Fertilization Effects on Physically Protected Light Fraction Organic Matter

Abstract

AbstractAggregation influences the decomposition of soil organic matter by limiting the accessibility of C substrate to microbes and fauna. We evaluated whether the turnover of light fraction organic matter (LF‐OM) in arable systems was affected by its location in the soil matrix and whether long‐term fertilization affected LF‐OM turnover. Organic C and natural 13C abundance were measured on the free and physically protected LF‐OM of soils that had been under continuous corn (Zea mays L.) for 32 yr. Fertilized corn soils had >2.5 times as much free LF‐OM material as their nonfertilized counterparts; most of this LF‐OM was derived from corn (a C4 plant). In contrast, most of the physically protected LF‐OM was derived from C3 vegetation, indicating that it had been incorporated into aggregates before the start of the experiment. Assuming that organic matter turnover follows first order kinetics, the estimated half‐life of C3‐C in the physically protected LF‐OM was twofold longer than that in the free LF‐OM. These data indicated that fertilization did not affect the turnover of either the free or the physically protected LF‐OM; however, the location of organic matter in soil aggregates was a key factor in its susceptibility to decomposition.