There is a global concern about the depletion in phosphorus (P) resources in the near future. Some attempts for effective reuse of P, including recovery from municipal wastes, have been conducted. However, a strong sorption of P onto some minerals may result in low P availability for crops. Therefore, it is necessary to understand the speciation of the chemical forms of P and to elucidate the relationships between P availability and chemical forms of P in soil. This study focuses on the variation in P speciation and the chemical forms of available P in a paddy soil. Incubation experiments with/without drainage, simulating the situation in a paddy field, were performed at a laboratory scale to evaluate the variation in speciation and P availability in soil. The speciation of P was analyzed according to Wilson’s sequential extraction method and measured using Bray No. 2 and Truog methods. Two kinds of chemical forms, i.e. , Fe and Mn (oxy)hydroxides (Fe-Mn-P) and organic and biogenic P (Org-P) were predominant in the soil, and they were easily interconverted by changing soil redox conditions. Available P using the Bray No. 2 method was increased in 21 days owing to the anaerobic condition; thereafter, it reached a constant value by the end of both the incubation experiments. However, a drastic decrease was detected in available P, using Truog-P. It occurred owing to the drying of soil, which suggested that some chemical form(s) of P other than Truog-P might be generated. A comparison between the concentrations of available P and that of each chemical form showed that available P included some Org-P, which might be less absorbed by plants compared to the exchangeable and pore water fraction (Ex-P) and Fe-Mn-P. We conclude that anaerobic soil conditions play an important role in the efficient consumption of P.
Phosphorus (P) is essential for plant growth and crop productivity. For the development of cropping systems, a large amount of P is applied to agricultural lands [
Since long, many studies have aimed to identify and evaluate the chemical forms of P available to plants (e.g., Olsen and Sommers [
In paddy fields, some drastic changes in the soil redox condition occur during the cropping period owing to water management for rice growth, which may alter the speciation and availability of P in soil. Therefore, the consumption of P in soil by rice cropping could be less efficient than that in other crops due to the changing chemical forms of P. It is important to elucidate the fluctuation in the status of P in paddy fields. However, there have been few studies on the transition of chemical forms and availability of P owing to the changes in the soil redox status.
The aim of this study was to elucidate the effects of the changes in the soil redox condition on the chemical properties of P by observing the chemical forms of P present in the soil, based on two incubation experiments. The findings of this study were expected to be useful for controlling the soil and watering conditions with the aim of promoting the efficient consumption of P. Moreover, this study was expected to provide valuable information on the identification of chemical forms of available P.
Simulating the situation in a paddy field, two different incubation experiments (with or without drainage) were performed separately on a laboratory scale to evaluate the speciation and the availability of P in soil. An experiment was conducted without drainage, i.e., under submerged conditions throughout the incubation period. The other experiment allowed drainage within the study period. The soil in this study was sampled from a paddy field at a depth of 0 - 15 cm in the lowland around Lake Biwa in Shiga Prefecture of Japan on October 29, 2015, after rice harvest. The sampling point was located within in approximately 1 km from the lake shore. The soil was prepared for incubation experiments by air-drying and sieving with a 2-mm sieve. The soil type was classified as gley lowland soil. Physical and chemical properties of the soil are shown in
All the chemicals used, except sodium hydrosulfite, were of Japan Industrial Standard special grade or better, purchased from Wako Pure Industries Ltd. (Osaka, Japan). Sodium hydrosulfite was of Japan Industrial Standard chemical pure grade.
In the incubation experiment without drainage, 40 g dry-soil was filled into a 110 mL screw glass bottle, and was submerged with distilled water in the screw bottle up to a depth of 7 cm during the incubation period. Flooding water and soil were sampled after 1, 3, 7, 14, 21, 28, 35, 42, and 63 days from the onset of treatment for the evaluation of the speciation and availability of P. In addition, oxidation-reduction potential (ORP) in the soil during the incubation period was measured with the reference electrode in saturated KCl solution (IM-32P, TOA DDK Co., Tokyo, Japan) in order to evaluate the soil redox status. The values of ORP in saturated KCl solution were approximately converted into those of ORP with standard hydrogen electrode (Eh) using Equation (1), by correcting for the electrode potential of the reference electrode. (Matsushita et al. [
Property | Means ± standard deviation (n = 3) |
---|---|
Soil particle density (g/cm3) | 2.46 ± 0.01 |
pH (1:5 H2O) | 5.5 ± 0.14 |
EC (1:5 H2O) (μS/cm) | 149 ± 33 |
CEC (cmolc/kg dry-weight) | 15.8 ± 0.07 |
Ignition loss (%) | 8.07 ± 0.10 |
Eh = ORP + 206 . (1)
In the incubation experiment with drainage, soil was submerged with distilled water for the first 35 days of incubation. Thereafter, the water was removed, and the soil was left unsubmerged for 14 days. The soil was again submerged with distilled water and stored by the end of the experiment (i.e., after 70 days of the incubation started). The duration of the incubation experiment was 70 days with sampling at days 7, 21, 35, 42, 49, 56, 63, and 70 days. Both incubation experiments were conducted at room temperature (20˚C - 25˚C) in triplicates.
One gram of wet soil was analyzed for fractionating P in chemical forms according to the sequential extraction method developed by Wilson et al. [
Two different indices commonly used as available P in paddy soils in Japan were used in order to investigate the fluctuation of the availability as well as chemical forms of P accompanied by the change in the soil redox condition. One index
(Bray2-P) was measured according to the Bray No. 2 method [
P concentrations in the water used for flooding were evaluated as total P (T-P) in order to observe leaching or precipitation, and fluctuation of the chemical forms of P in the soil. In the pretreatment of T-P, 20 mL of the water sample was autoclaved at 120˚C for 30 min with K2S2O8. The T-P concentration was measured by the spectrophotometric ammonium molybdate-blue method. In addition to T-P, dissolved P (D-P) concentration was determined by filtering the water sample with 5C quantitative ashless filter paper (ADVANTEC, Tokyo, Japan) before autoclaving. Moreover, particulate P (P-P) was estimated as the difference between T-P and D-P.
Total Fe concentrations (T-Fe) in the flooding water was measured in order to understand the behavior of Fe; T-Fe plays an important role in determining the behavior of P in the environment, including adsorption, eluviation, and precipitation [
Statistical analysis of the data was performed with Microsoft Excel 2016
Methods | Extraction procedure |
---|---|
Bray No. 2 | 0.03 mol/L NH4F + 0.1 mol/L HCl, 1 min shaking by hand, soil:extractant = 1:20 (w/v) |
Truog | 1 mmol/L H2SO4, 30 min shaking (165 rpm), soil:extractant = 1:200 (w/v) |
(Microsoft, San Francisco, CA, USA). In evaluation of the relationships among various concentration pairs, coefficient of determination (R2) and p value were used.
Little detection of Ex-P and little fluctuation of Min-P or Res-P occurred during the experiment period.
Approximately 60% of P in the soil existed steadily as Fe-Mn-P even within 7 days, which showed that more than half of P in the soil are absorbed or bound to Fe- and/or Mn-oxides. Thereafter, a comparatively steep decrease in Fe-Mn-P occurred 7 - 14 days from start of the experiment, whereas Org-P concentration increased in response to the decrease in Fe-Mn-P. This result showed the transformation in the chemical forms from Fe-Mn-P to Org-P, probably due to the transition of the soil redox status during this period. As shown in
It was reported that the reduction of Fe in the anaerobic condition of the soil
resulted in dissolution of oxides followed by the release of Fe and sorbed P by several studies (e.g., Kjaergaard et al. [
Both Bray2-P and Truog-P increased after 21 days from the onset of treatment by approximately 30% and 900%, respectively, implying that the bioavailability of soil P to plant increased. This increase in bioavailability probably resulted from the release of dissolved P under the anaerobic condition during submergence. After the increase of Bray2-P and Truog-P, both the values remained constant.
In addition, both of Bray2-P and Truog-P exceeded the sum of Ex-P and Fe-Mn-P from 14 days of the incubation, which showed that a portion of Org-P was also contained in available P. It is known that Bray2-P includes inorganic P; calcium-associated P, and some of the Al- and Fe-associated P, and that Truog-P includes acid-soluble P; and calcium- and magnesium-associated P [
Similar to the incubation experiment without drainage, little fluctuations in the concentration of Ex-P, Min-P, and Res-P were observed throughout the experiment, as illustrated in
Total P (=Ex-P + Fe-Mn-P + Org-P + Min-P + Res-P) concentration in the soil appeared to be almost constant. Thus, during the incubation period, a complementary fluctuation between Fe-Mn-P and Org-P occurred, which implied that these two forms easily interconverted. The relation of the sum of the three chemical forms (i.e., Ex-P, Fe-Mn-P, and Org-P) with Bray2-P or Truog-P before the drainage were similar to those in the experiment without drainage, which confirmed that bioavailable forms were included in these three chemical forms during the submergence period even in the experiment with the drainage. However, unlike the experiment without drainage, both Bray2-P and Truog-P decreased after 7 days from the beginning of the drainage period, which indicated the decline of P availability due to drainage followed by the dryness of the soil. In addition, the extent of decrease of Bray2-P was different from that of Truog-P. As shown in
should be included in available P (
In the experiment without drainage, a peak of T-P concentration of 6.9 P2O5 mg/L was found after 7 days from the submerged soil (
T-P concentration decreased gradually until 28 days, but it remained almost unchanged subsequently. The fluctuation pattern of P concentration was similar to that of Fe; therefore, the correlation coefficient between them was high (R2 = 0.76, p = 0.0045). Thus, it indicated that P had a similar pattern of leaching and precipitation as that of Fe in the soil. The increases of T-P, T-Fe, D-P, and D-Fe concentrations in the water used for flooding resulted from the releases of P and Fe in the soil. As shown in
The decreases in these two concentrations probably resulted from the transformation to some insoluble forms by the combination of P and Fe. Kyuma [
Additionally, in the experiment with drainage, although the maximum concentrations of P and Fe occurred after 7 days from the start of incubation, as in the experiment without drainage, the leaching 7 days after the re-submergence (i.e., after 63 days from the start) was much less. This could be attributed to the inadequate reduction (re-dissolution) on the soil surface, where the anaerobic state was weak. However, the detailed mechanism of the behavior of P and Fe in the water used for flooding is not understood to date. Further studies should address this issue in future.
In the present study, the incubation experiments with/without drainage provide novel insights into the fluctuation in P species and availability of P in the paddy field soil. The speciation of P in soil using the sequential extraction showed transformation in the chemical forms from Fe-Mn-P to Org-P. This was probably caused by the release of P from Fe and Mn oxide particles by the reduction and dissolution of Fe under anaerobic conditions. Eventually, some of the released P was probably formed into compounds, which were categorized as Org-P. We concluded that the anaerobic status in soil played some important roles in the efficient consumption of P.
In both the incubation experiments, the transformation of the chemical forms of P in the soil occurred only between Fe-Mn-P and Org-P., In addition, these experiments demonstrated that these two chemical forms of P are easily interconverted owing to changes in the soil redox status.
Based on the observation of P available to plants, Bray2-P and Truog-P, the availability of P was increased in soil submerged for 21 days. Thereafter, two different values were almost constant, which indicated that the availability was stable as long as the soil was submerged, i.e., under anaerobic conditions. However, in the incubation experiment with drainage, Bray2-P was almost constant throughout the experimental period, whereas a drastic decrease in Truog-P occurred during the drainage period, indicating that some chemical form(s) of P other than those included in Truog-P may be generated.
Comparison of the values of available P and those of the concentrations in each chemical form showed that available P concentrations often exceeded the sum of Ex-P and Fe-Mn-P. This indicated that available P may include some Org-P as well as Ex-P and Fe-Mn-P, which are likely to be transferred in soil and easily absorbed by plants. Further research is needed for the identification of the chemical forms of available P.
The authors declare no conflicts of interest regarding the publication of this paper.
Sakurai, S., Nishiura, Y., Horino, H. and Nakagiri, T. (2018) Variation in Speciation and Availability of Phosphorus in Paddy Soil. Open Journal of Soil Science, 8, 213-224. https://doi.org/10.4236/ojss.2018.89017