Organic water-soluble fertilizers promote cotton growth and phosphorus uptake by stimulating indigenous microbiota-mediated soil phosphorus activation

To elucidate the common mechanisms and concentration effects of six typical organic water-soluble fertilizers (OWSFs) on soil phosphorus (P) activation mediated by indigenous microorganisms, crop P uptake, and growth enhancement, a pot experiment with cotton in slightly saline soil was conducted. The study investigated two factors: 1) OWSF types (including six representatives: alginic acid powder, alginic acid aqueous solution, polyglutamic acid-based, chitin-based, fish protein-based, and molasses-based fertilizers) and 2) organic carbon (C) application rates (0, manufacturer-recommended dose, 40, 80, 160, and 240 mg·pot⁻¹). The effects of C rates within each OWSF type and across different OWSF types under the same C rate were evaluated for cotton seedling growth (plant height, leaf area dynamics, biomass, and P uptake), soil microbial activity (microbial biomass P and phosphatase activity), and soil available P content. The C priming effect (i.e., changes in microbial biomass P, phosphatase activity, bioavailable P, and plant P uptake per unit C input) of the six OWSFs was quantified. The key findings showed that 1) all six OWSFs enhanced soil P activation, cotton P uptake, and seedling growth in a C dose-dependent manner, with dose-response relationships well fitted by linear-plateau models. Threshold C rates (30~60 mg·pot⁻¹) for growth stimulation varied among OWSF types. 2) At equivalent C rates, OWSF types showed no significant differences in promoting cotton growth but exhibited distinct priming effects on microbial biomass P, phosphatase activity, and P availability (P < 0.05). This indicates that OWSFs enhance plant growth indirectly by stimulating microbially mediated soil P activation rather than through direct effects. 3) The efficiency of the priming effect of six OWSF-C sources varied significantly, with molasses-based OWSF demonstrating the highest priming effect on microbial biomass P among the six types. We further discuss the current definition of OWSFs, proposing that their "fertilizer" function is primarily achieved through water-soluble organic C priming microbial activity to activate soil P turnover.