Soil organic C sequestration and stabilization in karstic soils of Yucatan

Abstract

The tropical semiarid soils of northwestern Yucatan, Mexico, haveexceptionally high organic matter (OM) contents, between 50 and 150 g Ckg−1. The soils are formed on limestone and form amosaic of shallow black lithosols surrounding rock outcrops and deeper redrendzinas at slightly lower relief. Traditionally, these soils were managedunder shifting cultivation with an uncommonly short cultivation period of only2years followed by a long bush fallow. We examined OM mineral associationsusing size and density fractionations of two soils after 1 and 12 years offallow in a search for factors responsible for the high OM stability, shortcultivation period and poor productivity. Light-fraction OM accounted for up to38% of total soil OM and was responsible for almost all the OM accretion duringfallow. Red soils contained half as much OM as black soils. Lower average OMcontents of silt-size aggregates of red soils were due to a lower proportion ofOC-rich agregates, not to differences in composition of individual aggregateclasses. Expandable clays were practically absent in both soil types andsesquioxides were not related to OM contents or stability. Fine-sized secondarycarbonates, undetectable to X-ray, impreganted light fraction OM and stabilizedaggregates, and may be the principal cause of the OM accumulation.Mineralisation of coarse OM accumulated during fallow was impeded in bothsoils,but to a greater extent in the more calcareous black soils, so that relativelyundecomposed OM accumulates to well above the levels that are typical for othersemiarid tropical soils. Limited OM turnover will limit nutrient release, whichmay limit the agricultural productivity.