The 4 per 1000 goal and soil carbon storage under agroforestry and conservation agriculture systems in sub‐Saharan Africa

The 4‰ Initiative launched by the French government at COP21 in Paris in December 2015 aspires to increase global soil organic carbon (SOC) stocks at a rate of 0.4% per year. We conducted a systematic literature review on SOC storage under agroforestry and conservation agriculture systems in sub-Saharan Africa, where we reported 66 and 33 cases for both systems, respectively. The results showed that SOC storage rates were significantly higher than 4‰ yr-1 in fallows and in multistrata agroforestry systems (p-value = 0.0001 and 0.0178, respectively), but not in alley cropping and parklands systems. For conservation agriculture, only SOC storage rates in the system with all three principles applied, i.e. no- or minimum tillage combined with crop residue retention and intercropping or rotation, were significantly higher than 4‰ yr-1 (p-value = 0.0438). The data showed very large variability in SOC storage rates as the result of various factors, including previous land-use history, experimental set up and approach used to determine SOC storage, depth of soil sampling, soil type, type of cropping/agroforestry system and management, and duration of the experiment. SOC storage rates significantly decreased with time in the agroforestry systems (p-value = 0.0328). Our review clearly highlighted the scarcity of good quality data on SOC stocks in sub-Saharan Africa. We argue that there is a potential for SOC storage in agricultural soils of sub-Saharan Africa given the large areas of SOC-depleted soils, as illustrated by soil fertility gradients observed on smallholder farms. Low SOC levels are to a great extent the result of limited resources of most smallholder farmers in sub-Saharan Africa. The 4‰ initiative has to be implemented on the grounds of the positive impact on crop productivity rather than on climate change mitigation. The efficiency in doing so will depend on the specific situations and will need economic support to smallholder farmers