Response of cowpea to integrated mycoroot™ inoculation, biochar-compost mixture and soil variation: drought tolerance and performance.

Abstract

Cowpea (Vigna unguiculata L.) also known as black eye pea, is an annual heat-loving leguminous crops cultivated mostly in the semi-arid environments as grain and vegetable crop for human consumption and as animal feeds. It is a highly nutritious crop, rich in protein, fibre and other nutritional components such as vitamin, minerals and other secondary metabolites. Cowpea grain availability in the local market in South Africa remains very low due to poor soil fertility status, low soil moisture condition and low production level mainly by smallholder farmers. Adoption of improved production practices such as MycorootTM, a locally produced biofertilizer containing natural Arbuscular mycorrhizae fungi (AMF) have been used to enhance crop growth due to their ability to facilitate water and nutrients uptake and facilitate osmotic adjustments under adverse conditions such as moisture stress. Hence, a greenhouse pot experiment was conducted to examine the combined effect of MycorootsTM AMF inoculation with varied biochar-compost mixtures as an agronomic package to enhance cowpea growth, phenological and yield attributes in two soil textural types with different moisture levels. Trial consisted 2 soil textural types (sandy loam and loamy sand), 4 soil amendments comprising different mix ratios of biochar (BC) and compost (C), 2 AMF levels (inoculated and uninoculated) and 2 soil moisture regimes (adequate soil moisture and moisture stressed) as main treatment factors. The soil amendments comprised of 50% biochar 50% compost (50:50 BC/C), 75% biochar 25% compost (75:25 BC/C), 25% biochar 75% compost (25:75 BC/C) and a control with no amendment. The treatment factors were combined and laid out in a 2x2x4x2 factorial design fitted into RCBD with each replicated 4 times. Data collected included growth, phenological, yield attributes, grain nutrients and secondary plant metabolites. Results revealed a significant (p<0.05) soil textural types and moisture levels interaction effect on measured growth attributes while the effect of soil moisture stress on the measured growth attributes was less severe in sandy loam than loamy sand soil. AMF inoculation gave highest leaf length (13.8 cm) at reproductive stage than non-inoculated treatments. Integrated use of 75Bio25Comp as soil amendment gave the highest leaf length (14.12 cm). Variation in soil textural types exert significant (p<0.05) effect on all measured yield parameters with higher yields recorded under sandy loam than loamy sand soil. However, the measured cowpea growth and phenological attributes were higher in plants grown on loamy sand soil. Although AMF inoculation exerted inconsequential (p>0.05) effect on the measured yield parameters, it significantly (p<0.05) lowered the flavonoid content of cowpea grains. Application of 25:75 vi Biochar and Compost mix ratio as soil amendment resulted in increased anthocyanin contents in cowpea seeds. Interaction between soil moisture levels and AMF inoculation exerted a significant effect (p<0.05) on protein content. AMF inoculation increased cowpea seed protein (29.5%) and P content under moisture stressed condition. Integrated use of biochar, compost and MycorootTM product as soil amendment represents an important nutrients management strategy to mitigate the adverse effect of soil nutrients and moisture stress condition and enhance cowpea yield.