Spatio-temporal distribution of enzyme activities in cowpea rhizosphere – the role of plant growth stages and nodule senescence

Legume-soil interactions are well recognized for their role in ecosystem nutrient cycling, yet specific mechanisms such as nodule-senescence effects on soil nitrogen (N) and carbon (C) cycling remain poorly understood. Here, we investigated the effect of nodule senescence on soil enzyme activity and soil biochemical properties in the rhizospheres of cowpea ( Vigna unguiculata ) during plant growth. We conducted a rhizobox experiment using soil from the Kavango (loamy sand) and Omusati (sandy soil) regions in Northern Namibia under controlled temperature and optimum water conditions. To investigate spatial and temporal C and N release, in-situ zymography was conducted at early vegetative, flowering, and maturity stage (i.e. one day after the start of nodule senescence) with six replicates per soil. Three enzymes, representing the C (β-glucosidase, Chitinase) and N (Chitinase, Leucine-Aminopeptidase) cycle, were investigated. At each plant growth stage, three additional plants per soil were harvested to identify changes in soil properties, including soil organic carbon, total N, mineral N, and pH. Our results showed that enzyme activities did not vary significantly during plant growth in rhizospheres and at nodule and root surfaces. In contrast, enzyme activities significantly increased with plant growth in bulk soil, especially β-glucosidase and chitinase, with a peak at maturity stage. Particularly in the sandy soil, nodule senescence significantly increased enzyme activities. This indicates enhanced organic matter decomposition and nutrient release mainly from the nodule-influenced rhizosphere to the bulk soil and might be attributed to rhizodeposition and microbial responses to substrate availability. We conclude that nodule senescence of legumes is an important driver of enzyme activity and can be a crucial factor for managing soil properties in low-nutrient soils. • Bulk soil enzyme activities increased during plant growth. • Rhizospheres, root and nodule enzyme activities stayed stable during plant growth. • Senescing nodules released OM, triggering higher enzyme activity. • Higher bulk soil enzyme activity in loamy sand than in sandy soil.