Introduction

Farmers have been using macronutrients to fetch higher production as well as returns from agricultural land since the commencement of the green revolution. On the other hand, in developing countries, the cost of fertilizer can be significantly very high and it is often the limiting factor for food security, quality and supply and it had very low use efficiency1,2,3. Therefore, the application of secondary nutrients is the need of the hour. Among the secondary nutrients sulphur (S) is the most important nutrient because it is not applied in the required amount by the growers and it plays vital role in oilseed crops4. Sulphur is a constituent of amino acids, vitamins, and enzymes, and plays a vital role in photosynthesis, protein synthesis, and oil production5,6. Its deficiency can lead to stunted growth, yellowing of leaves, which ultimately leads to reduced photosynthesis and yield. Due to itsrole in the synthesis of proteins and enzymes, which are necessary for the intake and utilisation of other macro and micronutrients, sulphur fertilisation can increase the content and uptake of other micronutrients, including zinc, iron, and manganese6,7. A crucial component of many plant systems’ biochemical activities, sulphur (S) is a nutrient that is necessary for plant growth6,8. Traditional S fertilization methods have often led to inefficiencies, such as uneven distribution, and leaching, contributing to environmental concerns6. Moreover, in the quest for sustainable and efficient agricultural practices, researchers and agricultural experts have turned their attention towards innovative solutions to enhance crop yields while minimizing environmental impact9,10. One such ground-breaking advancement is the development and application of sulphur nano-particles (SNPs), an innovative technique with the potential to completely transform contemporary agriculture11,12. However, the development of SNPs presents a viable way to address these issues by utilising the special qualities of nanomaterials to give S to plants more efficiently13,14.

Sulphur nano particles involve the manipulation of S compounds at the nanoscale, typically in the range of 1 to 100 nm15. This nano-structuring imparts novel properties to the S particles, enhancing their solubility, reactivity, and ability to interact with plant roots16. By encapsulating or attaching SNPs to carrier materials, the controlled release of S can be achieved, ensuring a more sustained and efficient nutrient supply to plants throughout their growth stages17,18. This targeted delivery system minimizes wastage, reduces environmental pollution, and optimizes the utilization of S resources. The benefits of SNPs extend beyond improved nutrient delivery19,20. These nanoparticles can also serve as platforms for the controlled release of other essential nutrients. Additionally, the nanomaterials used in these fertilizers can potentially enhance the moisture-carrying capacity of soil, contributing to soil health and reducing the need for excessive irrigation21,22. Therefore, the use of SNPs has the potential to support more sustainable farming methods, lower input costs, and higher crop output23,24,25.

Groundnut (Arachis hypogea), also known as peanut, is a nitrogen-fixing crop that comes under the Fabaceae family. It is grown for its edible seeds, which develop belowground26,27. Groundnut have high amount of protein, oil, and other vital nutrients, it is a significant food and oil crop in the country as well as world27. India is one of the top three groundnut-producing nations in the world. It ranks second next to China while, in India, Gujarat, Rajasthan, and Tamil Nadu are the major producing states during the year 2020–21. A total of 10.24 Mt of groundnuts are produced annually on 6.02 million hectares of farmed land, with an average yield of 1703 kg/ha28. The area under groundnut constitutes approximately 3.3 per cent of the net sown area in India. In Tamil Nadu, groundnut productivity is the highest at 2502 kg/ha; in Gujarat, it is approximately 1911 kg/ha. In Gujarat, it is grown in about 2.16 M ha with a total production of about 4.13 Mt annually28. India is exporting 29,873 and 8493 Mt of groundnut oil meals in 2022–23 and 2023–24 respectively28.

Similarly, Indian mustard (Brassica juncea) is also an important oilseed crop29. Mustard seeds vary in colour and flavour based on the species, are widely used as condiments, spices, and as a source of edible oil30. Mustard is not only valued for its culinary applications but also for its potential health benefits because it contains phytochemicals and antioxidants31. India is next after Nepal, Canada and China in production of mustard in the financial year of 2021–22. While, In India, Gujarat is next after Rajasthan, Madhya Pradesh, Uttar Pradesh, Haryana, Jharkhand and Assam in area and production28. With an average yield of 1703 kg/ha and a total production of 11.96 Mt, mustard is grown on roughly 6.7 million hectares. The state of Gujarat has roughly 1976 kg/ha of mustard yield, whereas the maximum productivity is found in the state of Haryana, with 2028 kg/ha. In Gujarat, mustard is grown in an about 2.14 Mha area with a total production of about 4.24 Mt annually28. India is exporting 2,296,943 and 894,117 Mt of oil meals in 2022–23 and 2023–24 respectively28. To enhance the oil seed production as well as quality concern balanced fertilization is indispensable in these crops32.

Sulphur plays a critical role in the growth and productivity of oilseed crops, particularly groundnut and mustard33,34. It is essential for the synthesis of amino acids (cysteine and methionine), proteins, and enzymes, and directly influences oil content and quality35,36. Sulphur deficiency, widespread in Indian soils, often leads to reduced yields and poor oil quality in oilseeds. Mustard has a high sulphur demand due to its glucosinolate content, while groundnut requires adequate sulphur for pod development and seed protein37,38. Enhancing sulphur use efficiency is thus crucial for sustaining oilseed production39, and emerging nanotechnology-based sulphur fertilizers offer promising solutions to improve uptake, minimize losses, and boost crop performance under diverse agro-ecological conditions40,41. Despite the crucial role of sulphur in plant growth, its deficiency in soils due to imbalanced fertilization remains a significant challenge, particularly in sulphur-depleted regions. Traditional sulphur fertilizers often exhibit low use efficiency, leading to nutrient losses and environmental concerns. This study hypothesizes that sulphur nanoparticles (SNPs) can enhance nutrient uptake and sulphur use efficiency compared to conventional sources. Therefore, the objectives of this research are: (i) to evaluate the impact of sulphur application on the yield of groundnut and mustard, addressing the gap in optimized sulphur fertilization for improved productivity; (ii) to assess the influence of sulphur application on nutrient enrichment in the groundnut-mustard system, hypothesizing that SNPs enhance micronutrient availability and uptake; and (iii) to investigate the effect of sulphur application on sulphur dynamics and its use efficiency, filling the research gap on how nanoparticle-based sulphur fertilizers improve sulphur fractions and overall soil health.

Materials and methods

Investigation site and growing condition

Throughout the rabi (mustard) and kharif (groundnut) seasons of 2021–2022, the pot experiment was conducted in Polyhouse at the Centre for Advanced Research in Plant Tissue Culture, Anand Agricultural University, Anand (Gujarat). The experimental soil was loamy sand in texture, neutral in reaction with the pH (7.78), EC (0.30 dS/m)42, low in organic carbon 3.56 g/kg43, medium in available phosphorus 30.44 kg/ha44, and potassium 210.04 kg/ha45, low in available sulphur 5.16 ppm46, medium in Zn, Fe, Mn and Cu was 0.65, 4.46, 5.88 and 1.05 ppm respectively47.

Experimental design and treatments

The current study was carried out using a completely randomized block design with 11 treatment and four replications (Fig. 1). The treatment details of this experiment were as control (only recommended dose of fertilizer, RDF), RDF + elemental sulphur (ES) at the rate 8 mg S/kg soil, RDF + ES at the rate 8 mg S/kg soil (half at sowing and half as 1 MAS), RDF + SNPs at the rate 1 mg S/kg soil, RDF + SNPs at the rate 1 mg S/kg soil, (half at sowing and half as 1 MAS), RDF + SNPs at the rate 2 mg S/kg soil, RDF + SNPs at the rate 2 mg S/kg soil (half at sowing and half as 1 MAS), RDF + SNPs at the rate 3 mg S/kg soil, RDF + SNPs at the rate 3 mg S/kg soil (half at sowing and half as 1 MAS), RDF + SNPs at the rate 4 mg S/kg soil, RDF + SNPs at the rate 4 mg S/kg soil (half at sowing and half as 1 MAS). The RDF and sulphur treatments were applied through fertigation as per the treatments mentioned earlier and the flow rate of the dripper was kept at 4 L water per hour. The recommended dose of fertilizers (RDF) was applied using urea (46% N), muriate of potash (60% K₂O), and single super phosphate (16% P₂O₅, 11–12% S) as nutrient sources. The S treatments and RDF was applied in both the crops of kharif and rabi season as per treatment details.

Fig. 1
Fig. 1
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Treatment details.

Elemental sulphur (ES) and SNPs

The ES fertilizer was brought from SML (sulphur mills limited) having 90% ES on w/w basis (Fig. 2a). The sulphur nano particle was synthesized at AAU, Anand, Gujarat, via the green synthesis method48 in which 5 g neem leaf powder was taken in 100 mL milli Q (Ultra-pure) water and boiled it in microwave oven for 3–5 min. The boiled extract was cooled and filtered by a sieve to get clear supernatant after that it was filtered with Whatman No 42. Volume of this extract was made up to 100 mL in a 250 mL of beaker and it was put on magnetic stirrer. After that 15.81 g of 1 M Na2S4O7.5H2O was added in it pinch by pinch in 5% neem leaf extract under constant stirring condition. After the addition of sodium thiosulphate, it was allowed to stand for 20–30 min on a magnetic stirrer and then add 1N 1–2 mL HCl till the yellow colour changed. The solution was kept on stirrer for 10–15 min and after that 1 mL of prepared solution for DLS instrument. Synthesized SNPs were kept in a refrigerator for overnight and then dried at 60 °C in hot air oven till the complete removal of water and dried powder obtained was used without any modification (Figs. 2a, b). The synthesized SNPs powder was also characterized using the UV- XRD (Fig. 1) and found that the particle has a spherical shape. The synthesized SNPs were analyzed using di-acid digestion (HNO3:HClO4, 3:1 ratio) procedure for total sulphur estimation and it had 35% sulphur on a dry weight basis.

Fig. 2
Fig. 2Fig. 2
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(a) Systematic representation of SNPs preparation using neem leaves and sodium thiosulphate (b) Combined FTIR spectrum of sodium thiosulphate and synthesized sulphur nanoparticle.

Crop management

Groundnut (GG-34) and mustard (GM-4) were raised under a polyhouse condition. Groundnut was sowed on 1st July 2021 and harvested on 20th October 2021. Mustard was sown on 20th November 2021 and harvested on 09th March 2022. Both the crops were sown at 2–3 cm deep manually and harvested manually. The RDF for groundnut and mustard was 25–50-00 and 50–25-00 kg/ha of N, P and K, respectively. Seeds were treated with Agrosan GN fungicide not with rhizobium culture. Two hand weeding were done at 25 and 45 DAS, and drip irrigation was applied frequently with flow rate of 4 L/hour. Standard crop practices were followed, and dimethoate was sprayed twice on mustard for aphid control.

Content and uptake

Sulphur content was determined using the standard procedure49. The plant samples seed, haulm and stover were washed, dried and it was digested in di-acid mixture of HNO3: HClO4 till a white colour appeared. The digested solution was diluted to 50 mL and analyzed using spectrophotometer at 430 nm wavelength. The sulphur upta was calculated using given formula50.

$$Sulphur upke\left( {mg/pot} \right) = \frac{{Sulphur conent\left( \% \right) \times Dry matter yield\left( {g/pot} \right)}}{100}$$
(1)

Micronutrient uptake was calculated as per the given formulas50.

$$Micronutrient uptake\left( {mg/pot} \right) = \frac{{Nutrient content\left( {mg/kg} \right) \times Dry matter yield\left( {g/pot} \right)}}{1000}$$
(2)
$$SUE\left( \% \right) = \frac{{\begin{array}{*{20}c} {Sulphur uptakefromfertilized plot \left( {mg/pot} \right) - Sulphur uptake from} \\ {unfertilized plot\left( {mg/pot} \right)} \\ \end{array} }}{{Sulphur applied \left( {mg/pot} \right)}}$$
(3)

Sulphur fractions in soil

Sulphur dynamics (fractions) were determined using a destructive method for each fraction. The CaCl2 (0.15%) method was used for the determination of water soluble, heat soluble and adsorbed sulphate sulphur36. Organic and total sulphur fractions in the soil were determined using standard procedure51,52. Non-sulphate sulphur was determined by computing the difference between total sulphur and the sum of organic and sulphate sulphur46.

Data analysis and interpretation

The analyses of variance (ANOVA) of the parameters were performed using the F-test53. The treatment means comparisons were carried out by Duncan’s New Multiple Range test (DNMRT) using SPSS 16.0 for Windows (SPSS Inc., Chicago, IL, USA). The heat map of correlation was generated using Python software.

Results

Chlorophyll content of groundnut and mustard

Treatments, RDF + SNPs at the rate 4.0 mg S/kg soil, recorded significantly higher chlorophyll content than the rest of the treatments barring RDF + sulphur nanoparticles at the rate 4.0 mg S/kg soil, (half at sowing and half as 1 MAS), at 30 DAS, respectively, while, significantly higher chlorophyll content was registered with the application of RDF + sulphur nanoparticles at the rate 3.0 mg S/kg soil, (half at sowing and half as 1 MAS) at 60 DAS over rest of the treatments. Chlorophyll content increased by 20.02 and 17.93% under RDF + sulphur nanoparticles at the rate 4.0 mg S/kg soil and RDF + sulphur nanoparticles at the rate 3.0 mg S/kg soil, (half at sowing and half as 1 MAS), respectively over control in groundnut wherever, 18.76 and 17.0% higher was obtained under mustard.

Productivity of groundnut and mustard

Significantly remarkably pod yield /plant achieved with the application of RDF + sulphur nanoparticles at the rate 3.0 mg S/kg soil, with (half at sowing and half as 1 MAS) than rest of the treatments (Table 1). This novel approach resulted in pod yield measuring 12.50 g /plant, surpassing all other treatments. Meanwhile, lowest (11.04 g) pod yield reported at control. Increases in pod yield with the use of RDF + sulphur nanoparticles at the rate 3.0 mg S/kg soil; (half at sowing and half as 1 MAS) was in the tune of 13.22% higher over control. Notably, treatment RDF + sulphur nanoparticles at the rate 3.0 mg S/kg soil, (half at sowing and half as 1 MAS), exhibited significantly higher haulm yield/plant, which was 20.28 g than rest of the treatments barring RDF + sulphur nanoparticles at the rate 4.0 mg S/kg soil and RDF + sulphur nanoparticles at the rate 4.0 mg S/kg soil, (half at sowing and half as 1 MAS) (Table 1). The control treatment, without sulphur application recorded the lowest haulm yield. The increment in haulm yield with the fertilization of RDF + sulphur nanoparticles at the rate 3.0 mg S/kg soil, (half at sowing and half as 1 MAS) was 14.50% higher than the untreated pot.

Table 1 Effect of SNPs and elemental sulphur on chlorophyll concentration, yield attributes and yield of groundnut-mustard.
Full size table

Significantly higher seed yield of mustard /plant was obtained with the application of RDF + sulphur nanoparticles at the rate 3.0 mg S/kg soil, (half at sowing and half as 1 MAS) than the remaining treatments barring RDF + elemental S at the rate 8.0 mg S/kg soil, RDF + elemental S at the rate 8.0 mg S/kg soil, (half at sowing and half as 1 MAS), RDF + sulphur nanoparticles at the rate 3.0 mg S/kg soil, RDF + sulphur nanoparticles at the rate 4.0 mg S/kg soil and RDF + sulphur nanoparticles at the rate 4.0 mg S/kg soil, (half at sowing and half as 1 MAS). Minimum mustard seed yield was recorded under control (Table 1). The yield was registered 18.68% higher with the application of RDF + sulphur nanoparticles at the rate 3.0 mg S/kg soil, (half at sowing and half as 1 MAS) over control treatment.

Significantly higher stover yield was obtained under RDF + sulphur nanoparticles at the rate 3.0 mg S/kg soil, (half at sowing and half as 1 MAS), which was on par with treatments RDF + Elemental S at the rate 8.0 mg S/kg soil, (half at sowing and half as 1 MAS), RDF + sulphur nanoparticles at the rate 4.0 mg S/kg soil and RDF + sulphur nanoparticles at the rate 4.0 mg S/kg soil, (half at sowing and half as 1 MAS), differed significantly from rest of the treatments. The improvement in stover yield due to the application of RDF + sulphur nanoparticles at the rate 3 mg S/kg soil, (half at sowing and half as 1 MAS) was to the tune of 17.46% over control.

Sulphur uptake by groundnut and mustard

Application of RDF + sulphur nanoparticles at the rate 3.0 mg S/kg soil, (half at sowing and half as 1 MAS), resulted in higher sulphur content (Fig. S1) and uptake by seed and haulm of groundnut (Table 2), with a value of 113.72 and 120.42 mg /pot than rest of the treatments barring treatments RDF + elemental S at the rate 8.0 mg S/kg soil, RDF + elemental S at the rate 8.0 mg S/kg soil (half at sowing and half as 1 MAS), RDF + sulphur nanoparticles at the rate 3.0 mg S/kg soil, RDF + sulphur nanoparticles at the rate 4.0 mg S/kg soil and RDF + sulphur nanoparticles at the rate 4.0 mg S/kg soil, (half at sowing and half as 1 MAS) in case of seed, while RDF + sulphur nanoparticles at the rate 4.0 mg S/kg soil, (half at sowing and half as 1 MAS) being on par in terms of sulphur uptake by haulm. The control treatment registered significantly lower sulphur uptake of 89.85 and 94.93 mg /pot, respectively than rest of the treatments. The increase in sulphur uptake by seed and haulm of groundnut with the application of RDF + sulphur nanoparticles at the rate 3.0 mg S/kg soil, (half at sowing and half as 1 MAS) was 26.57 and 26.85% higher over control only RDF treated.

Table 2 Effect of SNPs and elemental sulphur on sulphur uptake and use efficiency.
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At treatment RDF + sulphur nanoparticles at the rate 3.0 mg S/kg soil, (half at sowing and half as 1 MAS), resulted in the highest sulphur uptake (Table 2) by seed and stover of mustard, with values of 69.34 and 78.63 mg /pot, respectively, which being on par with RDF + sulphur nanoparticles at the rate 4.0 mg S/kg soil, (half at sowing and half as 1 MAS), differed significantly from rest of the treatments. The control treatment, which consisted of pots treated solely with RDF, displayed a significantly lower sulphur uptake of 49.67 and 57.21 mg /pot, respectively than remaining treatments except for RDF + sulphur nanoparticles at the rate 1.0 mg S/kg soil and RDF + sulphur nanoparticles at the rate 1.0 mg S/kg soil, (half at sowing and half as 1 MAS) for seed and RDF + sulphur nanoparticles at the rate 1.0 mg S/kg soil, RDF + sulphur nanoparticles at the rate 1.0 mg S/kg soil, (half at sowing and half as 1 MAS), RDF + sulphur nanoparticles at the rate 2.0 mg S/kg soil and RDF + sulphur nanoparticles at the rate 2.0 mg S/kg soil, (half at sowing and half as 1 MAS) for stover. The increase in sulphur uptake by seed and stover of mustard with application of RDF + sulphur nanoparticles at the rate 3.0 mg S/kg soil, (half at sowing and half as 1 MAS) over control was 39.60 and 37.44%, respectively.

Micronutrient content and uptake of groundnut and mustard

Iron (Fe) content and uptake

At treatment combining the RDF with SNPs at the rate 3 mg S/kg soil (half at sowing and half as 1 MAS), resulted in a significantly higher seed and haulm Fe content and uptake, with a value of 50.1, 352.0 ppm, 1.25 and 14.29 mg/pot, respectively (Table 3, Figs. 4 and 5). At RDF + SNPs at the rate 3 mg S/kg soil, RDF + SNPs at the rate 4 mg S/kg soil and RDF + SNPs at the rate 4 mg S/kg soil (half at sowing and half as 1 MAS) for Fe uptake by seed and haulm. The control treatment, which consisted of pots treated solely with RDF, displayed a significantly lower seed Fe content (41.1 and 311.8 ppm) and uptake (0.91 and 11.04 mg/pot), respectively (Fig. S2). The percentage increase in Fe content and uptake of seed and haulm was 21.7, 12.8, 37.36 and 29.34% respectively under RDF + SNPs at the rate 3 mg S/kg soil (half at sowing and half as 1 MAS) over control.

Table 3 Effect of SNPs and ES on micronutrient content of groundnut and mustard.
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Application of RDF + SNPs at the rate 3 mg S/kg soil (half at sowing and half as 1 MAS), resulted in a significantly higher seed and stover Fe content and uptake, with a value of 25.07, 285.01 ppm and 0.31 and 6.71 mg/pot, respectively, over remaining treatments barring RDF + ES at the rate 8 mg S/kg soil, RDF + ES at the rate 8 mg S/kg soil (half at sowing and half as 1 MAS), RDF + SNPs at the rate 4 mg S/kg soil and RDF + SNPs at the rate 4 mg S/kg soil (half at sowing and half as 1 MAS) for Fe content and uptake by seed and RDF + ES at the rate 8 mg S/kg soil, (half at sowing and half as 1 MAS), RDF + SNPs at the rate 4 mg S/kg soil and RDF + SNPs at the rate 4 mg S/kg soil (half at sowing and half as 1 MAS) for Fe content in stover and RDF + SNPs at the rate 4 mg S/kg soil and RDF + SNPs at the rate 4 mg S/kg soil (half at sowing and half as 1 MAS) in terms of Fe uptake by stover of mustard (Table 3). The control treatment, which consisted of pots treated solely with RDF, displayed a significantly lower seed and stover Fe content and uptake of 20.68, 239.30 ppm, 0.22 and 4.79 mg/pot, respectively. The Fe content and uptake by seed and stover increased 21.2, 19.1, 43.9 and 40.0% under RDF + SNPs at the rate 3 mg S/kg soil (half at sowing and half as 1 MAS) over control.

Zinc (Zn) content and uptake

RDF with SNPs at the rate 3 mg S/kg soil (half at sowing and half as 1 MAS), resulted in significantly higher Zn content and uptake of seed and haulm, with a value of 31.4, 22.9 ppm and 0.79 and 0.93 mg/pot, respectively, than rest of the treatments except, RDF + ES at the rate 8 mg S/kg soil, RDF + SNPs at the rate 4 mg S/kg soil and RDF + SNPs at the rate 4 mg S/kg soil (half at sowing and half as 1 MAS) for Zn content in seed, while Zn content in haulm was significantly the higher under RDF + SNPs at the rate 3 mg S/kg soil (half at sowing and half as 1 MAS) and RDF + ES at the rate 8 mg S/kg soil, RDF + ES at the rate 8 mg S/kg soil (half at sowing and half as 1 MAS), RDF + SNPs at the rate 3 mg S/kg soil, RDF + SNPs at the rate 4 mg S/kg soil and RDF + SNPs at the rate 4 mg S/kg soil (half at sowing and half as 1 MAS) Zn uptake by seed and haulm. The Zn concentration and uptake by seed and haulm increased 20.7, 39.8, 36.2 and 60.34% higher under RDF + SNPs at the rate 3 mg S/kg soil (half at sowing and half as 1 MAS) over the control (Table 3, Figs. 3 and 4).

Fig. 3
Fig. 3
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Effect of SNPs and ES on micronutrient uptake of groundnut.

Fig. 4
Fig. 4
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Effect of SNPs and ES on micronutrient uptake of mustard.

The application of RDF + SNPs at the rate 3 mg S/kg soil (half at sowing and half as 1 MAS), resulted in significantly higher seed and stover zinc content and uptake, with a value of 37.57, 11.83 ppm, 0.47 and 0.28 mg/pot respectively, over rest of the treatments except RDF + ES at the rate 8 mg S/kg soil, RDF + ES at the rate 8 mg S/kg soil (half at sowing and half as 1 MAS), RDF + SNPs at the rate 4 mg S/kg soil and RDF + SNPs at the rate 4 mg S/kg soil (half at sowing and half as 1 MAS) in terms of Zn content and RDF + SNPs at the rate 4 mg S/kg soil and RDF + SNPs at the rate 4 mg S/kg soil (half at sowing and half as 1 MAS) in terms of Zn uptake by seed and stover uptake significantly higher. The control treatment only RDF, displayed a significantly lower seed and stover zinc content and uptake of 31.01, 9.76 ppm, 0.32 and 0.20 mg/pot respectively (Table 3, Figs. 3 and 4). The Zn content and uptake by seed and stover under RDF + SNPs at the rate 3 mg S/kg soil (half at sowing and half as 1 MAS) was to the tune of 21.15, 21.21, 43.8 and 42.6% over control.

Manganese (Mn) content and uptake

RDF with SNPs at the rate 3 mg S/kg soil (half at sowing and half as 1 MAS) resulted in a significantly higher seed and haulm Mn content and uptake, with a value of 20.0, 35.8 ppm and 0.45 and 1.45 mg/pot, respectively (Table 3, Figs. 3 and 4) than other treatments barring RDF + ES at the rate 8 mg S/kg soil, RDF + ES at the rate 8 mg S/kg soil (half at sowing &half as 1 MAS), RDF + SNPs at the rate 3 mg S/kg soil, RDF + SNPs at the rate 4 mg S/kg soil for Mn content in seed and RDF + ES at the rate 8 mg S/kg soil, RDF + ES at the rate 8 mg S/kg soil (half at sowing and half as 1 MAS), RDF + SNPs at the rate 4 mg S/kg soil and RDF + SNPs at the rate 4 mg S/kg soil (half at sowing and half as 1 MAS) for Mn content in haulm and RDF + ES at the rate 8 mg S/kg soil, RDF + ES at the rate 8 mg S/kg soil (half at sowing and half as 1 MAS), RDF + SNPs at the rate 3 mg S/kg soil, RDF + SNPs at the rate 4 mg S/kg soil and RDF + SNPs at the rate 4 mg S/kg soil (half at sowing and half as 1 MAS) for Mn uptake by seed and RDF + SNPs at the rate 4 mg S/kg soil (half at sowing and half as 1 MAS) for Mn uptake by haulm of groundnut. Seed and haulm of groundnut contained 32.2, 26.8, 49.4 and 45.0% more Mn content and uptake respectively, under RDF + SNPs at the rate 3 mg S/kg soil (half at sowing and half as 1 MAS) than the control (Table 3, Figs. 3 and 4).

The fertilization of RDF + SNPs at the rate 3 mg S/kg soil (half at sowing and half as 1 MAS), resulted in a higher seed and stover Mn content and uptake, with values of 49.99, 36.76 ppm, 0.62 and 0.70 mg/pot, respectively, over rest of treatments except RDF + ES at the rate 8 mg S/kg soil, RDF + ES at the rate 8 mg S/kg soil (half at sowing and half as 1 MAS), RDF + SNPs at the rate 4 mg S/kg soil and RDF + SNPs at the rate 4 mg S/kg soil (half at sowing and half as 1 MAS) for Mn content in seed and stover and Mn uptake by seed while, RDF + SNPs at the rate 4 mg S/kg soil (half at sowing and half as 1 MAS) in terms of Mn uptake by stover of mustard. The control RDF showed a significantly lower seed and stover Mn content and uptake of 41.25, 30.33 ppm, 0.43 and 0.61 mg/pot respectively. The increase in Mn content and uptake by seed and stover of mustard was 21.2, 21.2, 44.2 and 42.5% higher with the application of RDF + SNPs at the rate 3 mg S/kg soil (half at sowing and half as 1 MAS) over control.

Copper (Cu) content and uptake

Application of RDF along with SNPs at the rate 3 mg S/kg soil (half at sowing and half as 1 MAS) resulted in higher Cu content and uptake in the seed and haulm, with values of 12.1, 15.3 ppm, 0.30 and 0.62 mg/pot respectively, than other treatments, while it was on par with RDF + ES at the rate 8 mg S/kg soil, RDF + ES at the rate 8 mg S/kg soil (half at sowing and half as 1 MAS), RDF + SNPs at the rate 4 mg S/kg soil and RDF + SNPs at the rate 4 mg S/kg soil (half at sowing and half as 1 MAS) for Cu content in seed, for haulm it was significantly superior. The control RDF showed the lowest Cu content and uptake of Cu in seed and haulm of groundnut, with a value of 9.79, 10.9 ppm and 0.22 and 0.39 mg/pot, respectively. The percentage increase of Cu content and uptake by seed and haulm of groundnut was 24.2, 39.8, 36.3 and 58.9% respectively, with RDF + SNPs at the rate 3 mg S/kg soil (half at sowing and half as 1 MAS) over control (Table 3).

At RDF + SNPs at the rate 3 mg S/kg soil (half at sowing and half as 1 MAS), resulted in the higher Cu content and uptake in the seed and stover of mustard, with values of 11.87, 13.58 ppm, 0.15 and 0.32 mg/pot, respectively over rest of the treatments except RDF + ES at the rate 8 mg S/kg soil, RDF + ES at the rate 8 mg S/kg soil (half at sowing and half as 1 MAS), RDF + SNPs at the rate 4 mg S/kg soil and RDF + SNPs at the rate 4 mg S/kg soil (half at sowing and half as 1 MAS) in terms of Cu concentration in seed and stover while Cu uptake being at par with RDF + ES at the rate 8 mg S/kg soil, RDF + ES at the rate 8 mg S/kg soil (half at sowing and half as 1 MAS), RDF + SNPs at the rate 4 mg S/kg soil and RDF + SNPs at the rate 4 mg S/kg soil (half at sowing and half as 1 MAS) in case of seed while in case of stover RDF + SNPs at the rate 4 mg S/kg soil (half at sowing and half as 1 MAS) found on par. The control only RDF showed a significantly lower Cu content in seed and stover, with a value of 9.80, 11.20 ppm, 0.10 and 0.22 mg/pot respectively. The increase in Cu content and uptake in seed and stover with application of RDF + SNPs at the rate 3 mg S/kg soil (half at sowing and half as 1 MAS) was to the tune of 21.1, 21.2, 44.1 and 42.8% higher over control (Table 3, Figs. 3 and 4).

Sulphur dynamics

Significantly higher water soluble, heat soluble, organic, adsorbed sulphate sulphur with the value of 3.17, 5.48, 38.14 and 5.42 mg/kg, respectively (Table 4) was obtained under application of RDF + sulphur nanoparticles at the rate 4.0 mg S/kg soil, (half at sowing and half as 1 MAS) than the rest of the treatments barring RDF + elemental S at the rate 8.0 mg S/kg soil, RDF + elemental S at the rate 8.0 mg S/kg soil, (half at sowing and half as 1 MAS), RDF + sulphur nanoparticles at the rate 3.0 mg S/kg soil, (half at sowing and half as 1 MAS) and RDF + sulphur nanoparticles at the rate 4.0 mg S/kg soil for heat soluble, adsorbed sulphate and water soluble sulphur and RDF + elemental S at the rate 8.0 mg S/kg soil, RDF + sulphur nanoparticles at the rate 3.0 mg S/kg soil, (half at sowing and half as 1 MAS) and RDF + sulphur nanoparticles at the rate 4.0 mg S/kg soil for organic sulphur. The numerically lowest water, heat soluble, organic and adsorbed sulphate sulphur with the value of 1.97, 4.25, 35.5, and 3.44 mg/kg, respectively were obtained in the control treatment which didn’t receive sulphur throughout the cropping.

Table 4 Effect of SNPs and elemental sulphur on fractionation in soil after harvest of groundnut-mustard sequence (After harvest of Mustard).
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Highest non-sulphate sulphur was found with the value of 20.4 mg/kg with application of RDF + sulphur nanoparticles at the rate 3.0 mg S/kg soil. Moreover, the highest total sulphur of 61.89 mg/kg was registered with the application of RDF + sulphur nanoparticles at the rate 4.0 mg S/kg soil, (half at sowing and half as 1 MAS) followed by 61.51 mg/kg was obtained with RDF + sulphur nanoparticles at the rate 3.0 mg S/kg soil, (half at sowing and half as 1 MAS). Wherever, the lowest non-sulphate and total sulphur fraction was measured under control treatment with the value of 17.7 and 56.65 mg/kg.

DTPA extractable micronutrients in soil after harvest of groundnut and mustard

Available zinc content in soil at harvest of mustard varies from 0.60 to 0.67 ppm, with the highest value 0.667 mg/kg was obtained under RDF + SNPs at the rate 4 mg S/kg soil (half at sowing and half as 1 MAS) (Table 5). The available iron, manganese and copper in the soil at harvest vary from 4.19 to 4.63 ppm, 5.81 to 6.43 ppm and 1.03 to 1.14 ppm, respectively. Higher micronutrients 4.63, 6.43 and 1.14 mg/kg (Fe, Mn and Cu) respectively, were found under RDF + SNPs at the rate 4 mg S/kg soil (half at sowing and half as 1 MAS).

Table 5 Effect of SNPs and ES on DTPA-extractable micronutrients in soil after harvest of groundnut-mustard.
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Sulphur use efficiency

Significantly higher sulphur utilization efficacy 121.37 and 91.30%, was registered with the application of RDF + sulphur nanoparticles at the rate 1.0 mg S/kg soil, (half at sowing and half as 1 MAS) and RDF + sulphur nanoparticles at the rate 3.0 mg S/kg soil, (half at sowing and half as 1 MAS) over the rest of the treatments. Moreover, the lowest sulphur utilization efficacy was measured under control with the values 19.28 and 18.51% under RDF + elemental sulphur 8 mg/kg soil. It was also observed that the sulphur nano particles have more utilization efficacy than the elemental sulphur. The sulphur content and uptake were significantly higher so, the sulphur use efficiency was much higher in RDF + sulphur nanoparticles at the rate 3.0 mg S/kg soil, (half at sowing and half as 1 MAS).

Pearson’s correlation

Among various sulphur fractions (heat soluble sulphur, total sulphur, organic sulphur, adsorbed sulphate, water soluble sulphur, and non-sulphate) with crop yields (specifically groundnut pod yield, groundnut haulm yield, mustard grain yield, and mustard straw yield) (Table 6). Positive correlation values, having R2 = 0.91 to R2 = 0.97, revealed robust positive associations, implying that there is a tendency for crop yields to increase as sulphur percentages increase. It is worth mentioning that there is a notable positive association between groundnut pod yield (GPY) and groundnut haulm yield (GHY) with various sulphur fractions, suggesting a major favourable impact. In contrast, it can be shown that non-sulphate sulphur (NS-S) exhibits a negative correlation with both total sulphur and organic sulphur, indicating an inverse association between these variables. This information facilitates comprehension of the influence of various sulphur fractions on agricultural productivity within the framework of the provided dataset.

Table 6 Relationship between sulphur fractions and yield using Pearson’s correlation.
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Discussion

Sulphur is a crucial element for the growth and development of plants, as it is required for the synthesis of proteins, activation of enzymes, and the production of chlorophyll and it play a vital role in many physiological and biochemical processes within plants53,54,55,56. The sulphur plays an important role in the formation of chlorophyll that directly involves in photosynthesis. Chlorophyll plays a vital role in plants, serving as the pigment responsible for their green colour. Its importance lies in the process of photosynthesis, where plants convert sunlight into biochemical energy. Similar positive effect of sulphur nano particles was reported57,58, who observed that application of sulphur nanoparticles increased chlorophyll content and photosynthetic rate in mustard plants.

Significantly higher pods and siliqua/plant were found due to the application of sulphur. The application of RDF + sulphur nanoparticles at 3.0 mg S/kg soil, (half at sowing and half as 1 MAS) can be an optimum dose of sulphur through sulphur nano particles. Due to the optimum dose of sulphur as well as a split application, it was being available throughout the entire crop growth period that improves vegetative growth and development of both crops59. Findings found that the application of SNPs showed a significant increase in branches, pods per plant and yield of groundnut over control60,61,62.

These sulphur sources help to fulfil the sulphur requirements of the plants, leading to increase sulphur content in different plant parts as well as increase transformation and metabolism of sulphur in different plant parts (Fig. 5). Also, sulphur application improves the activity of sulphur related enzymes and transporters involved in sulphur uptake, assimilation and redistribution, thereby increasing the sulphur content in different plant parts (Fig. 6). Similarly, reported that the application of SNPs significantly increased the sulphur content of maize kernels and leaves by 20.6 and 36.8%, respectively63, compared to control (Fig. 7). These findings align with recent studies emphasizing the importance of integrating nutrient dynamics64, biochar amendments, and rhizosphere interactions to enhance nutrient use efficiency and stress resilience in cropping systems65,66.

Fig. 5
Fig. 5
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Overview and different growth stages of groundnut.

Fig. 6
Fig. 6
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Growth stages of groundnut and overview of mustard crop.

Fig. 7
Fig. 7
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Pod formation stage of mustard.

The application of SNPs and elemental sulphur as fertilizers can increase the sulphur uptake in both seed and haulm of groundnut and mustard plants. This is primarily due to enhanced sulphur availability, improved nutrient uptake, efficiency, and the facilitation of sulphur assimilation and transport within the plant (Figs. 5, 6 and 7). Thereby, plants assimilate more of the sulphur in different plant parts that’s why increased the plant uptake of sulphur. Rajput et al.67 and Karimi et al.68, documented analogous outcome, observing that the utilization of sulphur led to enhanced uptake of sulphur across various plant parts. RDF + sulphur nanoparticles at the rate 3.0 mg S/kg soil, (half at sowing and half as 1 MAS) have balanced nutrition of other essential nutrients for the groundnut and mustard, so the growth and yield parameters were more. So, obtained the highest sulphur uptake and use efficiency.

The content and uptake of micronutrients (Zn, Mn, Fe, and Cu) were considerably influenced by the application of sulphur. With the application of sulphur to soil, it may change the pH, solubility, microbial activity, and encourage root development which may affect the availability and uptake of micronutrients. Thereby, the availability of cationic micronutrients is affected by pH change. Higher micronutrient concentrations in plant tissues may come from better micronutrient uptake as a result of this69,70. As well as the experimental soil has neutral reaction that enhances the nutrients absorption. Sulphur can combine with specific micronutrients to generate complexes that increase their availability and stop them from precipitating in the soil. This makes it easier for plant roots to absorb them.

Numerous controllable and uncontrollable factors affect the growth and development of the plant, and one of the most important of these is well-balanced nutrition. Ferredoxin and acetyl-CoA have sulphur and are essential for the reduction of CO2 and the synthesis of organic molecules, the availability of sulphur may have an impact on the rate of photosynthetic71. The beneficial effects of RDF and S application on root growth and morphology have been demonstrated in several studies72,73,74,75.

Sulphur, for instance, has an impact on the production of certain molecules involved in the transport and absorption of nutrients76. The application of sulphur can modify the microbial populations in soil, which in turn can have an impact on nutrient availability and cycling77. Some soil bacteria help to solubilize and mobilise micronutrients78,79,80, which has an impact on how well plants absorb those minerals81,82.

The numerically lowest water, heat soluble, organic and adsorbed sulphate sulphur were obtained in the control treatment which didn’t receive sulphur throughout the cropping. The organic sulphur fraction was the dominant form of sulphur in soil. The variation in the organic sulphur is mainly due to mineralization and oxidation of sulphur and is also based on organic carbon content and clay fraction of soil. Adsorbed sulphur was extracted from the exchange complex of the soil by the extractant. The water soluble and adsorbed sulphur fractions were increased in the soil and it is mainly due to slow and steady releasing behaviour of the nano based fertilizer. Occluded sulphur or non-sulphate sulphur is mostly made up of sulphate occluded in and adsorbed on carbonates or insoluble sulphur compounds of iron and aluminium in soil which remains unextractable after removal of organic carbon and sulphate sulphur. The total sulphur content of the soil was increased with an increase in organic carbon content and finer fraction of the soil, while less affected by lower doses of sulphur application83,84,85,86,87. The findings unequivocally demonstrate that, while having no discernible impact, the smallest particle size of S increased the amount of DTPA extractable micronutrients in the soil relative to the baseline measurements88,89,90,91,92,93,94.

Conclusion

This study highlights the transformative potential of sulphur nanoparticles in enhancing crop productivity and soil nutrient dynamics (Fig. 8). The optimized application of SNPs significantly improved yield, nutrient uptake, and sulphur use efficiency in the groundnut-mustard system, offering a sustainable alternative to conventional sulphur fertilizers. Results suggest that application of RDF + sulphur nanoparticles at 3.0 mg S/kg soil, (half at sowing and half at 1 MAS) can be an optimum dose of sulphur for groundnut and mustard crops under controlled environment of pot experiment. The higher chlorophyll content, yield, nutrients content and uptake were obtained with the application of RDF + sulphur nanoparticles at the rate 3.0 mg S/kg soil, (half at sowing and half as 1 MAS), while the higher sulphur fractionation and DTPA extractable micronutrients in soil were obtained under RDF + sulphur nanoparticles at the rate 4 mg S/kg soil, (half at sowing and half as 1 MAS). Based on the findings, the recommended strategy for optimal sulphur management in sulphur-deficient soils is the application of RDF + sulphur nanoparticles (SNPs) at 3.0 mg S/kg soil, applied in a split dose (half at sowing and half at 1 MAS). This approach significantly enhances yield, nutrient uptake, and sulphur use efficiency in the groundnut-mustard cropping system. Additionally, higher sulphur fractions and DTPA extractable micronutrients were observed at 4.0 mg S/kg SNPs, indicating its role in improving soil fertility. The use of SNPs reduces reliance on conventional fertilizers, promoting sustainable and efficient nutrient management in modern agriculture.

Fig. 8
Fig. 8
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Nanotechnology boost groundnut-mustard systems sustainability and relevant SGDs.