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The gradually dwindling reserves of rock phosphate, the primary material used in the manufacturing of phosphatic fertilizers, encourages researchers to look for ways to exploit the accumulated fixed P pool in soil. Phosphate solubilizing microorganisms (PSM) could be a viable option for addressing the problem at a lower cost. Keeping these in mind, the present study was undertaken to evaluate the changes in the distribution of P in soil as affected by P fertilization, phosphate solubilizing fungi (PSF) and liming vis-à-vis the contribution of these fractions toward P nutrition of a test crop soybean (Glycine max L.). A bulk surface soil sample (0–15 cm) was obtained from Negheriting tea estate of Golaghat district of Assam, India (Ultisol, pH = 4.23) and after processing, three levels of P [0, 50, and 100% of recommended dose of P (RDP)], two levels of lime [No lime, 1/10th of Lime Requirement (LR)] and two levels of PSF (No-PSF, PSF) were applied in a completely randomized design with three replications. Sequential P fractionation was done in the post-harvest soil. On an average, the abundance of different P fractions in the soil, expressed as % of total P, followed the order: residual P (67.5%)> Fe bound P (12.1%)> reductant soluble P (8.85%)> Al bound P (4.04%)> occluded P (3.79%)> Ca bound P (3.11%)> soluble and loosely bound P (0.46%). All the inorganic P fractions except the residual P, increased significantly with P fertilization. Either liming or PSF application significantly increased the soluble and loosely bound P fraction and decreased the Al bound and Fe bound P fractions in soil. Positive growth response of soybean was obtained due to the application of P, lime, and PSF. Liming increased the P uptake by 30.4% and dry matter yield of soybean by 18.5% over no liming. On the other hand, PSF inoculation increased the P uptake by 16.7% and dry matter yield by 7.77% over no inoculation. So, it is evident that in short term, either liming or PSF was able to solubilize the native soil P. Phosphorus×lime and lime×PSF interactions should also be exploited in future endeavors.

Tropical soils are often deeply weathered and vulnerable to degradation having low pH and unfavorable Al/Fe levels, which can constrain crop production. This study aims to examine nitrogen-enriched novel biochar co-composts prepared from rice straw, maize stover, and gram residue in various mixing ratios of the biochar and their feedstock materials for the amelioration of acidic tropical soil. Three pristine biochar and six co-composts were prepared, characterized, and evaluated for improving the chemical and biological quality of the soil against a conventional lime treatment. The pH, cation exchange capacity (CEC), calcium carbonate equivalence (CCE) and nitrogen content of co-composts varied between 7.78–8.86, 25.3–30.5 cmol (p+) kg−1, 25.5–30.5%, and 0.81–1.05%, respectively. The co-compost prepared from gram residue biochar mixed with maize stover at a 1:7 dry-weight ratio showed the highest rise in soil pH and CEC, giving an identical performance with the lime treatment and significantly better effect (p < .05) than the unamended control. Agglomerates of calcite and dolomite in biochar co-composts, and surface functional groups contributed to pH neutralization and increased CEC of the amended soil. The co-composts also significantly (p < .05) increased the dehydrogenase (1.87 µg TPF g−1 soil h−1), β-glucosidase (90 µg PNP g−1 soil h−1), and leucine amino peptidase (3.22 µmol MUC g−1 soil h−1) enzyme activities in the soil, thereby improving the soil’s biological quality. The results of this study are encouraging for small-scale farmers in tropical developing countries to sustainably reutilize crop residues via biochar-based co-composting technology.