Supplementing chemical fertilizer with an organic component increases soil biological function and quality

• Bio-organic fertilizer improved soil biological activity and microbial community size under cotton.
• Partial substitution of NPK (mineral) fertilizer with an organic component alters soil biology.
• Suppression of cotton soil borne pathogens was highly correlated with increases in Bacillus.

The aim of this study was to investigate the impacts of organic-supplementation of a chemical fertilizer for improving soil biological activity, particularly towards disease suppression, in a drip-irrigated cotton system. Over a two-year field-based trial, our study characterized effects of partial substitution of chemical fertilizer with two types and amounts of organic components on the size, activity, carbon utilization potential, and culturable groups of pathogenic and antagonistic microbiota. The total microbial community size increased with application of chemical fertilizer (P < 0.05), and more so when chemical fertilizer was supplemented with organic amendments (P < 0.05). Alteration of microbial biomass carbon to nitrogen ratio, and the ratio of culturable bacteria and fungi, indicated that increases in the microbial community were underpinned by changes in bacteria. Enzymatic activity was greater in soils receiving fertilizer with organic supplement (P < 0.05). Multivariate analysis indicated that there was no influence of fertilizer treatments on carbon utilization profiles (CUPs; P > 0.05), but a strong year-effect was evident (P < 0.05). The population size of Verticillium dahliae and Fusarium oxysporum decreased with organic amendments (P < 0.05), whilst antagonistic Trichoderma spp. and Bacillus spp. were inversely correlated to the pathogen populations. Overall, the results of this exploratory study support partial substitution of inorganic fertilizer with organic manure or bio-organic fertilizer to promote suppression of cotton root diseases. Associated benefits on soil biological quality will further increase the sustainability of this agroecosystem.