Recently Deposited Organic Matter in Soil Water-Stable Aggregates

Some suggest that young labile soil organic matter accumulates preferentially in water-stable macroaggregates (>250 µm) where it acts as a transient binding agent. We determined the proportions of recently deposited C (<15 yr) in water-stable aggregate size fractions using ¹³C natural abundance. The experiment was performed on a silty loam (fine-loamy, mixed, frigid Aeric Haplaquept) on plots that were under either continuous corn (C₄) or a mixed C₄-C₃ rotation for 15 yr and that had previously been under a permanent meadow (C₃). Field-sieved (6 mm) air-dry soil samples (0-10 cm) were either dry sieved or wet sieved with slaking to obtain a range of aggregate sizes. Total organic C contents and natural ¹³C abundances were determined in each aggregate size fraction, which allowed calculation of the proportions of aggregate C derived from either C₃ or C₄ (recently deposited C). The proportion of water-stable macroaggregates in the meadow soil and their C₃-derived C contents decreased with corn cropping. The net loss of C₃-derived C was greater for larger macroaggregates (>2 mm) than for smaller ones. Assuming first-order kinetics, the estimated half-life of the C₃-derived C of stable aggregates >2 mm was 13 yr, which corresponds to that reported for macro organic matter in similar systems. Analysis of water-stable macroaggregates under corn showed that they were enriched in recently deposited C relative to microaggregates and to the whole soil, which partly compensated for their loss in C₃-C. On average, 20% of the C in water-stable aggregates >1 mm was derived from corn whereas this value was down to 9% in the whole soil and 1% in the microaggregates. The results of this study provide further quantitative evidence that slaking-resistant macroaggregates are enriched in, and probably stabilized by, recently deposited organic matter.