Organic matter is stabilized in soil, in part, by protection in aggregates. Macroaggregates offer some protection, but microaggregates, composed of very small clay-sized particles, offer better protection. According to the hierarchal theory, microaggregates form within macroaggregates. Aggregation and soil organic matter dynamics in tropical forests on highly-weathered soil are especially poorly known. Differences in parent material provide a way to examine geochemical control on aggregation. Replacing forest with agriculture has shown to disturb aggregates and destabilize soil organic matter, but the recovery of soil organic matter with tropical secondary forest is variable. We have been examining aggregation and organic matter in soils from forest sites on differing parent material within tropical semi-deciduous forests in the Canal Zone of the Republic of Panama. Parent materials show sharp contrasts from fine-grained andesite rocks to coarse-grained volcanic rocks to sedimentary conglomerates interbedded with marine carbonates. Soils included Oxisols and Inceptisols. Soils taken to a depth of 25 cm are separated by aggregate size fractions, and each fraction is analyzed for concentrations and natural abundance of stable isotopes of carbon and nitrogen. Land use is assessed by the amount of charcoal in the soil.
Results/Conclusions
Overall, concentrations of carbon and nitrogen in bulk soil are fairly similar across parent material. However, soils derived from the andesite flow have fewer macroaggregates, more fine clays containing carbon, and greater levels of nitrogen than soils derived from volcanic facies. Soils from the sedimentary conglomerate parent material have large amounts of protected microaggregates and high levels of carbon in primary aggregates. The soils contain charcoal, suggesting agriculture in the past, but the carbon isotope ratio of -25 per mill shows no lasting record of C4 crops, mainly, maize. The nitrogen isotope ratio shows a large increase with soil depth indicating a fast rate of organic matter decomposition. The patterns are consistent with the hypothesis that parent material influences soil aggregation in tropical forest soils and has important implications for organic matter stabilization and carbon dynamics following burning.