Leaching and vertical distribution of Fe and Zn citrate nanoparticles in Indian red soil

Abstract

This research aims to address common constraints in the effective utilization of plant nutrients in soil, such as fixation, mobility, leaching, and reactions with soil colloids. To mitigate these issues, Fe and Zn citrate nanoparticles were synthesized and applied as nanochelators in a reconstructed soil profile column. We evaluated the mobility, release, and leaching behaviors of these nanoparticles. Results revealed that, no leaching of Fe and Zn citrate nanoparticles occurred even after a 90-day incubation. The release profiles exhibited a peak at 60 and 90 days for Fe and Zn respectively. In the mobility studies, Fe and Zn availability was highest in the 0–15 cm soil depth. Fe citrate and Zn citrate nanoparticles demonstrated the highest availability at 264.7 mg/kg and 86.26 mg/kg of soil, respectively as compared with commercial samples. The superior performance of Fe and Zn citrate nanoparticles was observed in terms of reduced leaching and improved accessibility, indicating their potential as efficient and environmentally-friendly plant nutrient sources. The study concludes that Fe and Zn citrate nanoparticles are stable nutrient sources that can enhance plant use efficiency with minimal environmental impact.

Highlights

• •Environment friendly metal citrate nanoparticles.
• •No leaching of Fe and Zn citrate nanoparticles after a 90-day incubation period observed.
• •Release profiles peaked at 60 days for Fe and 90 days for Zn, indicating controlled release.
• •Highest Fe and Zn availability at 0–15 cm soil depth: 264.7 mg/kg and 86.26 mg/kg, respectively.
• •Superior performance of Fe and Zn citrate nanoparticles in reduced leaching and accessibility.

1. Introduction

Fertilizers containing macro- or micronutrients are crucial for enhancing plant growth and productivity. However, widespread use of conventional soil-applied enhancements, due to low nutrient use efficiency (NUE), has led to compensatory supplemental additions. This has resulted in environmental concerns, with economic losses ranging from 40 % to 70 %, and declines in soil fertility due to leaching and soil fixation [[1], [2], [3], [4], [5], [6]].

To address these issues and optimize nutrient utilization, there is a growing interest in sustainable alternatives. These alternatives ensure gradual nutrient release and enhanced plant nutrient efficiency using nanoparticles with a high surface-to-volume ratio [[7], [8], [9], [10], [11], [12], [13]].

In modern agriculture, nanofertilizers show significant potential to enhance soil fertility, reduce nutrient loss, increase crop yields, mitigate environmental pollution, and support beneficial microorganisms [14]. Understanding the fate and mobility of these nanomaterials in agricultural environments is crucial due to their potential environmental impact [[15], [16], [17], [18]]. On the other hand, chelate-based nanosized micronutrients incorporating ligands such as ascorbic acid and humic acid offer improved mobility and reduced reactivity in soil [19,20]. However, challenges such as variability in chelation effectiveness and susceptibility to soil conditions persist [[21], [22], [23]].

Nanoparticles have wide-ranging applications, including their use as plant nutrients. In this study, iron oxide (Fe₃O₄, Fe₂O₃) and zinc oxide (ZnO) nanoparticles were explored for their potential in enhancing nutrient delivery. However, these nanoparticles can aggregate and segregate within plants, affecting their efficiency. The novelty of this research lies in the development of partially water-soluble Fe and Zn citrate nanoparticles, a relatively unexplored area in soil nutrient management. The partial solubility is due to citrate ligands, which, combined with the nanoparticles’ size, results in reduced leaching and improved nutrient availability compared to conventional sources. This innovative approach offers a significant contribution to sustainable agriculture by enhancing nutrient use efficiency and minimizing environmental impact.

Based on this introduction, we pose two questions: 1) whether the leaching, release kinetics, and vertical mobility of nanonutrients differ from conventional commercial nutrients, and 2) whether chelated nutrients provide any advantage. Zinc and iron are vital micronutrients for plant growth [24,25]. Therefore, in our recent investigations [26], we reported a novel class of Fe and Zn citrate nanoparticles as promising plant nutrients. These nanoparticles were synthesized through a method involving solid-state grinding followed by ball milling. To gauge their efficacy, we conducted experiments with 15-day-old soybean seedlings using the white-sand technique, encompassing 40 treatments including individual citrates, combined citrates, and commercially available nutrient samples. Key parameters such as total fresh weight (g), leachate Zn (mg/kg), leachate Fe (mg/kg), and plant Fe content (mg/pot) were evaluated. The resulting dataset underwent principal component analysis (PCA), which identified crucial insights by determining sample effectiveness based on statistical analysis and physical observations [27].

To answer the posed questions, we present an analysis of leaching behavior, release patterns, and mobility of the five most promising samples using the soil column method. Additionally, we provide comparative data involving commercially available nano- and chelated nutrients under identical experimental conditions. This article is structured as follows: we present key findings including stability of nanoparticles, release profiles, and vertical distribution in soil columns, followed by discussion. In the discussion section, we interpret the results, compare them with existing literature, and demonstrate the superior performance of our synthesized samples in reducing leaching and enhancing nutrient availability. Finally, we conclude by highlighting the potential benefits of Fe and Zn citrate nanoparticles as plant nutrient sources and suggest directions for future research.

2. Materials and methods

Ferric nitrate hydrous (FN; ((Fe(NO₃)₃).9H₂O)), zinc nitrate hydrous (ZN; (Zn(NO₃)₂.6H₂O)), and citric acid anhydrous (CA; (C₆H₈O₇)) were procured commercially from SRL, India. The ferric nitrate had a purity of 98 %, zinc nitrate with a purity of 98 %, and citric acid with a purity of 99 %. The reagents were used as received without any additional purification, as they were already in a suitable state for experimentation.

The metal citrates were prepared as reported earlier [26,27]. Briefly, the citrate nanoparticles were prepared by solid-state grinding technique and followed by ball milling. As mentioned, the best samples from the previous study are being considered here and those are ball milled ferric citrate BFC(1:1)- 6; ball milled zinc citrate BZC(1:3)- 6; ball milled ferric and zinc citrate BFZ(4:6)- 8; ball milled ferric and zinc citrate BFZ(5:5)- 2; ball milled ferric and zinc citrate BFZ(5:5)- 6; ball milled ferric and zinc citrate BFZ(8:2)- 4; ball milled ferric and zinc citrate BFCZ(1:1:1)-6. For all samples the prefix “B” refers to ball-milled samples. The individual ball-milled citrates and were coded as BFC(1:1), BZC(1:3). The ratio indicates the mole ratio of metal to citric acid in the samples. For the combined citrates, the sample codes are designated as BFZ (x:y) where x and y are FC and ZC weight ratios used for the synthesis of combined citrates. And the number at the end of the sample code corresponds to ball milling duration in h = 2, 4, 6, 8, and 10 h. The other commercial nutrients FeGro® as chelated-Fe (Fe-EDTA); Zingap® as chelated-Zn (Zn-EDTA); Geolife® as Nano-Fe; and (Geolife®) as Nano-Zn were used in this study for comparison.

2.1. Water solubility measurements

One gram of the powdered metal citrate nanoparticles was mixed with 100 mL of deionized water, vortexed on an ultrasonicator for 1 min, then left at 25 °C for 1 h followed by keeping it at 4 °C for 15 min. The dispersion was centrifuged at 8000 rpm. The atomic absorption spectrophotometer (AAS) method was used to determine how much soluble Fe and Zn was present in the supernatant. The equation (eqn. (1)) was used to calculate the solubility.

$$Solubility(\%)=\frac{\text{metal}\left(\text{Fe}\text{or}\text{Zn}\right)\text{content}\text{in}\text{supernatant}}{\text{Fe}\text{or}\text{Zn}\text{content}\text{in}\text{sample}}X100$$ (1)

2.2. Size distribution measurements

A dynamic light scattering (DLS) instrument was used to examine the particle size distribution of the metal citrate nanoparticle samples. The samples were dispersed in pure deionized water at concentration of 0.01mg/100 ml. The water-dissolved metal citrates were added to the wet dispersion accessory. Malvern Mastersizer diffraction techniques were used to perform the hydrodynamic calculations (Malvern Instruments, UK). A refraction index of 1.57, 1.509, and 1.33 were considered for ferric citrate, citric acid and water respectively.

2.3. Soils

In this study, red soil (Loamy-skeletal, mixed, isohyperthermic, Typic Rhodustalfs), categorized as alfisol by the USDA, was sourced from Sangareddy field (17.6140° N, 78.0816° E) at depths of 0–20 cm. After processing and sieving (2 mm), physicochemical characteristics (Table 1) were analyzed using standard procedures. Soil properties, including dry weight (oven at 105 °C), pH (potentiometric measurement in 1 M KCl solution with 24-h incubation at a liquid-to-soil ratio of 2.5), and Total Organic Carbon (TOC) quantification (TOC-VCSH instrument), were determined following Griffith and Schnitzer's method. The nutrient content was assessed by extracting available nutrients with the Diethylenetriamine pentaacetate (DTPA) method, and total nutrient content was measured using AAS after digestion of the soil. The water-holding capacity of soil (WHC) was determined via a percolation test, while electrical conductivity (EC) was measured using equipment from EUTECH instruments, UK. Field Capacity (FC) and Permanent Wilting Point (PWP) were established using a pressure plate apparatus.

Table 1.

Physicochemical properties of the experimental soils.

Soil parameters	Typic Rhodustalfs (Red soil)
Field capacity (%)	27.9
Permanent wilting point (%)	10.47
pH	6.5
Electrical conductivity (dS/m)	0.27
Bulk density (g/cm3)	1.41
Organic carbon (g/kg)	4.1
Clay (%)	27
Sand (%)	18
Silt (%)	54
Amorphous Al (g/kg)	1.34
Available Fe (mg/kg)	21.7
Available Zn (mg/kg)	1.63
Total Fe (mg/kg)	1192.4
Total Zn (mg/kg)	8.3
Loamy sand	(Sand- 83.25 %; Silt- 6.25 %; Clay- 10.5 %)

2.4. Soil column leaching experiment

The leaching experiment was conducted with soil columns using optimized Fe and Zn metal citrate nanoparticles, referenced from prior work [27] and the details are outlined in Table 2. Commercial samples (Fe-EDTA, Zn-EDTA, nano-Fe, and nano-Zn) were included for comparative analysis. The experiments with untreated soils served as controls. Before the experiment, soils were saturated and thoroughly dried. The nutrients were introduced, and incubation lasted for six weeks with soil moisture maintained at 40 % of field capacity and temperature of 25–30 °C. Polyvinyl chloride columns (60 cm length and 10.12 cm diameter) coated with paraffin wax were filled with acid-washed gravel and sand of 2 cm depth. A triplicates of untreated soil columns were gradually filled to 50 cm (6.5 kg), and incubated Fe and Zn soil samples (0.5 kg each) were placed in separate columns at 0–5 cm maintained at 25–30 °C. The columns were filled with nitric acid-washed sand, and a layer of nutrient-treated soil was tightly pressed. The mean bulk density of the investigated soil replicated field conditions. A thin layer of fine sand was added on top which prevented the disturbance during water addition. The columns were stacked vertically for leachate collection, and a consistent leaching experiment was performed by adding 500 ml deionized water at seven intervals over 120 days (Scheme 1). The leachates were analyzed using AAS for Fe and Zn concentrations, providing detailed insights into soil leaching behavior and treatment effects on metal ion mobility [28].

Table 2.

Composition of various of iron (Fe) and zinc (Zn) samples and doses added for the experiments.

Code		Common name	Treatment	Fe(mg/kg)	Zn (mg/kg)
T1	Synthesized nutrients	Individual citrates	BFC (1:1)- 6	207	–
T2			BZC (1:3)- 6	–	298
T3		Combined citrates	BFZ (4:6)- 8	187	262
T4			BFZ (5:5)- 2	209	232
T5			BFZ (5:5)- 6	209	232
T6			BFZ (8:2)- 4	215	142
T7			BFCZ (1:1:1)- 6	187	258
T8	Commercial nutrients	Geolife	Nano Fe	240	–
T9			Nano Zn	–	240
T10		Fe Gro Zingap	Fe-EDTA	240	–
T11			Zn-EDTA	–	240
T12			Untreated	0	0

Scheme 1.

Schematic representation of soil column study for leaching and mobility.

2.5. Vertical mobility

In the vertical mobility experiment, columns were filled similar to the leaching experiment setup. The Fe and Zn samples were introduced at varying rates (Table 2) within the 0–5 cm range. The soil moisture was maintained at 40 % of field capacity to ensure optimal conditions. To collect the loosened soil with minimal disruption, a room temperature drying process was initiated. This drying stage was crucial for preserving the soil structure and composition. Subsequently, 1g of soil within the columns was collected at specified depths: 15 cm (upper), 30 cm (middle), and 45 cm (lower). The DTPA extraction technique provided critical insights into nutrient availability within the soil samples over the experiment duration. The collection of soil samples was performed at 10-day intervals over 90 days. The collected soil samples were then completely dried and processed using DTPA extraction, estimating Fe and Zn availability with AAS [29].

2.6. Release kinetics

This research aimed to evaluate the effectiveness of Fe and Zn citrates alongside commercial samples as nutrient sources, focusing on Alfisol in a laboratory setting. A100 grams of each of the soil sample was sieved to 2 mm and was placed in air-tight containers in triplicates for reliability. Fourteen samples in total were studied prepared including an untreated sample as a control (Table 2). Throughout the 30-day pre-incubation and 90-day incubation, the samples were maintained at 60 % soil moisture and temperature of 28 ± 2 °C. The study explored the release kinetics of Fe and Zn by analysing DTPA extracts with AAS at intervals (10, 30, 60, and 90 days).

2.7. DTPA extraction of Fe and Zn and AAS estimation

In the analysis of soil collected from vertical mobility columns and release kinetics, 10 g of soil was air-dried and the contents were extracted with 20 mL DTPA. The supernatant was filtered using Whatman 42 filter paper for subsequent AAS (PerkinElmer PinAAcle 900F, USA) analysis for Fe and Zn concentrations. The Fe analysis utilized a wavelength of 248.3 nm, lamp current of 12 mA, and a bandpass of 0.2 nm. For Zn, settings included a wavelength of 213 nm, lamp current of 12 mA, and a bandpass of 0.2 nm. The calibration standards ranging from 0.5 to 3.0 mg/L for Fe and 0.2–0.8 mg/L for Zn were used. The working standards were obtained by diluting a stock solution (1000 mg/L) of Fe and Zn (Merck Pure AAS standards) with deionized water. The AAS method accounts for Fe and Zn assessment in pure deionized water and digestive acid. The final quantification utilized WINLAB 32 AA Version 7.4.1 software with a minimum correlation coefficient of 0.995 assigned for the two-coefficient equation in quality control checks. Following the detection of an S-shaped calibration curve for standards, Fe and Zn nutrient measurements were conducted.

2.8. Characterization of metal citrate nanoparticles

After the synthesis process, the chemical composition and particle sizes of Fe and Zn citrate nanoparticles were examined. FTIR spectra were captured using a Thermo Nicolet (IS50) spectrometer equipped with a diamond ATR crystal. For Dynamic Light Scattering instrument (DLS) characterization, the samples were dispersed in pure deionized water, and the hydrodynamic size and zeta potential were determined using a Malvern Zeta-Sizer NanoZS (Malvern Instruments, USA). Before each measurement, the samples were allowed to stabilize for 2 min at a temperature of 25 °C.

2.9. Statistical data analysis

Statistical analysis involved calculating the average mean value from a given set, derived from three replicates. Error bars represent standard error (SE) of the mean estimated with generalized least squares.

3. Results

3.1. Water solubility and IR spectra of metal citrates

Table 3 presents data on the water solubility of individual Fe and Zn citrate nanoparticles. The water solubility of ferric citrates ranged from 67.6 % to 12.6 %, while zinc citrates ranged from 98.2 % to 82.5 %. Importantly, the solubility of these citrates decreased with increased ball milling duration. FTIR analysis (not shown) of the insoluble precipitates revealed bands matching those of the citrate samples before solubilization, confirming no conversion of citrates into other forms due to ball milling. The IR spectra of the supernatant solution also exhibited the same features as the metal citrate nanoparticles, affirming the retention of the chemical integrity of the samples in water presence. Fig. 1(a–d) displays the IR spectra of the ball-milled samples, specifically BFCZ(1:1:1)-6, BFZ(4:6)-8, BFZ(5:5)-6, and BFZ(8:2)-4 combined citrates respectively. The features observed around 1700 and 1630 cm⁻¹ can be attributed to symmetric and asymmetric COO stretching bands, consistent with those observed in the as-prepared combined citrates [27].

Table 3.

Water solubility (%) of metal citrates.

S.No	Different ball milling durations	Ferric citrate (FC)	Zinc citrate (ZC)
1.	0 h	67.6	98.2
2.	6 h	20.7	89.3
3.	10	12.6	82.5
4.	Commercial ferric citrate trihydrate	98.7	–

Fig. 1.

FTIR spectra a) BFCZ(1:1:1)- 6, b) BFZ(4:6)- 8, c) BFZ(5:5)- 6, and d) BFZ(8:2)- 4.

3.2. Size distribution of metal citrates in water

Table 4 depicts the particle size distribution in water for synthesized metal citrates, determined using the DLS technique. The particle sizes ranged from 70.9 nm (BFC(1:1)-6) to 387.3 nm (FZ(5:5)) for the examined metal citrates. As-prepared citrates varied in size from 123.8 nm (FZ(4:6)) to 387.3 nm (FZ(5:5)). Ball-milled citrates exhibited sizes ranging from 70.9 nm (BFC(1:1)-6) to 359.5 nm (BFZ(5:5)-2). It is important to note that the citrate samples exhibit partial solubility in water; therefore, the obtained sizes apply to the insoluble fraction of the citrate samples.

Table 4.

Particle distribution of metal citrates in water measured by DLS.

Category	Sample	Particle size in water (nm)
Un-ball milled individual citrates	FC (1:1)	151.0
	ZC (1:3)	158.1
Un-ball milled combined citrates	FZ (4:6)	123.8
	FZ (5:5)	387.3
	FZ (8:2)	167.4
	FCZ (1:1:1)	364.3
Ball milled individual citrates	BFC (1:1)- 6	70.9
	BZC (1:3)- 6	121.0
Combined ball milled citrates	BFZ (4:6)- 8	91.6
	BFZ (5:5)- 2	359.5
	BFZ (5:5)- 6	301.6
	BFZ (8:2)- 4	109.7
	BFCZ (1:1:1)- 6	320.8

3.3. Column leaching studies

In the investigated red soil, the concentrations of leached Fe and Zn showed significant variability among the samples. Notably, both individual and combined citrate nanoparticles exhibited no detectable leaching of Fe and Zn throughout all leaching events up to 120 days (Fig. 2(a) and (b)). The highest leaching of Fe was observed with Fe-EDTA, reaching 126.9 mg/L, with soluble iron content ranging from 29.1 to 126.9 mg/L. For Zn, the highest leaching occurred with Zn-EDTA and nano-Zn samples. In nano-Zn samples, leaching was initially low but increased steadily from the second leaching event onwards, maintaining a similar trend up to 120 days. Soluble Zn concentrations in Zn-EDTA ranged from 69.1 to 145.05 mg/L, whereas in nano-Zn, the concentration range was 0–30.83 mg/L. No leaching was observed for citrate nanoparticles at a soil column depth of 60 cm, across various durations.

Fig. 2.

Dynamics of leaching of (a) Fe and (b) Zn content in red soil at different days of incubation.

Entire data was analyzed for statistical analysis at 95 % confidence interval. Means of replications within a treatment was used for plotting of graph. The lines display error bars. Error bars represent standard error (SE) of the mean estimated with generalized least squares.

3.4. Release studies

Fig. 3(a–d) illustrates the periodic release patterns of Fe and Zn nutrients from soils treated with various metal citrate nanoparticles and other commercial nutrients over a 90-day incubation period. The release of Zn increased over time for individual and combined metal citrate nanoparticles, with the exception of the BFCZ(1:1:1)-6 sample. Nano-Zn initially exhibited a release of 157.97 mg/kg of soil at 10 days, which decreased gradually over time. Zn-EDTA showed significantly lower Zn release compared to other samples. Specifically, BZC(1:1)-6 consistently demonstrated increasing Zn release throughout the incubation period, reaching levels similar to nano-Zn at 90 days. The highest zinc release was observed in BFCZ(1:1:1)-6 at 94.2 mg/kg of soil at 10 days, followed by BZC(1:3)-6 at 92.54 mg/kg of soil at 90 days.

Fig. 3.

Release pattern (a, b, c, d) of nutrients (Fe and Zn) in the amended soil at different intervals of incubation.

Entire data was analyzed for statistical analysis at 95 % confidence interval. Means of replications within a treatment was used for plotting of graph. The lines display error bars. Error bars represent standard error (SE) of the mean estimated with generalized least squares.

The release kinetics of Fe during incubation showed a slight increase up to 30 days, followed by a more rapid increase at 60 days and a subsequent decrease at 90 days for all metal citrate nanoparticles and nano-Fe. Fe-EDTA exhibited minimal ferric content release compared to other samples. The highest release at 60 days was observed in BFZ(5:5)-2 (271.97 mg/kg of soil), followed by BFC(1:1)-6 (241.17 mg/kg of soil) and nano-Fe (222.10 mg/kg of soil).

Table 5 provides a detailed comparison of the measured values of Fe and Zn releases at different incubation days, compared with the initial nutrient application in the experimental soil. An excess accumulation of Fe exceeding 100 % at 60 days of incubation was observed, possibly due to citrates mobilizing Fe from the total Fe in the soil (1192.4 mg/kg). This excess Fe did not seem to impact the release of Zn in combined citrate treatments. Additionally, higher initial zinc content in combined citrates led to a more efficient release of Fe compared to cases with lower zinc content in the combined citrates.

Table 5.

Release of Fe and Zn (g/kg) from nutrients in the soil at different duration of incubation in comparison with initial concentration.

Treatment	10 days	30 days	60 days	90 days
Nutrient- Zn
BFZ(4:6)- 8	0.6	172.6	181.9	107.9
BFZ(5:5)- 2	1.1	146.4	161.9	124.2
BFZ(5:5)- 6	3.2	107.4	143.2	94.1
BFZ(8:2)- 4	57.5	63.7	89.6	33.7
BFCZ(1:1:1)- 6	365.1	330.4	247.9	21.4
BZC(1:3)- 6	0.4	246.9	271.3	310.5
Nano Zn	658.2	615.6	520.0	380.9
Zn-EDTA	167.1	98.8	65.7	54.4
Nutrient-Fe
BFZ(4:6)- 8	140.8	220.6	1276.1	324.6
BFZ(5:5)- 2	103.8	128.8	1301.3	312.9
BFZ(5:5)- 6	128.5	221.2	1074.8	301.7
BFZ(8:2)- 4	183.8	210.3	1057.7	146.0
BFCZ(1:1:1)- 6	187.2	222.9	1214.6	253.1
BFC(1:1)- 6	192.3	209.0	1165.1	280.0
Nano Fe	151.8	273.9	925.4	274.5
Fe-EDTA	119.9	71.7	571.0	37.1

The initial concentration of Fe and Zn added in this experiment was given in Table 2.

3.5. Vertical mobility

In the column study, significant variations in DTPA-extracted Fe and Zn were observed at different depths within the red soil columns. While there was slight movement of Fe and Zn among the three depths (0–15 cm, 15–30 cm, and 30–45 cm), the upper layer consistently exhibited the highest nutrient content, followed by the middle and bottom layers at all observed time intervals. Specifically, over a 60-day period, nano-Zn demonstrated the highest available zinc content at 134.16 mg/kg of soil, followed by 86.26 mg/kg of soil in the BZC(1:3)-6 sample. Similarly, the highest available Fe content at the 0–15 cm depth during the same period was observed in BFC(1:1)-6 (264.7 mg/kg of soil), followed by nano-Fe (222.8 mg/kg of soil).

Fig. 4(a, b, c, d) clearly illustrate that the DTPA-Fe and Zn content in the untreated soil column layer remained constant. This observation indicates significant movement and accumulation of Fe and Zn in all treated samples across depths, highlighting a distinct contrast with the control soil.

Fig. 4.

Vertical mobility (a, b, c, d) of Fe and Zn in red soil treated with citrate nanoparticles at different depths and days.

Legend indicates the nutrient availability on the particular day of sample collection. Entire data was analyzed for statistical analysis at 95 % confidence interval. Means of replications within a treatment was used for plotting of graph. The bars display error bars. Error bars represent standard error (SE) of the mean estimated with generalized least squares.

4. Discussion

In our previous investigations we performed comprehensive characterizations like electron microscopy and powder X-ray diffraction data analyses, substantiating the nanoscale dimensions and structural attributes of metal citrate nanoparticles [26,27]. The ball milling process affected the density of ferric citrate, increasing it from 0.84 g/cm³ to 1.1 g/cm³. The density of amorphous and crystalline Fe₂O₃ exhibited densities ranging from 4 to 5.2 g/cm³. Thermogravimetric analysis (TGA) patterns were consistent across samples and the decomposition trend match with the bulk/commercial ferric citrate. The solubility of metal citrate, denoting partial dispersion in water, serves as a crucial evaluation index for nutrient mobility, leaching, and plant availability. The ball-milling treatment induces a reduction in the solubility of metal citrates through mechanisms involving particle size reduction, surface chemistry modifications and also due to aggregation, potentially affecting water-binding sites as earlier reported in the case of proteins [30,31] and alumina [32]. These changes collectively contribute to the nanoparticles' enhanced stability in soil environments, facilitating their role as effective and sustainable nutrient carriers for agricultural purposes. Despite the alterations in solubility observed due to ball milling, the form of iron (Fe) in the samples remains unchanged from as-prepared samples. This lower solubility in metal citrates is advantageous as it prevents leaching, a highly desirable characteristic. The DLS data revealed the scattering from the particles in water, suggesting a decrease in water solubility. The combined influence of reduced water solubility, lower water content, nanoscale size, and particle aggregation resulted in no leaching and a controlled release pattern, crucial for supplying essential nutrients gradually to plants [26].

The focus of this study was on utilizing metal citrates in water to assess the mobility and release of Fe and Zn within the soil post-water application. Leaching in soils and agriculture is contingent upon factors such as nutrient mobility, interactions with soil particles, and solubility in water or irrigation [[33], [34], [35], [36]]. Soil characteristics, including type, structure, and composition, also play a crucial role in leaching dynamics [[37], [38], [39]]. The soil texture, characterized by sand (18 %), silt (54 %), and clay (27 %), determines water retention, drainage, and nutrient adsorption. Sandy soils leach nutrients more readily due to larger particles, while clayey soils retain nutrients better with their higher surface area and cation exchange capacity (CEC) [40,41]. Fe and Zn citrate nanoparticles in our study showed release peaks at 60 days (Fe) and 90 days (Zn), influenced by soil moisture and microbial activity. A soil pH (6.5) affects metal ion solubility, while organic matter (4.1 g/kg) enhances CEC and nutrient retention. These properties collectively optimize Fe and Zn availability, outperforming commercial samples in nutrient use efficiency.

The rewetting of dry soil is a significant trigger for leaching, as it facilitates the movement of nutrients through the soil profile, potentially resulting in the loss of vital nutrients [42]. The leaching process depends on nutrient mobility in soil and its interaction with soil particles [42]. Limited research exists concerning micronutrient leaching, especially in Indian soils [43,44]. The type of chemical entities (ligands) and their stability affect the leaching in soil [20]. The mobility of zinc depends on the nature of nutrient sources, with zinc oxysulfate being less mobile and zinc sucrate being immobile [19].

Leaching and excess fertilization of micronutrients pose limitations, which nanofertilizers and low-soluble fertilizers. Their lesser leaching capacity is attributed to strong bonding, minimal reaction with soil components, and high surface area [45]. This study suggests that due to the lesser mobility of metal citrate nanoparticles, even with excess irrigation and rainfall, there might be lesser leaching compared to commercial nutrients [45]. Nanoparticles of Fe and Zn citrates effectively mitigate leaching and excess fertilization of micronutrients through strong bonding and minimal reactivity with soil components. These nanoparticles form stable bonds via electrostatic interactions and surface complexation with soil particles, facilitated by citrate ligands that chelate metal ions. This stability prevents rapid nutrient dissolution and loss into the soil solution. Their high surface area enhances adsorption onto soil surfaces, facilitating a gradual release of nutrients to plants while minimizing leaching. Unlike water-soluble fertilizers, they show low reactivity with clay minerals and organic matter, maintaining nutrient availability in the root zone and reducing leaching potential even under excessive irrigation or rainfall [46]. Thus, the leaching behavior of Zn and Fe citrates are better than other the chelated nutrients sources used in this study.

Nutrient release observations during the 90-day incubation period correspond well with typical crop durations under Indian conditions. The citrate nanoparticles show an initial surge in nutrient release, likely due to the citrate groups facilitating mobilization and preventing reactions with soil components. Unlike commercial nano-Zn, which shows a decline, and Zn-EDTA, with minimal zinc content, the zinc release from citrate nanoparticles steadily increases over time. Similarly, for iron, while both commercial and citrate nanoparticles exhibit comparable trends, the release from citrate nanoparticles is markedly higher than that from nano-Fe. This release behavior for both Fe and Zn aligns with the carrier-based nutrient release reported by Das and Ghosh [47], underscoring the efficacy of citrate nanoparticles in nutrient delivery.

The extracts of Fe and Zn using DTPA are interpreted as available Fe and Zn content to plants. The depth-wise trends in soil columns reveal significant distribution patterns of Fe and Zn. Citric acid-based chelates demonstrate high mobility and effectiveness in delivering nutrients to plant roots, particularly in the 0–15 cm depth range [48]. Compared to other environmentally unfriendly chelators like EDDHA and EDTA [49,50], citric acid-based micronutrients may prove more advantageous. Utilizing citric acid as a chelator, owing to its stability and nanosize results in more available Fe and Zn in soil columns, particularly in the 0–15 cm depth range [47,48]. Efforts to enhance the delivery and plant use efficiency of slow-release fertilizers have led to the utilization of different ligands, including ascorbic acid, humic acid, and lignosulfonate, which chelate to zinc metal ions [[19], [20], [21], [22], [23]]. Thus, from the results obtained in this study, we conclude that an integrated approach for ligand-based nanomaterial is important in the design of effective suitable plant nutrient sources.

5. Conclusions

All citrate nanoparticles exhibited a notably high concentration of Fe and Zn contents within the 0–15 cm depth of soil columns. The DTPA availability data indicate its lesser reactivity despite being a water-soluble source. Remarkably, the results from metal citrate nanoparticles are comparable to commercially available nano-Fe and nano-Zn sources and also surpassed chelated forms available in the market. The solubility in water, a crucial property determining nutrient availability, mobility, and leachability, was impacted by ball milling in the case of metal citrate nanoparticles. Ball milling significantly increases the aggregation of particles due to the high surface energy generated during the process. Once aggregated, these particles are difficult to separate, leading to minimal interaction between water and nanocitrates. Additionally, the initial water content plays a crucial role in the solubility of the nanocitrates. The ball milling process affects the moisture content, and as a result, increased ball milling leads to decreased solubility. This reduction in solubility occurs through mechanisms such as particle size reduction, surface chemistry modifications, and alterations in crystallinity and structure. These changes enhance the stability of metal citrate nanoparticles in soil environments, making them effective and sustainable nutrient carriers for agricultural applications. The interaction between Zn and Fe in combined citrates reveals no mutual interference in nutrient absorption. In citrate complexes, both Fe and Zn are likely bound to citric acid molecules, forming stable complexes. These complexes may shield the metal ions from direct competition during uptake processes in plants. The specific chemical structure of the citrate complexes needs to be studied and confirmed in our future research. It is difficult to separate the effect of the soluble and insoluble part of the nutrient sources on leaching, release and mobility. The observed effects of ball milling on solubility and its effect on leaching, release and mobility in metal citrates warrant further investigation (at relatively higher nutrient amount). Besides, soil-plant-micronutrient interaction studies would also provide valuable insights into optimizing nutrient management practices, enhancing crop productivity, and ensuring sustainable agricultural practices. To conclude, the formulated citrate samples would emerge as promising nutrient sources with optimal mobility, no leaching, and good stability, making them suitable for plant through soil application. Further refinement of citrate nanoparticle formulations to enhance nutrient stability and availability.

Funding

No funding available.

Data availability

Not applicable.

Ethics approval

This article does not contain any studies with human participants or animals.

Consent to participate

All the authors mutually agreed to participate in this work.

Consent for publication

All the authors mutually agreed to publish the work.

CRediT authorship contribution statement

Kella Sri Venkata Poorna Chandrika: Writing – review & editing, Writing – original draft, Methodology, Investigation, Data curation. Ankita Singh: Data curation. Abdul Aziz Qureshi: Visualization, Validation. Balaji Gopalan: Writing – review & editing, Supervision, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.