Effects of agroecological practices on soil organic carbon sequestration using synchronic and diachronic approaches in Madagascar

Soil organic carbon (SOC) sequestration in agricultural systems is a key indicator of soil fertility improvement and climate change mitigation at the global scale. In Madagascar, the effect on SOC sequestration of converting from traditional practices to agroecological ones remains unclear. The objectives of this study were (i) to assess the impact of agroecological practices, such as agroforestry (AF), tree plantation (TP) and improved farming practices (IFPs), on SOC sequestration at the field level; and (ii) to use both synchronic and diachronic approaches to quantify SOC sequestration following the adoption of agroecological practices. For the diachronic approach, two sampling years (2014 and 2018) were used to assess the annual soil carbon sequestration rates with agroecological practices. For the synchronic approach, SOC sequestration arising from agroecological practices was compared to that on reference fields, such as fallow land (FL) and traditional farming practices (TFPs). Soil sampling were carried out on 36 fields with agroecological practices and 60 reference TFP and FL fields. The diachronic approach showed that SOC sequestration rates for AF, TP and IFPs were respectively, +2.8, +1.6 and -0.9 MgC.ha-1.yr-1. The synchronic approach showed that SOC stock in AF was higher (109.4 MgC.ha-1) than in reference TFP (73.8 MgC.ha-1) and FL (67.4 MgC.ha-1) fields. The SOC stock in IFP (74.8 MgC.ha-1) and TP (69 MgC.ha-1) presented no significant difference compared to reference fields. This study provided a better understanding of soil carbon dynamics at the farm level in Madagascar when agroecological practices are adopted in place of traditional practices.