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DIVISION S-5—PEDOLOGY

Dissolved Organic Carbon Retention in Soils

Comparison of Solution and Soil Measurements

^a USDA-NRCS, 11 Industrial Park Drive, Walpole, NH 03608
^b Dep. of Natural Resources, University of New Hampshire, James Hall, Durham, NH 03824
^c Geology Department, University of Wisconsin-Parkside, Kenosha, WI 53141-2000

* Corresponding author (kmccracken{at}nh.usda.gov)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Interactions between dissolved organic C (DOC) and mineral soils were examined in a laboratory column study. Specific emphasis was given to comparisons of soil and solution measurements of organic C (OC) retention in four soil parent materials. Soil columns were leached with one of three concentrations of forest floor (FF) leachate or distilled water every third day for 1 yr. The concentration of DOC in solution was analyzed before and after percolation through soil columns to assess net release or retention of DOC. Changes in soil chemical properties (pH, loss-on-ignition [LOI], extractable Al and Fe) following leaching were consistent with podzolization. Micromorphological investigation of the soil materials following leaching showed increased accumulation of organo-metallic coatings in materials treated with high concentrations of DOC relative to distilled water or low concentrations of DOC. The most dramatic changes in soil chemical properties were found in soils leached with high concentrations of DOC. Comparison of solution and soil measures of OC retention indicates that only 50% of the DOC lost from solution was measured as soil OC at the end of the experiment. This finding suggests that microbial decomposition is a significant factor regulating OC concentrations in soils and that soil solution data alone do not yield a complete picture of soil OC dynamics.

Abbreviations: DOC, dissolved organic C • FF, forest floor • FF-High, full strength • FF-Low, 1:9 forest floor leachate/distilled water • FF-medium, 1:1 forest floor leachate/distilled water • LOI, loss on ignition • OC, organic C • subscript p, sodium pyrophosphate extractable

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
ON GLOBAL AND REGIONAL SCALES, examination of the retention and turnover of DOC is useful in characterizing and quantifying the C storage capacity of forest soils. Dissolved OC in soil solution may be exported to groundwater or surface waters, utilized by microbes, or retained in the soil by abiotic mechanisms. It is generally accepted that DOC concentrations in soil solution decline with depth in mineral soils as a result of DOC retention by soil surfaces (McDowell and Likens, 1988; Moore et al., 1992; Vance and David, 1992). Qualls and Haines (1992) concluded that abiotic retention of DOC via adsorption to soil surfaces was primarily responsible for reduction in DOC concentrations because biological decomposition was too slow to account for the large reduction in DOC concentrations observed in field studies. They noted, however, that decomposers may facilitate adsorption processes by removing organic compounds held on the soil exchange complex, thereby opening more sites for additional adsorption (Qualls and Haines, 1992). Their work corroborated early work by McDowell and Wood (1984), who also concluded that abiotic adsorption was largely responsible for DOC retention. Most studies of DOC retention have focused on changes in the concentration and composition of DOC in soil solution. Examination of the changes in soil chemical properties and accumulation of grain coatings, coupled with solution chemistry would provide additional evidence to determine the relative importance of microbial utilization and abiotic sorption of DOC. Additionally, such research would improve understanding of the mechanisms of DOC retention and C sequestration in forest soils.

The objective of this study was to examine the relationship of DOC concentration to retention patterns of DOC in soils as evidenced by solution chemistry, soil chemistry, and micromorphology. Few studies have combined these approaches to examine the dynamics of DOC and its role in C retention and turnover in soils. Additionally, this investigation attempted to minimize the differences between laboratory and field investigations by using natural heterogeneous DOC solutions and by not pretreating the soils with strong acids and bases or grinding the soils. Previous investigators have pointed out the need for closing the gap between laboratory and field approaches to weathering and C retention (Davis, 1982; Sollins et al., 1996). Reducing differences between laboratory and field investigations of DOC retention facilitates application of laboratory results to field settings.

	MATERIALS AND METHODS

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Soil Characteristics
Soil parent materials with different lithologic compositions were collected from four locations in New Hampshire (Table 1). The Hermon (sandy-skeletal, isotic, frigid Typic Haplorthods) parent material was collected from the C horizon of a soil pit. The Marlow (Coarse-loamy, isotic, frigid Oxyaquic Haplorthods) and Success (sandy-skeletal, isotic, frigid, ortstein Typic Haplorthods) parent materials were collected by digging horizontally into C horizons of road cuts. The Lombard (Coarse-loamy, mixed, active, frigid Typic Dystrudepts) parent material was collected from the 2Cr of a road cut. The 0.053- to 11.2-mm size fraction of all soils was used in this investigation (Table 2). All soils were nearly 100% minerals with little to no organic content. The term parent material will be used to refer to samples collected from the field, whereas untreated soils will be used to refer to the sieved materials used in the investigation. All four parent materials had a very low percentage of <0.053-mm particles; however, the Lombard parent material is derived from phyllite that ranges from easy to moderately difficult to crush by hand. The fragile nature of this material makes measurements of particle-size distribution and surface area of the Lombard samples only approximate measurements. The chemical composition of each untreated soil was determined using x-ray fluorescence spectroscopy (XRAL Activation Services Inc., Ann Arbor, MI). BET soil surface area was measured using a krypton adsorbate (Micromeritics Instrument Corp., Norcross, GA).

Experimental Procedure
One hundred ninety grams (mean soil depth = 32 mm) of air-dried soil was packed into Falcon Bottle Top Filter (Becton Dickinson, Franklin Lakes, NY) units with a 38-mm nylon filter in the bottom outlet. Soil columns were kept under a laboratory hood to prevent airborne contamination.

Effects of four solution treatments were evaluated—dis-tilled water and three concentrations of FF extract: FF-high, FF-medium, and FF-low. Triplicate samples of each soil were prepared for each treatment. Packed columns received 0.1 L of solution (equivalent to 34 mm in depth) every 3 d for 1 yr; cumulative solution addition per column was 11.8 L (equivalent to 4000 mm in depth). Dissolved OC was measured in solution samples collected before and after percolation through the soil column using a Shimadzu TOC 5000 Total Organic Carbon Analyzer (Shimadzu Scientific Instruments, Inc., Columbia, MD) (high-temperature, Pt-catalyzed combustion). Solution samples were frozen prior to DOC analysis. The percentage of water yield following the first solution addition was 84 + 5, 84 + 6, 84 + 6, and 78 + 8 for the Hermon, Marlow, Success, and Lombard soils, respectively. The percentage of water yield was similar or slightly higher for subsequent solution additions. The solution chemistry was analyzed five times during the course of the experiment: Days 69, 165, 228, 285, and 357, corresponding to cumulative volumes of 2.3, 5.5, 7.6, 9.5, and 12.0 L of solution passed through each column, respectively. One-milliliter subsamples of the outflow from each leaching event were bulked during the last four collection periods for determination of DOC. The subsamples for individual columns were bulk sampled. The outflow from the first collection period was not bulk sampled; a single outflow sample was collected after 23 solution additions.

Following the 1-yr leaching treatments, the percentage of OC (Perkin Elmer CHN analyzer), the percentage of LOI (600°C for 16 h), and sodium pyrophosphate extractable Al, Fe, and C were determined for samples from each soil-solution treatment, and from untreated soils. Sodium pyrophosphate extractable Fe and Al (Al_p and Fe_p) were determined following the methods of Ross and Wang (1993). The Al_p and Fe_p are typically equated with organically complexed forms of these constituents. Concentrations of Fe and Al in pyrophosphate extracts were measured using a Beckman Direct Current Plasma Emission Spectrometer (Beckman Coulter, Inc., Fullerton, CA). Carbon content of the pyrophosphate extracts was measured using a Shimadzu TOC 5000 Total Organic Carbon Analyzer to assess the percentage of C_p. Three thin-sections were made from one column of each soil-solution treatment, and from a packed column of untreated soil materials. Ten randomly selected locations were identified on each slide and 81 points on a grid were counted at each location (total points counted per column = 2430) to determine the abundance of organo-metallic coatings (Drees and Ransom 1994).

Differences between soil–solution treatments were determined using ANOVA with Bonferroni multiple comparison test. Soil physical and chemical properties were examined with linear regression. A paired two-sample t-test was used to determine if solution and soil measures of OC accumulation were significantly different. Relationships among soil chemical and physical properties and between soil solution chemistry were assessed using Pearson's correlation. For all statistical analyses, statistical significance was established at P < 0.05.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Dissolved Organic Carbon Retention from Solution
All soils retained DOC at all levels of FF leachate treatment. The Hermon, Marlow and Lombard soils retained less DOC under FF-low concentrations than for the FF-medium or FF-high treatments (Fig. 1) . The difference in net retention between the FF-medium and FF-high treatment was significant only for the Lombard soil (P < 0.001). The Lombard soil retained significantly more DOC (245 mmol C g^-1 of soil) with the FF-high treatment than the other three soils (<158 mmol C g^-1 of soil) leached with FF-high (P < 0.05). This difference is consistent with the relatively high surface area of the Lombard soil.

View larger version (37K): [in this window] [in a new window]   Fig. 1. Dissolved organic C retention for each soil during the 1-yr weathering period. The error bars are one standard deviation. For all samples, n = 3.

Micromorphology
In thin sections of the soil columns, organo-metallic coatings appeared as orange-brown to dark brown deposits on the edges of grains. There were visible differences among solution treatments. The distilled water-treated soils had noticeably cleaner grains than the soils treated with FF leachate (Fig. 2a,b,c) . The soils treated with FF-low leachate had occasional small pockets of light orange-brown coatings (Fig. 2b). The FF-medium and FF-high treatments had larger pockets of coatings as well as thin coatings surrounding entire grains. In the FF-high soils, the organo-metallic coatings were thicker and generally darker brown than the other treatments (Fig. 2c). The distilled water treatments had low net (treatment–untreated soil) abundance of grain coatings (<0.5%). The abundance of organo-metallic coatings on grains increased with increasing concentration of DOC in the leachate solution (Fig. 3) . There were no significance differences among soils with the same solution treatment. The amount of grain coatings in FF-high treatments was significantly greater than in the distilled water and FF-low treatments for each soil (P < 0.01). Only the Lombard soil FF-high treatment had a greater amount of coatings than the FF-medium treatment (P < 0.001). Of the soil chemical properties investigated, net abundance of coatings was most strongly related to the net percentage of LOI and C_p (r > 0.60; P < 0.01). Abundance of coatings was also positively correlated with the net percentage of pyrophosphate Fe_p, and Al_p (r > 0.45; P < 0.05).

View larger version (73K): [in this window] [in a new window]   Fig. 2. Examination of thin-sections for the columns allowed for comparison of accumulation of organo-metallic coatings among solution treatments. The photomicrographs above are of the Success soil treated with (a) distilled water, (b) low concentration of forest floor leachate, and (c) high concentration of forest floor leachate.

View larger version (38K): [in this window] [in a new window]   Fig. 3. The net abundance of coatings (treated – untreated soil) on grains following the one year leaching experiment. The error bars are one standard deviation. For all samples, n = 3.

View larger version (19K): [in this window] [in a new window]   Fig. 4. The relationship between solution (µmol DOC retained per gram of soil) and the percentage of abundance of coatings as a result of leaching with DOC solutions (r = 0.84).

View larger version (25K): [in this window] [in a new window]   Fig. 5. The relationships between solution (net DOC retention) and soil measures of net organic C retention (net change in the percentage of loss on ignition, pyrophosphate extractable C and CHN-C). Percentage of loss-on-ignition is assumed to be 50% organic C in this investigation. See text for further details. The 1:1 line is the relationship expected if all C retained on the soil is present at the end of the experiment.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

Mayer (1994a)(b) has extensively examined the relationship between particle surface area and OC content of soils and sediments. The monolayer equivalent, a single-layer of OC covering all mineral surfaces, was calculated to be equivalent to 0.86 mg OC m^-2 of soil surface area (Mayer, 1994b). Following 1 yr of leaching, the soils retained between 0.05 and 0.92 mg OC m^-2, based on soil solution data, which assumes minimal microbial decomposition of DOC. The FF-medium and FF-high treatments for the Hermon, Marlow, and Success soils are at or near the level of monolayer coverage following the 1 yr of solution treatment. The Lombard materials, with at least twice the surface area of the other soils had only 0.48 mg OC m^-2. The net accumulation of OC based on soil chemistry was significantly lower (mean 0.2 mg OC m^-2; P < 0.01) than the estimates based on solution chemistry.

Our results confirm that changes in soil chemical properties are the direct outcome of leaching by DOC-rich solutions. Leaching with DOC, especially high concentrations, resulted in a significant increase in the percentage of LOI, CHN-C, and C_p. Examination of the changes in soil solution chemistry indicates high net loss of DOC from solution during leaching. Results of this investigation suggest that abiotic retention of DOC may not have been the only factor involved in removing DOC from solution (Fig. 5). The amount of OC accumulated on soil surfaces, based upon the percentage of LOI (LOI = 50% OC, this investigation; Nelson and Sommers, 1982; Goldin, 1987; David, 1988; Lowther et al., 1990), the percentage of C_p and CHN-C, was well below (32–46%) the amount of OC retained based upon solution chemistry. However, it must be noted that because of the sampling scheme during the first few months of the column study, our final calculations of DOC loss from solution may be overestimated by an average of 5% (range = 4–6%) (see McCracken, 1998 for additional details). Even with the correction for the possible overestimation of DOC retention on soil, DOC retention based upon solution chemistry was much higher than the OC content measured via LOI, C_p, and CHN-C. From our data, it is not possible to determine whether microbes utilized DOC in solution or if decomposition of sorbed DOC followed abiotic retention on surfaces. This missing C may have been released as CO₂ or may have been leached as carbonic acid. In any case, there is a significant discrepancy between the amount of OC removed from solution and the amount of OC on soil surfaces, thus clearly indicating that examinations of soil OC and DOC dynamics based entirely on solution chemistry are not a complete and accurate measure of C retention or long-term C storage capacity in soils.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
This investigation focused on interactions between organic and mineral soil constituents during initial stages of experimental pedogenesis by examining changes in soil and solution chemistry in a laboratory setting. This dual approach allows for a more complete understanding of the processes involved in soil formation than would either approach alone. Previous shorter-term investigations of DOC retention based on solution chemistry (McDowell and Wood, 1984; Qualls and Haines, 1992) indicate that abiotic retention is the major mechanism responsible for the decline in DOC concentrations in soil solution. The net loss of DOC from solution during percolation through the column (Fig. 1) coupled with the increase in OC in the soils confirms the importance of abiotic sorption in controlling DOC concentrations, yet our soil chemistry data also suggests an important role for microbial utilization in longer-term experiments. Examination of the dynamics of DOC and soil OC indicates that abiotic retention of DOC plays an important role in accumulation of OC in soils, yet because of the high potential for microbial utilization of DOC and soil OC, both solution and soil chemistry must be investigated. By pairing solution and soil chemistry data during examinations of C turnover and C sequestration, it is likely that we will gain a better understanding of the rate of these processes.

	ACKNOWLEDGMENTS

 
We thank Laura Morton, Jeff Merriam, Diana McKenzie, and Mindy Wardell for assistance with field and laboratory work. This research was funded in part by the Agricultural Experiment Station of the University of New Hampshire. We also thank three anonymous reviewers for thoughtful comments that improved the manuscript.

Received for publication March 20, 2000.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

• David, M.B. 1988. Use of loss-on-ignition to assess soil organic carbon in forest soils. Commun. Soil Sci. Plant Anal. 19:1593–1599.
• Davis, J.A. 1982. Adsorption of natural organic matter at the oxide/water interface. Geochim. Cosmochim. Acta 46:2381–2393.
• Drees, L.R., and M.D. Ransom. 1994. Light microscope techniques in quantitative soil mineralogy. p. 137–176. In J.E. Amonette and L.W. Zelazny (ed.) Quantitative methods in soil mineralogy. SSSA Misc. Publ. SSSA, Madison, WI.
• Goldin, A. 1987. Reassessing the use of loss-on-ignition for estimating organic matter content in noncalcareous soils. Commun. Soil Sci. Plant Anal. 18:1111–1116.
• Hatch, N.L. 1963. Geology of the Dixville Quadrangle New Hampshire. Bull. No. 1 of the New Hampshire Division of Economic Development. State of New Hampshire Department of Resources and Economic Development, Concord, NH.
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• Mayer, L.M. 1994a. Relationships between mineral surfaces and organic carbon concentrations in soils and sediments. Chem. Geol. 114:347–363.
• Mayer, L.M. 1994b. Surface area control of organic carbon accumulation in continental shelf sediments. Geochim. Cosmochim. Acta 58:1271–1284.
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• Moore, T.R., W. DeSouza, and J.F Koprivnjak. 1992. Controls on the sorption of dissolved organic carbon by soils. Soil Sci. 154:120–129.
• Qualls, R.G., and B.L. Haines. 1992. Biodegradability of dissolved organic matter in forest throughfall, soil solution, and stream water. Soil Sci. Soc. Am. J. 56:578–586.[Abstract/Free Full Text]
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