Adsorption of Dissolved Organic and Inorganic Phosphorus in Soils of a Weathering Chronosequence

lor, 1989; Wada, 1989). However, little is known about the adsorption characteristics of DOP. Only one study Leaching of dissolved organic matter (DOM) and associated nutrihas examined adsorption isotherms of DOP in natural ents can be a significant loss from developing ecosystems. We studied DOM for soils. Kaiser (2001) compared the adsorption how the adsorption of dissolved organic P (DOP) and PO4 changes characteristics of DOP to those of DOC and found that during the development of young andesitic soils and determined which soil characteristics are responsible for these changes. We sampled 77, the overall retention of DOP was smaller than that of 255, 616, and about 1200 yr-old andesitic soils at the 0to 10-, 30DOC. Qualls and Haines (1991) studied the DOC/DOP to 40-, and 140to 150-cm soil depths and performed adsorption ratios in soil solution in soil profiles of the Appalachian isotherm analyses that were described using a modified Langmuir Mountains. They found that the DOC/DOP ratio de(for DOP) or linear equation (for PO4). We also sampled soil solution creased with increasing soil depth, concluding that DOC at the 10to 20-, 40-, and 150-cm soil depths during the main snow is preferentially adsorbed from the soil solution. Howmelt period in 2001 and 2002 and analyzed it for DOP and PO4. The ever, no study has compared adsorption isotherms of ability of the soils to adsorb DOP and PO4 increased with soil age. natural DOP and PO4. Stepwise multiple regression analyses between the adsorption capacity Other authors have studied the differences in adsorpfor DOP, or the slope of the adsorption isotherm for PO4, and several tion characteristics of specific dissolved organic and insoil parameters showed that allophane concentrations control the adsorption of DOP and PO4 in these soils. Tests of preferential adsorporganic P compounds. Anderson et al. (1974) found tion of DOP vs. PO4 and DOC vs. DOP showed that the adsorption that inositol hexaphosphate, with multiple negatively strength increased in the following order: DOC DOP PO4. The charged PO4 ester groups, was adsorbed more strongly preferential adsorption of PO4 vs. DOP and DOP vs. DOC increased than inorganic orthophosphate in acid soils. Additionsignificantly with increasing soil development, whereas the soil depth ally, organic P depressed the adsorption of inorganic P, did not have a consistent and significant effect. Significant correlations whereas inorganic P did not depress the adsorption of between the null-point adsorption of DOP or PO4 and field soil organic P. The stronger adsorption of organic than inorsolution concentrations indicated that the results obtained in laboraganic P and the depression of inorganic P adsorption tory experiments were applicable to field conditions. Consequently, were more pronounced at pH 3 than at pH 6. Frossard the tendency of DOP and PO4 to leach from these andesitic soils et al. (1989), found that in forested Boralf soils, the decreases as soils and ecosystem develop. mobility of various P compounds increased in the following order: adenosine triphosphate orthophosphate anions choline phosphate. D organic matter plays an important role The contrasting results concerning the adsorption in soil development through the processes of illuvistrength of DOP compounds might be due to different ation and eluviation (Duchaufour, 1982). Movement of numbers of PO4–ester groups in the organic molecule. elements in DOM may conceivably be important in Little is known about the structure of natural DOP. depositing nutrient elements in pores that are inaccessiOnly one study has been published on 31P NMR of DOP, ble to bacteria or into adsorbed phases protected by in which the DOP was concentrated from seawater, complexation to metals. Elements in DOM may also be and 31P NMR gives limited information on whether the lost from the rooting zone through leaching (Hedin et functional groups are monoesters, diesters, or phosphoal., 1995; Seely et al., 1998; Qualls et al., 2002). Leaching nates (Kolowith et al., 2001). Consequently, studies on of DOP might play a role in the trends in the stocks of the adsorption characteristics of model compounds canP present in soil that have been observed during soil not be easily extended to natural DOM. development (Walker and Syers, 1976). However, until In an earlier study, we showed that adsorption of now, the emphasis on cycling and leaching of nutrients dissolved organic C (DOC) and N (DON) in an andesitic has largely been focused on inorganic nutrients. soil chronosequence in northern California increased Inorganic PO4 anions are known to be strongly adduring soil development, mainly due to increases in allosorbed by amorphous oxyhydroxides and aluminosiliphane concentrations (Lilienfein et al., 2003, 2004). In cates, which typically increase in concentration during this paper we focus on P and extend our earlier analysis soil development (Egawa, 1984; Schwertmann and Tayto DOP and PO4. Our objectives were to determine: (i) how the adsorption of DOP and PO4 of these young J. Lilienfein, R.G. Qualls, and S.M. Uselman, Dep. of Environmental andesitic soils changes with soil development, (ii) which and Resource Sciences, MS 370, Univ. of Nevada–Reno, Reno, NV soil characteristics control the adsorption of DOP and 89557; S.D. Bridgham, Ecology and Evolutionary Biology, Univ. of PO4 in these soils, (iii) if there are any differences in Oregon, Eugene, OR 97403. Received 29 May 2003. *Corresponding author ([email protected]). Abbreviations: DOC, dissolved organic C; DOM, dissolved organic Published in Soil Sci. Soc. Am. J. 68:620–628 (2004).  Soil Science Society of America matter; DON, dissolved organic N; DOP, dissolved organic P; SOC, soil organic C. 677 S. Segoe Rd., Madison, WI 53711 USA