Macro- and Micromorphology of Subsurface Carbon in Riparian Zone Soils

Soil organic matter contains fractions that range from very active to passive, relative to microbiological activity Soil organic matter (SOM) contains fractions that range from very (Schimel et al., 1985), suggesting that classification of active to passive, relative to microbial-driven ecosystem processes and functions. A classification system is needed that can test the hypothesis SOM forms may be helpful in understanding ecosystem that SOM can be separated by morphology into functionally meanprocesses. A number of systems have been devised to ingful fractions. The objectives of this study were to use macroand separate SOM into functionally equivalent categories micromorphological techniques to classify the various C forms present based on their chemical, physical, and biological charin saturated (or seasonally saturated) subsurface horizons of hydric acteristics. Sollins et al. (1984) divided SOM into two riparian soils, and to increase our understanding of their genesis. Nine broad categories: (i) mineral-free, partially decomposed soils formed in outwash or alluvium, located in Rhode Island riparian plant debris (light fraction), and (ii) SOM sorbed on minwetlands, were described and sampled up to depths of over 3 m. The majority of these soils had seasonally high water table levels at or eral surfaces or within organomineral aggregates (heavy above the soil surface. Thirty-four thin sections were constructed from fraction). The light fraction, defined originally by Greenundisturbed samples collected from subsurface horizons for microland and Ford (1964), is more labile and decomposes morphological investigation. Six C forms (roots, fragmental organic more quickly than does SOM in the heavy fraction (Solmatter [FOM], lenses, infillings, masses, and horizon C) and five rootlins et al., 1984). Organomineral fractions have been decomposition classes were identified. All C forms were more abunsubdivided based on the size of the aggregates: macrodant in the subsurface of alluvial soils than in the subsurface of outwash aggregates ( 250 m) and microaggregates ( 250 m) soils. Masses and roots were the most abundant C form identified. (Edwards and Bremner, 1967). Elliot (1986) and Jastrow Most masses have likely formed from dispersion of C associated with decomposed roots. Alluvial deposition has resulted in considerable et al. (1996) found that the SOM associated with macroC in subsurface riparian zone soils in the form of buried A and O hoaggregates is more labile than the SOM associated with rizons, lenses, and FOM. Illuvial-horizon C was found primarily in microaggregates. Other studies have used 53 m as the sandy horizons having outwash parent materials. Carbon dating sugsize break between the larger, more labile fraction (pargested that many of these C forms persist for thousands of years in ticulate organic matter, POM) and the smaller, relathe riparian subsurface. The variety of C forms that exist in riparian tively inactive mineral associated SOM (Cambardella zone subsoils suggests that understanding C morphology, and how and Elliot, 1992; Six et al., 1998, 1999). these forms are related, may prove useful for developing functionally different morphologic classes of soil C. Although the depth and breadth of C research is rich, there still exists a need for detailed morphologic studies focused on C within saturated or seasonally saturated subsurface horizons. Most micromorphological approaches S organic carbon supplies govern a number of ecological processes operating within riparian wetto classify SOM have focused on surface horizons (Heilands. One such process is denitrification, the conversion berg and Chandler, 1941; Jongerius and Schelling, 1960; of nitrate to nitrogen gases by primarily heterotrophic Barratt, 1964; Babel, 1975; Bullock et al., 1985). Albacteria (Knowles, 1982). High rates of denitrification though Bullock’s system of classifying SOM into three have been recorded in surface horizons of riparian soils main classes: plant residues, organic fine material, and (Pinay et al., 1993; Schipper et al., 1993). In many riparorganic pigment, can be applied to subsurface horizons, ian settings, however, only a small portion of the groundthe system was not designed for macroscale studies. Adwater moves through surface horizons (Hill et al., 2000; ditionally, in our focused studies on riparian zone soils Gold et al., 2001). The highest rates of denitrification in (Gold et al., 2001; Rosenblatt et al., 2001; Blazejewski, the subsurface riparian soils have been observed where 2002), we have recognized many more C forms than nitrate-laden groundwater interacts with supplies of oxithose identified in previous studies. Thus, a morphologidizable (i.e., active or labile) organic C (Robertson et al., cal approach to classify soil C is needed. This classifica1991; Hedin et al., 1998; Devito et al., 2000; Hill et al., tion system would provide the first step toward testing 2000). Thus, there is great interest in understanding the the hypothesis that SOM can be separated by morpholgenesis, processing, and lability of organic matter in the ogy into functionally meaningful fractions. In this study, subsurface of riparian wetlands. we (i) identified the forms of SOM present in the subsurface of riparian zone soils, (ii) classified the forms G.A. Blazejewski, M.H. Stolt, and A.J. Gold, Dep. of Natural Reof SOM based on their macroand micromorphology, sources Science, Univ. of Rhode Island, Kingston, RI 02881; P.M. and (iii) investigated the genesis of these forms. Groffman, Inst. of Ecosystem Studies, Box AB, Millbrook, NY 12545. Received 23 Apr. 2004. *Corresponding author ([email protected]). Abbreviations: AMS, accelerator–mass spectrometer; BP, before presPublished in Soil Sci. Soc. Am. J. 69:1320–1329 (2005). Wetland Soils ent; c/f, coarse/fine ratio; c/f100 m, coarse/fine ratio of the mineral component at a 100m limit; FOM, fragmental organic matter; PD, poorly doi:10.2136/sssaj2004.0145 © Soil Science Society of America drained; POM, particulate organic matter; SOM, soil organic matter; SWPD, somewhat poorly drained; VPD, very poorly drained. 677 S. Segoe Rd., Madison, WI 53711 USA 1320 Published online June 28, 2005