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

Abstract

Abstract Soil carbon (C) distribution, natural 13 C abundances and their changes as a consequence of cropping were studied in three neighboring areas on an Oxisol from Brazil. One site (T 0 ) was under forest, while the two other sites (T 12 and T 50 ) 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 T 0 was twice that of T 12 and T 50 , then decreased sharply with depth, to values similar to the other profiles. Delta 13 C had practically constant values of −25.1, −22.8, and −20.4‰, throughout the 0 to 0.30‐m layer of T 0 , T 12 , and T 50 respectively. These values increased in deeper layers, to about −17‰, due to increased humification and possibly to deposition of organic matter from a former 13 C‐rich vegetation. The 0.10‐ to 0.20‐m layer was separated into particle‐size fractions and alkaline extract. Carbon contents decreased from T 0 to T 50 in the sand‐size fractions and alkaline extracts, but did not change in the clay‐size fractions. Delta 13 C 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 T 12 and T 50 , respectively. It reached its maximum value (67 ± 3.7%) in the coarse sand fraction of T 12 and T 50 and decreased with decreasing fraction size, to 13.8 ± 9.4% and 30.5 ± 6.5% in the fine clay fractions of T 12 and T 50 , respectively. Thus, Cdff persisted mainly in the clay‐size fraction.