Phosphorus management through sources of phosphorus, FYM and phosphate solubilizing bacteria in transplanted autumn rice (Oryza Sativa L.) for enhancing productivity under Assam conditions

Introduction

Rice (Oryza sativa L.) is the most important cereal crop globally and serves as the staple food for more than half of the world's population. Globally, 755.47 million tonnes of rice are produced from 162.05 million hectares (FAOSTAT, 2019) [1] . Asia contributes nearly 92% of global rice production while accommodating 59% of the world's population. China is the highest rice producer, followed by India. With the global population projected to reach 9.8 billion by 2050 (Reetz, 2016) [2] , enhancing rice productivity is essential to meet future food demand. In India, rice occupies about 44 million hectares with an annual production of 172.58 million tonnes and an average productivity of 2.57 t/ha. In Assam, rice is the staple food and occupies nearly two-thirds of the gross cropped area. Due to agro-climatic diversity, Assam has three rice-growing seasons: autumn (ahu), winter (sali), and summer (boro). Autumn rice covers only 2.2 lakh ha with low productivity (Directorate of Economics and Statistics, 2016) [3] . Although autumn rice avoids flood damage common in winter rice, its productivity remains low due to water stress, weed infestation, and poor nutrient management. Phosphorus (P) is a crucial macronutrient involved in photosynthesis, ATP synthesis, root proliferation, tillering, flowering, and grain formation. However, only 0.1% of total soil phosphorus is available to plants due to fixation by iron and aluminium oxides in acidic soils (Pereira et al., 2014; Panda et al., 1984) [5, 4] . Acid soils of Assam exacerbate this problem, leading to low phosphorus use efficiency (PUE: 20-25%) and phosphorus recovery efficiency (PRE: ~10%) (Fageria et al., 2015; Von & Mutert, 1995) [6,7] . Excessive dependence on chemical P fertilizers not only increases production costs but also disrupts soil microbial diversity (Gyaneshwar et al., 2002) [8] . India imports nearly 90% of its phosphate fertilizers, making rice cultivation economically vulnerable (Bagyaraj et al., 2015) [9] . Rock phosphate (RP), a natural and lowcost P source, is more suitable for acidic soils due to its gradual dissolution and long-term nutrient release (Siddaramappa et al., 1991) [10] . India has over 260 million tonnes of low-grade RP deposits (Kumari et al., 2012) [11] ; however, its low solubility limits direct application. Farmyard manure (FYM) improves soil structure, enhances microbial activity, reduces P fixation by chelating Fe and Al ions, and increases available phosphorus through mineralization (Sharma et al., 1985; Guppy et al., 2005; Huang et al., 2014) [ 13, 12, 14] . Moreover, phosphate-solubilizing bacteria (PSB) such as Bacillus, Paraburkholderia, and Pseudomonas solubilize insoluble phosphates by organic acid secretion, enzyme production, and phytohormone synthesis like IAA, enhancing root growth and nutrient uptake (Chen et al., 2006; Ahemad et al., 2014) [16,15] .

Materials and Methods

The field experiment was conducted during the autumn seasons In PSB-treated plots, seedlings were root-dipped in a slurry containing 500 ml/ha of PSB inoculum. To study root growth parameters, a separate pot experiment was also conducted in both the years. For the pot experiment, fertilizer requirements were calculated considering a 1ha area containing 2.24 × 10 6 kg soil up to 15 cm depth. Therefore, for 5 kg soil, Urea @ 0.190g, MOP @ 0.074g, SSP @ 0.279g (100 %) and 0.1395g (50 %), RP @ 0.223g (100 %) and 0.112g (50 %), and FYM @ 4.4g were applied in respective pots. The application of manures and fertilizer in the pot experiments in both the years were done in the same way as in field conditions. The field was maintained at 5 cm standing water until flowering and then kept under saturated conditions. For weed and insect pest control the recommended package of practices were followed. For the purpose of studying roots, plants from their respective pots were removed, and roots were gently washed under a fine water jet to remove the soil from them. The root mean length was measured using a measuring scale from their bases to their tips. Root volume was measured using the displacement-based root volume technique (Mishra et al., 1987) [17] . For root fresh weight and dry weight, roots were collected, the soil was removed gently, and the fresh weight of roots was recorded. For dry weight, same samples were dried in sunlight followed by in an oven at 60 °C for 72 hrs before being weighed. The averages values were considered. For the field experiment, five plants were randomly chosen from each treatment to record growth and yield parameters. Plants from a net plot area of 6 m 2 and were harvested, sundried, threshed, and yields were recorded. All the data pertaining to the present investigation were statistically analysed for Factorial Randomized Block Design (FRBD). Critical differences (CD) at 5 per cent probability level was calculated when the F value was found out to be significant.

Results and Discussion

Plant Growth Parameters: A critical examination of the data (Table 1) revealed that growth parameters of transplanted autumn riceplant height, number of tillers/hill, leaf area index (LAI), and dry matter accumulation were significantly influenced by sources of phosphorus during both years. The combined application of single super phosphate (50% RDP) + rock phosphate (50% RDP) (P 3 ) resulted in markedly higher plant height (88.47 and 87.28 cm), number of tillers/hill (19.28 and 19.17), LAI (4.64 and 4.57), and dry matter accumulation (28.26 and 27.31 g/hill) in 2021 and 2022, respectively, compared to the sole use of either SSP or rock phosphate. The superiority of P 3 can be attributed to the complementary effects of soluble and slowly available phosphorus forms, ensuring a steady nutrient supply and efficient utilization throughout crop growth. The better performance of P 3 could be related to improved phosphorus availability, root proliferation, and higher photosynthetic efficiency, supporting vigorous plant growth-findings consistent with Sarkar et al. (2018) [18] . Treatment F 5 (FYM @ 2 t/ha + PSB I (Paraburkholderia tropica) + PSB II (Bacillus circulans)) produced significantly greater plant height (90.58 and 89.40 cm), number of tillers/hill (16.49 and 15.31), LAI (4.75 and 4.66), and dry matter accumulation (29.15 and 28.67 g/hill) in both years, compared with the control. The superior response of F 5 likely resulted from favourable soil conditions created by FYM and enhanced phosphorus solubilization by PSB, which stimulated nutrient uptake, hormonal activity (IAA, GA, cytokinins), and overall physiological function, as observed by Jalil et al. (2014) [19] , Bakhshandeh et al. (2015) [20] , and Biswakarma et al. (2018) [21] . The control (F 1 ) consistently recorded the lowest values for all parameters. Root Growth Parameters: Root growth characteristics-root length, volume, fresh and dry weights showed significant variation under different phosphorus sources (Table 2). The treatment P 3 (SSP 50% RDP + RP 50% RDP) recorded the highest root length (27.04 and 26.19 cm), root volume (79.32 and 78.16 cc), root fresh weight (14.10 and 13.01 g/hill), and root dry weight (4.51 and 4.22 g/hill) in 2021 and 2022, respectively. This improvement is likely due to the continuous availability of both soluble and slow-release phosphorus fractions, ensuring sustained nutrient supply and encouraging root proliferation. Similar observations were made by Sarkar et al. (2018) [18] , who reported that partial substitution of SSP with rock phosphate improved root development and phosphorus-use efficiency in cereals. Treatment F 5 (FYM @ 2 t/ha + PSB I + PSB II) produced the longest roots (27.57 and 26.63 cm), greatest root volume (82.69 and 82.14 cc), and highest fresh (15.92 and 14.84 g/hill) and dry weights (5.17 and 4.88 g/hill) during both years, followed by F 4 (FYM + PSB II). The control (F 1 ) registered the lowest values. The improvement under F 5 can be ascribed to the combined benefits of FYM and PSB, which improved soil structure, enhanced nutrient availability, and stimulated rhizosphere microbial activity. Phosphate-solubilizing bacteria increased phosphorus availability through secretion of organic acids and phosphatases, while FYM provided a conducive medium for microbial proliferation and root elongation. Enhanced root biomass under PSB inoculation might also result from the production of plant growth regulators such as indole acetic acid and gibberellins, promoting root elongation and branching. These results are in conformity with the findings of Hatia et al. (2000) [22] , Rahi et al. (2010) [23] , and Singh et al. (2014) [24] . Yield and Yield Attributes: Yield and yield-attributing characters viz. number of panicles/hill, panicle length, filled and unfilled grains/panicle, test weight, grain yield, straw yield and harvest index -varied significantly with phosphorus sources and organic amendments in both years, (Table 3). Application of P 3 (50% SSP + 50% RP) resulted in the highest number of panicles/hill (14.49 and 13.53), panicle length (22.98 and 22.29 cm), filled grains/panicle (92.95 and 91.00), and test weight (19.50 and 19.45 g) during 2021 and 2022, respectively. This treatment also produced highest grain yields (3287.17 and 3247.11 kg/ha) straw yields (4042.68 and 4037.71 kg/ha) and harvest index (44.83 and 44.55 %), followed by P 2 , whereas P 1 recorded the minimum. The improvement under P 3 may be attributed to balanced and prolonged phosphorus availability through the combined effect of readily soluble and slow-release forms, resulting in better nutrient uptake, root development, and efficient partitioning of photosynthates towards reproductive sinks. These observations agree with those of Sarkar et al. (2018) [18] . Treatment F 5 ( , Sarkar et al. (2018) [18] , and Akbari et al. (2010) [26] . The data about grain and straw yield as influenced the interaction of sources of P, organic amendments and phosphate solubilizing microbe has been presented in Table 4. Regarding the interaction effect, the sources of P, organic amendments and phosphate solubilizing microbe interacted significantly in respect of grain yield and straw yield in both the years of experimentation. The highest values of grain yield (3559.44 and 3521.18 kg/ha in 2021 and 2022 respectively), straw yield (4251.73 kg/ha and 4260.63 kg/ha in 2021 and 2022, respec-tively) were obtained in treatment combination of P 3 F 5 which caused in 49.05 % and 51.70 % increase in grain yield in both the years than that in P 1 F 1 combination. In P 3 F 5 , combined application of SSP and RP might have provided P throughout the growing season of rice crop; application of PSB might have further augmented the release P from RP in later growth stages, in addition to these, FYM application also boosted favorable soil condition for root proliferation and large biomass production resulted in better growth attributes leading to increase in grain yield and straw yield.

Conclusion

It can be concluded that P 3 F 5 combination proved to be the most effective approach in enhancing yield of transplanted autumn rice. The superior performance of P 3 F 5 was attributed to improved phosphorus availability from synergistic effects of water-soluble P from SSP, slow-release P from RP, enhanced microbial solubilization, and soil biological activity due to FYM and PSB application. So P 3 F 5 is a sustainable alternative to sole chemical P fertilization in acidic soil-based rice ecosystems of Assam, which also enhances productivity of transplanted autumn rice.

Acknowledgement

AcknowledgementAuthor is grateful to Department of Agronomy and Soil Science, College of Agriculture, Assam Agricultural University during the period of her research work.

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