Soil organic C sequestration and stabilization in karstic soils of Yucatan

The tropical semiarid soils of northwestern Yucatan, Mexico, have exceptionally high organic matter (OM) contents, between 50 and 150 g C kg(-1). The soils are formed on limestone and form a mosaic of shallow black lithosols surrounding rock outcrops and deeper red rendzinas at slightly lower relief. Traditionally, these soils were managed under shifting cultivation with an uncommonly short cultivation period of only 2 years followed by a long bush fallow. We examined OM mineral associations using size and density fractionations of two soils after 1 and 12 years of fallow in a search for factors responsible for the high OM stability, short cultivation period and poor productivity. Light-fraction OM accounted for up to 38% of total soil OM and was responsible for almost all the OM accretion during fallow. Red soils contained half as much OM as black soils. Lower average OM contents of silt-size aggregates of red soils were due to a lower proportion of OC-rich agregates, not to differences in composition of individual aggregate classes. Expandable clays were practically absent in both soil types and sesquioxides were not related to OM contents or stability. Fine-sized secondary carbonates, undetectable to X-ray, impreganted light fraction OM and stabilized aggregates, and may be the principal cause of the OM accumulation. Mineralisation of coarse OM accumulated during fallow was impeded in both soils, but to a greater extent in the more calcareous black soils, so that relatively undecomposed OM accumulates to well above the levels that are typical for other semiarid tropical soils. Limited OM turnover will limit nutrient release, which may limit the agricultural productivity.