Conjunctive use of composted leguminous shrub Caragana microphylla-straw and Bacillus cereus reduces nitrogen input but enhances nitrogen use efficiency and cucumber yields in alkaline soils

Fertilizer N use efficiency (NUE) is relatively low in most regions of the world, especially when a relatively large proportion of synthetic N fertilizer is used. To protect both the environment and crops, it is essential to improve NUE by reducing N input but maintaining or enhancing crop yields. In this study, we estimated the effects of composted leguminous shrub Caragana microphylla-straw (CM), Bacillus cereus (BC) and their combination on soil properties, cucumber plant growth, fruit yield, NUE and fruit quality under variable N-input rate conditions, through a study of four successive cropping seasons (CSs). The soil treatments considered were (i) traditional chicken manure applied as basal (control), (ii) CM plus 100% N from basal (CM-100% N), (iii) CM plus 75% N from basal (CM-75% N) and (iv) CM plus 50% N from basal (CM-50% N). The control and the CM-100% N treatment received the same amount of total N input according to local recommendations. Cucumber plants were inoculated without or with BC to create plant treatments that included (i) no inoculation (-BC) and (ii) inoculation with BC (+BC). Finally, a total of eight treatments (4 soil treatments x 2 plant treatments) were created. In general, CM application and BC inoculation enhanced soil quality (expressed by soil quality index (SQI)) mainly through reducing secondary salinization and autotoxin accumulation, both of which had detrimental effects on crop growth, and by increasing the utilization potential of amino acids, which had beneficial effects on crop growth. The SQI exhibited strong positive relationships with both fruit yield and NUE. The highest fruit yield was generally found under the CM-75% N + BC treatment in the first CS and under the CM50% N + BC treatment in other three CSs, while the highest NUE was found under the CM-50% N+ BC treatment in all four CSs. Our results infer that conjunctive use of CM and BC could enhance both crop yields and NUE under conditions with reduced N input.