Stable carbon in soils under rubber tree (Hevea brasiliensis) agroforestry systems in the south of Bahia, Brazil

Abstract

The early natural forest conversion to pasture, rubber tree plantations or agroforestry systems (AFSs) will not negatively affect soil organic carbon (SOC) accumulation due to the lack of soil tillage and the continuous contribution of vegetal residues to the soil, over the years, which would favor C occlusion in soil aggregates. The objectives of this study were: to evaluate the potential of AFSs with rubber trees to accumulate SOC up to 100 cm; to evaluate the δ13C variations over the soil profile after forest to pasture and rubber tree plantation (RTP) conversion; and to verify the C physical protection in aggregates as a mechanism of C stability in these soils. SOC was quantified up to 100 cm in whole soil and in three fraction-size classes (macroaggregates, microaggregates and silt + clay class). The occluded C was quantified in macro- and microaggregates by using an ultrasonic method. The relative contributions of C3 and C4 plants-derived C were quantified. The RTP system presented the highest SOC values, up to 100 cm depth, contributing with 20 Mg C ha−1 from belowground input, during 35-year old of system implantation. However, about 90% of SOC was not physically protected in the soil aggregates. The forest to rubber + cacao agroforestry system (AFS) conversion reduced SOC stock up to 100 cm approximately 72 Mg ha−1, and the reduction in SOC stock after forest to rubber tree + acai AFS was around 34 Mg ha−1. Rubber tree + cacao and rubber tree + acai AFSs had significant contributions of occluded C in soil aggregates.