Chromoleana odorata fallow-cropping cycles maintain soil carbon stocks and yam yields 40 years after conversion of native- to farmland, implications for forest conservation

Natural ecosystem conversion to agriculture is known to alter soil carbon dynamic. Following such conversion in many tropical areas, land undergoes fallow-cropping cycles where fallows are invaded by the pantropical weed Chromoleana odorata. This study was undertaken in the forest-savanna interface area of Côte d'Ivoire to evaluate the impact of these cycles on soil carbon stocks (SOCS) and yam yields, decades after natural ecosystem conversion. Trials involved four treatments including yam farms in forest (FOR, n = 10), in forest-derived C. odorata fallows (FoDCo, n = 7), in savanna (SAV, n = 3) and in savanna-derived C. odorata fallows (SaDCo, n = 3). Prior to turning plots to farms, soil was sampled in the 0-10, 10-20 and 20-40 cm layers for physical, chemical and microbial parameters. Since forest and savanna soils were different in granulometry, FoDCo was compared to FOR, and SaDCo to SAV. The soil organic matter and nutrient concentrations in the 0-10 cm soil layer in FoDCo and FOR were similar, except for available P which was higher in the former. SaDCo was higher than SAV in terms of SOC, available P, mineral N, and NO3-N:Mineral N ratio. With regard to SOCS, value in FoDCo equalled that in FOR regardless of soil layers (63.2 ± 4.4 and 63.7 ± 4.6 Mg ha−1 in 0-40 cm layer, respectively). However, SOCS significantly increased in SaDCo relative to SAV in the 0-20 cm (31.4 ± 2.7 vs. 24.2 ± 2.1 Mg ha−1). Consistently, yam yield in FoDCo was like that in FOR while it doubled in SaDCo compared to SAV, with SOC, CEC and mineral N as the controlling factors. The major finding of this study is that the predominance of C. odorata in fallow phases allows at least for maintenance of SOCS and yam yields decades after natural ecosystem conversion to farmland. Furthermore, evidence of the feasibility of the “4 per mille” was given. These results are highly useful in forest protection strategies since farmers usually cut forest for yam cropping.