Effect of Long-Term Organic Fertilization in Flooded Rice Soil on Phosphorus Transformation and Phosphate Solubilizing Microorganisms

Phosphorus (P) is a major limiting factor for plant growth. Long-term organic fertilization increases P availability by improving soil microbial and chemical properties. To study this, rice was grown under different treatments, namely, control, farmyard manure (FYM), green manure (GM), FYM + GM (1:1) (FYGM), FYM + Azolla (1:1) (FYAZ), rice straw (5 t ha−1) + GM (GMRSF), Azolla + GM (1:1) (GMAZ) and rice straw (2.5 t ha−1) + GM (GMRSH), with the aim of comparative evaluation of microbiological and chemical changes occurring in the rice soil after 15 years of rice-fallow cycles. The results showed that organic fertilization increased dehydrogenase activity by 12–50% over the control. Both acid (166.1 μg PNP g−1 h−1) and alkaline phosphatase (46.1 μg PNP g−1 h−1) activities were found highest in GMRSH. Long-term application of organic fertilization increased the pools of active/labile C (microbial biomass carbon, readily mineralizable carbon), N (microbial biomass nitrogen) and P (microbial biomass phosphorus) in the soil. Bacterial and fungal populations were observed higher in the FYM treatment than in the control. However, the populations of phosphate solubilizing bacteria (PSB) and fungi (PSF) were observed highest in GMRSH and FYAZ, respectively. Non-occluded Al and Fe-P (125.94 kg ha−1) was highest in GMRSH, while occluded P (6.46 kg ha−1) was highest in GMRSF. Thus, long-term (15 years) organic fertilization improves PSB and PSF populations, contributing to better P transformation in the soil. GMRSH and GMRSF treatments were found to be effective organic fertilization in increasing enzymatic activities, available phosphate fractions and microbial biomass in soil.