Impact of bambara seed residue biochar and NPK on soil fertility, aggregate carbon and nitrogen concentrations and yield of cucumber

The mechanisms of biochar effects on soil and crop yield are still elusive. Assessing the influence of biochar on total carbon (TC) and total nitrogen (TN) concentrations in hierarchical soil aggregates could provide an explanation. In a randomized complete block design were control, bambara seed residue biochar (BSB), NPK and NPK + BSB treatments superimposed on a two-year bambara seed residue amended Ultisols, in a four-year continuous cucumber cultivation. The effects of the treatments on optimizing cucumber yield and soil sustainability parameters, particularly the aggregate- (4.75-2.00, 1.00-2.00, 0.25-1.00 and < 0.25 mm) associated TC and TN concentrations were studied. The BSB and NPK + BSB treatments significantly (p ≤ 0.05) increased the concentrations of TC by 113.79 and 104.60%; TN by 100 and 87.50%; and available P by 172% and 415%, respectively. Furthermore, both treatments significantly reduced exchangeable Al3+ but increased Mg2+ and soil moisture content 4-6 weeks after planting. Exchangeable K+ was significantly increased by BSB. The BSB and NPK + BSB significantly improved TC and TN in all aggregate fractions with minimal effect in the 0.25-1.00 mm fraction. However, the overall percentile TC and TN increase by these treatments was more in the macro- (4.75-0.25 mm) than in the micro (< 0.25 mm) aggregates. Application of BSB and NPK + BSB significantly improved cucumber fruit weight by 12- and 24-fold, respectively, compared to NPK treatment. The study demonstrated that BSB enhanced soil fertility, while NPK + BSB further improved cucumber productivity. In addition, these treatments have the potential to substantially sequester carbon and nitrogen more in macro- than micro- aggregates.