Adsorption and Desorption Characteristics of Nitrogen and Phosphorus in Paddy Soils from Different Parent Materials

JIAN Yan, ZHU Jian, PENG Hua, XIONG Li-ping, CAI Jia-pei, JI Xiong-hui*

1. Hunan Institute of Agro-Environment and Ecology, Key Lab of Agro-Environment of Soil Heavy Metal Pollution in Hunan Province, Changsha 410125, PRC;

2. College of Longping, Graduate School of Hunan University, Changsha 410125, PRC

Abstract Reddish clayey soil (HH), alluvial sandy soil (HS),granitic sandy soil (MS) and purple clayey soil (ZS) were used as the test materials to reveal the adsorption-desorption characteristics of nitrogen and phosphorus nutrients in paddy soils from different parent materials. The results showed that the nitrogen desorption amount of each soil was greater than the nitrogen adsorption amount in the low nitrogen concentration range of 0～10 mg/L; in the high nitrogen concentration range of 20～50 mg/L, the soil nitrogen desorption rate gradually decreased with the increase of nitrogen concentration of the equilibrium liquid; when the soil nitrogen adsorption amount was-57.267～352.400 mg/kg, the nitrogen adsorption capacity of the paddy soils from different parent materials was HS>ZS>HH>MS; when the nitrogen desorption amount was 8.367～37.833 mg/kg, the nitrogen desorption capacity of the paddy soils from different parent materials was HH>HS>MS>ZS; the nitrogen adsorption isothermal curves of HH, MS, HS and ZS fitted the Linear model, the correlation coefficients were 0.928～0.978. At the same time, in the range of low phosphorus concentration (0～10 mg/L), the phosphorus adsorption amounts of 4 paddy soils were greater than their phosphorus desorption amounts.When the phosphorus concentration of the equilibrium solution exceeded 10 mg/L, phosphorus fixation capacities of 4 paddy soils weakened, meanwhile their phosphorus desorption increased, but their adsorption amounts were still greater than their desorption amounts.When phosphorus adsorption and desorption amounts of 4 paddy soils were -110.312～534.961 and 0.188～14.320 mg/kg respectively, the phosphorus adsorption and desorption capacities of 4 paddy soils were HS>HH>ZS>MS and ZS>MS>HS>HH, respectively. The phosphorus adsorption isothermal curves of 4 paddy soils fitted the Langmuir and Freundlich models, the correlation coefficient were 0.945～0.995. In general, paddy soils developed from different parent materials in Hunan Province have different adsorption and desorption characteristics for nitrogen and phosphorus. Purple clayey soil has the strongest nitrogen fixation capacity due to its stronger viscosity, which can reduce the risk of nitrogen loss by effectively holding nitrogen in the soil solution. On the contrary, being of strong sandy property, granitic sandy soil has the worst nitrogen fixation capacity and higher risk of nitrogen loss. The four paddy soils have strong adsorption capacity and low desorption rate of phosphorus, which indicates that the main paddy soils in Hunan Province have strong adsorption capacity for phosphorus and relatively small loss risk.

Key words Paddy soil; Nitrogen; Phosphorus; Adsorption; Desorption

1. Introduction

With the increasing demand for cereal since the early 1970 s, the application of nitrogen and phosphorus in agricultural production has increased significantly. Statistics show that our cultivated area is less than 1/10 of the world’s total, whereas the application of chemical fertilizer is the largest in the world, among which the nitrogen fertilizer takes up 1/3 of the world’s total. The application of chemical fertilizer in China is featured with“excessive nitrogen and phosphorus and insufficient organic fertilizer”. This unreasonable application structure has resulted in low utilization of chemical fertilizer. The utilization rate of nitrogen fertilizer is only 30%～35%, and the utilization of phosphorus fertilizer is only 10%～20%, indicating a massive loss of nitrogen and phosphorus and highlighting the threat of the non-point nitrogen and phosphorus pollution of farmland to the environment. Although our ability in recognizing and controlling agricultural nonpoint source pollution is getting stronger, the input in nitrogen and phosphorus, the accumulation of soil nutrient, and the loss of nitrogen and phosphorus are getting larger, and the proportion of agricultural nonpoint source pollution is increasing continuously.This kind of large-scale and high-volume non-point source pollution of farmland is hard to control,which seriously threatens the rural water, soil and air security. Therefore, controlling the application of nitrogen and phosphorus in agriculture and reducing the loss of nitrogen and phosphorus are the contradictions between agriculture and ecological environment that need to be solved urgently in China.At present, remediation and prevention of agricultural non-point source pollution have been included in the major strategic demand of China’s 13Five-Year Plan.

Nitrogen and phosphorus are the main sources of soil fertility for plants. They are also important products in soil nitrogen and phosphorus conversion and plant growth and development. The adsorption and desorption mechanism of nitrogen and phosphorus on the surface of solid phase determines the conversion rate of nitrogen and phosphorus between solid and liquid phase of soil, which has effect on plant’s absorption and utilization of nitrogen and phosphorus and may increase the risk of nitrogen and phosphorus loss in farmland. The study of the chemical composition of soil and the adsorption and desorption characteristics of nitrogen and phosphorus can reveal the law of nitrogen and phosphorus migration and transformation in soil, and help to evaluate the environmental capacity of farmland soil to nitrogen and phosphorus, provide guidance for the agricultural fertilization structure, and predict the loss of nitrogen and phosphorus during farmland drainage.

Previous studies on the adsorption and desorption characteristics of nitrogen and phosphorus in paddy soil mainly focused on the physical and chemical properties of soil, land use patterns, fertilization level,soil texture,

etc

. Only a very few of them reported the adsorption and desorption of nitrogen and phosphorus in different soil types. These soils studied were mainly red soil, brown soil, loess soil

etc

. Other paddy soils from different parent materials, such as alluvial sandy soil (HS), granitic sandy soil (MS), reddish clayey soil (HH) and purple clayey soil (ZS), were seldom discussed. Therefore, the author selected these four soils from Hunan Province as the test soil and studied their adsorption and desorption characteristics of nitrogen and phosphorus nutrients, in an attempt to explore the interaction mechanism between agricultural nitrogen and phosphorus pollutant losses and soil environmental capacity changes, identify the law of nitrogen and phosphorus losses, and improve the study on soil nitrogen and phosphorus adsorption and desorption. The results of the study are of great significance to the reasonable application of nitrogen and phosphorus, effectiveness improvement, and reduction of the damage of nitrogen and phosphorus losses to the surrounding ecological environment.

2. Materials and Methods

2.1. Soil types

The test soils include the reddish clayey soil (HH)developed in quaternary red soil in Chunhua Town,Changsha city, Hunan Province, the alluvial sandy soil (HS) developed from the alluvial deposits of the Langbohu River in Nanxian County, Yiyang City,the granitic sandy soil (MS) developed from granite weathering materials in Beishan town, Changsha City,and the purple clayey soil (ZS) developed from purple plate shale in Zhuzhou County, Zhuzhou City, Hunan Province. The basic physical and chemical properties of these test soils were shown in Table 1.

Table 1 Basic physical and chemical properties of test soils

2.2. Test design

2.2.1. Nitrogen adsorption and desorption test

(1) Nitrogen adsorption test of soil: use a 20-mesh sieve to screen out 2.50 g grated air-dried soil,put them into a 100 mL centrifuge tube, and repeat three times for each soil type; then add 50 mL solution with nitrogen concentration of 0, 10, 20, 30, 40 and 50 mg/L (with 0.01 mol/L potassium chloride as the equilibrium electrolyte and pH value of 7) prepared by ammonium chloride to the soil respectively; shake the soil at 25℃ for 2 h, and centrifuge at 5 000 r/min for 5 min; then take the supernatant to determine the nitrogen concentration in the solution with a continuous flow injector. (2) Nitrogen desorption test of soil: after the adsorption test, wash the test soil twice with saturated sodium chloride solution, and add 50 mL 0.01 mol/L potassium chloride solution with a pH value of 7; then shake the soil at 20～25℃ for 30 min, cultivate for 6 d, and centrifuge at 5 000 r/min for 5 min; at last, take the supernatant to determine the nitrogen concentration in the solution with a continuous flow injector.

2.2.2. Phosphorus adsorption and desorption test

(1) Phosphorus adsorption test of soil: use a 20-mesh sieve to screen out 2.50 g grated air-dried soil,put them into a 100 mL centrifuge tube, and repeat three times for each soil type; then add 50 mL solution with phosphorus concentration of 0, 10, 20, 30, 40 and 50 mg/L (with 0.01mol /L potassium chloride as the equilibrium electrolyte and pH value of 7) prepared by monopotassium phosphate to the soil respectively, add 3 drops of chloroform to each tube ; shake the tube at 25℃ for 30 min, and put it into a 25℃ incubator for cultivation and equilibrium for 6 d; during this period,shake the test sample in the incubator for 30 min every morning and evening; after cultivation, centrifuge at 5 000 r/min for 5 min, and measure the phosphorus concentration in the supernatant using Mo-Sb colorimetric method. (2) Phosphorus desorption test of soil: after the adsorption test, wash the test soil twice with 30 mL saturated sodium chloride solution, and add 50 mL 0.01 mol/L potassium chloride solution (pH value of 7); then shake the soil at 20～25℃ for 30 min,cultivate it in 25℃ incubator for 6 d; during this period,shake the test sample in the incubator for 30 min every morning and evening; after cultivation, centrifuge at 5 000 r/min for 5 min, and measure the phosphorus concentration in the supernatant using Mo-Sb colorimetric method.

2.3. Data analysis (calculation method)

Use the following formulas to calculate the adsorption amount, desorption quantity and desorption rate:

Adsorption amount:

Where,

C

is the initial nitrogen or phosphorus mass concentration (mg/L),

C

is the nitrogen or phosphorus mass concentration in adsorption equilibrium solution (mg/L),

V

is the equilibrium solution volume (L),

m

is the test sample mass (g), and

Q

is the nitrogen or phosphorus adsorption per unit of soil (mg/kg).

Desorption amount：

Where,

C

is the desorption solution concentration (mg/L),

V

is the desorption solution volume(L),

m

is the test sample mass (g), and

Q

is the nitrogen or phosphorus desorption per unit of soil (mg/kg).

Desorption rate：

Where,

Q

is the nitrogen or phosphorus desorption per unit of soil (mg/kg);

Q

is the nitrogen or phosphorus adsorption per unit of soil (mg/kg);

P

is the nitrogen or phosphorus desorption rate (%).

Apply Linear model, Freundlich model and Langmuir model to fit the adsorption isotherm.

The Linear adsorption isotherm model:

Where,

C

is the mass volume concentration of equilibrium solution;

K

is the adsorption efficiency of soil on nitrogen or phosphorus (L/kg);

Q

is the initial nitrogen or phosphorus content per unit mass of sample;

Q

is the nitrogen or phosphorus adsorption per unit of soil (mg/kg).

The Freundlich adsorption isotherm model:

Where,

K

is the Freundlich adsorption coefficient (L/kg); 1/n is the exponential factor related to the properties of the adsorption system, usually 0<

n

<2; others are the same as previously stated.

The Langmuir adsorption isotherm model:

Where,

K

is the Langmuir adsorption coefficient(L/kg);

Q

is the saturation adsorption per unit mass of sample (mg/kg); others are the same as previously stated.

The Temkin adsorption model:

Where,

a

and

K

are constants related to temperature and adsorbates; others are the same as previously stated.

2.4. Data processing

Microsoft Excel 2003 was used for data processing and graph drawing; Orgin 8.0 was adopted for data fitting.

3. Results and Analysis

3.1. Nitrogen adsorption and desorption characteristics of different soils

3.1.1. Nitrogen adsorption characteristics of different soils

As shown in Fig. 1, when the nitrogen concentration in equilibrium solution was 0～50 mg/L, the nitrogen adsorption of HS, MS and HH all increased with the increase of nitrogen concentration in the equilibrium solution without significant break point; as for ZS, when the nitrogen concentration in equilibrium solution was 0～40 mg/L, the nitrogen adsorption of ZS increased linearly, and the adsorption was significantly greater than that of the other three soils; when the concentration rose to 50 mg/L, the nitrogen adsorption of ZS went down accordingly. The average nitrogen adsorption of the four soils was HS>ZS>HH>MS in descending order.

Fig. 1 Isothermal nitrogen adsorption curves of different soils

3.1.2. Isothermal nitrogen adsorption equation of different soils

The fitting of the four models of Linear,Langmuir, Freundlich and Temkin (Table 2) showed that the models for nitrogen adsorption were different for soils developed from different parent materials.Among them, the correlation of HS to Temkin model was higher than that of the other three models, with

R

reaching 0.988; HH and MS fitted well with Linear model, with the

R

of 0.978, which was a very significant level. The above three soils all presented high nitrogen adsorption capacity, and the saturated adsorption was still not reached at the concentration of 50 mg/L. However, the fitting of ZS on the four models was not significant.

Table 2 Isothermal nitrogen adsorption parameters of different soils

3.1.3. Nitrogen desorption characteristics of different soils

It can be seen from Fig. 2 that the trend of isothermal desorption of nitrogen by four different types of soil was relatively gentle. The desorption ranged from 8.367 to 37.833 mg/kg. The amount of desorption increased when the nitrogen concentration in equilibrium solution exceeded 30 mg/L. Of the four soils, HS had the highest desorption amount, followed by HH and then MS and ZS.

Fig. 2 Isothermal nitrogen desorption curves of different soils

As shown in Fig. 3, HH and HS had an obvious turning in desorption rate when the nitrogen concentration in equilibrium solution reached 10 mg/L. The desorption rate first increased sharply during 0～10 mg/L and then reached the maximum at 10 mg/L and all the desorption rates were higher than 100%. This was mainly because the nitrogen content in the soils tested was higher than the nitrogen concentration in equilibrium solution; when the nitrogen concentration reached 20 mg/L, the desorption rate of HS and HH dropped rapidly to lower than 100% because at this time the soils started to absorb NH; for the time when the nitrogen concentration in equilibrium solution was higher than 20 mg/L, the desorption rate of nitrogen decreased gradually with the increase of the nitrogen concentration in equilibrium solution and showed a flat trend. The order of the four soils for desorption rate was HH HS>MS>ZS.

Fig. 3 Isothermal nitrogen desorption rate of different soils

3.2. Phosphorus adsorption and desorption characteristics of different soils

3.2.1. Phosphorus adsorption characteristics of different soils

As shown in Fig. 4, the four soils had a similar phosphorus adsorption trend which consisted of two stages: for phosphorus concentration in equilibrium solution lower than 10 mg/L, the slope of isothermal phosphorus adsorption curves was large; for phosphorus concentration larger than 10 mg/L, the slope of the isothermal adsorption curves was reduced and the adsorption presented a slowly increasing trend. The phosphorus adsorption capacity of soils developed from four different parent materials ranged from -110.312 to 534.961 mg/kg, and the order of different soils in phosphorus adsorption capacity was HS>HH>ZS>MS.

Fig. 4 Isothermal phosphorus adsorption curves of different soils

3.2.2. Isothermal phosphorus adsorption equation of different soils

In order to better describe the characteristics of phosphorus adsorption of soils tested, Freundlich and Langmuir models were used to fit the isothermal phosphorus adsorption curves. The results were shown in Table 3. As shown, the correlation coefficients between the phosphorus adsorption characteristics and the two models were 0.945～0.995, which all reached a significant level. According to the principle of maximum correlation coefficient (

R

) and minimum residual error (

S

), the phosphorus adsorption characteristics fitted the Langmuir model best.

Q

standed for the maximum phosphorus adsorption in soil.The

Q

of the four soils ranged from 290.698 mg/kg to 628.931 mg/kg. HS had the largest

Q

, followed by HH and then ZS and MS. Adsorption constant

K

was an important parameter to reflect the phosphorus adsorption capacity level of soil. According to Table 3,HS had the highest phosphorus adsorption capacity, the

K

of which was 0.173; HH had the lowest phosphorus adsorption capacity, the

K

of which was only 0.085;the maximum buffer capacity (MBC) was a good way to feature the phosphorus absorption of soil. When the soils have similar amount of phosphorus adsorption,the larger the MBC, the lower the energy state of the adsorbed phosphorus, and the easier the adsorption of phosphorus by crops. The order of the MBC of the soils tested was HS>HH>ZS>MS, which was basically consistent with the order of adsorption amount and

Q

.3.2.3. Phosphorus desorption characteristics of different soils

According to Fig. 5, the phosphorus desorption process of the three soils of HH, HS and MS can be divided into three stages of “rise slowly—rise quickly—rise slowly”, while the phosphorus desorption process of ZS consists of two stages of “rise slowly—rise quickly”, which indicated that the desorption amount increased with the adsorption amount. Of the four soils, ZS had the largest phosphorus desorption,followed by MS and then HH and HS. However, the phosphorus desorption of the four soils was much lower than the phosphorus adsorption, which was only 0.188～14.320 mg/kg.

As shown in Fig. 6, the four soils all had obvious turning point in phosphorus desorption rate, presentinga trend of “rapid increase-slow increase to stable-slow decrease”. It can be clearly seen from the phosphorus desorption rate that HH soil had the lowest desorption rate, indicating that HH had greater phosphorus adsorption capacity than other three soils. However,all the four soils had low phosphorus desorption rate,which ranged from 0 to 4.53%, indicating a higher phosphorus adsorption trend than desorption trend of soils.

Table 3 Isothermal phosphorus adsorption parameters of different soils

Fig. 5 Isothermal phosphorus desorption curves of different soils

Fig. 6 Relationship of phosphorus desorption and adsorption of different soils

4. Conclusion

(1) The nitrogen absorption of paddy soils from different parent materials rose linearly with the increase of nitrogen concentration in equilibrium solution. The amounts of nitrogen absorption were ranged from 57.267 mg/kg to 352.400 mg/kg, and the order of the nitrogen absorption capacity of the soils was HS>ZS>HH>MS. The fitting by isothermal nitrogen adsorption curves indicated that the nitrogen absorption of HS fitted the Temkin equation best, and the fitting of HH, MS and ZS with Linear equation had reached a significant level.

(2) The nitrogen desorption of the four soils increased with the increase of nitrogen concentration in equilibrium solution. The amounts of nitrogen desorption were ranged from 8.367 mg/kg to 37.833 mg/kg, and the order of the nitrogen desorption capacity of the paddy soils from different parent materials was HH>HS>MS>ZS. Further analysis on the nitrogen desorption rate of different soils showed that the nitrogen desorption contained two stages.First, the nitrogen desorption increased rapidly at low nitrogen concentration (0～10 mg/L), with the desorption rate higher than 100%. Second, as the nitrogen concentration increased, the desorption rate decreased to a certain extent. Of the four soils, HH had the highest desorption rate, almost 20 times of the rate of ZS, 5 times of the rate of MS, and twice of the rate of HS. This indicated that the ability of nitrogen adsorption of sandy soils such as alluvial sandy soil and granitic sandy soil had the worst nitrogen adsorption capacity and excessive application of nitrogen could cause loss of nitrogen and increase the risk of non-point source pollution.

(3) The phosphorus absorption of different paddy soils presented a trend of increasing first rapidly and then slowly with the increase of phosphorus concentration in equilibrium solution. Specifically,the phosphorus absorption of the soils increased rapidly for concentration lower than 10 mg/L, and slowed down for concentration higher than 10 mg/L.The phosphorus absorption of the four paddy soils ranged from -110.312 to 534.961 mg/kg, and order of their adsorption capacity was HS>HH>ZS>MS.The fitting of the isothermal adsorption curves showed that the four soils fitted well with Freundlich model and Langmuir model. The adsorption constant was 0.085～0.173. HS had the strongest adsorption capacity, while HH had the weakest. The maximum adsorption

Q

ranged from 290.698 mg/kg to 628.931 mg/kg. HS had the largest phosphorus absorption,followed by HH and then ZS and MS.

(4) The phosphorus desorption of the four soils increased with the increase of phosphorus concentration in equilibrium solution, which can be divided into three stages of “slow increase—rapid increase—slow increase”. The total amount of phosphorus desorption was 0.188～14.320 mg/kg.The order of the paddy soils from different parent materials in terms of phosphorus desorption capacity was ZS>MS>HS>HH. The phosphorus desorption was only 0～4.53% of the adsorption level, which indicated that the paddy soils in Hunan Province had strong adsorption and small risk of loss.

5. Discussion

5.1. Nitrogen adsorption and desorption characteristics of different soils

Current studies have shown that soils from different parent materials have different nitrogen adsorption-desorption capacities due to their physical and chemical property differences in clay mineral composition, clay component, organic component,organic matter content, water-soluble Kcontent in soil, water-soluble NHconcentration in soil,cation type, and pH value in soil,

etc

. In this study, purple clayey soil presented high adsorption and low desorption of nitrogen, while alluvial sandy soil was high in both adsorption and desorption of nitrogen, indicating that purple clayey soil had higher nitrogen capacity than alluvial sandy soil. In addition,the nitrogen in purple clayey soil was stable, while the loss of nitrogen in alluvial sandy soil was more common. Therefore, the application of nitrogen in alluvial sandy soil area should be controlled during agricultural production and fertilization. The nitrogen adsorption and desorption in reddish clayey soil and granitic sandy soil were both lower than purple clayey soil and alluvial sandy soil, with the nitrogen adsorption rate of reddish clayey soil being only above granitic sandy soil and the desorption rate being only secondary to alluvial sandy soil. The main reason may lie in the different parent materials of the two soils. Parent material had certain effect on the mineral composition of soil. The clay minerals of the alluvial sandy soil developed from river alluvium and the purple clayey soil developed from purple shale weathering products are mainly 2 ∶1 types such as vermiculite, illite and montmorillonite,

etc

. Therefore,these soils have high content of fixed ammonium. In comparison, the clay minerals of the reddish clayey soil developed from quaternary red soil and the granitic sandy soil developed from granite weathering products are mainly 1 ∶1 types, hence they contain relatively low fixed ammonium.

Previous studies divided the isothermal adsorption process of nitrogen in soil into two stages,rapid adsorption stage and slow adsorption stage. At low nitrogen concentration, the nitrogen adsorption was greatly affected by the change of concentration,which was the rapid adsorption stage. At high nitrogen concentration, the adsorption capacity was slightly affected by the change of solution concentration, and the adsorption trend was flat, which was the slow adsorption stage. The purple clayey soil tested in this study had similar performance with the above conclusion, but the nitrogen adsorption of other three soils increased linearly with the increase of nitrogen concentration in equilibrium liquid mainly due to the low background nitrogen content of the soil tested.Under the condition of this test, the soils presented strong capacity to hold nitrogen while not being saturated.

pH value also affects the nitrogen adsorption and desorption of in soil due to the adsorptive sites competition between NHand H. At low pH value,Hwould increase and would be easily adsorbed by the anionic group in the solution; as pH value increases,Hwould decrease and the anionic group would likely to adsorb more NH. Hence, the nitrogen adsorption would increase with the increase of pH value.A similar conclusion was drawn in this study. The adsorption capacity of nitrogen of the four soils from different parent materials changed with the pH value,and the order was HS>ZS>HH>MS. Meanwhile, for these soils, owing to different surface structures and surface energy, presented different adsorption forces on nitrogen. The adsorption of the soils can be divided into physical adsorption and chemical adsorption.Physical adsorption is not firm, resulting in high reversibility in desorption; chemical adsorption is firm, resulting in low reversibility in desorption.Therefore, the nitrogen adsorption amount of different soils is much lower than desorption amount, mainly due to the dominated chemical adsorption of the soils or the low background nitrogen content and the strong nitrogen retention ability of the soils tested.

A number of studies have suggested that Langmuir model and Freundlich model have the best fit for nitrogen adsorption of soil. However,the isothermal nitrogen adsorption curves of HH,MS and HS in this study showed the highest fitting degree with Linear equation because of the high background nitrogen content of the three soils. Under this experimental condition, the adsorption of nitrogen of soils was unsaturated, as shown in the law of the adsorption amount linearly increasing with the increase of the nitrogen concentration in equilibrium solution.

5.2. Phosphorus adsorption and desorption characteristics of different soils

Most of the phosphori in soil were transformed in the form of POduring the adsorption and desorption process, and some were transformed into organic state through biological action. Different types of soils varied greatly in terms of phosphorus adsorption and desorption capacity. Even for soils in the same category, the environmental capacity of soils to phosphorus also varied a lot because of different parent materials and ambient environments. Test results showed similar law of isothermal phosphorus adsorption curves of different paddy soils, which can be divided into two stages: at low concentration, the phosphorus adsorption increased dramatically; at high concentration, the phosphorus adsorption increased slowly with the phosphorus concentration, which was consistent with the findings of WANG Y

et al

..The main reason for this law was that at low concen tration, the iron, aluminum and clay particles in soil were chemically and covalently adsorbed with phosphorus in equilibrium solution, raising the bond energy at phosphorus adsorption sites in soil. As the phosphorus concentration in equilibrium solution increases, the bond energy of phosphorus adsorption in soil would decrease, the phosphorus desorption would increase, and the adsorption curve would become flat. Therefore, at high phosphorus concentration,the adsorption curve was expected to be flat as the phosphorus adsorption of soil was saturated.Phosphorus desorption is considered to be more important than phosphorus adsorption in soil, because it not only involves the phosphorus recycling capacity of soil, but it also reflects the impact of phosphorus desorption on the environment. The desorption process of phosphorus is not a simple reverse process of adsorption. Different adsorption stages represent different energy levels of adsorption and different bonds with the soil during phosphorus desorption process. The higher the adsorption capacity, the securer the adsorption of phosphorus. With the increase of phosphorus concentration in equilibrium solution, the process of phosphorus desorption showed a “slow—rapid—slow” desorption trend,which is basically the same as the adsorption process.This trend might be connected with the stability of the phosphorus after adsorbed in soil. According to YUAN R X

et al

., the rapid stage mainly referred to the phosphorus desorption by VDW (Van der Waals’ force) and being static in soil, while the slow stage mainly involved the phosphorus adsorption of covalent bond or higher bond energies as well as the phosphorus desorption in soil colloid. This study also revealed the following relationship between phosphorus adsorption and desorption of the four soils from different parent materials: HS and HH have high phosphorus adsorption level and low desorption rate; ZS and MS have low adsorption level and high desorption rate. This indicated that HS and HH paddies had large environmental capacity and strong adsorption capacity for phosphorus, which was not easy for phosphorus desorption. In comparison, ZS and MS paddies had small environmental capacity for phosphorus, which was easy for phosphorus desorption and would cause the loss of phosphorus.There are mainly 4 types of models for isothermal phosphorus adsorption fitting, namely Linear,Langmuir, Freundlich and Tempkin. Results showed that Langmuir model and Freundlich model fitted well with the phosphorus adsorption of soils from different parent materials, all reaching a significant level. Therefore, both models can be used to describe the adsorption characteristics of phosphorus in soil. The maximum adsorption

Q

is a symbol to represent the soil phosphorus pool level. The higher the

Q

, the stronger the adsorption P. Some studies compared the

Q

of different soils and concluded the order of limestone>plate shale>quaternary red clay>purple sand shale parent material>purple plate shale for

Q

; QIU Y

Q

et al

.concluded the order of quaternary red soil>alluvial soil of the Yangtze River>purple sand shale purple soil for

Q

. These studies indicated that soils from different parent materials had different

Q

. As for this study, the order of the four soils for

Q

was HS>ZS>HH>MS.It meant that the

Q

of HS developed from river alluvium was larger than that of HH developed from quaternary red soil and MS developed from granite weathering materials, whereas it was smaller than that of ZS developed from purple plate shale. This was because the same parent soil may have different soil properties as a result of different climate conditions,farming management and fertilization levels. These differences could lead to different soil pH values, clay content, organic content, active iron and aluminum content, and phosphorus content, affecting the phosphorus adsorption and desorption of soils.

K

was a soil phosphorus adsorption affinity constant,which revealed the difficulty of soil’s spontaneous adsorption. In this study, HS had the highest

K

,indicating the highest spontaneous adsorption degree of the soil. The reason for HS’s stronger phosphorus adsorption degree than other three soils may due to its highest organic content among the soils tested.Organic matters can dissolve, complex and reduce crystallized iron and aluminum compounds, improve their activation grade, and enhance the phosphorus adsorption capacity of soil. MBC is a comprehensive parameter of phosphorus adsorption strength and capacity, which is quite stable. The larger the MBC,the stronger the phosphorus storage capacity of the soil. The MBC of the soils in this study followed the same law with

Q

. The HS developed from river alluvium had better phosphorus adsorption capacity than other the three soils, while MS developed from granitic shale had the poorest phosphorus fixation capacity, making it easier for the loss of phosphorus.