Organic Matter and Natural Carbon‐13 Distribution in Forested and Cultivated Oxisols

Soil carbon (C) distribution, natural 13C abundances and their changes as a consequence of cropping were studied in three neighboring areas on an Oxisol from Brazil. One site (T0) was under forest, while the two other sites (T12and T50) had been deforested, then cultivated with sugar cane for 12 and 50 yr, respectively. Soil morphological, chemical and mineralogical characteristics in all three sites were very similar. Total C content of the 0.06‐m layer of T0was twice that of T12and T50, then decreased sharply with depth, to values similar to the other profiles. Delta 13C had practically constant values of −25.1, −22.8, and −20.4‰, throughout the 0 to 0.30‐m layer of T0, T12, and T50respectively. These values increased in deeper layers, to about −17‰, due to increased humification and possibly to deposition of organic matter from a former 13C‐rich vegetation. The 0.10‐ to 0.20‐m layer was separated into particle‐size fractions and alkaline extract. Carbon contents decreased from T0to T50in the sand‐size fractions and alkaline extracts, but did not change in the clay‐size fractions. Delta 13C values were used to estimate the proportions of C derived from forest (Cdff) and from sugar cane (Cdfc). Carbon derived from sugar cane represented 17.3 ± 3.2% and 40.5 ± 2.2% of total C in T12and T50, respectively. It reached its maximum value (67 ± 3.7%) in the coarse sand fraction of T12and T50and decreased with decreasing fraction size, to 13.8 ± 9.4% and 30.5 ± 6.5% in the fine clay fractions of T12and T50, respectively. Thus, Cdff persisted mainly in the clay‐size fraction.