Introduction

An extremely vital element for the growth of agricultural crops, phosphorus is also a key constituent of biologically important compounds, including DNA, RNA, and phospholipid1. It plays a crucial role in the worldwide food crop production and is integral to the sustainability of agricultural practices2. However, it is estimated that two-thirds of the world’s agricultural soils are affected by phosphorus deficiency3. As global food demand continues to rise, the use of chemical phosphorus fertilizers has dramatically increased4. Consequently, phosphorus accumulation in soils has become an increasingly prominent environmental issue. In soil, 20-60% of inorganic phosphorus is adsorbed and fixed by soil particles5, reducing its bioavailability and limiting phosphorus utilization6. Most phosphorus not absorbed by plants enters rivers and lakes through surface runoff, causing environment pollution7. Phosphorus, as a key factor in farmland surface pollution, has thus become a major focus of research8.

The adsorption-desorption characteristics of soil phosphorus are a critical for understanding its environmental behavior. These characteristics are particularly important for evaluating soil phosphorus fertility, assessing the risk of environmental phosphorus loss, and understanding associated environmental impacts9. Adsorption processes are known to limit phosphorus availability, while desorption bioavailability. Exogenous phosphorus enters soil primarily through nonspecific adsorption (ion exchange adsorption) and specific adsorption (ligand adsorption)10,11. Nonspecific adsorption, characterized by weak binding affinity, contrasts with specific adsorption involving strong interactions at specific binding sites12. Langmuir adsorption isotherm parameters-including maximum adsorption capacity (Qm), adsorption saturation, and adsorption energy constant-are frequently used to describe phosphorus adsorption characteristics. These parameters exhibit significant regional variation due to differences in soil formation conditions, mineral composition, and human activities13,14,15. In the northeast black soil area, high organic matter content, clay minerals and iron/aluminum oxides enable exchange with phosphorus to form stable complexes, resulting in strong soil-phosphorus affinity16. Therefore, for sustainable agricultural development and the management of phosphorus pollution in water bodies, an in-depth study of phosphorus adsorption mechanisms in this particular region is crucial.

The northeastern black soil region is renowned for its high fertility, making itself suitable for agriculture and recognized globally for its exceptional production potential17. Nevertheless, the prolonged use of unsustainable agricultural methods has led to reduced soil organic matter and degradation soil structure18. Conservation tillage practices-such as straw return and no-tillage/reduced tillage-have been shown to mitigate soil disturbance, reduce runoff/erosion, enhance organic matter content, and improve soil structure19,20. These practices not only increase the utilization of straw and reduce the environmental impacts of straw burning but also represent a sustainable and protective agricultural technology21. The positive effects of conservation tillage on soil properties - including bulk density, porosity, moisture content, and organic matter - are well-documented. Yuan et al.22 demonstrated that no-tillage practices enhance soil organic matter, improve granular structure, accelerate nutrient cycling, and reduce erosion, all of which positively influences phosphorus retention. Similarly, Yang et al.23 found that conservation tillage decreased phosphorus adsorption capacity in North China Plain tidal soils. Soil pH and nitrogen content significantly influence phosphorus adsorption: acidic conditions, positively charged soil surfaces, and high anion concentrations reduce adsorption24. The presence of nitrogen in the soil can lead to the production of H+ by nitrobacteria, which lowers soil pH and consequently decreases the soil’s phosphorus adsorption capacity. As nitrogen levels rise, nitrogen competes with phosphorus for adsorption sites, further limiting phosphorus retention. While conservation tillage increases phosphorus levels in the soil, it also elevates the risk of phosphorus loss. The potential for phosphorus release from soils is typically assessed using indices such as the Environmental Risk Index (ERI), Degree of Phosphorus Saturation (DPS), and Phosphorus Sorption Index (PSI). These indices help assess the soil’s adsorption capacity for phosphorus, predict dynamic changes in soil phosphorus, and reveal the potential for phosphorus fixation and release25,26.

Despite the extensive research on soil structure and fertility under conservation tillage practices, there remains limited systematic assessment of phosphorus adsorption capacity, its underlying mechanisms, and the associated release risks. Therefore, this study investigates three types of soils in the Jiutai district of Jilin Province: (1) no-tillage with straw off the field(CK), (2) no-tillage with straw crushed and mulched(FS), and (3) no-tillage with straw high-stubble mulched (GH). Batch experiments were conducted to investigate the phosphorus adsorption mechanism, assess the risk of phosphorus release, and evaluate the comprehensive effects of conservation tillage on soil phosphorus behavior. This research aims to provide scientific data basis to support soil management, agricultural production, and environmental pollution control.

Materials and methods

Sample collection

The study was conducted at the conservation tillage demonstration site in Jiutai District, Changchun City, Jilin Province (N 44°10′93.92″, E 125°39′28.23″). The positioning test began in May 2011 in a locale characterized by a temperate continental monsoon climate and an average annual precipitation of 566.6 mm. The growing season of the crop extends from April to September. The soil is representative of the black soil area, and maize is the predominant crop cultivated. The experiment included three distinct treatments: (1) no-tillage with straw off the field (CK), (2) no-tillage with straw crushed and mulched (FS), where straw is crushed and returned to the field during the autumn maize harvest, and (3) no-tillage with straw high-stubble mulched (GH), where stubble is left at a height of 40–50 cm during the autumn maize harvest, and straw is crushed and returned to the field above the stubble. In the spring of the following year, since no-tillage is applied, the straw will be returned to the rows, with rotary tillage in the seedling zone. The fertilization rates for all three treatments were the same.

Test samples were collected in October 2023 following the maize harvest. Using a five-point S-shaped distribution method, Each plot was sampled from the topsoil layer (0–20 cm). With the stones and plant residues removed, the samples were also thoroughly mixed. Since a portion was used for aggregate determination, the remainder of the soil samples was placed in a well-ventilated area for drying. After drying, the mesh sizes of 2 mm, 1 mm, and 0.15 mm is used to sieve the soil, respectively, and then set aside. The soil’s physicochemical attributes were evaluated according to the Soil and Agricultural Chemistry Analysis(3rd edition)27.

Phosphorus adsorption features of soil

Adsorption kinetics

Three soil samples (1 ± 0.0005 g) were weighed into polyethylene centrifuge tubes, and 20 mL of 100 mg·L− 1 KH₂PO₄ was added which is a kind of phosphorus standard solution, with 0.01 mol·L− 1 KCl solution serving as the background solution. After adding three drops of toluene in order to inhibit microbial activity, shake the samples at 298 K. The samples were then centrifuged at 4000 rpm for 10 min, and the resulting fluid was filtered through a 0.45 μm microporous filter membrane. Samples were collected at 1, 5, 10, 20, 30, 60, 90, 120, 240, and 480 min, respectively, and subjected to centrifugation at 4000 rpm for 10 min. The resulting filtrate was then passed through a 0.45 μm microporous filter membrane, and the phosphorus content in the filtrate was determined using molybdenum-antimony colorimetry. The equilibrium adsorption time for phosphorus was determined, and the pseudo-fist-older kinetic, pseudo-second order kinetic, and intra-particle diffusion models were applied to model the kinetics of phosphorus adsorption.

Adsorption isothermal tests

Three soil samples, each weighing (1 ± 0.0005) g, were placed into polyethylene centrifuge tubes The methodology employed for achieving adsorption equilibrium followed the procedure outlined in Sect. 2.2.1, including centrifugation and filtration. The determination method stayed consistent, employing the Langmuir and Freundlich equations to model the phosphorus adsorption isotherms.

To determine the adsorption thermodynamic parameters, the reaction temperature was varied (288 K, 298 K, and 308 K), and the above-mentioned procedure was repeated.

Impact of environmental conditions on phosphorus adsorption

The pH of the adsorbent solution (100 mg·L− 1 KH2PO4) was set to 4.0, 5.0, 6.0, 7.0, 8.0, and 9.0 to examine the influence of pH on the phosphorus adsorption capacity of the samples. 20 mL of 100 mg·L− 1 KH2PO4 containing phosphorus (P) with a mass concentration of N (prepared from CH4N2O) of 0, 3, 5, 7 and 12 mg·L− 1 were added. The experiment involved the use of a KH2PO4 mixed solution containing 100 mg·L− 1 of phosphorus (P) to investigate the effect of pH on the amount of phosphorus adsorbed. The test method was the same as 2.2.2.

Data analysis methods

Pseudo-fist-older kinetic model assumes that the adsorption process is mainly controlled by a diffusion step28. Pseudo-secondary-older kinetic models are used in order to describe the chemisorption mechanism, i.e., the process of covalent bonding and ion exchange between the adsorbate and adsorbent by electron transfer or electron pairing28. The Elovich kinetic equation model is used to describe chemisorption on heterogeneous surfaces and is particularly suitable for processes with significant changes in activation energy29.

  1. (1)

    Adsorption kinetic model.

Pseudo-first order kinetic equation:

$$\:{\text{q}}_{\text{t}}\text{=}{\text{q}}_{\text{e}}\text{(1-}{\text{e}}^{\text{-}{\text{k}}_{\text{1}}\text{t}}\text{)}$$
(1)

Pseudo-second order kinetic equation:

$$\:\frac{\text{1}}{{\text{q}}_{\text{t}}}\text{=}\frac{\text{1}}{\text{t}{\text{k}}_{\text{2}}{\text{q}}_{\text{e}}^{\text{2}}}\text{+}\frac{\text{1}}{{\text{q}}_{\text{e}}}$$
(2)

Elovich equation:

$$\:{\text{q}}_{\text{t}}\text{=}\text{alnt+b}$$
(3)

Intra-particle diffusion model serves as a mathematical construct to characterize the diffusion dynamics of materials inside solid particles. This model is applicable for examining the diffusion of pollutants in porous media and pinpointing the rate-determining steps30.

Intra-particle diffusion model:

$$\:{\text{q}}_{\text{t}}=\text{C}+{\text{k}}_{\text{i}}{\text{t}}^{0.5}$$
(4)

Where: qe is the equilibrium adsorption of soil phosphorus (mg·kg− 1); qt is the adsorption at time t (mg·kg− 1); k1, k2 are the adsorption rate constants; a, b are constants; t is the adsorption time (min); and ki is the rate constant for the intra-particle diffusion model (mg·kg− 1·min0.5);C is the intraparticle diffusion constant.

  1. (2)

    Adsorption isotherm equation.

Langmuir equation:

$$\:{\text{q}}_{\text{e}}\text{=}\frac{{\text{K}}_{\text{L}}{\text{Q}}_{\text{m}\text{ax}}{\text{C}}_{\text{e}}}{\text{1+}{\text{K}}_{\text{L}}{\text{C}}_{\text{e}}}$$
(5)

Freundlich equation:

$$\:{\text{q}}_{\text{e}}\text{=}{\text{K}}_{\text{F}}{\text{C}}_{\text{e}}^{\text{1}/\text{n}}$$
(6)

Where: Ce is the phosphorus concentration in the equilibrium solution (mg·kg− 1); qe is the adsorbed amount per unit of adsorbate (mg·kg− 1); maximum adsorbed amount of phosphorus (mg·kg− 1) appeared to be Qmax; KL is the adsorption affinity constant; KF is the equilibrium constant; n is a constant31.

(3)Adsorption thermodynamic equation:

$$\Delta {\text{G=-RTlnK}}$$
(7)
$$\Delta {\text{G=}}\Delta {\text{H-T}}\Delta {\text{S}}$$
(8)
$${\text{ln}}{{\text{k}}_0}{\text{=}}\frac{{\Delta {\text{S}}}}{{\text{R}}}{\text{-}}\frac{{\Delta {\text{H}}}}{{{\text{RT}}}}$$
(9)

Where: R is the ideal gas constant, 8.314 J·(40 mol·K)−1; T is the Kelvin temperature, K; and a straight line is made between lnk0 and 1/T, with the slope and intercept of the line corresponding to the values of ΔH and ΔS, respectively.

(4)Maximum buffer capacity of soil (MBC)32:

$$\:\text{M}\text{B}\text{C}={\text{K}}_{\text{L}}\times\:{\text{Q}}_{\text{m}\text{a}\text{x}}$$
(10)

(5)Environmental Risk Index (ERI).

Degree of Phosphorus Saturation (DPS):

$$\:\text{D}\text{P}\text{S}=(\text{O}\text{l}\text{s}\text{e}\text{n}-\text{P}/{\text{Q}}_{\text{m}\text{a}\text{x}})\times\:100\text{\%}$$
(11)

Phosphorus Sorption Index (PSI):

$$\:\text{P}\text{S}\text{I}=\frac{\text{X}}{\text{l}\text{g}\text{C}}$$
(12)

Phosphorus Release Risk Index (ERI):

$$\:\text{E}\text{R}\text{I}=(\text{D}\text{P}\text{S}/\text{P}\text{S}\text{I})\times\:100\text{\%}$$
(13)

In this formula: Olsen-P is the effective phosphorus content of soil (mg·kg− 1); X is the adsorbed amount of phosphorus (mg·kg− 1); C is the concentration of dissolved phosphorus in the filtrate (umol·L− 1). ERI < 10, low risk; 10 < ERI < 20, medium risk; 20 < ERI < 25, high risk; 25 < ERI, high risk32,33.

Data processing

Execl 2021 was used to calculate the experimental data; SPSS 26.0 is used for data differences analysis, while the method of one-way analysis of variance (ANOVA) and LSD was approached to compare the characteristic parameters and physicochemical properties, the differences in adsorption capacity at different pH and nitrogen concentrations (p < 0.05) of the three test soils; Origin 2021 was used for adsorption fitting treatment, adsorption capacity at different pH and nitrogen concentrations, and the rendering of correlation heat graph; contribution rate analyzing is supported by “Vegan” pack of the R language.

Results and discussions

Adsorption properties of phosphorus

Phosphorus equilibrium time and adsorption kinetics

The duration required for achieving phosphorus adsorption equilibrium and the associated kinetic processes for three different soil treatments: no-tillage with straw off the field(CK), no-tillage with straw crushed and mulched(FS), no-tillage with straw high-stubble mulched(GH) were evaluated at 288 K under a 100 mg·L-1 KH2PO4 phosphorus concentration. The adsorption time ranged between 0 and 480 min, as shown in Fig. 1. As demonstrated in the figure, the phosphorus adsorption trends for CK, FS, and GH soils were generally similar with increasing adsorption time. During the initial 0–30 min, phosphorus adsorption increased rapidly, with adsorption rates reaching 13.634 mg·(kg·h)-1, 11.426 mg·(kg·h)-1, and 10.480 mg·(kg·h)-1, respectively. Between 30 and 240 min, the adsorption rate continued to increase gradually but at a slower pace, followed by a slight decline. The final adsorption rates after 240 min were 0.978 mg·(kg·h)-1, 0.954 mg·(kg·h)-1, and 0.944 mg·(kg·h)-1 for CK, FS, and GH, respectively. The adsorption equilibrium was achieved at 240 min, with the phosphorus adsorption amounts ranked as CK (496.557 mg·kg-1) > FS (480.655 mg·kg-1) > GH (398.097 mg·kg-1). This variation reflects the multi-stage nature of phosphorus adsorption. In the initial stage, high phosphorus concentration in the solution and abundant soil adsorption sites allowed electrostatic attraction to dominate, characterized by nonspecific anion-exchange adsorption34. As time elapses, the adsorption sites on the soil surface reach saturation, which results in a considerable decrease in the quantity of accessible adsorption sites. Consequently, the rate of phosphorus diffusion into the inner structure of the soil declines, and adsorption becomes more specific, involving OH⁻ plasma exchange and ligand adsorption as the primary mechanisms of specialized adsorption35. Finally, equilibrium was reached when these adsorption processes stabilize. Compared with CK, straw returned treatments (FS and GH) significantly reduced soil phosphorus adsorption. This because the carbon decomposed by the straw after returning the straw to the field would mask the soil pore space, reduce the adsorption sites in the soil, and decrease the rate of adsorption of phosphorus by the soil36. To ensure complete phosphorus adsorption equilibrium, 480 min was selected as the adsorption equilibrium time.

Fig. 1
figure 1

Adsorption kinetics of phosphorus. (a) CK; (b) FS; (c) GH; (d) Intra-particle diffusion model.

Full size image

The fitting parameters for the adsorption kinetics are shown in Table 1. As demonstrated in the Table, the maximum adsorption amount (Qm) fitted by the pseudo-second-order kinetic model is more closely aligned with the actual measured values. Additionally, the fitting correlation coefficients R2 (0.8892–0.9520) are significantly higher than those of the pseudo-first-order kinetic model. This suggests that in the process of phosphorus adsorption, chemical adsorption serves as a dominant factor in the phosphorus adsorption process. This result is further supported from the excellent fitting coefficient of Elovich kinetic equation, where R2 values span from 0.9640 to 0.9823, offering a more accurate representation of the phosphorus adsorption data for the three soils.

Table 1 Fitting parameters for adsorption kinetics.
Full size table

As illustrated in Fig. 1(d), the intra-particle diffusion of phosphorus adsorption by those three different soil samples, along with the associated fitting parameters, are provided in Table 2. An analysis of Fig. 1(d) and Table 2 reveals that phosphorus adsorption occurs in three distinct stages, none of which intersect the origin of the coordinates. This finding implies that intra-particle diffusion is not the only step determining the rate37. The adsorption rate constants follow the order Ki1> Ki2> Ki3, with Ki2(GH) > Ki2(FS) > Ki2(CK). This indicates that the phosphorus adsorption rate from the soil during the 30–240 min period is highest in GH, followed by FS and CK. Parameter C is the adsorbent boundary layer thickness constant, the larger the value of C the thicker the adsorbent boundary layer thickness, the smaller the adsorption rate. With the growth of adsorption time, the boundary layer thickness C gradually increased, indicating that the adsorption rate of soil on phosphorus is larger at the beginning of the reaction. This is mainly because abundant free adsorption sites on the soil surface for phosphorus adsorption, which is favorable for phosphorus adsorption. However, as the reaction proceeds, the adsorption sites are gradually saturated, and the resistance of phosphorus to enter the soil pores increases, resulting in a slower adsorption rate23.

Table 2 Fitting parameters of the intraparticle diffusion model.
Full size table

Study of adsorption isotherms

Adsorption isotherm

Adsorption isotherms are widely used tools in the analysis of solid/liquid systems to elucidate the interactions between adsorbents and adsorbates38. Figure 2 illustrates the quantity of phosphorus adsorbed by the test samples at varying temperatures (288 K, 298 K, and 308 K), as well as the adsorption isotherms obtained by adjusting initial phosphorus concentration (5, 10, 20, 30, 50, 70, 100, 150, 200 mg·L-1). Though the treatments were different, the phosphorus adsorption curves exhibited a consistent pattern: in the first stage a rapid increase (≤ 50 mg·L-1), in the second stage a slower rise (50–100 mg·L-1), in the third stage stabilization (100–200 mg·L-1) as the concentration increased. Phosphorus adsorption in soil is generally divided into two stages: chemical or physical adsorption. At low phosphorus concentrations, the number of available adsorption sites is higher, the binding energy is stronger, and chemical adsorption predominates. Nevertheless, with the rise in phosphorus concentration, the chemical adsorption sites tend towards saturation, leading to a transition to low-energy ad sorption, in which physical adsorption takes precedence39. The phosphorus adsorption capacity of the various treatments was in the order of CK > FS > GH. Compared with the no-tillage treatment with the removal of field straw, FS and GH reduced the amount of phosphorus adsorbed by 14.68% and 28.11%, respectively. The organic matter content of the soil was GH > FS > CK, and the organic matter occupied the adsorption sites for phosphorus, effectively reducing the physicochemical adsorption potential of soil mineral colloids for phosphorus40, and decreasing the adsorption capacity of FS and GH soils for phosphorus.

Fig. 2
figure 2

Phosphorus adsorption isotherms.

Full size image
Adsorption characterisation parameters

Describing the adsorption of an adsorbate onto an ideally homogeneous monomolecular layer of adsorbent, primarily for chemisorption processes41, we can use the Langmuir equation. The Freundlich equation, on the other hand, is typically employed to describe adsorption onto a non-homogeneous, multimolecular layer of adsorbent, encompassing physisorption, chemisorption, and solution adsorption42. The fitted parameters for the phosphorus adsorption isotherms for the different test samples are presented in Table 3. As it is shown in Table 3, both the Langmuir and Freundlich equations provide good fits to the experimental data; however, the R2 values for the Langmuir equation (ranging from 0.9769 to 0.9982) are higher and statistically significant. This indicates that chemisorption on a monomolecular layer mainly governs the adsorption process, with multilayer adsorption acting as a supplementary factor. These results align with the findings of Li et al.43.

The adsorption parameters obtained from the Langmuir fitting equation were selected for further analysis. The parameter KL indicates the strength of the soil’s affinity to phosphorus adsorption26, with higher values reflecting a stronger adsorption affinity and a lower risk of phosphorus loss. The KL values (0.048–0.245) for CK soils were greater than those for FS and GH soils, suggesting that the risk of phosphorus loss is the lowest in the CK soils. The maximum buffer capacity (MBC) is obtained by multiplying KL and Qm, and higher MBC values indicating a stronger adsorption capacity for phosphorus and a reduced risk of phosphorus loss16. As shown in Table 3, MBC values increased with rising temperature. The MBC values for CK soils (63.649–250.180) were higher than those for FS and GH, indicating that higher temperatures enhance the soil’s ability to adsorb phosphorus.

Table 3 Fitting parameters of the adsorption isotherm of phosphorus in the tested soil.
Full size table
Adsorption thermodynamic parameters

Three temperatures, 288 K, 298 K and 308 K, were chosen to investigate the effect of temperature on the adsorption of phosphorus in soil by the Gibbs free energy (ΔG), standard enthalpy change (ΔH) and standard entropy change (ΔS). ΔS value refers to the measure of the disorder of the system before and after the adsorption, and as it can be seen in Table 4, ΔS is a positive value indicating that the process of adsorption of phosphorus by the sample of the test is a disordered reaction43. ΔG is a negative value which indicates that the adsorption reaction process is a spontaneous reaction process44, and ΔG gradually decreases with the increase of temperature, indicating that the increase of temperature is favorable to the adsorption reaction. ΔH, standard enthalpy change of the reaction, is used to indicate the change of internal energy of the material system45, and a positive value of ΔH indicates that the adsorption process is a heat-absorbing reaction, and the increase of temperature is favorable to the reaction, which is in agreement with the results of ΔG. The reason for the heat-absorbing reaction may be that phosphorus breaks the original structure through ligand exchange and needs to absorb heat to overcome the bond energy46.

Table 4 Thermodynamic parameters of phosphorus adsorption.
Full size table

Soil characterization

Based on the FTIR spectra (Fig. 3) and EDX mapping (Fig. 4), changes in surface functional groups and elements of three soils (CK, FS, GH) before (XFQ) and after (XFH) phosphorus adsorption were analyzed to determine. Figure 3 shows minimal differences in functional group types before and after adsorption, but the intensity variations of characteristic peaks indicate changes in functional group abundance. The peaks at 3700 cm-1, 3625 cm-1, and 3400 cm-1 correspond to -OH stretching vibrations47, which may originating from inherent clay minerals or hydroxyl-containing organic matter in soils. The altered peak intensity at 3700 cm-1 in FS and GH after adsorption suggests potential interactions between -OH groups and phosphorus. In CK soil after phosphorus adsorption, new peaks emerged at 2925 cm-1 and 2853 cm-1, assigned to asymmetric and symmetric stretching vibrations of aromatic -CH bonds respectively48. This phenomenon may result from phosphorus binding with soil -OH groups, displacing originally coordinated organic matter and releasing free -CH groups49. The peak at 1640 cm-1 represents aromatic C = C stretching vibrations50, characteristic of lignin. Peaks at 1035 cm-1, 786 cm-1, and 690 cm-1 correspond to clay mineral characteristics: 1035 cm-1 to Si-O-Si stretching vibrations in silicates, while 786 cm-1 and 690 cm-1 to Si-O bending vibrations51,52. Reduced peak intensities at these positions in FS and GH after adsorption indicate silicon involvement in adsorption. The 535 cm-1 peak corresponds to metal oxide vibrations (Fe-O, Al-O, Ca-O)53. EDX mapping (Fig. 4) identified major elements: Si, O, C, Al, Fe, P, N, Ca, S. These results corroborate FTIR observations. The high silicon content derives from maize being a silicon-accumulating plant - roots absorb soluble silicon to form phytoliths54. Through straw returning, silicon re-enters soils, promoting silicon accumulation. Soil SiO32- competes with phosphorus for adsorption sites, reducing phosphorus adsorption capacity55. Elevated C, O, and N levels reflect high organic matter content, while Al/Fe derive from black soil parent material weathering56.

Fig. 3
figure 3

FTIR before and after phosphorus adsorption in CK, FS, and GH soils.

Full size image
Fig. 4
figure 4

EDX before and after phosphorus adsorption in CK, FS and GH soils.

Full size image

Effect of background liquid nitrogen content on phosphorus adsorption

Nitrogen is also a vital nutrient for the growth of plant, and its appropriate application through fertilization has been shown to significantly enhance crop yields. Both straw return and nitrogen fertilizer application have been reported to influence soil pH, subsequently affecting the soil’s phosphorus adsorption capacity. This study investigates the effect of varying nitrogen concentrations (0–12 mg·L-1) on phosphorus adsorption, using urea (CO(NH2)2) as the nitrogen source (Fig. 5). Data revealed a consistent adsorption pattern: initial increase followed by decline. Maximum adsorption capacities occurred at 3 mg·L⁻¹ nitrogen, with CK, FS, and GH showing 21.93%, 22.59%, and 24.81% higher adsorption compared to 12 mg·L⁻¹, respectively. The hydrolysis of urea produces NH4+, which is further converted to NH3- by nitrifying bacteria, a process that releases H+ and lowers soil pH. The generated H⁺ compete with phosphorus for adsorption sites on the soil surface, resulting in reduced phosphorus adsorption57. A proportional relationship was observed between increasing nitrogen concentrations and the decline in both soil pH and phosphorus adsorption capacity. Moreover, it was observed that incorporating straw into the soil increased nitrogen content, which in turn heightened the competition between nitrogen and phosphorus for adsorption sites58. Among the tested treatments, the phosphorus adsorption capacity followed the order CK > FS > GH.

Fig. 5
figure 5

Effect of added nitrogen on phosphorus adsorption. * indicates significant at p < 0. 05, ** indicates significant at p < 0.01, and *** indicates significant at p < 0.001, as below.

Full size image

Effect of background liquid pH on phosphorus adsorption

Changes in pH affect soil phosphorus cyclic precipitation dissolution processes and variable charge surface charge properties, which in turn affect phosphorus adsorption behavior (Figs. 6 and 7). The effect of changing the pH (4–9) of the background solution on the amount of phosphorus adsorbed is shown in Fig. 6 and the adsorption mechanism is shown in Fig. 8. Adsorption initially increased and then decreased with rising pH, peaking at pH 7. Compared to pH 4, CK, FS, and GH soils exhibited 33.39%, 38.77%, and 41.89% higher adsorption at pH 7, respectively. As pH increased from 4 to 7, H+ concentration decreased, reducing H+ occupation of soil adsorption sites59. Phosphorus is bound to positively charged soil colloids by electrostatic adsorption, resulting in increased phosphorus adsorption60. In neutral pH ranges, phosphorus primarily exists as HPO42− and H2PO4−, which are readily adsorbed by mineral surfaces. At low pH, PO43− reacts with Al3+, Fe2+ / Fe3+, Ca2+ in the soil to produce AlPO4·Al(OH)3, Fe(H2PO4)2 and Ca10(PO4)6(OH)2 precipitates, which reduces the amount of phosphorus adsorption, with the following reaction formula: PO43− + Al3+ → AlPO4·Al(OH)3↓; PO43− + Fe2+/Fe3+ → Fe(H2PO4)2↓; PO43− +Ca2+ → Ca10(PO4)6(OH)2↓61. When the pH is alkaline (> 7), as OH− increases in the soil, OH− competes with phosphate ions for adsorption sites, leading to a decrease in phosphorus adsorption62. The behavior of returning the straw to the field has been demonstrated to have a significant impact on the composition of soil aggregates, with an increase in the proportion of large aggregates (> 2 mm) being observed. This has been demonstrated to lead to a decrease in the specific surface area and a corresponding reduction in the soil’s phosphorus adsorption capacity. Furthermore, at the same pH the adsorption of phosphorus was CK > FS > GH.

The zero-point charge diagrams of the soils are shown in Fig. 7. The pHpzc is the pH value when the net surface charge of the adsorbent is zero, at which time the anion-exchange capacity is equal to the cation-exchange capacity63. The zero-point charges of the three types of soils, CK, FS, and GH, are 7.66, 7.71, and 7.76, respectively. When pH < pHpzc, the surfaces of the soils, CK, FS, and GH, are positively charged, which have strong electrostatic adsorption capacity for the solution with the negatively charged phosphate has a strong electrostatic adsorption capacity, increasing the amount of phosphorus adsorbed in the soil. When pH > pHpzc, the soil surface is negatively charged, and negatively charged phosphate ions produce electrostatic repulsion, and the adsorption amount decreases.

Fig. 6
figure 6

Effect of pH on phosphorus adsorption.

Full size image
Fig. 7
figure 7

Zero point charge of three test soils.

Full size image

Mechanism of adsorption

As mapped in Fig. 8, the soil phosphorus adsorption process is a complex phenomenon governed by both physical and chemical processes. Not only physicochemical properties of the soil influenced it but also the existence of competing ions such as CO32- and SO42-38. Physical adsorption is a non-specific interaction whereby phosphorus binds to positively charged soil colloids via electrostatic forces. When the adsorption sites on the colloid surface are saturated, phosphorus diffuses into the soil colloid pores until equilibrium is established64. The incorporation of straw into the soil has been found to promote the formation of larger soil aggregates, which reduces the specific surface area of soil particles and thereby lowers the capacity for phosphorus adsorption65. In contrast, chemical adsorption involves specific interactions, where ions such as Fe3+, Al3+, and Ca2+ on the soil colloid surface chemically react with PO43-, to form precipitates that become fixed to the soil66. Straw, as a significant source of Ca2+, plays a key role in this process. Long-term straw return facilitates the transfer of Ca2+ from straw to soil, where it reacts with PO43- to form calcium phosphate precipitates. This precipitation reduces the soil’s overall phosphorus adsorption capacity by chemically immobilizing phosphorus in a form unavailable for adsorption67.

Fig. 8
figure 8

Adsorption mechanism.

Full size image

Risk of conservation tillage on soil physicochemical properties and phosphorus release

Characteristics of soil physics and chemistry

Understanding soil fertility necessitates an appreciation of the critical role played by soil physicochemical properties, which are fundamental for enhancing agricultural productivity and assessing soil-environmental health. The study of soil physicochemical properties under conservation tillage practices, as shown in Table 5. As shown in the table, FS and GH treatments resulted in pH reductions of 0.26 and 0.42 units, respectively, compared to CK. Simultaneously, organic matter content has increased by 23.39% and 33.23% in FS and GH, respectively. The high organic carbon content in straw facilitated its decomposition within the soil, thereby contributing to an increase in the organic matter content of the soil. Prolonged straw return promotes the accumulation of organic carbon, while microbial decomposition of organic matter produces organic acids, leading to a reduction in soil pH. These observations align with those of Shao et al.68, who reported that FS and GH treatments significantly enhanced soil nitrogen, phosphorus, and potassium levels. Additionally, FS and GH increased total phosphorus (TP) content by 12.23% and 24.46% and Olsen-P content by 17.26% and 23.68%, respectively. These consistently increasing values were attributed to the direct input of phosphorus through straw return and the stimulation of soil phosphatase activity, which enhanced phosphorus availability69. Moreover, the GH treatment demonstrated superior water and moisture retention, reduced wind and sand erosion, and contributed to improved soil nutrient content and nutrient fixation70. The decomposition of straw in the soil released organic substances, such as humus and polysaccharides, which facilitated the binding of soil particles and the transformation of micro-aggregates into macro-aggregates. Straw return also promoted microbial growth, and microbial secretions, including extracellular polysaccharides, played a key role in binding soil particles. Consequently, the FS and GH treatments significantly increased the proportion of macro-aggregates (e.g., > 2 mm) in the soil.

Table 5 Physicochemical properties of soils.
Full size table

Disclosure of risk

The primary metrics for assessing the risk of soil phosphorus release include the Degree of Phosphorus Saturation (DPS), Phosphorus Sorption Index (PSI), and Environmental Risk Index (ERI). The risk of phosphorus released from conservation tillage is illustrated in Table 6. DPS is shown to be related to maximum phosphorus adsorption and effective phosphorus content in soil. It is therefore considered an important index by which to measure the level of phosphorus and phosphorus fixation capacity in soil71. Higher DPS values indicate reduced soil phosphorus fixation capacity. Straw incorporation into the field led to a rise in the soil’s available phosphorus content, as certain sites on the soil surface became occupied by available phosphorus, consequently reducing the phosphorus adsorption capacity. The DPS values of both FS and GH were higher than those of CK. As the PSI value increased, the soil’s phosphorus buffering capacity strengthened, its phosphorus fixation capacity enhanced, and the risk of phosphorus loss from the soil diminished72. The PSI values followed order: CK (26.859–32.260) > FS (23.411–28.286) > GH (21.234–24.817). The increase of temperature PSI value increased, which indicated that straw return reduced the phosphorus fixation capacity of the soil, and the increasing temperature was favourable to the adsorption of phosphorus by the soil, which was consistent with the results of the expression of DPS value. Huang et al.33 proposed ERI as the ratio of DPS to PSI to evaluate phosphorus release risk. Data revealed the following order of phosphorus release risk: GH (14.628 -19.646%) > FS (9.699 -13.533%) > CK (6.622 -9.280%). To sum up, the practice of returning straw to the field heightened the likelihood of phosphorus being released from the soil. Notably, the soil from the GH treatment exhibited the most significant risk of phosphorus release, which was categorized as being at a moderate level.

Table 6 Risk parameters of soil phosphorus release under three conservation tillage practices.
Full size table

Influence of soil physicochemical properties on the risk of phosphorus adsorption and release

Figure 9 elucidates the relationship between soil physicochemical characteristics and the risk of phosphorus adsorption and release. As depicted, maximum phosphorus adsorption capacity (Qmax) shows a significant positive correlation with soil pH. At low pH levels, an increase in pH leads to protonation of hydroxyl groups (OH⁻) on soil colloid surfaces, forming positively charged groups that bind with negatively charged phosphorus ions, thus enhancing phosphorus adsorption73. Conversely, Qmax exhibits a significant negative correlation with available nitrogen (AN). The increase in AN vied with phosphorus for adsorption sites on the soil surface, thereby reducing phosphorus adsorption24. Significant positive correlation between DPS and both AN and Olsen-P (p < 0.05), whereas microbial biomass carbon (MBC) shows significant negative correlations with AN and Olsen-P (p < 0.05). This can be attributed to straw return, which enhanced phosphorus availability in the soil. The associated increase in AN reduced phosphorus adsorption, elevating DPS and increasing phosphorus mobility. The adsorption affinity constant (KL) is negative correlation (p < 0.05) with total phosphorus (TP). Straw return increases TP content, adsorption sites of the soil surface gradually saturated and thereby weakening the adsorption affinity. Furthermore, the relationship between soil aggregate size and phosphorus adsorption characteristics revealed that large aggregates (> 2 mm, 2–0.25 mm) were negatively correlated with Qmax, MBC, and KL, while positively correlated with DPS. In contrast, microaggregates (0.25 –0.053 mm, < 0.053 mm) demonstrated an inverse correlation. This phenomenon stems from the fact that larger aggregates exhibited a reduced specific surface area, thereby limiting their phosphorus adsorption potential.

Fig. 9
figure 9

Heat map of correlation between soil physicochemical properties and adsorption capacity.

Full size image

The physical properties of the soil have been shown to exert an effect on the adsorption process by altering the moisture, heat and porosity. Chemical characteristics of the sample had been recognized as crucial elements influencing the adsorption process. The role of these physicochemical properties under conservation tillage in the adsorption process is illustrated in Fig. 10. As demonstrated in the figure, physicochemical interactions emerge as the predominant factor contributing to the adsorption amount and characteristics, accounting for 87.13% of the total contribution. Chemical properties contribute 12.09%, with a residual unexplained proportion of 0.69% potentially linked to microbial activity.

Fig. 10
figure 10

Contribution of soil physico-chemical properties to sorption in conservation tillage soils.

Full size image

Conclusions

In this paper, we study the dynamic changes of phosphorus adsorption on soil by conservation tillage and explore the adsorption mechanism. The adsorption kinetics of phosphorus on soil was divided into three stages, and the adsorption equilibrium was reached at 480 min. The amount of adsorption was shown as straw off-farm no-till (CK) > no-tillage with straw crushed and mulched (FS) > no-tillage with straw high-stubble mulched (GH). Soil adsorption of phosphorus was consistent with the Elovich equation and Langmuir model. Soil adsorption of phosphorus was dominated by chemical adsorption in a single molecular layer and showed a spontaneous and disordered heat absorption reaction. Appropriate increase in soil pH and decrease in soil nitrogen content can help soil adsorption of phosphorus.

There was a significant positive relationship (p < 0.05) between the Qmax of soil and pH, whereas a significant negative association (p < 0.01) was observed with AN. Conservation tillage improved the soil environment and increased the nutrient content in the soil, but at the same time reduced the soil’s ability to fix phosphorus and increased the risk of phosphorus release from the soil, which was manifested as GH (medium risk) > FS (low-moderate risk) > CK (low risk). Physicochemical interactions exerted the most substantial influence on adsorption. These discoveries made in this investigation augment the knowledge regarding the adsorption properties and release perils of phosphorus in soil maintained through conservation tillage, thereby supplying a theoretical underpinning for further explorations in this subject.