Forest Soil Organic Horizon Acidification: Effects ofTemperature, Time, and Solution/Soil Ratio1

BRUCE R. JAMES AND SUSAN J. RiHA2

ABSTRACTA forest soil organic horizon (Oe) was acidified with HNO, (0-5

cmol kg"') in the laboratory at three solution/soil ratios (10, 50,100), three temperatures (4,14, 30°C), and for up to 162 h to com-pare effects of added H on pH, soluble Ca, Al, and C with effectsof environmental conditions likely to vary seasonally in the forestfloor. Soil suspension pH reached a steady state within 48 h, re-gardless of quantity of H+ added, solution/soil ratio, or temperature.Calcium concentrations also reached a steady state with soil solidphases quickly, and additions of H' increased quantities of Ca (mmolkg ') in solution as did increasing solution/soil ratio. Shaking timeand temperature had smaller effects on Ca than did H+ additionsand solution/soil ratio. In contrast, reactions controlling dissolvedorganic carbon (DOC) and total soluble aluminum (Al,) were slowerthan those for pH and Ca. Quantities of DOC and Al going intosolution remained nearly constant, so that increasing the solutionsoil ratio diluted concentrations. Dissolved organic C and soluble Alincreased with longer shaking times and higher temperature. Theresults have implications for designing realistic laboratory experi-ments on forest floor acidification, for evaluating seasonal and yearlyvariations in soil solution composition in the field, and in developingaccurate and reliable simulation models of H+ reactions in forestsoils.

Additional Index Words: acid precipitation, dissolved organic car-bon, pH, fulvic acid, kinetics.

James, B.R., and S.J. Riha. 1987. Forest soil organic horizon aci-dification: Effects of temperature, time, and solution/soil ratio. SoilSci. Soc. Am. J. 51:458-462.

TO ASSIST IN RESOLVING THE CONTROVERSY OVCFwhether anthropogenic acid precipitation or nat-

ural soil processes are responsible for contemporaryacidification of forest soils (Krug and Frink, 1983;Johnson et al., 1984; van Breemen et al, 1984), weneed more information from controlled laboratory ex-periments to compare the relative effects of temper-ature, solution/soil ratio, reaction time, and H+ ad-ditions on soluble Al, Ca, and organic C (DOC) in theforest floor (James and Riha, 1984; Krug and Isaac-son, 1984; DeWalle et al., 1985). Variability in soiltemperature, volume of leaching water, flow rates forsoil water, and litter decomposition may have signif-icant impacts on solubility of these constituents of theforest floor soil solutions, especially during springsnowmelt and summer drought periods (Johnson andSiccama, 1983; Hooper and Shoemaker, 1985). Theforest floor contains many fine roots (Lyford, 1975)and comprises the first horizons of the soil profile toreact with snow meltwater, throughfall, and rain. Thesehorizons also exhibit more variation in temperature,water content, and residence time of soil water thando underlying mineral horizons because of their po-

1 Contribution from the New York State Agric. Exp. Stn., Ithaca.Received 21 Apr. 1986.2 Research Associate and Assistant Professor, Agronomy pep.,Cornell Univ., Ithaca, NY 14853. Senior author is now AssistantProfessor of Soil Chemistry, Agronomy Dep., Univ. of Maryland,College Park, MD 20742.

sition at the surface of the soil. Variability with depth(James and Riha, 1984) and with distance from theboles of trees (Riha et al., 1986) also make these ho-rizons a heterogeneous part of forest soil profiles.

Information on soil conditions likely to vary sea-sonally regardless of the acidity of atmospheric dep-osition should aid in developing accurate simulationmodels of natural and acid precipitation-induced aci-dification of soils and freshwater systems (Bloom andGrigal, 1985; Cosby et al., 1985). Extension to the field(James and Riha, 1984) and better ways to predict thekinetics of soil acidification should be possible (Pohl-man and McColl, 1986).

Our previous experimentation on acidification offorest floor organic horizons showed consistent in-creases in soluble Ca and decreases in DOC followingaddition of increasing quantities of HNO3, but solubleAl concentrations increased and decreased in the ho-rizons used (James and Riha, 1984, 1986). Since weconducted these experiments at constant temperature,solution/soil ratio, and equilibration time, we ob-tained no information on how changing these condi-tions would affect soluble Al, Ca, and DOC at differentlevels of added H+. In the research reported here, weacidified an Oe forest floor horizon using three solu-tion/soil ratios at one temperature or using three tem-peratures at one solution/soil ratio, all with ionicstrength held constant. We simulated the range in con-ditions likely to occur seasonally in the northeasternUSA, and the levels of added H+ simulated additionsto the forest floor in the Northeast during the springsnowmelt period (James and Riha, 1984).

METHODSSoil

The soil horizon we used was physically and chemicallyrepresentative of partially decomposed material composingthe forest floor in northeastern forest soils previously studiedor sampled (James and Riha, 1984, 1986; Table 1). We sam-pled it in the fall of 1982, when soil water potential wasapproximately —10 kPa and soil temperature was 14°C. Afine root mat ramified the horizon, and mycorrhizae hadinfected many lateral roots of the dominant vegetation, 50-yr old red pine (Pinus resinosa Ait.). Since little understory

Table 1. Characteristics of forest floor Oe horizon, tSampling site Adirondack region (Russia) of New York StateParent material Glacial outwash sandVegetation Pinus resinosa Ait. (age -50 yr)Classification

(suborder) PsammentDrainage class Excessively drainedpH 3.1JOrganic matter 910 g kg~'§Depth 5-7 cmCEC 8 cmol (NHJ) kg-' at pH 3.7Ca saturation 76%1t James and Riha, 1986. Site 2.| 4:1 solution/soil ratio in 10 mM CaCl,.§ Loss on ignition, 450 °C.1 Ca extracted by 1.0 M NH.CiyCEC.

458

JAMES & RIHA: FOREST SOIL ORGANIC HORIZON ACIDIFICATION 459

vegetation was present, we assumed the horizon developedalmost entirely from decomposition of red pine litter androot material. We sieved the field-moist soil using a 4-mmpolyethylene sieve and stored it at 4°C in a 25-nm thickpolytheylene bag approximately 3 yr.

Acidification ProcedureWe acidified the Oe horizon in weighed, 50-mL polycar-

bonate centrifuge tubes containing the weight of field-moistsoil equivalent to 3.0, 0.60, or 0.30 g oven-dried soil (105°C)and 0.30 mL l.OMNH4NO3. We added volumes of 10 mMHNO3 equivalent to 0, 1, 3, or 5 cmol (H+) kg-' soil beforeadding distilled, deionized water by weight to give a totalsolution volume of 30.0 mL in each tube. The final solution/soil ratios were 10, 50, and 100 for the three soil weights.Each solution/soil ratio X acid added treatment was repli-cated nine times. We capped the tubes and shook them ona reciprocating shaker (110 cycles min~') for 1, 48, or 120h at 30 ± 2°C in a constant temperature room. At the endof each time, we centrifuged three tubes from each treatment(35 000 X g, 40 min., 25°C), filtered the supernatant liquids(0.4 /am polycarbonate membranes) into polyethylene bot-tles, and stored them at 4°C.

To identify temperature effects, we repeated this proce-dure using solution/soil ratio = 50 and 12 replicates of eachacid treatment. After 19, 43, 65, and 162 h shaking times at4 ± 2°C, 14 ± 2°C, or 30 ± 2°C, we centrifuged, filtered,and stored three soil suspensions for each acid added Xtemperature treatment, as described above.

Analysis of FiltrateIn all filtrates, we measured total soluble Al by the 8-

hydroxyquinoline method (James et al, 1983), Ca by atomicabsorption, and pH in equilibrium with the atmosphere tothe nearest 0.01 pH unit using a Chemtrix type 62 pH meterand a Corning flat surface combination electrode. The pHmeter was standardized daily using pH 7.00 and pH 4.00phosphate and phthalate buffers, respectively. Dissolved or-ganic C was measured by a modification of the Mebius pro-cedure using 42 mM K2Cr2O7 and 50 mM Fe (NH4)2(SO4)2and heating the Cr(Vl)-H2SO4-sample mixture 0.5 h at 130-140°C (Nelson and Sommers, 1982).

RESULTS AND DISCUSSIONSoil Suspension pH

With increasing contact time between solution andsoil from 1 to 120 h, pH of filtrates of unacidified soiltreatments (no H+ added) showed no consistent trendand varied <0.03 pH units around the means at eachsolution/soil ratio (Table 2). The pH's did increase

Table 2. Soil suspension pH (no H* added) and pH buffercapacity (/3) in a forest soil organic horizon acidified

with 0 to 5 cmol (H*) kg'1 at three solution/soilratios or three temperatures.!

Solution/soil ratio (at 30 °C) pH 0 [mmol(rT) kg-' (pH unit)-']1050

100

4.00 ± 0.034.07 ± 0.024.16 ± 0.02

Temperature (°C)(at solution/soil ratio = 50)

4 4.22 ± 0.0114 4.24 ± 0.0130 4.21 ± 0.01

113 ± 8t148 ± 7154 ± 10

140 ± 2§130 ± 4148 ± 6

approximately 0.16 pH units, however, with increas-ing solution/soil ratio. We observed this increase inpH at each level of H+ added (data not shown), andwe measured increasing pH buffer capacities (/8) from114 to 154 mmol (H+) kg~' (pH unit)"1. In contrast,changes in pH and 0 due to increasing temperaturewere small, suggesting that seasonal variability in for-est floor temperature would not affect changes in soilsolution pH or buffer capacity.

These data showed that reactions of H+ added tothe soil reached a steady state within 48 h, regardlessof solution/soil ratio, level of added H+, or temper-ature.

Dissolved Organic CarbonWe present the results for DOC, Al, and Ca (Fig.

1-4) as mole quantities based on soil weight, insteadof concentrations based on solution volumes. Express-ing the results as quantities rather than concentrationsaccounts for dilution of soluble species as solution/soil ratio increases and permits direct comparisons be-tween different ions. These units also permit directreference to field conditions where variable volumesof water leach forest floors of a given weight and thick-ness.

Dissolved organic C levels (cmol 100 g"1) remainedroughly constant with increasing solution/soil ratio af-ter a given shaking time and level of H+ added (Fig.1). This constancy in DOC level based on soil weightindicated that as solution/soil ratio increased from 10to 100, DOC concentration measured in filtrates waslowered by dilution. This dilution probably reflectsthe relative kinetic inertness in dissolution of organic

t Ionic strength = 0.01 (NH.NO,) in all treatments.j Mean ± 1 SE for shaking times 1, 48, and 120 h, n = 9.§ Mean ± SE for shaking times 19, 43, 65, and 162 h, n = 12.

I 3 501 3 501 3 5

H+ added (cmol /kg)Fig. 1. Soluble organic carbon (DOC) vs. HNO3 added to an Oe

horizon at 3 solution/soil ratios and after 3 shaking times. A stan-dard error (SE) bar is shown where 1 SE> point size.

460 SOIL SCI. SOC. AM. J., VOL. 51, 1987

matter, as was observed in repeated, sequential leach-ings of other forest floor organic horizons (James andRiha, unpublished data). Levels of DOC did, how-ever, increase with increasing equilibration time at agiven solution/soil ratio and generally decreased withadded H+, as observed previously .(James and Riha,1984). The decreases in DOC with increasing quantityoff H+ added were balanced by increases due to in-creasing solution/soil ratio, shown most clearly at 48li shaking time (Fig. 1). The gradual increases in DOCwith contact time were in contrast to rapid H+ re-moval by the soil.

Microbial decomposition and metabolism of var-ious forms of organic C in solution and solid phasesof the soil should be considered in order^to better eval-uate mechanisms for these observations of effects ofH+, solution/soil ratio, and temperature on DOC lev-els. We need a better understanding of the kinetics ofbiological, as well as chemical, reactions controllingforms and solubility of organic C in the forest floor.

At 4 and 14°C, these effects of time and H+ addi-tions in increasing and decreasing DOC solubility weresmaller than at 30°C (Fig. 2). At 4°C, addition of H+

resulted in almost no change in DOC (Fig. 2a); at 14°C,H+ had greater effects (Fig. 2b). These results sug-gested that increasing the volume of water and quan-tity H+ leached through an organic horizon in the fieldat 4°C may not change the DOC leached out of theforest floor, but greater leaching of DOC may occurat higher temperatures, especially if contact time be-tween soil and solution is long. At each of the fourequilibration times (Fig. 2), the effect of added H+ inlowering DOC (ADOC/AH+) was positively and lin-

early correlated with temperature (r2>0.90), suggest-ing that a given pulse of H+ added to an organic ho-rizon will lower DOC concentration more at warmertemperatures than at colder ones.

In contrast to DOC, quantities of soluble Ca in-creased with increasing solution/soil ratio, suggestingthat equilibria between sparingly-soluble Ca solidphases, such as Ca-oxalate (Graustein et al., 1977;Chandler, 1937) and soluble Ca were maintained ateach solution/soil ratio. This difference in effect of in-creasing solution/soil ratio on quantities of soluble Cand Ca in the organic horizon suggests differences intypes of solid phases controlling their solubility or inkinetics of their dissolution. Since concentrations oforganic C and Ca in the forest floor are larger than inmineral soils (Bockheim et al., 1983), such solubilitydifferences are important in evaluating leaching phe-nomena between the forest floor and mineral hori-zons, particularly regarding effects of Ca and DOC onAl solubility and speciation (James and Riha, 1984,1986).

The increases in Ca levels in response to adding 5cmol (H+) kg"' were the same as those due to increas-ing solution/soil ratio from 10 to 50 (Fig. 3a, b). Sim-ilar though less consistent increases were measuredwhen solution/soil ratio was increased to 100 (Fig. 3c).Increasing temperature from 4 to 14°C did not in-crease Ca solubility, and only small increases were ob-served at 30°C. Time had only small effects on Calevels, similar to effects of time on pH, and in contrastto DOC (Table 2, Fig. 1 and 2).

These major effects of H+ and solution/soil ratioand relatively minor effects of shaking time and tem-

10

O)oo

oEo

0 1 5 0 1 5 0 1added (cmol/kg)

Fig. 2. Soluble organic carbon (DOC) vs. HNO3 added to an Oehorizon at three temperatures and after four shaking times. Astandard error bar is shown where 1 SE> point size.

0 1 3 5 0 1 3 5 0 1

H+ added (cmol/kg)Fig. 3. Soluble Ca and Al vs. HNO3 added to an Oe horizon at three

solution/soil ratios and after three shaking times. A standard er-ror bar is shown where 1 SE> point size.

JAMES & RIHA: FOREST SOIL ORGANIC HORIZON ACIDIFICATION 461

perature on Ca suggest that rapid kinetics control Casolubility in this organic horizon, and are coupled toH+ removal by the soil. We observed previously thatCa was the dominant cation in equilibrium solutionsof six acid organic horizons and its concentrationchanged more than other cations in response to aci-dification (James and Riha, 1986). These facts indi-cated the importance of acidity and volume of influentH2O in predicting quantities of Ca that may be leachedfrom the forest floor during different times of the year.We need to identify the relative contributions of H+-Ca2+ exchange and solid phase dissolution if mecha-nisms responsible for Ca solubility and leaching areto be better understood.

AluminumPatterns in solubility of total Al (Fig. 3 and 4) were

similar to those of DOC, except levels of Al were 3mmol kg-' and those of DOC were 200 to 1000 mmolkg~', The correlation coefficents (R2) for a linearregression model between DOC and Al, increased from0.16 at 4°C to 0.34 at 14°C and 0.90 at 30°C, whenvalues for DOC and Alr at all times and levels of H+

added were pooled at each temperature. The im-proved correlation between DOC and Al with increas-ing temperature suggested that control of Al solubilityin the forest floor by DOC could vary according tosoil temperature (James and Riha, 1984).

ImplicationsThe results demonstrated that changes in volume

of H2O leaching the forest floor, residence time of soilwater, and soil temperature affect soluble species in

1 3 5 0 1 3 5 0 1 3 5

H+ added (cmol /kg)Fig. 4. Soluble Ca and Al vs. HNO, added to an Oe horizon at three

temperatures and after four shaking times. A standard error baris shown where 1 SE> point size.

an organic horizon differently and will interact withH+ to control drainage water chemical composition.Therefore, quantities of organic C, Ca, and Al solublein forest floors depend on environmental conditionsthat vary temporally in forest soils. Recognition shouldbe given to these factors in designing and evaluatinglaboratory and field experiments on acidification offorest floors, in constructing simulation models of for-est floor nutrient dynamics, and in trying to predictlong- and short-term temporal changes in forest soilsolution composition due to acid precipitation.

The different responses of Ca, DOC, and Al indi-cated that reaction kinetics controlling dissolution ofsparingly-soluble precipitates and exchange reactionsbetween H+, Ca2+, and A13+ should be investigatedfurther to elucidate mechanisms responsible forchanges in chemical composition of forest floor so-lutions observed in this and other studies.

ACKNOWLEDGMENTThe research described in this article has been funded in

part by the EPA-NCSU Acid Precipitation Program (a co-operative agreement between the U.S. Environmental Pro-tection Agency and North Carolina State Univ.). It has notbeen subjected to EPA's required peer and policy review andtherefore does not necessarily reflect the views of the Agencyand no official endorsement should be inferred.

462 SOIL SCI. SOC. AM. J., VOL. 51, 1987

Nelson, D.W., and L.E. Sommers. 1982. Total carbon, organic car-bon, and organic matter. In A.L. Page et al. (ed.) Methods of soilanalysis, Part 2. 2nd ed. Agronomy 9:539-580.

Pohlman, A.A., and J.G. McColl. 1986. Kinetics of metal dissolu-tion from forest soils by soluble organic acids. J. Environ. Qual.15:86-92.

Riha, S.J., B.R. James, G.P. Senesac, and E. Pallant. 1986. Spatialvariability of soil pH and organic matter in forest plantations.Soil Sci. Soc. Am. J. 50:1347-1352.

van Breemen, K, C.T. Driscoll, and J. Mulder. 1984. Acidic dep-osition and internal proton sources in acidification of soils andwaters. Nature 307:599-604.