Coupling of soil carbon and water dynamics in two agroforestry systems in Malawi

Consequences of climate change are likely to pose severe challenges on agriculture in Southern Africa. Agroforestry systems (AFSs) can potentially alleviate some of the adverse effects and offer adaptation solutions to a sustainable land use. Positive effects of AFSs may include increasing soil carbon (C) and nitrogen concentrations, sustaining favourable nutrient cycling, protection against erosion and increased carbon sequestration. The influence of the AFS tree component on the soil water storage and thus water availability for the crops, however, is still relatively unknown. In this study we assessed the influence of Gliricidia sepium-maize intercropping on carbon cycling and water fluxes compared to maize as a sole crop at two well-established long-term experiments in central and southern Malawi, run by the World Agroforestry (ICRAF). Utilizing the field experiments of different durations (>10 and >30 years) at the two sites provided information regarding soil-specific impacts of gliricidia on water dynamics. We examined soil C contents and density fractionation as proxy for organic matter stability, soil physical and soil hydrological characteristics. We also monitored soil moisture and matric potential in different depths, determined retention curves on samples in the lab and from field data and analysed soil moisture responses to rainfall events to assess the influence of the AFS on water fluxes. Our results show a clear increase in C contents and stability as a result of the gliricidia impact compared to the control at the site with the generally lower baseline C contents. At this site, the treatment effect was not visible in soil physical characteristics such as porosity and bulk density, but in saturated hydraulic conductivity, which is rather a structural soil property. The soil water dynamics were influenced by several additional factors such as soil texture and interception. The gliricidia treatment showed greater soil water storage capacities and retained overall more water, while generally none of the plots neither control nor treatment were under severe water stress during the observation period. We also noticed a protective effect against soil drying below the topsoil potentially by more immediate/macropore infiltration into the subsoil under gliricidia. We conclude that, from a methodological point of view, assessing the effects on water fluxes requires respective field measurements as they cannot be deduced from soil physical characteristics directly. Overall, the AFS treatment of adding gliricidia into maize cultivation can have a considerable effect on nutrient and water dynamics in the system, however, this effect is also dependent on initial site conditions. A sensible AFS implementation can not only support carbon accumulation and stabilization but also increase the efficient use of available water, thus supporting different aspects towards sustainable agriculture in Malawi.