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iii Maize (Zea mays) is the principle staple crop of Malawi, and provides the livelihood for the majority of the population, both in terms of caloric intake and income generation. Over the last several years, maize yields have become stagnant and are not enough to support the growing population. In older, highly-weathered soils typical of Malawi, phosphorus (P) is often the main factor limiting crop production. Phosphorus is an essential macronutrient for plant growth, yet sources of rock phosphate, the raw material for mineral P fertilizers, are nonrenewable and in limited supply worldwide. However, most soils contain a large amount of P, yet only a small proportion is in a plant-available form. As such, numerous strategies for using this abundant P source are being explored to reduce dependency on P fertilizers while still maintaining global crop yields. To begin, the effective use of soil P for crop production requires the understanding of how P pools are stabilized and cycled within soil aggregates, as this is a key driver for soil nutrient storage and availability for plants. While much is known regarding the effect of soil structure on carbon and nitrogen cycling, much less is known about how changes in soil aggregation affect P pools and plant availability. In addition to the influence of soil processes, it is well known that certain plant species possess the ability to access soil P pools typically unavailable to other crops through a variety of complex rhizospheric processes. For example, pigeon pea (Cajanus cajan), a nutritious grain legume commonly grown in Malawi, is one such species that is adapted to use sparingly soluble soil P pools through the exudation of phosphatase enzymes and organic acids. However, much less is known about how these processes are effected within an intercropping system, nor if the neighboring intercropped species can benefit from this increased P availability. The overall goal of this project was thus to understand how P is cycled within soil aggregates, and how intercropping maize and pigeon pea can influence these complex interactions as a means to improve P availability and crop yield in Lixisols of Malawi.

Phosphorus cycling in soil aggregate fractions in maize-pigeon pea intercropping systems of Malawi