Humic acid and phosphate solubilizing bacteria led phosphorous bioavailability for enhancing photosynthetic efficiency and productivity of direct-seeded rice

Phosphorus deficiency is a major limitation in rice production ecosystems across the globe, restricting photosynthetic productivity and overall crop yields in spite of its very vital function in energy transfer and metabolic processes. Direct-seeded rice tends to experience phosphorus fixation in the soil, which lowers the bioavailability of this vital nutrient and restricts its key functions in ATP synthesis and photosynthesis. In the view of these concerns, field experiment conducted during the kharif seasons of 2022 and 2023 at Govind Ballabh Pant University of Agriculture and Technology, Pantnagar, Uttarakhand (India), with 12 treatment combinations tested in factorial randomized block design. The treatments consisted of 3 phosphorus levels (control, 50%, and 100%, corresponding to 0, 30, and 60 kg of P2O5 ha-1) and 4 phosphate solubilizers (control, humic acid (HA) @ 10 kg ha-1, carrier-based phosphate solubilizing bacteria (PSB) biofertilizer @ 10 g kg-1 seed, and HA @ 10 ha-1 + carrier-based PSB biofertilizer @ 10 g kg-1 seed). Results showed that higher phosphorus application significantly enhanced Normalized Difference Vegetation Index (NDVI) values across all growth stages, with notable increases of 9.33% at 60 days after sowing (DAS) and 26.53% by 120 DAS under 100% phosphorus (60 kg P2O5 ha-1) compared to the control. Similarly, chlorophyll-a content improved by 19.17% and 22.81% at 60 and 90 days after sowing (DAS), respectively, while chlorophyll-b and total chlorophyll content exhibited comparable enhancements. The application of phosphate solubilizers also enhanced NDVI values, with the combination of humic acid and PSB yielding the highest NDVI of 0.82 at 60 DAS, representing an 11.11% increase over the control. Regarding chlorophyll content, humic acid + PSB application resulted in chlorophyll-a levels of 1.38 mg g⁻¹ fresh weight (FW) at 60 DAS, marking a significant increase of 12.19% compared to the control. The same treatment also resulted in 13.51% higher chlorophyll-b content. These positive findings demonstrate the potential of incorporating phosphorus management with natural solubilizers to significantly improve photosynthetic efficiency and overall performance in direct-seeded rice, providing an auspicious, real-world solution for farmers confronted with nutrient constraints.