieved (< 4 mm) and used to fill the bottom 15-cm of PVC columns [33 cm x 10 cm (i.d.)]. Treatments were mixed with homogenized soil at the specified rate and then used to fill the top 15 cm of columns to a bulk density of ~ 0.8 g cm^-3. Columns were inserted into holes in soil with ~2.5 cm of the column above ground level. Columns were removed at 0 (24 hr), 3, 6, and 12 months after treatment application and separated into three depths (0-5, 5-15, and 15-30 cm) prior to drying (65°C) to constant weight and sieved (< 4 mm). Subsamples (50 g) from each depth were separated into three size fractions (>250, 250 – 53, and <53 µm) using a rotary sieve and dry sieving. Separated size fractions were weighed and ground prior to elemental (total C and N) and isotopic analyses (δ¹³C and δ¹⁵N). The discussion will focus on significant differences in aggregate formation, C source and storage in aggregate fractions as affected by LEA and other treatments over the study period.