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a Dep. of Environmental and Resource Sciences, MS 370, Univ. of Nevada–Reno, Reno, NV 89557
b Ecology and Evolutionary Biology, Univ. of Oregon, Eugene, OR 97403
* Corresponding author (qualls{at}unr.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: DOC, dissolved organic C • DOM, dissolved organic matter • DON, dissolved organic N • DOP, dissolved organic P • SOC, soil organic C
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Inorganic PO4 anions are known to be strongly adsorbed by amorphous oxyhydroxides and aluminosilicates, which typically increase in concentration during soil development (Egawa, 1984; Schwertmann and Taylor, 1989; Wada, 1989). However, little is known about the adsorption characteristics of DOP. Only one study has examined adsorption isotherms of DOP in natural DOM for soils. Kaiser (2001) compared the adsorption characteristics of DOP to those of DOC and found that the overall retention of DOP was smaller than that of DOC. Qualls and Haines (1991) studied the DOC/DOP ratios in soil solution in soil profiles of the Appalachian Mountains. They found that the DOC/DOP ratio decreased with increasing soil depth, concluding that DOC is preferentially adsorbed from the soil solution. However, no study has compared adsorption isotherms of natural DOP and PO4.
Other authors have studied the differences in adsorption characteristics of specific dissolved organic and inorganic P compounds. Anderson et al. (1974) found that inositol hexaphosphate, with multiple negatively charged PO4 ester groups, was adsorbed more strongly than inorganic orthophosphate in acid soils. Additionally, organic P depressed the adsorption of inorganic P, whereas inorganic P did not depress the adsorption of organic P. The stronger adsorption of organic than inorganic P and the depression of inorganic P adsorption were more pronounced at pH 3 than at pH 6. Frossard et al. (1989), found that in forested Boralf soils, the mobility of various P compounds increased in the following order: adenosine triphosphate < orthophosphate anions < choline phosphate.
The contrasting results concerning the adsorption strength of DOP compounds might be due to different numbers of PO4–ester groups in the organic molecule. Little is known about the structure of natural DOP. Only one study has been published on 31P NMR of DOP, in which the DOP was concentrated from seawater, and 31P NMR gives limited information on whether the functional groups are monoesters, diesters, or phosphonates (Kolowith et al., 2001). Consequently, studies on the adsorption characteristics of model compounds cannot be easily extended to natural DOM.
In an earlier study, we showed that adsorption of dissolved organic C (DOC) and N (DON) in an andesitic soil chronosequence in northern California increased during soil development, mainly due to increases in allophane concentrations (Lilienfein et al., 2003, 2004). In this paper we focus on P and extend our earlier analysis to DOP and PO4. Our objectives were to determine: (i) how the adsorption of DOP and PO4 of these young andesitic soils changes with soil development, (ii) which soil characteristics control the adsorption of DOP and PO4 in these soils, (iii) if there are any differences in the adsorption characteristics of DOP vs. PO4 or DOC vs. DOP, and (iv) whether adsorption isotherm parameters correlate with concentrations of DOP and PO4 in soil solution collected from the field.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The parent material of all flows consists of ground rocks of hornblende-andesite composition (Dickson and Crocker, 1953). In 2001, the soils of the four different mudflows were 77, 255, 616, and 1200+ yr old (Lilienfein et al., 2003). The age of the oldest flow is not exactly known and was arbitrarily designated as 1200+ yr (Dickson and Crocker, 1953). Soils of the 77- and 255-yr-old mudflows were classified as Vitrandic Haploxerepts, and those of the 616- and 1200+ yr-old mudflows were Humic Haploxerands (Peter Van Susteren, U.S. Forest Service, McCloud Ranger District, personal communication, 2003). For the young soils the subgroup name "Vitrandic" was based on the content of volcanic glass (Dickson and Crocker, 1954).
On each of the flows we established five to six plots, which were randomly selected along transects. Areas of obvious disturbance, for example by fire or bark beetles (Ips pin or Dendroctonus spp.), were rejected. More detailed information concerning the study sites is given in Dickson and Crocker (1953) and Lilienfein et al. (2003).
Equipment and Sampling
To compare field soil solution concentrations with the results of laboratory experiments, we installed three acid washed ceramic suction cup lysimeters (Soilmoisture Equipment Corp., Santa Barbara, CA) in each of the plots. The ceramic cups were model B02M2, made from a high fire silica body, with maximum pore size of 1.3 µm. Preliminary tests using forest floor solution indicated no significant adsorption of DOC or PO4. In each plot, one lysimeter was installed at the lower boundary of the A horizon (at 10-cm soil depth in the 77- and 255-yr-old soils, at 16-cm soil depth in the 616-yr-old soil, and at 20-cm soil depth in the 1200+ yr-old soil). A second lysimeter was installed at the lower boundary of the B horizon at the 40-cm soil depth, and the deepest suction cup lysimeter was installed at a 150-cm soil depth to represent the bottom of the rooting zone. Soil solution was sampled for five consecutive days per month during the time when the main water fluxes occurred in the soil due to snowmelt (February to May) in 2001 and 2002, for a total of eight soil solution events (Lilienfein et al., 2004). Additionally we took solid soil samples from 0- to 10-, 30- to 40-, and 140- to 150-cm soil depth. Because organic matter content tended to be more variable at the 0- to 10-cm soil depth (Lilienfein et al., 2004), three samples were taken per plot at the 0- to 10-cm depth and then composited for analyses.
Chemical Analyses
Soil samples for chemical analyses were dried at 40°C and passed through a 2-mm sieve for homogenization. Organic C content was determined on ground subsamples by dry combustion with a PerkinElmer 2400 CHN analyzer (Perkin Elmer, Norwalk, CT). Soil pH was measured in water with a soil/solution ratio of 1:1 and was reported as the arithmetic average (Baker et al., 1981).
Oxalate soluble Al (Alo), Fe (Feo), and Si (Sio) were determined with the method of Schwertmann (1964). Total pedogenic Fe oxides (Fed) were extracted with dithionite-citrate-bicarbonate (DCB, Holmgren, 1967), and organically bound Al (Alp) was extracted with pyrophosphate (McKeague, 1967). Metal concentrations in the extracts were determined using a PerkinElmer Plasma 1000 inductively coupled plasma emission spectrometer (Perkin Elmer, Norwalk, C.T.).
Concentrations of crystalline Fe oxides were calculated as follows:
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Allophane concentrations, in terms of g kg–1 soil, can be calculated by as follows (Dahlgren, 1994):
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The factor f depends on the Si/Al molar ratio and distinguishes Si-rich allophane (Al/Si, 1:1) and either Al-rich allophane or imogolite (Al/Si, 2:1). The factor f can be determined as follows:
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For an Al/Si ratio of 1:1, the factor is 5, and for a ratio of 2:1, the factor is 7. We found an Al/Si ratio of 2:1, which suggests that we were not overestimating allophane content due to dissolution of Si from other soil materials (Lilienfein et al., 2003). In this paper we refer to allophane concentrations calculated from the dissolution analysis, but it should be understood that it could also include imogolite.
Specific surface area was determined by the ethylene glycol monoethyl ether (EGME) method according to Carter et al. (1986). This method requires drying of the soil, which could conceivably introduce artifacts.
Soil solution was analyzed for DOC with a TOC analyzer (TOC 5050 A, Shimadzu Corp. Columbia, MD), and for orthophosphate with the molybdate blue method (Murphy and Riley, 1962) using a spectrophotometer (Shimadzu UV-1201, Shimadzu Corp.). Total P was measured after digestion with persulfate (Wetzel and Likens, 1991) as orthophosphate. Concentrations of DOP were calculated as the difference between total P and orthophosphate.
Sorption Experiments
Fresh field-moist soil taken from 0- to 10-, 30- to 40-, and 140- to 150-cm soil depths was used for the adsorption experiment. Forest floor materials from the youngest and the oldest mudflows, were used to make up stock DOM solution. Forest floor material was placed on netting and sprayed periodically with deionized water. The solutions that had percolated through from the forest floor material from both mudflows were mixed with a 1:1 ratio. The solution was filtered through 0.45-µm cellulose acetate membrane filters (GN-6, Pall Corp., Ann Arbor, MI) to remove particulate organic matter from the solution. The stock solution contained 292 µg DOP and 696 µg PO4. For the sorption experiment, initial solutions containing 0, 8, 18, 41, 292 µg DOP L–1 and 4, 19, 51, 103, 696 µg PO4 L–1, respectively, were prepared by diluting the stock DOM solution with a solution containing similar inorganic ion composition as the stock solution (pH 5.2, 0.45 mmol L–1 for K, 0.72 mmol L–1 for Ca, 0.07 mmol L–1 for Mg, 0.13 mmol L–1 for Na, 0.12 mmol L–1 for SO42–, and 1.86 mmol L–1 for Cl–, for a total ionic strength of 2.16 mmol L–1). The concentration range was chosen to represent the concentrations of the forest floor leachate in the field.
For the sorption experiments, 30 mL of each of the five initial solutions were added to 3 g of each of the three soil depths in 21 plots and shaken for 24 h in an end-over-end shaker with one rpm. The low revolution rate was chosen to avoid a breakdown of the fragile soil structure of wet allophanic soil (Parfitt, 1990). The suspensions were centrifuged and the supernatants were then filtered through 0.45-µm filters and analyzed for PO4 and total P as above.
Sorption Isotherms
Sorption of DOP and PO4 was analyzed by using sorption isotherms in which the equilibrium (i.e., concentration after 24 h) DOP or PO4 concentration in solution is plotted against the mass of DOP or PO4 adsorbed per gram of dry soil. Adsorption isotherms data of DOP were fit to a modified Langmuir curve rather than a straight line because the fit to the Langmuir curve was better than to a straight line. Because we were working with soil samples that contained native adsorbed organic matter, we added a parameter a in the Langmuir equation, which allows for a nonzero y-intercept (Lilienfein et al., 2004).
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The PO4 adsorption isotherms were fit to a straight line (using linear regression) because the fit was rather better than to the Langmuir curve.
We also determined the null-point concentration, which is defined as the DOP or PO4 equilibrium concentration at which there is no net adsorption or release of DOP or PO4, calculated as the x-intercept of either the Langmuir or the linear equation.
Preferential Adsorption of PO4 vs. DOP and DOC vs. DOP
To test whether the degree of adsorption of DOP is different from that of DOC and PO4, we compared the PO4/DOP and the DOC/DOP ratios in the solution before and after the batch adsorption experiment. A difference in the ratio before and after adsorption was used to indicate a preferential adsorption of one or the other form. For example, a higher DOC/DOP ratio in the equilibrium solution than in the initial solution would indicate that more DOP is removed from the solution and therefore that it is preferentially adsorbed over DOC.
Statistical Analyses
A multiple comparison of means of soil organic C (SOC), allophane, Feo, crystalline Fe concentrations, pH, and specific surface area in soils of different ages, within each soil depth, were done using Tukey's honest significant difference (HSD) test. A multiple comparison of means was also used to study the influence of soil age and depth on the PO4/DOP and the DOC/DOP ratio in the equilibrium solution for the highest level of addition of PO4 and DOP. All multiple comparisons of means were performed on log-transformed data (except for pH) to ensure equal variances. To compare the PO4/DOP and the DOC/DOP ratio of the DOM solution before and after the batch experiment, a two-tailed single sample t test was performed. Ratios were log transformed to normalize the data. Linear regression analyses were used (i) to fit lines to the PO4 adsorption isotherms, (ii) to test the influence of soil age on the adsorption parameters of DOP and PO4, and (iii) to test the relationship between the null-point concentration of DOP or PO4 of the soils and the DOP or PO4 soil solution concentrations. To determine the soil characteristics that explain the variation in the adsorption of DOP and PO4 in these soils, we performed simple and stepwise multiple regression analyses between parameters of the adsorption curves (as the dependent variables) and various soil characteristics as the independent variables. For all statistical analyses, significance was set to P 
0.05 and statistical analyses were performed with SPSS 11.5 (SPSS, Inc., 2000).
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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For both DOP and PO4, the results of the stepwise multiple regression analysis showed a highly significant correlation between the parameters of the adsorption isotherm and the allophane concentration in the soil (Fig. 4) . None of the other independent variables included in the stepwise multiple regression analysis significantly improved the coefficient of determination. Therefore, we conclude that the adsorption of DOP and PO4 in these soils is controlled predominantly by allophane in these young andesitic soils. This can be explained by the very high adsorption capacity of allophane for PO4 of 6.2 to 18.6 g P kg–1 allophane (Wada, 1989).
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Preferential Adsorption of PO4 or DOP
To determine whether PO4 or DOP was preferentially adsorbed from solution, we compared the PO4/DOP ratios of the equilibrium concentrations of the highest initial concentration for all three soil depths. All samples showed a net adsorption of PO4 and DOP from that solution (Fig. 1 and 3). The PO4/DOP ratio of the initial solution was 2.4 (Fig. 5)
. A lower PO4/DOP ratio in the equilibrium than in the initial solution would indicate that PO4 is preferentially adsorbed. For all except one (77-yr-old soil, 0–10 cm) of the soil samples, the PO4/DOP ratio in the equilibrium solution was significantly lower than in the initial solution (Fig. 5), indicating preferential adsorption of PO4. The initial solution contained more PO4 than DOP, whereas the equilibrium solution of all samples from the 255, 616, and 1200+-yr old soils contained more organic than inorganic P. A multiple comparison of means showed a significant influence of soil age on the PO4/DOP ratio in the equilibrium solution (Fig. 5). The ratio significantly decreased with increasing soil age. This means that the adsorption of inorganic P increases more strong with soil age than the adsorption of organic P. The soil depth, however, did not consistently have a significant influence on the PO4/DOP ratio in the equilibrium solution (Fig. 5).
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The preferential adsorption of PO4 over the organic forms may also be related to the reason we observed linear isotherms for PO4. With increasing concentrations of PO4, the high affinity of allophane surfaces for PO4 (Wada, 1989) may result in the displacement of other adsorbed species, while DOM molecules are unable to displace other adsorbed species and exhibit saturation of the adsorption isotherms.
Preferential Adsorption of DOC or DOP
We also tested if there is any preferential adsorption of DOC vs. DOP. Therefore, we performed the same statistical analyses to compare DOC/DOP ratios as we did for PO4 and DOP. A higher DOC/DOP ratio in the equilibrium solution than in the initial solution would indicate that DOP was preferentially adsorbed. All samples of the 77 and 255-yr old soils, except the 0- to 10-cm depth of the 255-yr old soil, showed no significant differences in the DOC/DOP ratio between the initial and equilibrium solutions (Fig. 6)
. However, all samples of the 616 and 1200+ yr-old soils showed a significantly higher DOC/DOP ratio in the equilibrium than in the initial solution, indicating a preferential adsorption of DOP vs. DOC. Thus, DOP was preferentially adsorbed in the older soils but not in the younger soils. A multiple comparison of means among soils of different ages, within each depth, showed that the ratio also significantly increased with increasing soil age (Fig. 6) indicating that the effect of preferential adsorption of DOP over DOC increases with the soil age. However there was no consistent trend in the DOC/DOP ratio with soil depth. The lack of a trend with the soil depth might mainly be explained by the fact that the allophane and ferrihydrite concentrations also show a much stronger trend with soil age than with soil depth.
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Correlation with Soil Solution Concentrations
We tested whether the results of the adsorption experiments were correlated with soil solution concentrations during snowmelt in the field. Average DOP and PO4 soil solution concentrations are presented in Table 2. The null-point concentration is the concentration for which the soil neither releases nor adsorbs PO4 or DOP, respectively. If the soil solution is in equilibrium with the soil solid phase, the null-point concentration should be correlated to the soil solution concentration of the corresponding soil depths. Therefore, we performed regression analyses between the null-point concentrations of PO4 or DOP at the 0- to 10-, 30- to 40-, and 140- to 150-cm soil depth and the measured PO4 or DOP concentrations in the soil solution at 10-, 16- or 20-, 40-, and 150-cm soil depth, respectively.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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A comparison of the ratios of PO4/DOP and DOC/DOP in the initial and equilibrium solution indicated that the adsorption strength increased in the following order DOC < DOP < PO4, and these differences in adsorption increased with soil age.
Because adsorption of DOP, DOC, and PO4 increased with soil age the tendency to leach from the root zone is also likely to decrease with soil age. The finding supported this conclusion that concentrations of DOP, DOC, and PO4 in soil solution at 150 cm were higher in the younger soils (Table 2, and Lilienfein et al., 2004). In addition, the finding that PO4 was more strongly adsorbed than DOP and DOC also suggests that DOP and DOC are more susceptible to leaching. The factors controlling the leaching of organic and inorganic nutrients have been compared and classified as biological, geochemical, and hydrological (Qualls, 2000). Phosphate, and perhaps some small organic P compounds may be directly taken up by roots and microbial cells. Dissolved organic P may also be biologically mineralized. Geochemical factors include adsorption. Hydrological factors include the water flux through the soil and four factors that may prevent soil solution from coming into equilibrium with soil surfaces (Qualls, 2000). Solution may fail to reach equilibrium due to lack of time to equilibrate with soil surfaces. Diffusion through particles may limit the time to reach equilibrium. Preferential flow may short circuit exposure of the entire soil surface (e.g., Akhtar, 2003). On a larger scale, flow paths through a watershed may bypass the most strongly adsorbing horizons, for example, surface flow, lateral flow, and throughfall on stream channels. However, in our study the concentrations of DOP, PO4, and DOC, at the 150-cm depths in general corresponded to the relative degree of adsorption.
Like many volcanic soils, the soils of the chronosequence underwent rapid weathering (Lilienfein et al., 2003) and increased in adsorption capacity in a relatively short-time period. However, many soils that weather far more slowly, characteristically increase in the adsorption capacity for PO4 due to formation of pedogenic oxides, clays, and short-range order aluminosilicates (Jenny, 1980). In one dolomitic pedogenic sequence, increases in adsorption strength for PO4 occurred largely because of concentration of Fe and Al as Ca and Mg leached from the soil (Carreira et al., 1997). Such increases in adsorption strength during pedogenesis are not likely universal however, since extreme podzolization (e.g., Walker et al., 1981) or pedogenesis under hydric conditions may result in decreasing adsorption capacity. In addition, increases in clay content, lowering hydraulic conductivity, and development of blocky structure may increase the occurrence of preferential flow (Aktar et al., 2003) during pedogenesis. The relationships of changing adsorption capacity during soil and ecosystem development have been widely appreciated in the case of PO4 but are also very important in controlling leaching of DOP and DON.
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ACKNOWLEDGMENTS |
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Received for publication May 29, 2003.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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