Authors: Murugan Kumar, Savita Santosh, Arjun Singh
Introduction
Phosphorus is the second most important plant nutrient which is diffusion limited and essential for plant growth and yield. Rock phosphate a major ingredient for manufacture of phosphatic fertilizers is found in large quantities in India. However much of it is of low grade with 25-30% P 2O5 and hence are not suitable for manufacture. Thus India is dependent on imported phosphatic fertilizers and an estimate quotes that almost 90 % of the phosphatic fertilizers used in India are imported (Bagyaraj et al., 2015). Even after adding phosphatic fertilizers to soil, a major portion may not be absorbed by plants because P can easily get bound in soil and sparingly soluble, thus warranting addition of large quantities of phosphatic fertilizers. In a typical agriculture system average uptake of P by plants is only 15-20% of applied phosphates (Sawers et al. 2017). Also a significant part of fixed phosphorus can then cause environmental issues like eutrophication. It is widely believed that a vast diversity of agriculturally important microorganisms can enhance use efficiencies of phosphorus and can be used as a major component in integrated nutrient management systems for sustainable crop production.
Microbial inoculants to enhance phosphorus use efficiency
The concept of enhancing P availability to plants by microbes is well established. Microorganisms can enhance P absorption capacity of plants through various mechanisms. Major three mechanisms are; 1) Increased root growth, 2) alteration of sorption equilibria resulting in increased net transfer of phosphate ions in soil solution and 3) induction of metabolic processes that are effective in solubilizing and mineralizing P from inorganic and organic sources respectively. Although such mechanisms are widely demonstrated under laboratory conditions and in some glass house conditions, unambiguous evidence regarding operation of such mechanisms to supply P under field condition remain elusive and hitherto remains poorly understood and quantified. Nonetheless results under field conditions have shown that microbial inoculants can increase phosphorus use efficiency (Richardson and Simpson 2011). Arbuscular mycorrhizal fungi (AMF) a group of fungi in the phylum Glomeromycota which form efficient symbioses with roots of higher plants have been shown to increase P uptake in diverse crop plants (Owen et al. 2015). Arbuscular mycorrhizal (AM) symbioses is a special kind of mutualistic interaction in which AMF helps in P uptake by plants and its mobility inside plants and in return the plants provide AMF with sugars and habitat. AM symbioses, because of its large surface area can contribute to plant nutrition uptake especially phosphorus uptake. Since phosphorus is highly immobile element the left over phosphates which are not absorbed by plants are easily absorbed in the bulk soil and hence a phosphate free zone occurs in the rhizosphere. But, the extraradical mycelium formed by fungal partner can extend beyond this phosphate free zone and help plants in absorbing phosphate available at a far off distance from roots thereby making the otherwise unavailable phosphates to available phosphates (Roy-Bolduc and Hijri, 2011). Glomus,Gigaspora, Scutellospora, Acaulospora and Entrophospora are the most commonly occurring AM fungi (AMF) and they are obligate symbionts (Bagyaraj et al., 2015). The role of AM fungi in improving P use efficiency and general growth parameters is established in different crops by various research groups (Table 1).
Table 1: Effect of AM fungi inoculation on P use efficiency of different crop plants.
| Crop | AM fungi inoculated | Response | Research group |
| Wheat | Glomus mosseae | Increased grain yield of up to 25.8 %; Increased protein content and wet gluten content | Kumar et al., 2011 |
| Rice and Mung bean in intercrop | Glomus caledonium | Intercropping and AM inoculation increased total P uptake by 57% in rice, total P and N acquisition by 65% and 64% respectively in mung bean, and nodulation by 54% in mung bean | Li et al., 2009 |
| Maize | Glomus intraradices | Increased available phosphate in the rhizosphere and in the bulk soil | Cozolino et al., 2013 |
| Green gram | Glomus fasiculatum, Glomus mosseae | Grain yield (by 14.8 and 13.5%) and grain P content was significantly increased (by 21.95 and 20.97%), respectively. | Bhat et al. 2011 |
| Tomato | Glomus intraradices | Increased N and P uptake in both drought stress and normal conditions | Subramanian et al., 2006 |
| Chickpea | Glomus fasciculatum | Increase plant yield and nutrient uptake when AM fungi was inoculated with Rhizobium and a Phosphorus solubilizer Pseudomonas striata | Zaidi et al., 2003 |
Conclusion
Phosphorus is one among the major limiting factors of crop production. Considering its importance in crop production and negative effects of fixed P and leaching thereafter microbial intervention through AMF and phosphorus solubilizing microorganisms (PSM) seems to be an effective way to improve P use efficiency. Hence both AMF and PSM can influence the benefit cost ratio positively. To achieve the dreams of doubling farmers’ income by 2022, microbial intervention to improve nutrient efficiency especially P use efficiency can be included as one of the major intervention points.
References
1. Bagyaraj DJ, Sharma MP, Maiti D (2015) Phosphorus nutrition of crops through arbuscular mycorrhizal fungi. Curr. Sci. 108:1288-1293.
2. Bhat MI Bangroo SA, Ali T, Yadav SRS, Aziz MA (2011) Combined effects of rhizobium and vesicular arbuscular fungi on green gram (Vigna radiata L. Wilczek) under temperate conditions. Res J Agric Sci 2:17â€"20.
3. Cozzolino V Di Meo V, Piccolo A (2013) Impact of arbuscular mycorrhizal fungi applications on maize production and soil phosphorus availability. J. Geochem Exploration 129:40-44.
4. Kumar A, Sharma KD, Gare R (2011) Arbuscular Mycorrhizae (Glomus mosseae) symbioses for increasing the yield and quality of wheat (Triticum aestivum). Indian J. Agri. Sci. 81:478-480.
5. Li Y, Ran W, Zhang R, Sun S, Xu G (2009) Facilitated legume nodulation, phosphate uptake and nitrogen transfer by arbuscular inoculation in an upland rice and mung bean intercropping system. Pl. Soil 315:285-296.
6. Owen D, William AP, Griffith GW, Withers PJA (2014) Use of commercial bio-inoculants to increase agricultural production through improve phosphorus acquisition. Appl. Soil Ecol. 86:41-54.
7. Richardson AE, Simpson RJ (2011) Soil Microorganisms Mediating Phosphorus Availability Update on Microbial Phosphorus. Pl. Physiol. 156:989-996.
8. Roy-Bolduc A, Hijri M (2011) The Use of Mycorrhizae to Enhance Phosphorus Uptake: A Way Out the Phosphorus Crisis. J Biofertil. Biopestici. 2:104.
9. Sawers RJH, Svane SF, Quan C, Gronlund M, et al. (2017) Phosphorus acquisition efficiency in arbuscular mycorrhizal maize is correlated with the abundance of root-external hyphae and the accumulation of transcripts encoding PHT1 phosphate transporters. New Phytol. 214: 632-643.
10. Subramanian KS, Santhanakrishnan P, Balasubramanian P (2006) Responses of field grown tomato to arbuscular mycorrhizal fungal colonization under varying intensities of drought stress. Scientia Horticulturae 107:245-253.
11. Zaidi A, Khan MS, Amil MD (2003) Interactive effect of rhizotrophic microorganisms on yield and nutrient uptake of chickpea (Cicer arietinum L.). Eur. J. Agron. 19:15-21.
About Author / Additional Info:
I am a Scientist in Agricultural Microbiology working with ICAR-National Bureau of Agriculturally important Microorganisms.