Optimizing soil aggregate turnover for enhanced soil carbon sequestration and nitrogen availability through beneficial interactions between management and soil macrofauna in West and East African cropping systems

Due to the physical protection capacity of soil aggregates, their formation and breakdown govern to a large extent the N availability and C sequestration in agricultural soils. Tillage, organic residue quality and mineral N use have an important impact on macro- and microaggregate turnover and on soil fauna diversity. The intimate relation between aggregate turnover and soil organic matter (SOM) and N dynamics implies that, to improve and sustain crop production in West and East Africa, management options should lead to an optimal balance between the protective (SOM stabilization) versus productive (N release) functions of soil aggregates. In addition, different functional groups of termites and earthworms incorporate organic residues of different quality into the mineral soil and contribute directly to the formation of stable biogenic macro- and microaggregates. The effects of climate and soil type on aggregate and SOM dynamics and the relative abundance on the diversity of soil macrofauna make that optimal management practices vary with the agro-ecological zone. We aim to determine the impact of interactions between soil management practices (organic matter inputs, rotations, tillage) and soil macrofauna on macro- and microaggregate turnover, C sequestration and N utilization by the crop. Our overall hypothesis is that management representing less disturbance, integrated residue management and diversification through crop rotations result in intermediate aggregate turnover rates thereby optimizing the balance between C stabilization and crop N utilization. This is partly due to the increase in soil macrofauna diversity under such conditions. The optimal management options in West and East Africa, however, will be modified by local climate and soil conditions. Microplot experiments using isotopic labeling and soil fauna exclusion techniques will be conducted to determine C and N dynamics in relation to management, while at the same time quantifying the role of soil macrofauna. Residues from plants with different isotopic composition will be used to study the mechanisms and turnover rates of SOM and N in association with macro- and microaggregate turnover. At harvest samples will be taken for soil physical fractionation and chemical characterization and stable isotope analyses of the different organic matter and N pools. Secondly, this project will handle the integration and upscaling of the results obtained in the three complementing multidisciplinary projects through database development and crop growth and ecosystem modeling. Finally, the post-doc will also be responsible for the overall coordination of the programme.