Organic carbon and 13C contents in soils and soil size-fractions, and their changes due to deforestation and pasture installation in eastern Amazonia

Abstract

In soils of the eastern Amazonian forest, modifications in soil organic matter (SOM) contents as a consequence of deforestation and pasture installation were investigated. Profile distribution of total organic carbon (C) and nitrogen (N), and of 13C isotope abundance (expressed in δ 13C%o units) were compared. The two soils, one under native forest and the other one after ten years under pasture of Pennisetum purpureum had similar C/N values, which slightly decreased with increasing depth, from 13.6 to 11.9–12.7 within the first 40 cm. In the pasture soil, the C content was slightly lower than in the forest soil, and reached 29 t ha −1 compared with 31 t ha −1, and 15 t ha −1 compared with 16 t ha −1, in the 0–20 and 20–40 cm layers, respectively. The δ 13C values reflected the origin of SOM, from either the forest (which had a C3 photosynthetic cycle), or the pasture (which had a C4 cycle). They were higher by at least 6.5 and 1.5 units, in the respective 0–10 and 10–20 cm layers of the pasture soil, than in the corresponding layers of the forest soil. These δ 13C values were used to estimate the proportions of C derived from the forest (Cdf) and from the pasture (Cdp). The calculations clearly indicated a strong input of Cdp, which reached 46–49% and 21–24% of total C in the respective layers. Particle size fractionation showed that SOM changes were predominant in the upper soil layer (0–10 cm), and vanished with increasing depth: in the pasture soil, the coarse organic residues accumulated, whereas the amount of C in the finest fractions decreased, which suggested a slowing down in humification processes. In the forest soil, the δ 13C values were lower in the coarsest than in the finest fractions, the latter being less affected than the coarsest ones by the vegetation change; in the 0–10 cm layer, these values increased by about 7–10 units in the sand-size fraction, and only 4–5 units in the clay-size fraction. Thus, the replacement of Cdf by Cdp was greater in the sand-size fraction (55–65% of total C), than in the clay-size fraction (34–45% of total C). Based on 14C dating, the oldest pool (i.e. that of mean age 5000 years) of forest SOM was calculated at each depth. In the soil surface, it was still smaller than the more labile fraction of Cdf, and represented about 17% and 26% of total C in the 0–10 cm and 0–20 cm layers, respectively.