Biochar and leguminous cover crops as an alternative to summer fallowing for soil organic carbon and nutrient management in the wheat-maize-wheat cropping system under semiarid climate

Cereal-based cropping systems under arid and semiarid climates have typically low soil organic matter and fertility, and natural fallowing during summer gap could have negative effects on soil quality and health. We tested an alternative approach of using biochar and leguminous cover crops to replace natural summer fallowing of about 75 days in the wheat-maize-wheat cropping systems with a view of studying the effects on legume productivity, soil organic carbon (C), and soil nutrients over the 2-year experiments. The two-factor completely randomized block experimental design consisted of biochar, developed from acacia tree biowaste and applied at 0, 5, and 10 t ha−1 rate, and leguminous cover crops (cowpea, mungbean, and sesbania) with natural fallow were used to test soil fertility and crop productivity. Treatments were laid out following completely randomized block design and each treatment had three replicates leading to 36 experimental plots in total. Pearson’s correlation coefficients were calculated to find the relationships between plant and soil variables. Regression analysis was performed to study correlations of total organic C with soil available N, P, and K contents. Results from the 2-year experiment indicated that plant height, pods plant−1, grains pod−1, 1000-grain weight, fresh and dry biomass, and grain yield were higher at 10 t ha−1 biochar rate; however, the nodule density plant−1 of cowpea and sesbania was the highest at 5 t ha−1 biochar. Integrating legumes with biochar further conserved and improved soil C which significantly positively correlated with soil fertility indicators. Despite linear positive changes in soil pH and electrical conductivity (EC) with biochar rate, increase was minor and was never significant. We observed significant positive relationships of biochar rate with soil organic C contents and soil N and K contents whereas soil P contents were higher at 5 t ha−1 biochar rate. However, not always significant difference between 5 and 10 t ha−1 biochar rates for legume and soil productivity suggested flexibility of choosing the biochar application rate. Our study emphasized that adopting integrative biochar-leguminous cover crop fallowing to replace natural summer fallowing could yield beneficial effects in terms of conserving soil organic carbon, sustaining soil fertility, and improving soil quality. In addition to positive supplementary effects on the succeeding crops, this approach also allows at utilizing and recycling on-farm biowaste more efficiently and environment friendly.