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1.
Estimation of the phosphorus sorption capacity of acidic soils in Ireland   总被引：4，自引：0，他引：4  
The test for the degree of phosphorus (P) saturation (DPS) of soils is used in northwest Europe to estimate the potential of P loss from soil to water. It expresses the historic sorption of P by soil as a percentage of the soil's P sorption capacity (PSC), which is taken to be α (Alox + Feox), where Alox and Feox are the amounts of aluminium and iron extracted by a single extraction of oxalate. All quantities are measured as mmol kg soil?1, and a value of 0.5 is commonly used for the scaling factor α in this equation. Historic or previously sorbed P is taken to be the quantity of P extracted by oxalate (Pox) so that DPS = Pox/PSC. The relation between PSC and Alox, Feox and Pox was determined for 37 soil samples from Northern Ireland with relatively large clay and organic matter contents. Sorption of P, measured over 252 days, was strongly correlated with the amounts of Alox and Feox extracted, but there was also a negative correlation with Pox. When PSC was calculated as the sum of the measured sorption after 252 days and Pox, the multiple regression of PSC on Alox and Feox gave the equation PSC = 36.6 + 0.61 Alox+ 0.31 Feox with a coefficient of determination (R2) of 0.92. The regression intercept of 36.6 was significantly greater than zero. The 95% confidence limits for the regression coefficients of Alox and Feox did not overlap, indicating a significantly larger regression coefficient of P sorption on Alox than on Feox. When loss on ignition was employed as an additional variable in the multiple regression of PSC on Alox and Feox, it was positively correlated with PSC. Although the regression coefficient for loss on ignition was statistically significant (P < 0.001), the impact of this variable was small as its inclusion in the multiple regression increased R2 by only 0.028. Values of P sorption measured over 252 days were on average 2.75 (range 2.0–3.8) times greater than an overnight index of P sorption. Measures of DPS were less well correlated with water‐soluble P than either the Olsen or Morgan tests for P in soil.  相似文献   

2.
This study shows that mobilization of phosphate from soils under anaerobic conditions can be intimately coupled with reductive dissolution of iron from iron oxides. Among four soil samples from the reclaimed Skjernå estuary in Denmark incubated anaerobically and amended with glucose, 28–39% of the dithionite-citrate-bicarbonate-extractable iron and 10–25% of the oxalate-extractable phosphorus (Pox) were released to the soil solution after 31 days. Significant correlation (r = 0.992**) between the molar ratio Pox/(Feox + Alox) for the aerobic samples and (PP sol/Fesol) (the molar ratio between phosphate and iron in solution during anaerobic incubation), indicates that the phosphate saturation status of the soil is an important determinant of the amount of phosphate released during flooding of moderately acid soils.  相似文献   

3.
As repeatedly reported, soil flooding improves the availability of P to rice. This is in contrast with an increased P sorption in paddy soils. The effects of soil flooding on the transformation of Fe oxides and the adsorption/desorption of P of two paddy soils of Zhejiang Province in Southeast‐China were studied in anaerobic incubation experiments (submerging with water in N2 atmosphere). Soil flooding significantly increased oxalate‐extractable Fe (Feox), mainly at the expense of dithionite‐soluble Fe (FeDCB), as well as oxalate‐extractable P (Pox), but decreased the ratio of Pox/Feox. Flooding largely increased both, P adsorption and the maximum P adsorption capacity. The majority of newly sorbed P in the soils was Pox, but also more newly retained P was found to be not extractable by oxalate. Flooding also changed the characteristics of P desorption in the soils. Due to a decrease of the saturation index of the P sorption capacity, P adsorbed by flooded soils was much less desorbable than that from non‐flooded soils. There are obviously significant differences in the nature of both, the Feox and Pox fractions under non‐flooded and flooded conditions. The degree of the changes in Feox, Pox, P adsorption and P desorption by flooding depended on the contents of amorphous and total Fe oxides in non‐flooded soils. Our results confirm that the adsorption and desorption behavior of P in paddy soils is largely controlled by the transformation of the Fe oxides. The reasons of the often‐reported improved P availability to rice induced by flooding, in spite of the unfavorable effect on P desorbability, are discussed.  相似文献   

4.
In Russia, iron is chemically fractionated according to a parallel scheme. Pyrophosphate-soluble iron (Fepyr) is considered to participate in organomineral complexes, oxalate-soluble iron (Feox) is believed to enter amorphous + poorly crystallized compounds, and dithionite-soluble iron (Fedit) is meant to represent the free (nonsilicate) compounds. However, the investigations prove that the commonly used subtraction operations (Feox ? Fepyr) and (Fedit ? Feox) are invalid because of the nonadditive action of the reagents in the parallel scheme of extraction. The low selectivity of reagents requires a new interpretation of chemically extracted iron compounds. In automorphic soils, the content of oxalate-soluble iron should be interpreted as the amount of Fe(III) capable of complexing with organic ligands; in hydromorphic soils with a stagnant moisture regime, it should be interpreted as the amount of iron (III) capable of being reduced in a short time. The content of dithionite-soluble compounds should be regarded as the amount of iron (III) within both (hydr)oxides and silicates potentially prone to reduction.  相似文献   

5.
Phosphate sorption was studied in samples (0 - 20 cm depth) of five soils from Egypt (pH 7.4 - 8.7), four soils from Ethiopia (pH 3.9 - 5.3) and six soils from Germany (pH 3.3 - 7.2). Sorption parameters were calculated according to Pagel and Van Huay (1976) and according to Langmuir (Syers et al., 1973). Phosphate sorption parameters and oxalate extractable Fe and Al (Feox, Alox) were related to the phosphate uptake by young rye plants in Neubauer pot experiments. P sorption parameter after Pagel and Van Huay (A) correlated significantly positively with the Feox and Alox content in acid (r = 0.73) as well as in calcareous soils (r = 0.89) if the whole equilibrium concentration range (0 - 14 mg P/L) was considered. The relations calculated after Langmuir (B) were similar. P uptake by rye in acid soils was negatively correlated with the affinity constant n (r = ?0.76, (A)). In calcareous soils, a negative correlation between P uptake and affinity constant was calculated in the lower P equilibrium range (0 - 2.8 mg P/L) only for (B). Thus, P uptake decreased with increasing strength of P bonding to soil. From these results it is concluded that phosphate sorbed to Fe/Al oxides is an important P source for plants in acid and calcareous soils.  相似文献   

6.
High gradient magnetic separation was used to fractionate the clay from some tropical soils. Acid-oxalate-extractable iron (Feox) and aluminium (Alox) and total carbon were measured in the whole clay, the magnetic fraction and the tailings. The magnetic separation resulted in a wider range of concentrations of these elements than in the whole clays. In each of the clays Feox was greater in the magnetic fraction than in the tailings; Alox was more variable. Carbon was also concentrated in the magnetic fraction suggesting that it is associated more with Feox than Alox. The relationships between Feox, Alox and carbon depend on soil classification and soil age.  相似文献   

7.
The sorption and desorption of phosphorus (P) from eroding soil particles in land runoff are important processes contributing to agriculturally‐driven eutrophication. We investigated the P‐exchange properties and related chemical characteristics of contrasting European agricultural soils and sediment material eroded from them under indoor (small‐scale) and outdoor (larger‐scale) rainfall simulations. Quantity‐intensity (Q/I) relationships revealed large variation in equilibrium P concentrations at zero net P sorption (EPC0) (0–10.3 mg l−1) and instantly labile P (−Q0, the amount of P to be desorbed to obtain a P equilibrium concentration of 0 mg l−1) (2–75 mg kg−1), both correlating closely with Al‐bound P and the P saturation degree of Al oxides (DPSAlox). Maximum P sorption (Qmax) (43–515 mg kg−1) also correlated most closely with Alox. The indoor and outdoor rainfall simulations produced sediments with different P sorption properties: in the indoor simulation (less kinetic energy, constant slope), the sediments had larger EPC0 values, and usually larger −Q0 values, than the sediments in the outdoor simulation (greater kinetic energy, variable slopes). Furthermore, the P exchange properties of the sediments differed from those of the bulk soil depending on the enrichment of soil P‐sorption components (Fe/Al oxides, clay). The outdoor simulation indicated that sites with gentle slopes produced sediments that were more enriched with Alox, Feox, Mnox and organic C than those with steeper slopes. In this study, when the bulk soil had an initial EPC0 greater than 1.3 mg l−1, the outdoor rainfall simulation produced sediment with smaller EPC0 and vice versa, indicating that, depending on the P status of the bulk soil, the sediment material was acting as source or sink for P during transport. However, on the basis of their EPC0 values, most eroding sediments might be expected to desorb, rather than adsorb, P when entering surface water.  相似文献   

8.
Inositol phosphates are abundant organic phosphates found widely in the environment. The sorption and desorption of organic phosphate (Po) are important processes in controlling the mobility, bioavailability and fate of phosphorus (P) in soil and sediment. The desorption characteristics of myo‐inositol hexakisphosphate (IHP) and inorganic phosphate (Pi) from goethite were studied by pre‐sorption of IHP or Pi followed by desorption by KCl, H2O, and citrate. Batch experiments and in situ attenuated total reflectance Fourier transform infrared (ATR‐FTIR) spectroscopy were used to investigate the desorption of IHP/Pi. The desorption percentage of IHP/Pi by citrate was much higher than that by H2O/KCl. The desorption of P by citrate was mainly achieved through ligand exchange, and the desorption increased with decreasing pH. Desorption by H2O was slightly greater than that by 0.02 M KCl because the electrostatic repulsion between the P molecules is larger in H2O. Due to the higher affinity of IHP for goethite than that of Pi, the maximum desorption of IHP was lower than that of Pi. Desorption curves (desorption concentration in solution vs. sorption density) of IHP or Pi on goethite by KCl or H2O was well fitted by an exponential equation, while those by citrate were well fitted by a linear equation. The desorption amounts of P in the first cycle account for more than 58% of the total desorption followed by substantial decreases in the second and third cycles. There was a re‐sorption of Pi from solution in the late stage of desorption by KCl and H2O, resulting in a sharp decrease in desorption. Re‐sorption of IHP did not occur, which is probably due to its poor diffusion into goethite. The initial desorption rate of Pi with KCl and H2O decreased with increasing pre‐sorption time, whereas that of IHP was opposite. This study indicates that strong sorption on and weak desorption of IHP from iron (hydr)oxides may explain the accumulation of IHP in soils.  相似文献   

9.
Sorption and desorption characteristics of four organic phosphates (OPs) with different molecular sizes and structures (glycerophosphate, GP; glucose‐6‐phosphate, G6P; adenosine triphosphate, ATP; myo‐inositol hexakisphosphate, IHP) and inorganic phosphate (Pi) on three aluminium (Al) (oxyhydr)oxides (amorphous Al(OH)3, boehmite and α‐Al2O3) were investigated. The maximum sorption amounts of OPs and Pi increased with decreasing crystallinity of the minerals on a per mass basis: α‐Al2O3 < boehmite < amorphous Al(OH)3. With an exception of IHP sorption on amorphous Al(OH)3, the maximum surface area‐based sorption densities increased with decreasing molecular weight (MW) of OPs and Pi: IHP < ATP < G6P < GP < Pi. Despite having the largest MW, IHP had greater sorption amounts on amorphous Al(OH)3 than the other OPs because of the transformation of surface complexes to surface precipitates. Sorption kinetics of OPs was first a rapid sorption followed by a long and slow sorption process. Of the three Al (oxyhydr)oxides, amorphous Al(OH)3 had the greatest first rapid sorption density and initial sorption rate of OPs within 5 minutes, both factors decreasing with increasing MW of OPs. The initial desorption percentages of OPs by KCl generally increased with decreasing MW of OPs, whereas the maximum desorption percentages of OPs by citrate were four to five times those achieved with KCl. Overall, strong specific sorption of OPs occurs on the surface of Al (oxyhydr)oxides, and molecular structure and size of OPs, as well as crystallinity and crystal structure of the minerals, are the key factors affecting the interfacial reactions and environmental behaviour of OPs.  相似文献   

10.
Submerged macrophytes are commonly used for the environmental engineering of the controlling of shallow lake eutrophication, and are also an effective and valid alternative for the remediation of eutrophic water bodies, not only under experimental conditions but also under natural conditions. Therefore, the effects of submerged macrophytes on the improvement of shallow lake water quality have been intensively investigated. But the mechanism was not well understood, especially the mechanism of the effects of submerged macrophytes on the exchange of nutrients at sediment–water interface in shallow lakes. This study selected a familiar submerged macrophyte Hydrilla verticillata in China and evaluated the effects of H. verticillata on the phosphate retention and release at the lake sediment–water interface in a simulated condition. The effects of H. verticillata on the phosphate sorption isotherm, phosphorus (P) availability were investigated and the subsequent kinetics of P release was also measured by repeated extraction with CaCl2 solution. Exchangeable Ca and ammonium oxalate-extractable Fe (Feox) and Al (Alox) of the sediments were also determined. The results show that the contents of organic matter, cationic exchange capacity (CEC), Ca, Fe, Al, exchangeable Ca, Feox and Alox of the sediments with H. verticillata were higher than those of the control sediments, and the contents of total phosphorus (TP), Olsen-P and reactive dissolve phosphorus (RDP) were lower. The sediments with H. verticillata had stronger P sorption ability and weaker ability of P release. H. verticillata did not significantly affect the trends of the sorption isotherms and kinetics of the released P on the sediments. H. verticillata can significantly increase the ability of P sorption, decrease in the ability of P desorption on sediments was one of the mechanism that maintained lower P levels of the overlying water through affecting the contents of organic matter, CEC, Ca, Fe, Al, exchangeable Ca, Feox and Alox in sediments.  相似文献   

11.
Abstract

Phosphate (P) sorption characteristics of six natural Ghanaian Oxisols, selected because of their hydrological and topographical suitability for agriculture, were evaluated. Availability of P appears to be adequate for half of the soils as suggested by the Bray P1 test and determination of the standard P requirement (SPR), i.e., the amount of P sorbed at a concentration of 0.2 ppm P (6.46 μM). The SPR was found to be very closely related to Pmax (Langmuir P sorption capacity), which in turn, was significantly correlated with oxalate‐extractable aluminum (Al) (Alo) and iron (Fe) (Feo) and related (not significantly) to the difference between dithionite‐citrate‐bicarbonate‐extractable Fe (Fed) and oxalate‐extractable Fe. Accordingly, Pmax is fairly well predicted by the model of Borggaard: Pca]e=0.211#lbÀlo+0.115#lbFeo+ 0.05#lb(Fed‐Feo)+0.3, except for one soil strongly enriched in Fe oxides, mainly goethite. This goethite was found by X‐ray diffraction analysis to consist of crystals larger than normally found for pedogenic Fe oxides. The difference between Pmax and Pcalc for this soil could, therefore, be attributed to the occurrence of these large Fe oxide crystals, because P sorption will decrease with increasing crystal size (decreasing specific surface area).  相似文献   

12.
Fourteen soils from Colombia and Brazil provided a wide range of sorption characteristics. Curves of sorbed phosphate that was exchangeable to 32P were described by Freundlich's equation, and of non-exchangeable phosphate by Temkin's equation. Exchangeable phosphate was associated with aluminium in poorly-crystalline oxides and in organic complexes. Non-exchangeable phosphate was related to aluminium in organic complexes, and especially to the ratio of AI/C in them. In Nigerian soils similar mechanisms controlled sorption of phosphate but oxides and organic complexes of iron were important. The concentration of phosphate in solution when affinities of soil for exchangeable and non-exchangeable phosphate are equal, and the importance of organic matter, are discussed in relation to soil management and to responses of crops to fertilizer phosphate. The results indicate that sorption curves should not be split into sections.  相似文献   

13.
The phosphate adsorption capacity (Pmax) of samples from various horizons of five Danish podzolized soils were investigated before and after organic matter removal. Removal of organic matter had no direct influence on Pmax suggesting that organic matter did not compete with phosphate for adsorption sites. In the soils investigated aluminium and iron oxides were the main phosphate adsorbents. Thus, more than 96% of the variation in Pmax could be accounted for by poorly crystalline aluminium and iron oxides (extractable by oxalate) and by well-crystallized iron oxides (taken as the difference between dithionite-citrate-bicarbonate-extractable iron and oxalate-extractable iron). Organic matter affected phosphate adsorption indirectly by inhibiting aluminium oxide crystallization. The resulting poorly crystalline oxides had high Pmax. In contrast, the influence of organic matter on the crystallinity of the iron oxides, and therefore on their capacity to adsorb phosphate, seemed limited.  相似文献   

14.
The influence of iron oxides on phosphate adsorption by soil   总被引：3，自引：0，他引：3  
Soils from Denmark and Tanzania were extracted with ammonium acetate (controls), EDTA to dissolve amorphous iron oxides, and dithionite-EDTA (DE) to dissolve crystalline iron oxides. The phosphate adsorption capacities of the extracted soils were taken as the maximum quantity of phosphate adsorbed computed from the Langmuir equation. The decreases in the phosphate adsorption capacity following EDTA extraction and DE extraction were attributed to the removal of iron oxides. Close correlations (P<0.001) were found (i) between EDTA-extractable iron (amorphous iron oxides) and the decrease in phosphate adsorption capacity following EDTA extraction, and (ii) between the difference between DE-extractable iron and EDTA-extractable iron (crystalline iron oxides) and the further decrease in phosphate adsorption capacity following DE extraction. The phosphate adsorption capacity, estimated to be approximately 2.5 μmol P m?2, was in good agreement with the capacity of various synthetic iron oxides. The calculated phosphate adsorption capacity of soil iron oxides, obtained from the contents and specific surfaces of amorphous and crystalline iron oxides together with the phosphate adsorption capacity per m2 for synthetic iron oxides, compared favourably with the measured phosphate adsorption capacity.  相似文献   

15.
Gleying and enhancement of hydromorphism in wetland soils due to Fe(III) reduction entail a series of degradation processes. The resistance of wetlands to degradation can be calculated from the content of potentially reducible iron, Fe(III)pr, which is found from the van Bodegom equation taking into account the contents of oxalate-soluble iron Feox and dithionite-soluble iron Fedit in the soil. In addition, this makes it possible to distinguish relict and actual gleysols. The van Bodegom equation is applicable to soils from which the oxalate solution extracts only amorphous and poorly crystallized iron compounds, which are quickly reduced by Fe-reducing bacteria. These soils have a low proportion of Fe(II) (no more that 15% of the total iron), as well as an accumulative profile distribution of Feox. The van Bodegom equation is unsuitable for calculating the Fe(III)pr content in soils with a high proportion of Fe(II) and a nonaccumulative profile distribution of Feox.  相似文献   

16.
Original and published data on the contents of X-ray amorphous oxalate-soluble compounds of Al, Fe, and Si in mesomorphic eluvial soils of cold, moderately cold, and moderately warm continental humid and semihumid regions are generalized. The groups of soils developed from mafic igneous, metamorphic, and pyroclastic rocks are considered. It is shown that the content of oxalate-soluble oxides (OSOx) in the horizons of their maximal accumulation varies from less than 1% to 20–30%; the Alox/Feox ratio varies from 1 to 6.5. The leading factor dictating the amount and quality of the OSOx in the soils is the presence or absence of volcanic glass in the parent materials. The boundary between the soils with and without volcanic glass corresponds to the OSOx content of 5% and the Al2O3ox/Fe2O3ox ratio equal to 2. These criteria are more reliable than the Alox/Feox ratio used by foreign soil scientists to specify Andosols (Alox/Feox > 2). The contents of oxalate-soluble oxides of Al and Fe do not depend on the total contents of these oxides in the parent material and seem to be related to the presence of these elements in minerals with different resistance to weathering. Under the natural conditions described in this paper, the content of OSOx shows a very weak response to zonal (temperature-controlled) climatic changes and/or to changes in the degree of humidity and the continentality of the climate.  相似文献   

17.
Technogenically contaminated urban soils contain a substantial amount of magnetite Fe3O4, whereas another ferrimagnetic, i.e., maghemite αFe2O3, more often prevails in unpolluted soils. The content of magnetite may exceed the content of another iron oxide, hematite, in contaminated soils. In the town of Chusovoi, where emissions from a single enterprise, a metallurgical plant, predominate among pollutants, the upper soil horizons are contaminated with magnetite of one type. In the much larger city of Perm, the polluting sources are diverse, which results in a wide variation of magnetic susceptibility of technogenic magnetite. The difference in magnetite properties may depend on the composition and the content of heavy metals associated with this mineral. A considerable amount of oxalate-soluble magnetite in technogenically contaminated soils produces two important consequences. Schwertmann’s criterion Feox: Fedit as a gleying index turns out to be overestimated and, therefore, does not work in technogenically contaminated soils. The second consequence is that Tamm’s reagent is inapplicable to extracting heavy metals bound to amorphous iron compounds from contaminated soils. On the other hand, a high solubility (30–60%) of technogenic magnetite by oxalate favors the use of Tamm’s reagent for the complete extraction of iron (hydr)oxides and heavy metals bound to them.  相似文献   

18.
As a result of the important role played by phosphorus (P) in surface water eutrophication, the susceptibility of soils to release P requires evaluation. The degree of phosphorus saturation, assessed by oxalate extraction (DPSox), has been used as an indicator. However, most laboratories do not include DPSox in routine soil tests because of cost and time. This study evaluates the suitability of the ammonium acetate extraction in the presence of EDTA (AAEDTA), the standard soil test P (STP) in Wallonia (Southern Belgium), to predict DPSox; we also compared it with the Mehlich 3 extraction. Ninety‐three topsoil samples were collected in agricultural soils throughout Wallonia. Good correlations were found between the AAEDTA and the Mehlich 3 methods for P, Fe and Al (r = 0.85, 0.77 and 0.86, respectively). An exponential relationship was found between PAAEDTA and DPSox. Results of principal component analysis and regression demonstrated that STP can be used to predict DPSox (r = 0.93) after logarithmic transformation. Soil test Al was also a good indicator of the P sorption capacity (PSCox) of soils (r = 0.86). Including the clay fraction in regression equations only slightly improved the prediction of PSCox (r = 0.90), while other readily available data (such as pH or organic carbon) did not significantly improve either DPSox or PSCox predictions.  相似文献   

19.

Purpose

The aim of the research was to determine the effect of lithogenic and pedogenic processes on the formation of Luvisols from the area of Vistula glaciation on the base of profile distribution of iron oxides and total iron in relation to texture and physicochemical properties. The indices of weathering of the soil material in genetic horizons were calculated, and changes in the content and forms of iron oxides were evaluated.

Materials and methods

The predominant type of soil in the study area is Luvisols under agricultural use, formed from silt formations on loam. The analyses were made applying the following methods: grain size composition using the sieve method and hydrometer method, the interpretation of the results was performed according to the World Reference Base for Soil Resources classification, the pH of soils was measured with the potentiometric method, C-organic with the Walkley-Black dichromate method, the content of the following iron forms was determined (total iron (Fet) after the mineralization of soils in the mixture of HF and HClO4 acids), free iron oxides were extracted using dithionite-citrate-bicarbonate method, and amorphous iron oxides after the ammonium oxalate extraction (using the Philips 9100PU apparatus). The clay mineralogy was estimated by X-ray diffraction analysis.

Results and discussion

It was observed that total iron enrichment occurs in argic horizons accompanied by iron depletion in luvic horizons, while the profile distribution of iron is similar to the distribution of clay. The (Fed/Fet) ratio indicates a low degree of weathering; the highest values were observed in argic (Bt) horizons, which confirms the effect of the process of pedogenesis on the value of that index. In the soils investigated, crystalline iron oxides generally dominate over the amorphous forms. The mineralogical composition of clay fraction separated from the upper part of soils was different as compared to the underlying material.

Conclusions

The results of the study showed that iron contents (together with the other indicators) and its forms can be used to distinguish soil layers of different origin. The depth distribution of Fed, Feo and Fet within soil profiles indicates that the soil material may be of different lithogenic origin in the studied pedons.
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20.
The regime of observations revealed that the Eh dynamics in soddy-podzolic and alluvial soils in the Middle Cis-Urals region depends not only on the rate of iron (hydr)oxides reduction but also on the rate of opposite reactions in the gleyed horizons. Both processes depend on the temperature. The Eh value decreases on heating in automorphic soils, when the reduction of Fe(III)-(hydr)oxide particles accelerates. On the contrary, in gley soils, the Eh decreases on cooling, probably, because of the reactions opposing the reduction of Fe(III)-(hydr)oxide particles, including Fe(II) fixation on the surface of mineral particles. Fe(III)-(hydr)oxides are, for the most part, preserved in gleyed soils of the Cis-Urals; the content of (Fe2O3)dit reaches 3.3% with iron minerals being usually represented by goethite. The increase in moistening influences the soil parameters (i.e., the redoxpotential rH and the content of conventional red pigment Hemconv) in an intricate manner. Both direct and reverse branches on the curve of the Hemconv-rH dependence point to the equilibrium and nonequilibrium conditions in the soil. The reverse branch probably stands for the initial phase of gleying in strongly humified soils, where, despite extra electrons in the solution, the brown pigment in the form of Fe(III)-(hydr)oxides is preserved.  相似文献   

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