Native soil phosphorous solubilization and enhancement of nutrient use efficiency by using bio fertilizers in core rainfed groundnut growing alfisols

Introduction

Groundnut (Arachis hypogaea L.) is a major rainfed legume crop cultivated extensively in semiarid regions, including the Alfisol tracts of southern India. Due to strong fixation by iron and aluminum oxides, poor organic content, and coarse texture that limits nutrient retention, alfisols usually have low to moderate plant-available phosphorus (P) (Sanyal & De Datta, 1991; Kamprath, 1984) [10,5] . Although phosphorus is necessary for groundnut root growth, nodulation, nitrogen fixation, pod formation, and successful reproduction, P deficiency continues to be a significant yield-limiting factor in dryland settings (Rego et al., 2007) [8] . Due to unpredictable rainfall and decreased nutrient absorption in dry soils, the problem of low P availability is made worse under rainfed circumstances (Venkateswarlu & Shanker, 2009) [15] . Furthermore, smallholder farmers are constrained by high fertilizer costs and restricted access to chemical fertilizers, which makes native soil P mobilization options more important and financially feasible (Subba Rao et al., 2015) [13] . The ability of microbial inoculants like phosphate-solubilizing bacteria (PSB), phosphatesolubilizing fungi (PSF), and arbuscular mycorrhizal (AM) fungi to solubilize sparingly soluble soil P through mechanisms like organic acid secretion, phosphatase release, chelation and increased rhizosphere activity is well known (Rodríguez & Fraga, 1999; Sharma et al., 2013) [9,11] . By creating vast hyphal networks that expand the root absorptive surface, AM fungi also enhance the uptake of P and micronutrients (Smith & Read, 2008) [12] . It has been demonstrated that combining microbial inoculants with organic additions like farmyard manure (FYM) improves nutrient-use efficiency (NUE) in dryland agriculture, promotes nutrient cycling, and creates a beneficial soil biological environment (Bolan et al., 1990; Reddy et al., 2016) [2,7] . Particularly in nutrient-deficient Alfisols, such integrated nutrient management techniques improve soil health and crop output while reducing reliance on chemical fertilizers (Vassilev & Vassileva, 2003) [14] . In light of this, the current study was carried out to characterize native phosphorus fractions in Alfisols that grow groundnuts, analyze the impacts of microbial P-mobilizing inputs on soil phosphatase activities, and evaluate their effects on groundnut development and pod yield under rainfed conditions.

Materials and Methods

Site description

The experiment was conducted at the ANGRAU, Agricultural Research Station, Anantapur, which is situated in a core rainfed groundnut growing Alfisol zone (latitude, 14o 41' 218" N, longitude, 77O 40' 115" E), from kharif 2013 to kharif 2016. With 36 wet days and an average annual rainfall of 590 mm, the location has a semi-arid climate. Red sandy-loam Alfisol with pH 6.2, organic carbon 0.48%, and accessible phosphorus (Olsen P) of 6.5 mg kg⁻¹ (poor) was found in the pre-sowing soil analysis (0-15 cm).

Experimental design and treatments

The experiment was set up on a 5 m × 5 m area in a randomized complete block design with three replications. T 1 (control); T 2 (half recommended fertilizer dose, HRDF: 10:20:20 kg NPK/ha); T 3 (HRDF + PSB + AM fungi); T 4 (HRDF + PSF + AM fungi); T 5 (HRDF + PSB + PSF + AM fungi); T 6 (STBF, soil test-based fertilizer); T 7 (STBF + PSB + AM fungi); T 8 (STBF + PSF + AM fungi); T 9 (STBF + PSB + AM fungi only). PSB (Bacillus megatherium var. phosphaticum) at 5 kg/ha, PSF (Aspergillus awamori) at 5 kg/ha, and AM fungi (arbuscular mycorrhiza) at 12.5 kg/ha were the microbial inoculants. A beginning dosage of farmyard manure (FYM) was administered at 500 kg/ha. Seeds of groundnut K6 variety were sown at recommended spacing. Standard agronomic methods for controlling weeds, pests, and diseases were consistently applied in accordance with ANGRAU recommendations. The PSB consortium included well-characterized strains, such as Bacillus megatherium var. phosphoticum, that are known to produce organic acids and phosphatases and to solubilize phosphate. The soil application techniques adhered to known protocols, and the inoculum amount was around 10 8 CFU g⁻¹ seed. Rock phosphate (RP) was obtained locally; FYM was well-decomposed; PSF (Asperiligillus awamori) enhanced soil P mobilization; and AM fungus (Arbuscular mycorriha) transported P into roots.

Measurements and sampling

Soil analyses involved baseline and post-harvest sampling from 0-15 cm depth to determine pH, organic carbon, available P (Olsen method), and available N and K. Plant growth and yield parameters measured included plant height (cm), number of branches per plant, number of pods per sq. m, pod yield (kg ha⁻¹), and haulm yield (kg ha⁻¹). Agronomic efficiency of P was calculated as (yield in treated plot -yield in control) / P applied (kg seed yield per kg P applied). All samples were analyzed following standard protocols (e.g., Jackson 1973; Olsen et al. 1954). Data underwent ANOVA, with means compared using LSD at P = 0.05; correlation and regression analyses explored relationships among soil P fractions and yield parameters.

Results

Plant Growth Parameters

Plant height, number of branches, root characteristics and podbearing capacity varied significantly between treatments. Microbial inoculation treatments (T 3 -T 10 ) showed a modest improvement over control (T 1 ), with plant heights ranging from 22.40 cm (T 5 ) to 24.11 cm (T 10 ). T 8 (STBF + PSF + AM fungus) had the most branches per plant (4.52), followed by T 7 (4.30), demonstrating the beneficial effects of combined organic and microbial nutrition management. Strong responses to biological inoculants was demonstrated by root nodulation; T 9 (STBF + PSB + PSF + AM fungus) recorded the highest number of nodules (68 nodules/plant), about twice as many as the control (36.87). With very slight increases in infected treatments, root length remained within the 7.54-8.78 cm range.

Soil Chemical Properties

The highest values of soil organic carbon were found in T 5 (0.54%), T 2 , T 8 , T 9 (0.50%), and T 10 (0.47%), as opposed to 0.46% in the control, indicating modest gains under microbial and STBF-based treatments. Under microbial treatments, available nitrogen improved from 149 kg ha⁻¹ (T 1 ) to 179.25 kg ha⁻¹ in T 9 , indicating enhanced root symbiosis and greater mineralization.

Because of fertilizer application, available phosphorus levels increased significantly in treatments receiving PSB/PSF/AM fungus. T 2 had the highest values (80 kg ha⁻¹), followed by T 10 (68 kg ha⁻¹) and T 4 (68.25 kg ha⁻¹). Under microbial inoculation, the availability of potassium was maximum in T 2 (419.5 kg ha⁻¹) and T 4 (421 kg ha⁻¹), while the availability of sulfur and micronutrients improved moderately.

Micronutrient Availability (Cu, Fe, Zn, Mn) Significant differences were observed among the treatments for micronutrient availability in soil at harvest.

Copper (Cu)

The copper level of the soil varied from 0.61 to 0.78 ppm, with T 2 having the lowest value (0.61 ppm) and T 10 having the highest (0.78 ppm). In contrast to the control (0.66 ppm), treatments with microbial inoculants, specifically AM fungus and PSF, such as T 7 (0.74 ppm) and T 8 (0.74 ppm), demonstrated enhanced Cu availability. Cu solubility was probably improved by increased root exudation and organic acid generation during microbial activity.

Iron (Fe)

Available iron showed notable variation, ranging from 4.99 ppm in T 4 to 8.33 ppm in T 2 , which recorded the highest Fe content. Microbial treatments such as T 10 (7.48 ppm) and T 8 (6.83 ppm) also enhanced Fe availability, likely due to chelation and rhizosphere acidification processes. The control recorded 5.86 ppm, which was lower than most inoculated treatments.

Zinc (Zn)

The lowest Zn availability was observed in T 9 (1.45 ppm) and T 3 (1.49 ppm), while the highest was recorded in T 8 (2.72 ppm). AM fungi in T 8 may have contributed to improved Zn uptake and mobility through enhanced root surface area. The control (1.79 ppm) was intermediate, indicating that specific microbial combinations (especially AMF + PSF) were more beneficial for Zn solubilization (Table 2).

Discussion

This study demonstrated the importance of microbial Psolubilization and integrated nutrient management in enhancing native soil P availability and improving groundnut productivity in rainfed Alfisol systems. The combined application of PSB, PSF, and AM fungi-particularly when integrated with STBF and FYM-significantly enhanced plant growth, nutrient uptake, and pod yield.

Effect of Bio-inoculants on Growth and Nodulation

The pronounced increase in root nodulation, especially in T 9 ,

indicates effective colonization by PSB, PSF, and AM fungi and improved rhizosphere biological activity. AM fungi enhance P uptake through extraradical hyphae, while PSB and PSF release organic acids and phosphatases that mobilize sparingly soluble P pools. Enhanced nodulation further improves N fixation, reflected in higher available N levels in biologically treated plots.

Yield improvement through enhanced P Mobilization

The significant increases in pod and haulm yield under combined microbial treatments corroborate earlier findings that Psolubilizing inoculants improve nutrient-use efficiency and yield under low-P conditions typical of Alfisols. Treatments T 5 and T 9 , receiving the full microbial combination (PSB + PSF + AM fungi), consistently outperformed the chemical fertilizer treatments (T 2 ), emphasizing the capacity of biological agents to mobilize native soil P and improve physiological efficiency. The improvement in pod number and seed filling may be attributed to improved energy metabolism, better root architecture, and balanced nutrient supply, especially during pegging and pod development stages.

Soil Fertility Enhancement

The role of organic acids, enzymatic activity, and improved nutrient cycling is confirmed by the rise in accessible N, P, and K under microbial treatments. Increased organic carbon in T 5 indicates that microbial biomass and FYM breakdown have cumulative benefits. Additionally, PSB/PSF's chelation and organic acid release enhanced micronutrient availability.

The moderate acidification brought on by microbial secretions is reflected in lower soil pH in some treatments (e.g., T 7: pH 5.43), which is advantageous in Alfisols for mobilizing P that would otherwise be fixed by Fe and Al oxides.

The results of this study unequivocally show that, in rainfed Alfisol conditions, the combined application of PSB, PSF, and AM fungus greatly enhances native soil phosphorus mobilization, nutrient uptake, and groundnut pod yield. Through mechanisms like organic acid secretion, phosphatase synthesis, and chelation of Fe/Al-bound phosphates-processes that have been thoroughly described by Rodríguez and Fraga (1999) [9] and Sharma et al.

(2013) [11] -microbial inoculants are known to increase P availability. The significant rise in root nodulation observed in treatments T 5 and T 9 is consistent with previous findings that PSB and AM fungi work in concert to improve rhizobial colonization, nutrient absorption, and root activity (Gyaneshwar et al., 2002) [4] . The higher pod number and yield in T 5 and T 9 can be attributed to improved P uptake, as phosphorus is essential for peg formation, pod setting, and energy metabolism in legumes. Similar enhancements in pod yield with microbial consortia were reported in groundnut by Reddy et al. (2016) [7] and Aher et al. (2015) [1] . The integration of FYM further supported microbial activity and nutrient mineralization, consistent with findings by Bolan et al.

(1990) [2] on organic matter improving microbial-mediated P cycling.

Micronutrient enhancement (Cu, Fe, Zn) under microbial treatments may be linked to organic acid-mediated solubilization and AM fungal facilitation of micronutrient uptake. This is supported by the work of Smith & Read (2008) [12] , who documented that AM fungi improve Zn and Cu acquisition through expanded hyphal networks. Increased Fe availability in treated plots is consistent with chelating effects of fungal metabolites, as observed by Kucey (1988) [6] . The results confirm that chemical P alone (T 2 ) is insufficient in Alfisols due to strong fixation by Fe/Al oxides, a phenomenon previously described by Sanyal and De Datta (1991) [10] . In contrast, microbial consortia effectively overcame P-fixation barriers and promoted higher nutrient-use efficiency. Such biological approaches are recognized as sustainable alternatives for P management in dryland systems, as also emphasized by Vassilev & Vassileva (2003) [14] .

Overall, the study supports a growing body of evidence that microbial consortia + FYM represent a viable, eco-friendly strategy for improving P availability, soil health, and productivity in rainfed groundnut-based farming systems.

Implications for Rainfed Alfisols

Given the inherently low P supply in Alfisols and the high cost of chemical fertilizers for rainfed farmers, the results highlight a sustainable pathway for improving groundnut productivity. Microbial consortia reduce dependency on external P fertilizers, improve soil health, and allow for stable yields even under moisture-stress conditions typical of rainfed regions.

Conclusion

The research showed that the utilization of phosphate-solubilizing bacteria (PSB), phosphate-solubilizing fungi (PSF), and arbuscular mycorrhizal (AM) fungi in conjunction with organic inputs greatly enhanced the nutrient availability, plant growth, and productivity of groundnuts cultivated in rainfed Alfisol conditions by mobilizing native soil phosphorus. With the highest pod numbers per square meter, maximum pod yield, enhanced root nodulation, and improved soil nutritional status, treatments T 5 (HRDF + PSB + PSF + AM fungi) and T 9 (STBF + PSB + PSF + AM fungi) continuously outperformed the others. The availability of vital micronutrients like copper, iron, zinc, and manganese was also improved by these treatments, suggesting improved nutrient cycling and heightened microbial activity in the rhizosphere. The findings unequivocally show that microbial consortia are essential for releasing fixed phosphorus, enhancing the effectiveness of nutrient usage, and maintaining production in conditions of moisture stress that are common in rainfed systems. For smallholder farmers in Alfisol regions, integrated nutrient management techniques utilizing PSB, PSF, AM fungus, and FYM present a viable, affordable, and environmentally responsible substitute for chemical fertilizers. Adopting such biological nutrient mobilization techniques can improve the resilience of rainfed agriculture and increase the sustainability of groundnut output.

References

1. Aher SB, Lakhdar A, Patil H. Effect of phosphorus levels and biofertilizers on yield and nutrient uptake of groundnut. Journal of Oilseeds Research. 2015;32(2):145-150.
2. Bolan NS, Hedley MJ, White RE. Processes of soil acidification during nitrogen cycling with emphasis on legume-based pastures. Plant and Soil. 1990;134:53-63.
3. Bolan NS, White RE, Hedley MJ. A review of the use of organic amendments to enhance phosphate availability. Plant and Soil. 1990;124(1):93-106.
4. Gyaneshwar P, Kumar GN, Parekh LJ, Poole PS. Role of soil microorganisms in improving P nutrition of plants. Plant and Soil. 2002;245:83-93.
5. Kamprath EJ. Crop response to lime on soils in the tropics. Soil Acidity and Liming. 1984;2:349-368.
6. Kucey RMN. Phosphate-solubilizing bacteria and fungi in various cultivated and virgin Alberta soils. Canadian Journal of Soil Science. 1988;68:75-85.
7. Reddy BN, Sreenivasarao C, Srinivasarao M. Influence of phosphate-solubilizing microorganisms on groundnut productivity under rainfed conditions. Legume Research. 2016;39(6):1030-1036.
8. Rego TJ, Rao PP, Sahrawat KL. Nutrient balance in rainfed cropping systems of India. Journal of SAT Agricultural Research. 2007;5:1-9.
9. Rodríguez H, Fraga R. Phosphate solubilizing bacteria and their role in plant growth promotion. Biotechnology Advances. 1999;17:319-339.
10. Sanyal SK, De Datta SK. Chemistry of phosphorus transformations in soil. Advances in Soil Science. 1991;16:1120.
11. Sharma SB, Sayyed RZ, Trivedi MH, Gobi TA. Phosphate solubilizing microbes: Sustainable approach for managing phosphorus deficiency in agricultural soils. Springer Plus. 2013;2:587.
12. Smith SE, Read DJ. Mycorrhizal Symbiosis. 3rd ed. Academic Press; 2008.
13. Subba Rao A, Rupa TR, Srivastava AK. Phosphorus management in dryland agriculture. Indian Journal of Dryland Agricultural Research. 2015;30(2):1-12.
14. Vassilev N, Vassileva M. Biotechnological solubilization of rock phosphate on media containing agro-industrial wastes. Applied Microbiology and Biotechnology. 2003;61(56):435-440.
15. Venkateswarlu B, Shanker AK. Climate change and dryland agriculture. Journal of Agrometeorology. 2009;11:1-8.