SOIL AGGREGATE STABILITY AND SOIL ORGANIC-MATTER FRACTIONS UNDER AGROPASTORAL SYSTEMS ESTABLISHED IN NATIVE SAVANNA

An area of native savanna on an Oxisol in the Eastern Plains of Colomb ia was opened and sown to various rotations of grass or grass-legume p asture with rice. After 4.5 years, the soil was sampled for studying t he effect of land conversion on soil aggregation and on the distributi on of total and particulate soil organic matter across the aggregate s ize classes. The size distribution of undisturbed aggregates did not v ary among treatments. Five different methods were used to measure wet aggregate stability (WAS). The choice of method affected the WAS avera ge across treatments as well as the differences among treatments. The only consistent observation was the lower WAS under monocropped rice c ompared with the other treatments. Inclusion of a legume in a pasture hardly affected aggregate stability. In contrast to the WAS measuremen ts, which were carried out with soil aggregates of 1-2 mm, wet sieving of whole-soil samples revealed additional differences among treatment s: large macroaggregates (>2 mm) proved less stable under those treatm ents that involved soil disturbance through ploughing and harvesting. Total soil C and N content did not vary among treatments, despite cons iderable differences in plant production levels. The C concentration, but not the N concentration, declined with decreasing aggregate size. The distribution of whole-soil C and N content across aggregate size c lasses depended more on the amount of soil in a certain size class tha n on the size class's C or N concentration. Those treatments that invo lved frequent soil disturbance had a smaller fraction of large macroag gregates (>2 mm) and, as a consequence, less C and N in the large macr oaggregate fraction. The particulate organic matter (POM) fraction acc ounted for only 6.2-8.5% of total soil carbon. The small size of this pool makes it unlikely that POM can serve in these Oxisols for estimat ing the amount of soil organic matter with medium turnover rate, as su ggested by others.