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

Abstract. 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 which have been shown 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 water cycling of the crops, however, is still relatively unknown.

In this study we assessed the influence of gliricidia-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). The controlled setup and different durations of the experiments (>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, especially pronounced at the site with the generally lower baseline C contents. The treatment effect is also visible in soil physical characteristics such as porosity and bulk density, which is mirrored by a greater saturated hydraulic conductivity. These treatment effects were, however, not directly translatable into soil water dynamics as the latter were influenced by several additional factors such as soil texture and interception. The gliricidia plots showed greater soil water storage capacities and retained overall more water, while generally both treatments were not under severe water stress during the observation period. We also noticed a protective effect against soil drying below the topsoil facilitated by the gliricidia. Furthermore, infiltration shifted towards more immediate/macropore infiltration under gliricidia.

We conclude that the AFS treatment of adding gliricidia into maize cultivation has a considerable effect on nutrient and water cycling in the crop, while the effect on water fluxes is not straightforward. While the differences in soil moisture and matric potential never lead to a shortage of water for the crops, a detailed examination of water fluxes require respective measurement in the field as they cannot be deduced from soil physical characteristics directly. AFS can thus not only support carbon accumulation and stabilization, but a sensible combination of trees and crops can also be beneficial for a more sustainable use of the available water.