FERTILIZATION EFFECTS ON PHYSICALLY PROTECTED LIGHT FRACTION ORGANIC-MATTER

Aggregation influences the decomposition of soil organic matter by lim iting the accessibility of C substrate to microbes and fauna. We evalu ated whether the turnover of light fraction organic matter (LF-OM) in arable systems was affected by its location in the soil matrix and whe ther long-term fertilization affected LF-OM turnover. Organic C and na tural C-13 abundance were measured on the free and physically protecte d 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 materi al as their nonfertilized counterparts; most of this LF-OM was derived from corn (a C-4 plant). In contrast, most of the physically protecte d LF-OM was derived from C-3 vegetation, indicating that it had been i ncorporated into aggregates before the start of the experiment. Assumi ng that organic matter turnover follows first order kinetics, the esti mated half-life of C-3-C in the physically protected LF-OM was twofold longer than that in the free LF-OM, These data indicated that fertili zation did not affect the turnover of either the free or the physicall y protected LF-OM; however, the location of organic matter in soil agg regates was a key factor in its susceptibility to decomposition.