Fate of Carbon in Water-Stable Aggregates during Decomposition of 13C-Labeled Corn Straw

Fundamental knowledge about decomposition, fate of crop residue, and allocation of residue-derived carbon (C) in soil aggregates is essential to understand the C dynamics in soil. The incorporation of C derived from corn residue in water-stable aggregate fractions, particulate organic C (POC), and mineral-associated C (MAC) in soil were examined using the C-13 tracer technique. Soil was treated with corn straw at the rate of 1% dry mass of soil brought to 66% of field capacity and incubated for 70 days at 25 degrees C. Samples were taken at 20, 35, and 70 days and analyzed for water-stable aggregates. Values for POC and MAC were analyzed for total C and C-13 enrichment. The addition of corn straw caused a shift in the distribution of recoverable particles with significant decreases in <53-m silts and clays, microaggregrates (53-250 m), and smaller macroaggegates (250-2000 m); however, the large macroaggegates (>2000 m) increased significantly. Macroaggregates contained greater amount of C than microaggregates. The proportion of C-13 recovered in the fractions <53 m (silt and clay), 53-250 m, and 250-2000 m increased during decomposition of corn straw, whereas there was no significant change in >2000-m fraction. Most (70-76%) of the soil organic C was affiliated with MAC (<53 m). Carbon (C-13) derived from corn straw decreased in POC but increased in MAC as decomposition proceeded. In the long term, microaggregate fraction appears to be involved in storage and stabilization of the C derived from corn straw and is important for soil quality and soil C sequestration point of view.