Role of DOC in the environment

At the interface between water and soil, dissolved organic matter (DOM) is an important and dynamic fraction of soil organic matter (SOM). Taking into account DOM movement through soils is essential to understand the distribution and stabilisation of soil carbon as well as the activity of microorganisms within soil profiles (McDowell and Wood 1984; Trumbore et al. 1992). DOM includes dissolved organic forms of carbon (DOC), nitrogen (DON) and phosphorus (DOP) (Michalzik et al. 2001). Many of the studies of DOM sources, transport and fate have focused on DOC. Recently, increasing attention has been paid to the dynamics of DOC, DON and DOP (Mattsson et al. 2009; Stutter et al. 2008). DOM‟s significance is largely due to its mobility, both within the soil and from the soil into groundwater or surface water bodies. At a soil scale, DOM serves as a substrate and carbon (C) source for microorganisms (Marschner and Kalbitz 2003; Maurice and Leff 2002; Neff and Asner 2001). DOM is also a significant source of carbon and energy in stream ecosystems (Ludwig et al. 1996a; Ludwig et al. 1996b; Maurice and Leff 2002; Tipping et al. 1997) and it filters solar UV and visible radiation (Dahlén et al. 1996; Zuo and Jones 1997). DOM also enhances the mobility of pollutants such as trace metals and hydrophobic organic compounds (Cabaniss and Shuman 1988; Piccolo 1994). The organic acids present in DOM can act as chelating agents, which enhance the mobilisation of toxic heavy metals (Kalbitz and Kaiser 2003). In addition, DOM forms a critical link in the biogeochemical cycling of carbon, nitrogen and phosphorus, which are the basis of all life and which regulate climate. DOM is a vector for the loss of C, N, and P from ecosystems. Over long time scales, small but consistent losses of DOM containing limiting or essential elements can reduce the capacity of ecosystems to support primary productivity (Hedin et al. 1995; Vitousek et al. 1998).

DOC fluxes are an important component of the biogeochemistry of terrestrial ecosystems (Figure I-1). DOC comprises about 20 to 25% of the 1 x 1015 g of carbon that is annually transferred by rivers from the continents to the oceans (Ludwig et al. 1998; Ludwig et al. 1996a; Ludwig et al. 1996b; Meybeck 1993). Jobbagy and Jackson (2000) estimate that carbon stored in the upper metre of mineral soils is around 1.5 x 1015 kg, which is about twice the amount in the atmosphere. DOC mainly comes from the degradation of organic matter in the soil (Bishop and Pettersson 1996; Hongve 1999; Ludwig et al. 1996a; Ludwig et al. 1996b) and, to a lesser degree, from the contribution of biological processes arising in the stream (Meybeck 1993). DOC fluxes in soils are several times larger than stream DOC fluxes, and in some cases soluble C transport from terrestrial environments represents a substantial component of the ecosystem C balance (Kling et al. 1991; Waddington and Roulet 1997). DOC transfer is influenced by climate (Moore 1989a; b; Tipping et al. 1997; Worrall et al. 2004), soil type (Dawson et al. 2001; Grieve and Marsden 2001), vegetation cover (Hongve 1999; Moore 1989a; b; Neal et al. 2005; Ross et al. 1999) and microbial activity (Aiken et al. 1985).

Definition

The easiest way to estimate DOM concentration is to measure DOC concentrations. The boundaries between dissolved, colloidal and particulate organic matter are not well defined. As described by Baldock and Nelson (2000) “the term „dissolved‟ refers to materials in solution that do not settle out under the influence of gravity. The definitions are usually made operationally. DOC is generally defined as organic carbon that has passed through a particular suction cup or filter or that occurs in the supernatant after centrifuging a soil suspension at a given relative centrifugal force for a given period.” Filtration pore sizes are usually 0.2-0.45 μm, but other pore sizes have been used, from 0.2 μm up to 1.2 μm, and even up to 25 μm (Bolan et al. 2004; Michalzik et al. 2001). Filtration separates the particulate organic fraction which is retained by filters and the dissolved fraction which passes through the filter (Figure I-2). Grossman and Udluft (1991) added that “these operational parameters should be clearly stated in discussions of DOC in soil, and it is important to note that processes such as adsorption onto or clogging of filters or suction cups may significantly influence the nature and amount of material obtained”. In soil studies the term, „water soluble organic matter‟ (WSOM) or „water extractable organic matter‟ (WEOM) is also used. These terms represent the fraction of the SOM extracted with water or dilute salt solution that has passed through a 0.45 μm filter (Zsolnay 2003).

Transport and fluxes

Only few data are available on fluxes of DOC in the upper layers of the forest floor (Oi = litter and Oe = fermented layer) and in the A horizon. In their review, Michalzik et al. (2001) also compiled DOC fluxes, which ranged from 40 to 160 kg DOC ha-1 y-1 in throughfall and 100 to 400 kg ha-1 y-1through the Oa horizon (Figure I-4). Neff and Asner (2001) reviewed further data, and quoted a range of 20 to 840 kg ha-1 y-1 for DOC fluxes through soils under a variety of vegetation cover, although mostly eucalyptus, coniferous and deciduous forest. Monitoring 21 sites during the period 2000-2006, Buckingham et al. (2008) also found DOC fluxes ranging from 22 to 222 kg ha-1 y-1 with a mean of 111 (± 57) for moorland sites, from 13 to 163 kg ha-1 y-1 with a mean of 85 (± 47) for forest sites and from 281 to 435 kg ha-1 y-1 with a mean and standard deviation of 350 (± 88) for peat bog sites at 10 and 15 cm depth. Fluxes decreased rapidly from the forest floor to the A horizon, where they were within the range of those in B and C horizons. DOC fluxes leaving the B horizons of these soils were often one or two orders of magnitude lower than in the O horizon and range between 10 to 200 kg ha-1 y-1 (Michalzik et al. 2001). Both recent litter and humified organic matter contribute significantly to the leaching of DOM from the O horizon (Fröberg et al. 2005; Sanderman et al. 2008). Fröberg et al. (2005) noticed a removal of DOC from the Oi and Oe horizons and a substantial production of DOC in the Oa horizon. DOC leaving the Oe horizon to a large extent had its origin within the Oe horizon itself (Fröberg et al. 2003). The Oi, Oe and Oa horizons contributed respectively approximately 20, 30 and 50% to the overall leaching of DOC from the O layer (Fröberg et al. 2003; Fröberg et al. 2005).

DOC in mineral soil is not simply the fraction of surficial leachates that have not been adsorbed or decomposed. Rather, exchange reactions with a portion of the more stabilised SOM pool exert the strongest control on both the concentration and composition of DOC found in these soils (Sanderman et al. 2008). DOC leaching plays a significant role in transporting carbon from surface horizons and stabilising it within the mineral soil. Processes of adsorption/desorption and dissolution/precipitation are governed by the nature and concentration of DOC as well as other solutes, pH and the nature of the solid phase material (Kalbitz et al. 2005). Sorption is greatest at low pH. Large or hydrophobic molecules are preferentially adsorbed compared to small or hydrophilic molecules (Kaiser et al. 1996). In soils with high specific surface area or high clay content, concentrations of DOC are kept low by adsorption of organic molecules onto mineral surfaces (Nelson et al. 1993). DOC concentrations are kept low because organic complexes of multivalent cations do not ionise readily and are of relatively low solubility where multivalent cations (Fe, Al, Ca) are abundant. In soils with low specific surface area or in soils with a high proportion of monovalent cations, a higher proportion of organic carbon tends to be dissolved (Nelson and Oades 1998).

Climate (temperature, precipitation and moisture)

In most temperate forest ecosystems, litter fall peaks in autumn and winter, resulting in an increased substrate supply at the coldest time of the year. Even in ecosystems where litter input is constant throughout the year, the available substrate is depleted faster over the warmer summer months and accumulates over the cooler winter months with less decomposer activity (Kirschbaum 2006). Concentrations of DOM in the forest floor are controlled mostly by leaching of freshly disrupted biomass debris in winter and spring and by the decomposition processes in summer and autumn (Kaiser et al. 2001a). The chemical composition of DOM differs markedly between seasons. Kaiser et al. (2001a) indicated that the chemical composition of DOM in forest floor seepage water in winter and spring shows greater mobility and degradability, and less interaction with metals and organic pollutants than that released during summer and autumn. In summer and autumn, the decomposition of DOM results in the production of strongly oxidised, water-soluble aromatic and aliphatic compounds whereas in winter and spring DOM is composed mostly of bacterial and fungal-derived carbohydrates and amino-sugars (Kaiser et al. 2001a). Moreover wetter incubation conditions increased the proportion of hydrophobic acids, whereas warmer incubation conditions increased the proportion of hydrophilic acids (Christ and David 1996b). Soil moisture is a key factor influencing the amount and composition of DOC in incubations. DOC concentrations and C mineralisation rates increase in soils subjected to wet-dry cycles during incubation (Chow et al. 2006; Kalbitz et al. 2000; Lundquist et al. 1999). Hypotheses have been proposed to explain this phenomenon. Christ and David (1996b) proposed that an increase in DOC production with moisture is the result of DOC accumulation in the forest floor, or to lysis of cells during extremely dry periods. For Tipping et al. (1999) warming and drying can accelerate the production of DOM which are qualitatively similar than DOM leached under current ambient conditions. For these authors, the quantities of DOM released after a wet-dry cycle could be positively correlated with the intensity of the dry conditions and the temperature. Thus, DOC content of soils should increase after a drying event.