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

Agroforestry systems may increase carbon storage of agricultural land, while simultaneously offering the potential for improved nutrient availability. The extent to which trees integrated into agricultural land and the accompanying potential increase of carbon input influence soil structure with regard to hydrologically relevant parameters, and thus water dynamics, storage, and availability, remains unclear.

In a case study in Malawi, two similar agroforestry experiments of the World Agroforestry (ICRAF) at different locations and of different durations (>10 and >30 years) were investigated. The systems consist of maize and Gliricidia sepium, which accumulate nitrogen in the soil as well as carbon through the incorporation of cut leaves and branches into the soil. Measurements were taken from soil samples and combined with 3-month measurement series to record the temporal dynamics of soil water fluxes. The same sampling scheme and measurement setup were used to compare maize control plots and agroforestry plots: Carbon concentrations and density fractionation were used to estimate the stability of the organic matter, along with soil physical and hydrological properties (e.g. saturated hydraulic conductivity), soil water content and matrix potential at various depths, water retention curves, and responses to precipitation events.

A significant increase in carbon concentrations and carbon stability was observed in the soil of the agroforestry plot. This effect was considerably greater in the system that had a lower initial carbon content before the start of the agroforestry experiment. However, the differences in carbon stability did not have immediate effects on soil hydrological properties such as porosity or bulk density, and therefore, no direct effects on soil water fluxes were detectable, which were also influenced by factors such as interception.
The agroforestry plot showed a greater soil water storage capacity and was able to retain more water overall. Additionally, a protective effect against topsoil desiccation was observed in the agroforestry plot, possibly due to macropores and resulting faster infiltration. A well-considered and site-adapted combination of plants can play an important role in improving water use. In particular, improving storage capacity can be crucial in arid regions or during dry periods.