Environ. Sci. Technol. 1980, 23, 1021-1024
NOTES Generation of Soil Solution Acid-Neutralizing Capacity by Addition of Dissolved Inorganic Carbon Mark B. David" and George F. Vance
Department of Forestry, Unlversity of Illinois, 110 Mumford Hail, 1301 W. Gregory Drive, Urbana, Illinois 61801 H A Spodosol B horizon (base saturation of 5.4%) collected
at the Watershed Manipulation Project site at Lead Mountain, ME, was used to examine soil solution chemistry in response to increasing solution levels of dissolved inorganic carbon (DIC). Acid-neutralizing capacity (ANC), determined by Gran titration, increased from -5 to 163 pequiv L-' in response to increasing DIC, with a corresponding increase in base cations (Ca2+,Mg2+,K+, and Na+). For the negative ANC solutions, degassing increased solution pH (in equilibrium with atmospheric COP)slightly from 4.94 to 5.14, whereas solutions with positive ANC showed large pH shifta (e.g., ANC of 69, pH shift from 4.73 to 6.81). Under equilibrium assumptions and log KA1determined from 2.66pH - pAl, measured values for ANC, sum of cations, pH, and degassed pH were found to be in agreement with predictions from a chemical equilibrium model. Results illustrate the importance of pC02 levels and cation exchange from the solid phase in generating solution ANC and determining surface water pH. Environmental implications and limitations in the use of chemical equilibrium models are discussed.
Introduction Many soil equilibrium models use relationships between soil C02 partial pressures (pC02), dissolved inorganic carbon, cation exchange, Al solubility reactions, and anion concentrations to predict solution acid-neutralizing capacity (ANC) (1-4). These models are used to both understand and predict soil and surface water chemistry with respect to inputs of strong acid anions from acidic deposition (5). Reuss and Johnson (1,2)indicated the importance of sulfate concentrations and soil pC0, in determining solution ANC by using a simplified equilibrium model. The significance of positive versus negative ANC in determining the pH of degassed surface waters was clearly illustrated. For example, model predictions for pH of soil solutions with negative ANC levels would remain relatively unchanged when degassed, but the pH of solutions with positive ANC increase upon degassing. This has important implications with respect to surface water chemistry. Reuss and Johnson (1, 2 ) based their discussion on theoretical considerations alone. David et al. (6) applied the model to soil chemistry data collected from watersheds in Maine and New York. This work again supported the importance of pC0, and other factors in determining ANC and its influence on surface water chemistry after solution equilibrium with atmospheric COP Negative ANC levels were produced in all E horizons (-69 to -37 pequiv L-l) at each pCOz level used (0.3-2%), whereas ANC in B horizons was found to be dependent on pC02 levels (6). In B horizons at low pCOz levels, ANC was negative. 0013-936X/89/0923-1021$01.50/0
However, as pC0, levels increased, positive ANC values were generated, thus indicating the importance of pC0, in controlling ANC. Unfortunately, there are few measurements of pC02 available for forest soils to make accurate predictions using the above mentioned models. Fernandez and Kosian (7) have reported values in the range of 0.348% pC0, in soil horizons of a Spodosol in Maine. Besides the lack of data on pC0, levels, no actual tests (laboratory or field) have examined ANC response to changes in pCOP In this study, we examined the response of the soil solution of a Spodosol B horizon to various levels of dissolved inorganic carbon by varying the pC02. Our purpose was to compare predicted ANC levels from the Reuss and Johnson ( I , 2 ) model to those experimentally measured, along with changes in pH and cations.
Experimental Section A composite Spodosol B horizon collected at Lead Mountain, ME, was used for this study. The soil was a Typic Haplorthod, with a 0.01 M CaC1, pH of 4.3, and an organic C level of 3.7%. After collection, the soil was passed through a 2-mm sieve and maintained at 3 OC in field moist condition. To apply the Reuss and Johnson (1)model, soil chemistry data were needed to assess cation-exchange and A1 solubility relationships. Exchangeable cations and cation-exchange capacity (CEC) were measured on field moist samples with unbuffered 1 N NH4C1 (8). Aluminum potential (2.66pH - pAl, designated log KA1)was estimated by measuring the total A1 concentration and the pH of a 0.002 M CaC1, soil extract (by using a mechanical vacuum extractor) after passage through a GF/F glass fiber filter (8). A Gaines-Thomas selectivity coefficient for Ca2+/A13+ (log K,) exchange was calculated from Ca2+in the 0.002 M CaC1, extract, neutral salt CEC, and the NH,Cl extractable bases (6). Total A1 in the 0.002 M CaC1, extract and in leachates was determined by using a pyrocatechol violet (PCV) procedure modified from Dougan and Wilson (9). Filtered aliquots of extracts and leachates were acidified (HN03) and analyzed for A1 after 24 h by using a 4-min reaction time with PCV. To examine the effects of various levels of DIC on soil solution ANC, 50 g of moist B horizon soil (equivalent to 35 g of oven dry mass) was added to a vacuum extractor syringe. Each syringe was then leached with 100 mL of a simulated throughfall solution (Table I), followed by another 50 mL of throughfall equilibrated with a COz-air gas mixture by bubbling the gas through the solution. This 150-mL leaching was conducted over a 2-3-h period. Following this pretreatment, 55 mL of throughfall was equilibrated with a gas mixture [3% COPin air (low COP),
0 1989 American Chemical Society
Environ. Sci. Technol., Vol. 23, No. 8, 1989
1021
Table I. Chemistry of Simulated Throughfall Solution ion
concn, pequiv L-l
so42-
80
c1-
NOgH+ NHA+
24 44 17
ion
concn, pequiv L-'
K+ Na+ Ca2+ Mg2+
34 30 37 17
13
100% COP (high COz) and an intermediate mixture of these two gases (medium COP)]or room air (control COz) and leached through the soil over an 18-h period. The throughfall solutions equilibrated with the gas mixtures were continuously bubbled during the leaching. The COP levels in the gas were used to obtain a wide range of DIC in the input solution (0.2-25 mmol of C L-l). All gas combinations were conducted in triplicate with only their averages reported and used in model prediction calculations. Although the Reuss and Johnson (1,2) model used to predict solution chemistry assumes equilibrium, we realize that over the 18-h contact time a true equilibrium may not have been reached. However, the soil chemistry models (1-4) used to predict surface water chemistry assume equilibrium. Therefore, we feel that the method used here provides a useful comparison to model outputs, regardless of whether a true equilibrium was established. Assumptions of equilibrium are commonly made in field studies (10-12),with results often used for thermodynamic comparisons with known chemical species. In addition to the 80 pequiv of HzSOt- L-l throughfall solution, we also conducted an intermediate COz-air mixture treatment with an additional 300 pequiv of HzS042-L-' (remaining ions unchanged, designated medium+S). This examined the effect that an increased sulfate input has on the final solution chemistry. With the vacuum extractor, soil solution leached from the soil was not exposed to the atmosphere before analysis. Subsamples of each replicate were transferred to sealed containers for measurement of pH and ANC with an Orion 960 Autotitrator. A Gran function was used to compute ANC from the acid titration data. Another subsample was used to measure degassed pH, following bubbling with NP, and equilibration with atmospheric COz. Filtered (GF/F) samples were analyzed for Sot-, NO3-, and C1- by ion chromatography, cations (Ca2+,Mg2+,K+, Na+) by atomic absorption, total A1 by the PCV method described earlier, and NH4+by use of a Wescan ammonia analyzer. DIC was also determined on unfiltered, sealed samples with a Dohrmann DC-80 carbon analyzer. The Reuss and Johnson ( 1 ) model was then used to calculate pH, degassed pH, ANC, Ca2+(as a surrogate for base cations plus NH,+), and HC03- levels in solutions. Input parameters included measured solution anion concentrations, log KA1,log K,, percent base saturation, and various pCOz levels to generate levels of HC03- similar to those observed in solution. In this way we could then compare predicted versus measured ANC, pH, degassed pH, and Ca2+levels. Details of the model used are presented in Reuss and Johnson (1, 2). This soil and water equilibrium model integrates a few soil processes to help understand acidic deposition effects on soil and solution chemistry. Basic processes included are cation exchange, A1 solubility, and soil air pC0, levels. The solution chemistry is first calculated in contact with the soil matrix. Then by use of the linked water model, the solution is equilibrated with atmospheric COPin the absence of the soil matrix. The 1022
Environ. Sci. Technol., Vol. 23, No. 8, 1989
Table 11. Characteristics of the B Horizon Soil from Lead Mountain. ME concn parameter
or value
cation-exchange capacity, mequiv 100 g-I exchangeable cations, mequiv 100 g-' Ca2+ Mg2+ K+ Na+ base saturation, 70 log KMo log K,"
6.25 0.17 0.04 0.07 0.06 5.4 8.5 3.5
Olog KAI and log K, calculated from 0.002 CaCl, extracts. The log KAI = 2.66pH - pAl.
soil and water models are based on a charge balance described by equations including H+, base cations, inorganic A1 species, strong acid anions, carbonate species, and OH-. Cation-exchange processes are incorporated into the soil model by use of the Gaines-Thomas equation for determining A1-Ca selectivity, which is assumed to be a surrogate for all Al-cation exchange. All exchangeable A1 is assumed to be trivalent so that it is equal to 1 - exchangeable Ca2+. Results of variables determined by the soil system model are used as input to the water model and recalculated by assuming degassing of C02to atmospheric levels. Thus, the chemical equilibrium model determines changes that would occur under different soil pCOz conditions. David et al. (6)present an application of the model to soil chemistry data from Maine and New York.
Results and Discussion Soil Characteristics. The soil used in this work had low base cations and base saturation (Table 11). Values for log KA1and log K , were calculated by using an A1 activity relationship based on 2.66pH - pAl, rather than the 3pH - pAl originally used in the Reuss and Johnson ( I ) model and by David et al. (6). Adjustment of the pH exponent was done in order to improve the fit of the data to the equilibrium model. Justification for the adjustment of the A1 activity relationship is discussed below. Studies involving soil lysimeters and laboratory experiments with soils suggest the theoretical solubility relationship involving Al(OH), does not conform to solution A1 levels (12, 13). This would indicate either undersaturated A1 solution levels with respect to the Al(OH)3mineral species, A1 activity governed by a different A1 mineral specie, or possibly kinetic constraints that control dissolution and precipitation involving Al. Because B horizons of Spodosols also have complexes of Al-organic matter (14, 15), these too may result in A1 activity relationships that do not conform to pure mineral solubility reactions. Surface waters may also not conform to the 3pH - pAl relationship as shown by Goenaga and Williams (IO),who found an exponent of 2.5 conformed to their data. Soil chemical reactions are known to respond at various rates: some are rapid, whereas others proceed so slowly that equilibrium is never attained (16). Modifications used for pKAlcalculations and results of field studies are therefore useful for particular systems but may not be representative of true thermodynamic equilibria, something that is rarely attained in ecosystems. We used the A1 activity relationship of 2.66pH - pAl and modified the Reuss and Johnson ( I ) model to take into consideration reactions dependent upon this modification. This alteration produced better results because model runs with an exponent of 3 gave consistently high pH values
Table 111. Chemistry of Soil Leachate Solutions of Varyihg Initial Levels of Dissolved Inorganic Carbon COz level
DIC, mmol of c L-1
control
0.21
low
1.1
medium high medium+S
4.3 13.7 4.5
HCO,"
SO>-
NO
