Biomass and soil carbon stocks of indigenous agroforestry systems on the south-eastern Rift Valley escarpment, Ethiopia

Biomass carbon (C) and soil organic carbon (SOC) stocks in three indigenous agroforestry (AF) systems (Enset [Ensete ventricosum, Musaceae], Enset-Coffee and Fruit-Coffee) practiced on the Rift Valley escarpment of Ethiopia are presented and evaluated. These unique AF systems are characterised by a high proportion and diversity of trees. Data were collected from six smallholdings per AF system. Above- and belowground biomass of trees (fruit and non-fruit), enset and coffee plants were calculated using allometric equations while the biomasses of herbs, litter and fine roots (< 2 cm) were determined by destructive sampling techniques. SOC stocks (0-30 and 30-60 cm) were calculated from measured C contents and bulk density values. Smallholding total biomass C stocks averaged 67 Mg ha(-1) with trees accounting for 39-93 %. Herbs accounted for < 4 % of aboveground biomass. Coffee accounted for 11 % and enset 9 % of total biomass C on average. SOC stocks (0-60 cm) were 109-253 Mg ha(-1) (52-91 % of total C stocks) and uncorrelated to biomass C stocks. Biomass C or SOC stocks did not significantly differ between AF systems but biomass C stocks were significantly correlated to elevation. The C stocks of the three studied AF systems were found to be amongst the highest reported for tropical forests and agroforestry systems. These unique AF systems are therefore well suited to REDD+ projects. However, the C stocks were more determined by individual smallholder management practice than by AF system or climate (elevation).