Recently Deposited Organic Matter in Soil Water‐Stable Aggregates

Abstract

Abstract 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 13 C natural abundance. The experiment was performed on a silty loam (fine‐loamy, mixed, frigid Aeric Haplaquept) on plots that were under either continuous corn (C 4 ) or a mixed C 4 ‐C 3 rotation for 15 yr and that had previously been under a permanent meadow (C 3 ). 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 13 C abundances were determined in each aggregate size fraction, which allowed calculation of the proportions of aggregate C derived from either C 3 or C 4 (recently deposited C). The proportion of water‐stable macroaggregates in the meadow soil and their C 3 ‐derived C contents decreased with corn cropping. The net loss of C 3 ‐derived C was greater for larger macroaggregates (>2 mm) than for smaller ones. Assuming first‐order kinetics, the estimated half‐life of the C 3 ‐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 C 3 ‐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.