Co-application of wheat straw biochar, phosphorus solubilizing Bacillus tropicus, and compost for spinach production under reduced chemical fertilizers

The overuse of chemical fertilizers in agriculture is a major cause of subsurface water contamination and greenhouse gas emissions. As a result, cutting back on the use of chemical fertilizer is crucial without compromising on agricultural yield. In this context, plant growth promoting rhizobacteria (PGPR), compost, and biochar (BC) can offer a more affordable and sustainable option. We evaluated the effect of wheat derived biochar and municipal solid waste compost on spinach under 100% or 50% of the recommended fertilizers(RF), in the presence or absence of a PGPR i.e., Bacillus tropicus. Spinach was grown on a sandy clay loam soil in pots for 48 days. Biochar and compost were mixed at 1% (w: w basis) with soil in all possible combinations of 100% or 50% of the recommended fertilizers. Plants receiving 100% and 50% of recommended fertilizers served as control. Biochar and PGPR significantly increased shoot-root length, with the maximum increase of 42.14-43.51% where BC+PGPR under 50RF were added. Moreover, co-application of biochar with PGPR further increased the root biomass, with the maximum increase of 150% when biochar and PGPR were co-applied while sole application of biochar significantly increased the shoot biomass by 58.15% under 50RF regime. Co-application of BC and PGPR further increased soil nitrate content with the maximum increase of 107.72% found under 50% fertilizer regime. The ammonium content was significantly increased 278.18% and 757.14% when compost was applied alone under 100RF and 50RF regimes. Addition of compost increased the microbial biomass C by 61.06% and 156.87% under 100RF and 50RF regimes, respectively. Overall, all the three amendments improved plant production and/or soil health indicators. However, the improvement in soil health indicators translated significantly into higher spinach biomass only in co-applied biochar and Bacillus tropicus treatment. Our results indicate that biochar and PGPR worked in tandem. PGPR solubilized phosphorus presumably from biochar, whereas the latter provided suitable environment for growth of PGPR. We recommend that this combination can be used to partially replace chemical fertilizers to produce spinach.