Sorption of dissolved organic carbon in soils: effects of soil sample storage, soil-to-solution ratio, and temperature

Abstract

Experiments on the sorption of dissolved organic carbon (DOC) in soils were mainly conducted in batch approaches. Because varying setups were used in these studies, comparison of the results requires knowledge on the effects that different experimental conditions may have on the sorption of DOC. This investigation evaluated the DOC sorption of soils using differently pretreated soil samples (field-fresh (two sampling dates), air-dried, stored at 3°C and −18°C), at different soil-to-solution ratios (1:40, 1:20, 1:10 and 1:5 w/v) and different temperatures (5°C, 15°C, 25°C and 35°C). The sorption of DOC was analyzed using the initial mass (IM) approach, which regressed the initial amount of sorbate (normalized to soil mass) against the sorbed amount (normalized to soil mass). The DOC release — when a solution without DOC was added — strongly increased with temperature and soil-to-solution ratio. Among the different types of sample storage and preparation, air-drying resulted in the largest DOC release. The smallest release was from the field-fresh samples. Freezing and storage at 3°C resulted in intermediate DOC release with freezing having the greater effect. The release from air-dried samples exceeded that of field-fresh samples by a factor of four at maximum. In contrast, none of the experimental setups influenced the slope of the IM isotherms. Thus, it seems possible to compare directly the binding affinity of DOC to different soils as determined at varying experimental conditions.

Introduction

Release and retention of dissolved organic carbon (DOC) affects the filter function and nutrient status of soils. It controls DOC-induced mobilization and translocation of metals and organic pollutants Berggren et al., 1990, Totsche et al., 1997 and the leaching of organic nutrient species (Donald et al., 1993). Binding of DOC also changes the properties of the sorbing soil material itself. Organic coatings on mineral particles improve their sorption capacity for metal cations and organic xenobiotics (Murphy and Zachara, 1995), reduce the further binding of organic matter (Kaiser and Zech, 1998) and anions (Sibanda and Young, 1986), affect the water retention Chittleborough et al., 1992, Jozèfaciuk et al., 1996, and increase the stability against acidic dissolution (Smeck and Novak, 1994). Finally, DOC release and retention are the driving forces of podzolization Dawson et al., 1978, McDowell and Wood, 1984.

These large environmental and pedological impacts induced an intensive research on the release and retention of DOM in soils. Besides some field studies Cronan and Aiken, 1985, McDowell and Likens, 1988, Guggenberger and Zech, 1993, the majority of experiments was carried out under controlled laboratory conditions. The objectives of these studies varied. Some of them aimed at the release of organic material from the forest floor Christ and David, 1996, Gödde et al., 1996, others tried to identify the parameters the DOM retention in mineral soil relates to. Investigated factors were soil constituents Jardine et al., 1989, Moore et al., 1992, Kaiser et al., 1996, solution composition Jardine et al., 1989, David and Zech, 1990, and DOM composition (Kaiser et al., 1997). Two principal approaches were used for the sorption studies: (i) batch experiments (e.g., Jardine et al., 1989) and (ii) column experiments (e.g., Qualls and Haines, 1992). Depending on the setup, both approaches may give similar (Qualls and Haines, 1992) or different findings (Totsche et al., 1997). Even within the batch studies, there were various differences in the experimental conditions such as differing soil-to-solution ratio, temperature, contact time, agitation mode, and preparation and storage of soil samples, making it difficult to compare the results.

In this study, batch sorption experiments were conducted at varying soil-to-solution ratios, temperatures, and soil sample storage methods. The objective was to evaluate how different experimental conditions may affect the sorption of DOM.