Grass vs. tree origin of soil organic carbon under different land-use systems in the Brazilian Cerrado.

Background and Aims: Silvopastoral management of tree plantations is becoming popular in Brazil. The impact of this practice on carbon dynamics of these soils is unknown, and predicting it is difficult because historical land-use records of the region do not exist. The objective of the study was to quantify the relative soil organic carbon (SOC) contributions of C3 and C4 plants in different land-use system. Methods: We evaluated total δ13C, the contribution of C4 and C3-derived soil organic C in three fraction-size classes of soils in six land-use systems (Eucalyptus hybrid plantations established in 1985 and 2005; a native forest; silvopasture stands of Brachiaria brizantha under eucalyptus established in 1994 and 2004; and an open pasture), and undertook carbon dating (C) for three of those systems, an Oxisol in Minas Gerais, Brazil. From each system, soil samples were collected from four depths (0-10, 10-20, 20-50, and 50-100 cm); samples were fractionated into 250-2000, 53-250, and <53 μm size classes, and their δ13C determined. Carbon dating (C analysis) was done for whole soil samples from 0 to 10 cm and 50-100 cm depth classes of three land-use systems. Results: The δ13C values increased (showing increase in C contribution by C4 plants, i.e., grasses) with soil depth at all sites and all size-classes. The older systems showed a higher contribution of C4-derived SOC at all depths. Carbon dating indicated that the area had been cleared about 300 years ago. Conclusions: Silvopastoral and pasture systems can be considered as good carbon sinks. We infer that the study site was grassland with high proportion of C4 plants in the past, not a forest as it is today. [ABSTRACT FROM AUTHOR]