Spatiotemporal distribution of enzyme activities in the rhizosphere of cowpeas (Vigna unguiculata)

To retain soil fertility and build up soil organic matter, cultivation of nitrogen-fixing legumes is a promising tool in low-nutrient soils in southern Africa. Legumes can potentially improve soil nitrogen (N) status, particularly in the rhizosphere, through N fixation in nodules. This affects soil microbial and enzymatic activities and thus soil organic carbon (SOC) turnover and nutrient availability. During plant growth, legumes provide photosynthetic carbohydrates for bacteria living in the nodules. When legumes reach maturity stage, developmental senescence begins and N-fixing nodules dissolve. This process represents a plant metabolic switch from a C sink to a C and nutrient source for the soil, as nutrients are released. Few studies exist on legume-soil interaction during plant growth and it is unknown if and how nodule senescence contributes to soil nutrient availability and SOC turnover. Therefore, the aim of this study is to investigate spatial and temporal legume-soil interactions in the rhizosphere, regarding nodule development, N release, and soil enzyme activity during legume growth.

We conducted a rhizobox experiment using two different soils from the Kavango (loamy sand) and Omusati (sand) regions in North Namibia. Cowpea (Vigna unguiculata), a common legume in southern Africa, was grown under controlled temperature and water availability. To investigate spatial and temporal C and N release as well as soil enzyme activity, in-situ zymography was conducted at early vegetative stage, flowering development, and one day after nodule senescence. Three enzymes, representing the C (β-glucosidase, Chitinase) and N (Chitinase, Leucine-Aminopeptidase) cycles were investigated. At each plant growth stage, three plants were harvested to identify changes in soil properties, including SOC, N, mineral N, and pH, over time.

Preliminary results indicate that β-glucosidase activity is generally higher in the rhizosphere and around nodules compared to bulk soil in both soils. During plant growth, β-glucosidase activity varied. It decreased from vegetative to flowering stage but slightly increased during nodule senescence in both soils. At nodule senescence, β-glucosidase activity in bulk soil increased by 17% in the loamy sand compared to the early vegetative stage and generally higher than in the sandy soil. With further image analysis, we aim to understand how β-glucosidase, Chitinase, and Leucine-Aminopeptidase activities are interlinked and influence nutrient availability during plant growth in soil.