FATE OF PARTICULATE ORGANIC-MATTER IN SOIL AGGREGATES DURING CULTIVATION

Particulate organic matter (POM) is a labile fraction of soil organic matter which is thought to be physically protected from biodegradation when within soil aggregates. We have developed a fractionation method to separate POM located outside stable soil macroaggregates (>200 mu m) and microaggregates (50-200 mu m) from that within them, and applie d it to a cultivation sequence of humic loamy soils. The natural abund ance of C-13 was used to determine the amounts of POM derived from for est and that derived from crop in the free and occluded fractions. In the forest soil the free and occluded POM fractions had the same compo sition, morphology and isotopic signature. On cultivation the amounts of POM decreased sharply. The loss of C in the POM from forest was mai nly from POM outside the aggregates. The POM occluded within microaggr egates was found to turnover slowly. This may be due either to its rec alcitrant chemical nature or to its physical protection within microag gregates.