Balancing biogeochemical gains and surface albedo shifts: climate impacts of no-tillage and mulching in Southern Africa

Conservation agriculture (CA) practices, such as no-tillage and mulching, can contribute to climate change mitigation by enhancing soil organic carbon (SOC) stocks and by influencing nitrous oxide (N₂O) emissions. However, their impacts on surface albedo and overall climate benefits, remain underexplored, particularly in Africa. This study tries to better address the net climate impacts of no-tillage and no-tillage with mulching compared to conventional tillage through two long-term experiments conducted in Zimbabwe - one established on an abruptic Lixisol soil (DTC site), the other one on a xanthic Ferralsol soil (UZF site). Over two years, measurements included SOC concentrations to a depth of 1 m, N₂O emissions, and surface albedo. The ICBM soil carbon model was employed to predict SOC stocks over 30 years of CA practices. Results indicated that no-tillage with mulching significantly increased SOC in the topsoil (0–30 cm), with stocks projected to reach 0.41 Mg C ha^-1y^-1 at DTC and 0.56 Mg C ha^-1y^-1 at UZF after 30 years. Conversely, no-tillage without mulching resulted in slight SOC losses at DTC, with predicted losses of approximately 0.036 Mg C ha^-1y^-1 over 30 years, while at UZF, SOC stocks increased by 0.11 Mg C ha^-1y^-1. Both sites exhibited very low N₂O emissions, indicating minimal climate impacts from this source. Net climate impacts were evaluated using the Global Warming Potential (GWP) approach at 20- and 100-year time horizons to assess short- and long-term climate effects. Results showed that no-tillage without mulching increased surface albedo on both soil types, inducing net cooling effects of -2.56 Mg CO₂ eq ha^-1 y^-1 and -0.65 Mg CO₂ eq ha^-1 y^-1, with surface albedo contributing 90% and 86%, respectively, on the Lixisol over 20 and 100 years. On the Ferralsol, no-tillage without mulching generated cooling effects of -1.25 Mg CO₂ eq ha^-1 y^-1 and -0.77 Mg CO₂ eq ha^-1 y^-1, with surface albedo contributing 52% and 23%, respectively, over the same periods. In contrast, mulching had contrasting effects at the two sites. On the Ferralsol, mulching enhanced surface albedo, contributing to net cooling effects of -1.82 Mg CO₂ eq ha^-1 y^-1 over 20 years and -1.57 Mg CO₂ eq ha^-1 y^-1 over 100 years, with surface albedo contributing approximately 20% in the short term and 5% in the long term. Conversely, on the Lixisol, mulching reduced surface albedo, offsetting 100% of SOC benefits and resulting in a near-neutral climate effect of +0.09 Mg CO₂ eq ha^-1 y^-1 over 20 years and +0.55 Mg CO₂ eq ha^-1 y^-1 over 100 years. This study underscores the necessity of integrating biogeochemical and biogeophysical effects when assessing the climate mitigation potential of CA practices, particularly in regions with diverse soil types and climatic conditions.