Organic Matter and Natural Carbon-13 Distribution in Forested and Cultivated Oxisols
1989
Vitorello, V. A. | Cerri, C. C. | Victória, R. L. | Andreux, F. | Feller, C.
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 (T₁₂ and 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 T₀ was twice that of T₁₂ and 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 T₅₀ respectively. 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 T₀ to T₅₀ in 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 T₁₂ and T₅₀, respectively. It reached its maximum value (67 ± 3.7%) in the coarse sand fraction of T₁₂ and T₅₀ and decreased with decreasing fraction size, to 13.8 ± 9.4% and 30.5 ± 6.5% in the fine clay fractions of T₁₂ and T₅₀, respectively. Thus, Cdff persisted mainly in the clay-size fraction.
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