Density Fractionation of Forest Soils: Methodological Questions and Interpretation of Incubation Results and Turnover Time in an Ecosystem Context
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Biogeochemistry (2007) 85:69–90 DOI 10.1007/s10533-007-9100-8 ORIGINAL PAPER Density fractionation of forest soils: methodological questions and interpretation of incubation results and turnover time in an ecosystem context Susan E. Crow Æ Christopher W. Swanston Æ Kate Lajtha Æ J. Rene´e Brooks Æ Heath Keirstead Received: 11 March 2006 / Accepted: 27 November 2006 / Published online: 15 March 2007 Ó Springer Science+Business Media B.V. 2007 Abstract Soil organic matter (SOM) is often common density-based methods for dividing soil separated by physical means to simplify a com- into distinct organic matter fractions. Further, we plex matrix into discrete fractions. A frequent directly address the potential effects of dispersing approach to isolating two or more fractions is soil in a high density salt solution on the recov- based on differing particle densities and uses a ered fractions and implications for data inter- high density liquid such as sodium polytungstate pretation. Soil collected from forested sites at (SPT). Soil density fractions are often interpreted H. J. Andrews Experimental Forest, Oregon and as organic matter pools with different carbon (C) Bousson Experimental Forest, Pennsylvania was turnover times, ranging from years to decades or separated into light and heavy fractions by floa- centuries, and with different functional roles for C tation in a 1.6 g cm–3 solution of SPT. Mass and nutrient dynamics. In this paper, we discuss balance calculations revealed that between 17% the development and mechanistic basis of and 26% of the original bulk soil C and N content was mobilized and subsequently discarded during S. E. Crow Á K. Lajtha density fractionation for both soils. In some cases, Botany and Plant Pathology Department, Oregon the light isotope was preferentially mobilized State University, Corvallis, OR, USA during density fractionation. During a year-long incubation, mathematically recombined density C. W. Swanston Center for Accelerator Mass Spectrometry, Lawrence fractions respired ~40% less than the bulk soil at Livermore National Laboratory, Livermore, CA, both sites and light fraction (LF) did not always USA decompose more than the heavy fraction (HF). Residual amounts of tungsten (W) present even J. R. Brooks Western Ecology Division, U.S. Environmental in well-rinsed fractions were enough to reduce Protection Agency, Corvallis, OR, USA microbial respiration by 27% compared to the control in a 90-day incubation of Oa material. H. Keirstead However, residual W was nearly eliminated by Crop and Soil Science Department, Oregon State University, Corvallis, OR, USA repeated leaching over the year-long incubation, and is not likely the primary cause of the Present Address: difference in respiration between summed frac- & S. E. Crow ( ) tions and bulk soil. Light fraction at Bousson, a Department of Earth & Atmospheric Sciences, Purdue University, West Lafayette, IN, USA deciduous site developed on Alfisols, had a e-mail: [email protected] radiocarbon-based mean residence time (MRT) 123 70 Biogeochemistry (2007) 85:69–90 of 2.7 or 89 years, depending on the interpreta- summarize trends in the density fractionation tion of the radiocarbon model, while HF was literature, emphasizing the expansion of work 317 years. In contrast, both density fractions from since the introduction of sodium polytungstate H. J. Andrews, a coniferous site developed on (SPT) as a high density liquid, and consider the andic soils, had approximately the same MRT link between conceptualized organic matter frac- (117 years and 93 years for LF and HF). At H. J. tions produced during density fractionation and Andrews the organic matter lost during density the actual physical, chemical, and biological separation had a short MRT (19 years) and can properties of the isolated fractions. account for the difference in respired CO2 In addition to providing a brief summary of between the summed fractions and the bulk soil. density fractionation as a method, we seek to Recognition and consideration of the effects of illustrate the interaction of the method with soils the density separation procedure on the recov- from two different ecosystems and then look ered fractions will help prevent misinterpretation more closely at potential gaps in interpretation. In and deepen our understanding of the specific role particular, we address the effect of density frac- of the recovered organic matter fractions in the tionation on C and N loss during separation, and ecological context of the soil studied. discuss the implications for data interpretation with regard to isotopic and C and N cycling and Keywords Density fractionation Á Heavy mass balancing. fraction Á Incubation Á Light fraction Á Mean residence time Á Radiocarbon Á Sodium polytungstate Á Soil organic matter Development of density fractionation methodology Introduction Density fractionation emerged initially as a means to separate both primary (Pearson and Truog Density fractionation of soil, which divides mate- 1937) and clay (Halma 1969; Francis et al. 1972) rial by floatation or sedimentation in a solution minerals. During the separation of soil minerals, it according to particle density, has been used for was necessary to disrupt soil aggregates and nearly 50 years to physically separate soil organic remove organic material. In the following dec- matter (SOM) into discrete fractions thought to ades, density fractionation techniques were devel- have differing stability. Christensen (1992) pro- oped with the specific goal to divide SOM into two vided a detailed review of soil physical fraction- discrete fractions (Monnier et al. 1962; Greenland ation techniques, which included both size and Ford 1964) that represented different stages separation and density fractionation. However, of degradation, i.e. recent, partially decomposed in the years since, there has been a substantial organic matter versus ‘‘thoroughly degraded’’ increase in the use of density fractionation meth- organic matter already associated with mineral ods across a variety of ecological research fields surfaces (Ford and Greenland 1968, Richter et al. and applications. As with all methods that 1975). A number of early studies were conducted attempt to operationally define organic matter with soil separated at ~2.0 g cm–3, since most pools, there have been shortcomings and con- primary and secondary soil minerals are denser cerns, particularly with respect to the relationship than 2.0 g cm–3. The use of a separation density of between the conceptual organic matter pools and 2.0 g cm–3 continued for some applications (Dalal the actual characteristics of the resulting soil and Mayer 1986; Trumbore and Zheng 1996); fractions. However, the development of density however, a lower density 1.6–1.8 g cm–3 eventu- fractionation as a means for physically separating ally became more common as a way to exclude the soil has implications for identifying mechanisms most mineral and organo-mineral material from of organo-mineral interactions and quantifying the LF while maximizing recovery of plant-like pools of organic matter with different ecological particulate organic matter (Ladd et al. 1977; roles that are relevant to SOM research. Here we Scheffer 1977; Young and Spycher 1979). 123 Biogeochemistry (2007) 85:69–90 71 Many heavy liquid solutions have been used intra-aggregate (‘‘occluded’’) LF separated from for soil fractionation, i.e. bromoform ethanol remaining sediment by sonication and floatation, (Monnier et al. 1962), bromoform-petroleum and a residual organo-mineral fraction (Golchin spirit mixture (Greenland and Ford 1964), and et al.1994b; see also Swanston et al. 2005). A NaI (Spycher and Young 1979; Sollins et al. 1983, more complex method follows the steps of the Boone 1994); see Christensen (1992) for a review first, but subsequently separates the residual of methods. In the early 1980s, sodium polytung- organo-mineral fraction by sequential fraction- state, H2Na6O40W12, (SPT, SOMETU, Berlin) ation into multiple fractions by increasing density was introduced as a safe, alternative high density in increments (Golchin et al. 1994a). Sequential solution (Plewinksy and Kamp 1984). Previously, fractionation of the same soil at increasing den- heavy liquid solutions were mostly halogenated- sities divides soil primarily based on mineralogy hydrocarbons that were highly toxic to humans (Basile-Doelsch et al. in press); however, Arnar- (Torresan 1987) while SPT is a non-corrosive, son and Keil (2001) and Sollins et al. (2006) also non-flammable, safe alternative (Skipp and used sequential fractionation specifically to con- Brownfield 1993). Sodium polytungstate is highly sider how organic coatings on different minerals viscous at densities >2.7 g cm–3, which can reduce could alter the particle density of the organo- the ease of use; however, with care SPT can be mineral complexes (i.e., sequential density made to densities as high as 2.9 g cm–3 and used fractions), and how this might relate to C successfully. Another alternative heavy liquid stabilization. was developed: LudoxTM, an aqueous colloidal Many methods combine physical fractionation dispersion of silica particles in which the soil is methods, i.e. size and density, in an attempt to suspended (Hassink 1995a, b; Meijboom et al. target multiple, spatially explicit SOM pools that 1995; Magid et al. 1996; van den Pol-van relate to stable aggregates. Based in part on Dasselaar 1999; Accoe et al. 2004). Ludox is highly earlier work by Cambardella and Elliot (1994), viscous, which prevents porous particles from Golchin et al. (1994a), and Jastrow (1996), Six becoming saturated and results in floatation. The et al. (2000) developed a complex method in maximum density of Ludox is 1.4 g cm–3 (around agricultural soils using a combination of size and the lower end of most organic debris) and, since density fractions to isolate organic matter within saturation of the pore space does not occur, the soil aggregates that have distinct functional roles separation density is more representative of the in nutrient cycling and dominant stabilization soil bulk density than of particle density. Thus, it mechanisms.