Arbuscular mycorrhizal fungi enhance phosphorus uptake and grain yield of pearl millet in phosphorus-deficient calcareous soil

Introduction

Pearl millet (Pennisetum glaucum L.) is the sixth most important cereal globally and the primary food grain for 90 million people in arid Africa and South Asia [1] . Its production on calcareous soils is severely constrained by low phosphorus availability: calcium carbonate precipitates P into insoluble calcium phosphates, reducing plant-available P to 3-8% of total soil P [2,3] . Arbuscular Mycorrhizal Fungi (AMF) extend the root's absorptive surface through extraradical hyphae that explore soil volumes beyond the P depletion zone, effectively increasing the plant's P acquisition capacity [4,5] . AMF can also solubilize occluded P through organic acid exudation and phosphatase enzyme activity [6] . While AMF benefits for cereals are well documented in neutral to acidic soils, fewer trials have quantified their effectiveness in calcareous systems where P fixation is dominated by carbonate chemistry rather than iron/aluminium oxides [7,8] . This research tested three AMF species and their consortium on pearl millet to determine which mycorrhizal strategy best overcomes P limitation in calcareous soil.

Material and Methods

Research area

The pot experiment was conducted at the Faculty of Agriculture, Cairo University, Giza (30.0131°N, 31.2089°E) during kharif 2022 (June-October). Mean temperature during the trial was 31.4°C with negligible rainfall. Pots (30 cm diameter, 35 cm depth) were filled with 12 kg of field-collected calcareous soil and maintained at 60% field capacity with drip irrigation [9] .

Soil analysis

Soil was a Typic Calcixerept with CaCO 3 content of 24.7%, pH 8.3, EC 1.4 dS/m, OC 0.41%, total P 487 mg/kg but available P (Olsen) only 4.8 mg/kg confirming severe P deficiency [10] . Clay fraction contained 31% smectite, 18% palygorskite. Soil P was fractionated into available (NaHCO 3 -extractable), organic (NaOH-extractable organic), and occluded (HCl-extractable) pools at 0, 1, 2, 3, and 4 months after AMF inoculation using the Hedley sequential extraction [11] .

Methods

Five treatments in RCBD with four replications: T1-Control (no AMF), T2-Glomus mosseae (200 spores/pot), T3-Glomus intraradices (200 spores/pot), T4-Rhizophagus irregularis (200 spores/pot), T5-Mixed consortium (T2+T3+T4, 200 total). Pearl millet cv. ICTP-8203 was sown at 5 seeds/pot, thinned to 3.

Basal fertilizer: 40 kg N/ha equivalent, no P applied. Root colonization was assessed by the gridline intersect method after trypan blue staining at harvest [12] . P uptake, growth parameters, and grain yield were recorded. Data analysed by ANOVA in R 4.3.1 [13] .

Results

Comprehensive interpretation

The mixed consortium achieved the highest values across all parameters: 71.8% colonization, 31.2 kg P/ha uptake, and 2.71 t/ha grain yield (Table 1). This was 119% above the control yield (1.24 t/ha). Among single species, G. intraradices (T3) was most effective with 67.2% colonization and 2.47 t/ha yield, followed closely by R. irregularis (T4). P-use efficiency peaked at 35.6 kg grain/kg P in the mixed consortium. Root colonization and P uptake were linearly related (r = 0.987, p<0.01) (Figure 1), confirming that the extraradical hyphal network directly drives P acquisition. Each 10% increase in colonization corresponded to approximately 3.5 kg/ha additional P uptake. AMF inoculation progressively increased available P and reduced occluded P (Figure 2). Available P rose from 14.3 to 24.7 mg/kg by month 3 (+73%), then declined slightly to 22.1 mg/kg at harvest as crop uptake intensified. Occluded P decreased from 47.5 to 25.5 mg/kg, indicating AMF-mediated solubilization of calcium-bound phosphates.

Discussion

The 119% yield increase from the mixed consortium is among the highest AMF responses reported for cereals, reflecting the extremely low baseline P availability (4.8 mg/kg Olsen P) that maximizes the relative benefit of mycorrhizal P acquisition [4,5] . G. intraradices outperformed G. mosseae as a single inoculant, consistent with its reputation for extensive extraradical hyphal networks [7] . The 37% reduction in occluded P confirms that AMF actively solubilize calcium phosphates through organic acid production, a mechanism previously demonstrated only in pot studies with legumes [6,8] . The strong colonization-uptake correlation (r = 0.987) validates root colonization as a reliable predictor of AMF benefit in calcareous soils and supports its use as a screening criterion for selecting effective AMF strains for arid-zone agriculture [12,13] .

Conclusion

AMF inoculation increased pearl millet yield by up to 119% in P-deficient calcareous soil. The mixed consortium was most effective (2.71 t/ha, 71.8% colonization), followed by G. intraradices (2.47 t/ha). Root colonization strongly predicted P uptake (r = 0.987). AMF shifted soil P from occluded to available fractions, demonstrating biological solubilization of calcium-bound phosphates. Mixed AMF inoculation merits adoption as a biofertilizer strategy for millet production on calcareous soils.

References

1. Yadav OP, Rai KN. Genetic improvement of pearl millet in India. Agricultural Research. 2013; 2(4):275-292.
2. Holford ICR. Soil phosphorus: its measurement, and its uptake by plants. Australian Journal of Soil Research. 1997; 35(2):227-239.
3. Hinsinger P. Bioavailability of soil inorganic P in the rhizosphere as affected by root-induced chemical changes. Plant and Soil. 2001; 237(2):173-195.
4. Smith SE, Read DJ. Mycorrhizal Symbiosis. 3rd ed. London: Academic Press; 2008.
5. Jakobsen I, Abbott LK, Robson AD. External hyphae of vesicular-arbuscular mycorrhizal fungi associated with Trifolium subterraneum L. New Phytologist. 1992; 120(3):371-380.
6. Tarafdar JC, Marschner H. Phosphatase activity in the rhizosphere and hyphosphere of VA mycorrhizal wheat. Soil Biology and Biochemistry. 1994; 26(3):387-395.
7. Cavagnaro TR, Smith FA, Smith SE, Jakobsen I. Functional diversity in arbuscular mycorrhizas. Mycorrhiza. 2005; 15(2):135-144.
8. Bolan NS. A critical review on the role of mycorrhizal fungi in the uptake of phosphorus by plants. Plant and Soil. 1991; 134(2):189-207.
9. Cairo University. Weather Station Data: Faculty of Agriculture. Cairo: CU Press; 2022.
10. Page AL, Miller RH, Keeney DR. Methods of Soil Analysis. Part 2. 2nd ed. Madison: ASA-SSSA; 1982.
11. Hedley MJ, Stewart JWB, Chauhan BS. Changes in inorganic and organic soil phosphorus fractions. Soil Science Society of America Journal. 1982; 46(5):970-976.
12. Giovannetti M, Mosse B. An evaluation of techniques for measuring vesicular arbuscular mycorrhizal infection in roots. New Phytologist. 1980; 84(3):489-500.
13. R Core Team. R: A language and environment for statistical computing. Vienna: R Foundation for Statistical Computing; 2023.