More Crop per Drop, More Cropping per Dropping: Optimizing the Interactions between Organic Resources, Soil Macrofauna Biodiversity and Soil Structure for Enhanced Water and Nitrogen Use Efficiency in West and East African Cropping Systems

Water and nutrient stress are major constraints to agricultural production in East and West Africa. A better understanding of the impact of soil fertility management on the interactions between soil organic matter (SOM), soil structure, and nutrient and water use efficiency is pivotal to the optimal exploitation of scarcely available organic resources and the design of sustainable cropping systems. Although different functional groups of soil macrofauna greatly affect soil structure, SOM, soil water and nutrient dynamics, quantitative understanding and optimization of their role is lacking. Our proposal comprises an interdisciplinary approach to investigate the effects of agricultural management on soil quality and nitrogen- and water use efficiency, emphasizing the role of soil macrofauna biodiversity and activity across soil types and climates in East and West Africa. The general hypothesis of PhD project 1 is, that any link between soil macrofauna biodiversity and soil functioning will be established through the diversity of soil structures that are produced by functional groups of earthworms and termites, in particular soil aggregates derived from excrements. In a survey of 11 long-term (> 15 seasons) field trials across the sub-humid ? semi-arid agro-ecological zones of West and East Africa, PhD student 1 will systematically quantify soil macrofauna biodiversity and relate this to environmental (including soil) characteristics. PhD student 2 will analyse soil samples for aggregate stability and size distribution. Combined with soil macrofauna data from PhD project 1 this will provide information on the interrelations between agricultural management, soil macrofauna diversity and structural stability of surface soils. In in-depth studies in 6 of the trials (3 in Burkina Faso and 3 in Kenya), PhD student 1 will quantify how management (organic resource quality +/- inorganic N use, rotation, tillage) affects soil fauna functional group diversity and how this is reflected in the diversity of biogenic (=soil fauna-formed) structures, the relative abundance of biogenic and physicogenic (= formed by abiotic factors, e.g. wetting and drying) macrostructures and the degree of SOM association with the mineral soil fraction. PhD student 2 will investigate how changes in soil physical and hydrological properties, induced by differences in management and associated macrofaunal biodiversity, determine the water and nitrogen use efficiencies in these cropping systems. The general hypothesis is that increased structural stability, porosity, infiltration rate and water retention capacity lowers evaporation, percolation and runoff losses, thereby increasing plant-available water. Data on plant water use will be combined with crop-N data to relate management-induced changes in the partitioning of the rainfall over soil evaporation, plant transpiration and runoff and deep percolation to nitrogen use efficiency (NUE) and water use efficiency (WUE). Recent studies revealing the intimate relation between aggregate turnover and SOM and N dynamics indicate the need for agricultural management that balances macro- and microaggregate formation and breakdown (aggregate turnover) in order to optimize their protective (C sequestration) versus productive (N release) functions. In a postdoc project we will test the general hypothesis that agricultural practices representing less disturbance, integrated residue management and crop diversification (rotations) result in intermediate aggregate turnover rates, thereby optimizing the balance between C stabilization and N availability and maximizing plant N use. Application of residues of different quality derived from 15N labeled C3 plants to soils that have been under long-term C4 vegetation in field microplots, with or without exclusion of soil macrofauna, will enable us to directly link aggregate turnover and C and N dynamics and asses the contribution of the soil macrofauna. The postdoc will also be responsible for integration and upscaling of the results of the three projects through database development and simulation modeling. The results of this programme will show how and to what extent the productivity and sustainability of tropical agro-ecosystems can be improved by stimulating the soil macrofauna (especially earthworm and termite) biodiversity and activity through the judicious use of organic resources and soil cultivation. If the overall hypothesis of the programme ? the diversity of earthworms and termites and their resulting biogenic soil structures will optimize SOM formation and dynamics with consequent positive impacts on N and water availability ? is corroborated, then we will have a better understanding of how we can utilize soil macrofaunal diversity to increase water and nutrient use efficiencies and crop performance in West- and East-African agroecosystems.