Effects of acid anion additions (trifluoroacetate and bromide) on soil solution chemistry of a northern hardwood forest soil
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EFFECTS OF ACID ANION ADDITIONS (TRIFLUOROACETATE ANDBROMIDE) ON SOIL SOLUTION CHEMISTRY OF A NORTHERN
HARDWOOD FOREST SOIL
TORSTEN W. BERGER and GENE E. LIKENSInstitute of Ecosystem Studies, Millbrook, New York 12545-0129, U.S.A.
( author for correspondence, e-mail: firstname.lastname@example.org; fax: +43 1 4797896; address forcorrespondence: Institute of Forest Ecology, Universitt Bodenkultur, Peter Jordanstrae 82,
A-1190 Vienna, Austria)
(Received 19 January 1998; accepted 19 November 1998)
Abstract. Experimental plots within the Hubbard Brook Experimental Forest, NH, were treatedwith sodium trifluoroacetate (TFA) and lithium bromide (Br), to study the impact of TFA alone andin the presence of increased anion concentrations (e.g. acid deposition) on the soil solution chemistryof a northern hardwood forest soil. Trifluoroacetate is a major atmospheric degradation product ofreplacement compounds of chlorofluorocarbons (CFC) and Br is widely used as a hydrologic tracer.Calculated drainage losses via soil water flow were less than 60% of inputs, added during the summer,and TFA and Br were temporarily retained in the soil until fall. The initial indication of an acid inputof the treatments (HTFA, HBr) in the Bs2 horizon, which reflects stream water chemistry as well,was an increase of base cations in the soil solution, decreasing the soils acid neutralizing capacity.Thereafter, trifluoroacetate and Br concentrations peaked after the peak in base cations, synchronouswith peaks in HC and Al concentrations. Organic anions, nitrate and chloride played the major rolein accompaning base cations out of the solum. Sulfate retention at soil adsorption sites was increasedby the presence of TFA and Br, reducing its role as a mobile anion of base cations in this experiment.Relative retention of anions for the whole profile of this northern hardwood forest soil was estimatedby correlation analyses and input-output balances in decreasing order on an equivalant basis: SO4 >TFA = Br Cl> NO3 > organic anions. Recovery from acid additions were recorded within severalweeks after the treatments were stopped. Evaluating the impact of added chemical compounds to soilsmust be considered within the context of linkages among element cycles and pools.
Keywords: acidification, base cations, bromide, forest ecosystems, mobile anion, soil solution, TFA,trifluoroacetate
The mobile anion concept is a model for understanding base cation biogeochem-istry. According to that concept the concentration of anions in solution will controlthe total concentrations of cations, while the composition of cations in solutionshould be controlled by equilibration with what is usually a large pool of cationsadsorbed on soil particles (Reuss and Johnson, 1986; Christ et al., 1997).
Research on the effects of acid rain in North America and Europe has focusedprimarily on the biogechemistry of sulfur and of nitrogen (Johnson and Lindberg,
Water, Air, and Soil Pollution 116: 479499, 1999. 1999 Kluwer Academic Publishers. Printed in the Netherlands.
480 T. W. BERGER AND G. E. LIKENS
1992; Likens et al., 1996; Ulrich, 1988) and the scientific community has madesignificant progress in defining the mechanisms of repsonse to these chemicalinputs. Atmospheric inputs of S and N not chemically or biologically taken upin the soil moves through the soil solution primarily as NO3 or SO4 anions andare accompanied by cations to maintain charge neutrality (e.g. Fasth et al., 1990;Rustad et al., 1996; Rutherford et al., 1985). As a result large quantities of the basecations Ca, Mg, K, and Na have been lost from the soil complex and exported bydrainage water. Base cations play essential roles in forest ecosystems and in thequality of surface water (Lawrence et al., 1995; Likens et al., 1996).
Trifluoroacetate (TFA) must be considered an additional mobile anion in theacid rain issue, although little is known of its effect on soil solution chemistry.Average global concentration of TFA in rainfall is predicted to be 0.16 g L1 by2010 (Tromp et al., 1995), but recent survey data already suggest levels of TFA inthe range predicted for 2010 (Frank et al., 1996). Trifluoroacetate is formed as animportant breakdown product by atmospheric degradation of chlorofluorocarbon(CFC) replacements (Franklin, 1993).
Trifluoroacetate is highly soluble in water and is transported to Earths surfacein precipitation (Franklin, 1993; Frank et al., 1995). Trifluoroacetic acid (molecularweight, 114 g mol1) is an acidic organic compound that predominantly occurs asan anion under environmental conditions.
Expected environmental concentrations of TFA are of little concern with re-spect to human health (Ball and Wallington, 1993), but there are reports of growthinhibitions of algae by TFA at low concentrations (100300 g L1, Thompson,1994).
Bromide (Br), widely used as a hydrologic tracer (e.g. Flury and Papritz, 1993),was used to compare transport via soil water in combination with TFA, and tosimulate effects of increased anion concentrations (e.g. sulfate from atmosphericdeposition) in the soil solution. The main objective of this paper is to evaluatepossible effects of strong anion additions, which currently are present in only tracequantities in the soil system, on solute concentrations and fluxes of major elementsfrom the soil. Two experimental plots were treated with A) sodium TFA and lithiumBr and B) sodium TFA only. This study was performed to address the followingquestions by calculating sources or sinks of analyzed ions within the soil profile,performing correlation analyses among these ions and studying changes of soilsolution chemistry over time: (1) Recent investigations (Berger et al., 1997; Richeyet al., 1997; Likens et al., 1997) showed retention of TFA in soils, but what are thepossible retention mechanisms? (2) How do additional anions fit into the mobileanion concept? Are different anions equivalent in mobilizing base cations? (3) Arethe biogeochemical changes caused by the strong anion treatments reversible andhow rapidly do small experimental plots recover?
EFFECTS OF ACID ANION ADDITIONS 481
2. Study Sites and Methods
2.1. STUDY AREA
The experiment was conducted in the Norris Brook watershed at an elevationof about 320 m within the Hubbard Brook Experimental Forest (HBEF) in NewHampshire (Likens and Bormann, 1995). The HBEF has northern hardwood vege-tation growing on Typic Haplorthods that developed in sandy till. The average pHof the humus layer (Oa horizon) is 3.9. The mineral soil pH increases from 4.2 to4.7 with depth (Johnson et al., 1991). Soil characteristics for the Oa-, Bhs- and Bs2-horizon of the study area are: organic matter (79.5, 12.6 and 0.5%), clay (8, 5 and3%) and cation exchange capacity (17.2, 7.4 and 0.05 cmolc kg1), respectively(Richey et al., 1997). Bhs is a combination of Bh and Bs1 horizons, which arethin and frequently discontinuous in this area of the HBEF. The main tree speciesin the study area are American beech (Fagus grandifolia; 60% of stems), yellowbirch (Betula alleghaniensis; 15%) and sugar maple (Acer saccharum; 13%). Meanannual precipitation during the study was 1230 mm. More detailed informationabout the study area is given by Christ et al. (1995).
2.2. PLOT INSTALLATIONS
During the spring of 1995 two lysimeter plots (Plots A and B, each 1.5- along thecontour x 1-m upslope) were installed within 10 m of each other on a southwestfacing slope in the Norris Brook watershed. Both plots were equipped with ceramiccup tension lysimeters (Soilmoisture Equipment Corp.; 5 replications each in 10-,30- and 50-cm depth; applied suction: 40 kPa). Fixed soil depths at 10-, 30- and50-cm enabled hydrologic flux calculations and matched approximately the aver-age lower boundary of the three soil horizons, Oa, Bhs and Bs2. All lysimeters wereset into cleared, vertical upslope faces of the two pits (just below the treated areas),which were then backfilled. The small area of the plots was free of soil vegetation.The study sites were equipped with 3 bulk samplers for collecting throughfall. Soilmoisture tensions were recorded by tensiometers (6 replications per horizon).
Trifluoroacetate additions (Table I) were calculated to meet the detection limitof an ion chromatograph (0.5 mol L1 TFA), representing a rather worst casescenario situation of expected TFA deposition by 2010 (Tromp et al., 1995).Plot A received three additions of sodium TFA (total of 0.81 g TFA m2) andlithium Br (total of 10 g Br m2). Similar amounts of added Br were used inother experimental studies (e.g. Jemison and Fox, 1991; Kung, 1990; Schnabel etal., 1995, Owens and Edwards, 1992). Trifluoroacetate and Br data of Plot A wereused previously by Berger et al. (1997), where Plot 2 is identical with Plot A of thisstudy. Plot B was treated twice with TFA only (total of 0.54 g TFA m2), to separate
482 T. W. BERGER AND G. E. LIKENS
Additions of trifluoroacetate (TFA) and bromide (Br) in mmolc m2 (c: charge; addedas sodium TFA and lithium Br). Amounts of collected throughfall during the singletreatment periods and total period are given in mm
Treatment Plot A Plot B ThroughfallNo. Date (day of the year) TFA Br TFA Br per period
1 08 July 1995 (189) 2.39 42.2 2.39 1822 12 August 1995 (224) 2.39 42.2 2.39 443 15 September 1995 (258) 2.39 42.2 192
08 July01 November 7.17 126.6 4.78 418
between effects of TFA and Br (questions 2 and 3). On occasion of treatments, theplots were covered just before expected precipitation and treated directly after thecessation of rain. Sodium TFA (98% NaTFA, provided by E. I. DuPont de Nemours& Co.) and lithium Br were mixed with water from nearby Mirror Lake (seeLikens, 1985), because of lack of actual throughfall. The volume of water addedvaried for each treatment corresponding to the amount of the previous precipitationevent, indicated by throughfall collectors. Therefore, no additional input variablewas necessary to run the hydrologic model (see below). Volume-weighted elementconcentrations (molc L1) were higher in Mirror Lake water (Likens, 1985) thanin measured throughfall (this study) for Ca (Mirror Lake vs. throughfall: 119 vs.22), Mg (41.1 vs. 12.1), SO4 (119 vs. 36.9) and Cl (30.7 vs. 13.2). Because of smalladded volumes the impact of Mirror Lake water on the soil solution chemistry isconsidered negligible (treatment no. 1: 1.5 mm, no. 3: 2.0 mm) or small (treatmentno. 2: 17.6 mm), when compared to collected throughfall during the total period ofthe experiment (418 mm).
2.4. SAMPLING AND ANALYSIS
Soil solutions were sampled twice a week from 19 June 95 to 1 November 95 (firsttreatment on 8 July), except for two weeks after each treatment when samples werecollected 3 times a week. During the dry summer of 1995, sampling was limited oc-casionally in the upper horizons because of high soil moisture tensions (Figure 1).Lysimeters were emptied into clean, high-density polyethylene bottles and storedat 4 C until analysis. Concentrations of TFA, Br, SO4, Cl and NO3 were measuredby ion chromatography, Ca and Mg by inductively coupled plasma emission (ICP)spectrophotometry, Na, Li and K by atomic absorption spectroscopy and NH4 byauto-analyzer. pH was determined electrometrically. Total monomeric and organ-ically complexed monomeric Al was measured by automated colorimetry usingpyrocatechol violet (Mc Avoy et al., 1992). The equivalents of charge of inorganic
EFFECTS OF ACID ANION ADDITIONS 483
Figure 1. Throughfall (mm), soil moisture (kPa) and concentrations of TFA, Na, Br, Li, Ca, Mg and K(molc L1) in Plot A from 12 June to 1 November 1995. Upside down triangles indicate additionsof TFA and Br according to Table I.
484 T. W. BERGER AND G. E. LIKENS
monomeric Al (AlnC), which was removed from solution by ion exchange column(difference between total monomeric and organically complexed monomeric Al),was calculated at the measured pH of the sample (based on 3 equilibrium equationsfor Al3C, Al(OH)2C, Al(OH)2C and Al(OH)4; Schecher and Driscoll, 1987).
Throughfall was collected twice a week and analyzed for SO4, Cl, NO3, Ca, Mg,K, Na, NH4 and pH as described for the soil solution samples. Throughfall fluxeswere calculated according to measured solution volumes per area of the collector.
The amounts of elements transported through the soil profile were estimated bymultiplying the element concentration in each sample of soil water by the wateroutput from the related soil horizon and time. The Brook2 model (Federer andLash, 1978a, b), developed and parameterized for the HBEF (adjacent watershedW3), was used to calculate drainage from the rooting zone as described by Bergeret al. (1997).
Brook2 requires daily precipitation and daily mean air temperatures as input.These data for the simulation were measured at the U.S.D.A. Forest Service RobertS. Pierce Ecosystem Laboratory site and at rain gage 22, some 0.9 km distant. Thevariable Edrain, which represents drainage from the rooting zone, was interpretedas the amount of water flowing past the plane of the lysimeters at 50-cm soil depth(Bs2 horizon), even though a few roots may be found below this plane. Their effectis assumed to be negligible. Water flux through each horizon was calculated fromEdrain according to Yanai (1990), that is, transpiration was distributed through thesoil profile according to the distribution of fine root biomass (Fahey et al., 1988).
3. Results and Discussion
3.1. SOIL SOLUTION CHEMISTRY
Volume-weighted means of element concentrations in the soil solution are given inTable II, which were calculated from modeled fluxes between the horizons. Sulfatedominated the soil solution chemistry at Plot B, while Br was quantitatively themost important anion at Plot A. Lithium decreased more rapidly with soil depththan Br, indicating much higher soil retention of this cation. Potassium and bothAl species (inorganic and organic Al) declined with increasing soil depth, withthe exception of an inorganic Al (AlnC) peak at 30-cm soil depth (Plot A). Nitrateconcentrations remained elevated in the lower horizons and were higher in Plot A.
Throughfall, mean soil moisture tensions, and concentrations of TFA, Na, Br,Li, Ca, Mg and K for Plot A are given in Figure 1. Sulfate, Cl, pH, AlnC and orgAl concentrations for Plot A, as well as NO3 concentrations and the differencesbetween analyzed cations minus analyzed anions (Cat-An) for both plots are plot-ted in Figure 2. Trifluoroacetate and Br concentrations show similar patterns for
EFFECTS OF ACID ANION ADDITIONS 485
Figure 2. pH, org Al (mol L1), concentrations of NO3, SO4, Cl, AlnC (molc L1) and thedifferences between analyzed cations minus analyzed anions (Cat-An, molc L1) for Plot A (threetreatments with TFA and Br are indicated by upside down triangles according to Table I), as well asNO3 and Cat-An concentrations for Plot B (two treatments with TFA only).
486 T. W. BERGER AND G. E. LIKENSTABLE II
Volume-weighted means of eleme...