S O I L S O L U T I O N C H E M I S T R Y IN A S O U T H E A S T A L A S K A

S P O D O S O L S U G G E S T S P O S I T I V E L Y C H A R G E D

O R G A N I C C O M P O U N D S

J. D. S T E D N I C K

Department of Earth Resources, Colorado State University, Fort Collins, CO 80523, U.S.A.

(Received September 7, 1983; revised February 13, 1984)

Abstract. Soil solution chemistry was sampled for 2 yr in a mature Sitka spruce - western hemlock (Picea sitchensis (Bong) Carr. Tsuga heterophylla (Raf.) Sarg) forest on a Dystric Cryandept (Spodosol) soil. Electroneutrality balances indicated a cation deficit at all soil solution sampling points. Calculated soil solution electrical conductivity was less than measured electrical conductivity, with differences greater than expected measurement errors. Soil solutions and streamflow were colored by organics, as measured by P.C.U.s. It is suggested that organic acids or other organic compounds may be positively charged in northern Spodosols.

1. Introduction

Nutrient cycling and soil solution composition are most often analyzed by sampling the hydrologic cycle. The hydrologic budget may be sampled as precipitation, soil water, or as streamflow. Lysimeter studies have enabled soil scientists to define the mechanisms controlling soil solution chemistry.

The currently accepted principle controlling soil solution chemistry is the mobile anion-cation association. The electroneutrality of any solution must balance, i.e., the sum of the positive charge (cations) must equal the sum of the anion charge (expressed as equivalents per liter).

Solution chemistry research in forested soils to date has regarded any difference in the cation-anion balance as being attributed to organic acids, if soil solutions were colored (Johnson, 1975; Johnson et al., 1977). Published research generally indicates an anion deficit that is attributed to organic acids. Most forest ecosystems are acidic in the near surface horizons and the mobile anion pool is dominated by organic acids with a shift to bicarbonate (HCO3-) lower in the soil profile, as the pH rises.

Southeast Alaska is characterized by a cool, wet maritime climate due to its proximity to the North Pacific warm stream. The area has more annual precipitation and a cooler growing season than the Pacific Northwest. Summer droughts are uncommon (Alaback, 1982).

The physiographic features in Southeast Alaska are the result of the northwest orientation of faults, bedrock strikes, and lineaments. The overall geological pattern is a northwesterly eugeosyncline in Paleozoic rocks (Loney et aL, 1975).

Most soils on forested sites are classified as Spodosols, having thick O2(Oe), thin A2(E) and B21(BA) horizons, extremely to very strongly acid sola, weak structure and thixotropic (Heilman and Gass, 1974). The 1981 soil classification system is in paren-

Water, Air, and Soil Pollution 23 (1984) 263-269. 0049-6979/84/0233-0263501.05. © 1984 by D. Reidel Publishing Company.

264 J. D. STEDNICK

thesis. These soils strongly attract and hold water, remaining moist throughout the year (Patric and Stephens, 1968).

The Indian River study area showed incoming precipitation was acidified upon passing through the forest canopy and 02(0) litter layer (Stednick, 1981b). Concurrent increases in solution color and Fe imply that organic acids (probably fulvic acids) were being leached from foliage and litter, transporting chelated Fe from A2(E) to B2hir(Bhs) horizons (Johnson etal., 1977). These observations are consistent with accepted theories concerning podzolization processes (Kononova, 1966), although particulate transport may play a major role in Fe and organic matter migration also (Ugolini et al., 1977). These organic acids with their chelated Fe and A1 precipitate in the B2hir(Bhs) horizon (Dawson etal., 1978) allow a pH rise and bicarbonate dominance of soil solutions beneath the B2hir(Bhs) (Johnson et aL, 1977).

The objective of this study is to present soil solution data that suggest positively charged organic Compounds in a northem Spodosol.

2. Site Description

Indian River streamflow is distributed in a bimodal pattern. Approximately 40 ~o of the annual valley precipitation of 2,500 mm, which is seemingly not subject to anthropogenic influences, occurs as rainfall in September and October. With decreasing air tempera- ture, precipitation as snow accumulates through March or April. Fall peakflows are reduced as snow accumulates. Winter low flows occur until May or June when rain and/or snow melt events may create peakflows. Decreasing rainfall through summer months results in a generally receding hydrograph. Fogs and mists are common and contribute an additional but undetermined quantity of water. Average annual stream- flow is 2000 mm (Stednick, 1981a).

2.1. GEOLOGY

Freshwater Bay, northeast of Indian River, is the northeast fold of a syncline in sedimentary and volcanic rocks that range in age from Silurian to Mississippian (Loney et al., 1975). The valley bottom is composed of unconsolidated alluvium, colluvium and glacial deposits. Subsurface water may flow through these sediments. Deep percolation of groundwater is prevented by a marine till which underlies the valley bottom. The valley follows the northwest-southeast strike of the Indian River fault.

2.2. VEGETATION

The experimental site (Lat 57 ° 49' 50" N, Long 135 ° 16' 00" W) was at an elevation of 105 m. The mature coastal Sitka spruce-western hemlock forest (Picea sitchensis (Bong) Carr. and Tsuga heterophylla (Raf.) Sarg) with subordinate vegetation of Vaccinium ovalifolium - V. alaskensis and Menziesia ferruginea (Viereck and Dyrness, 1980) was 400 to 600 yr old and approximately 70~o hemlock by tree volume.

SOIL SOLUTION CHEMISTRY IN A SOUTHEAST ALASKA SPODOSOL 265

TABLE I

Chemical data for a Dystric Cryandept soil over limestone Southeast Alaska (Stephens et al., 1969; Heilman and Gass, 1972; Stednick, 1984)

Horizon Depth pH N Available Ca t ion Exchangeable cations Base (cm) ( ~o ) P exchange saturation

(ppm) capacity Ca Mg K Na ( ~o )

meq i00 g- 1 O2(Oe) 13-0 3.8 1.43 50 115 10.8 7.2 2.1 1.5 19 A2(E) 0-1 3.9 0.39 1 45 1.2 0.9 0.2 0.3 6 B21(BA) 1-6 4.2 0.54 3 81 1.0 0.8 0.2 3.8 7 B22(B) 6-41 5.1 0.16 2 24 0.4 0.2 0.1 2.7 14 B23(BC) 41-48 7.1 0.47 2 74 46.4 1.6 0.2 7.1 75

2.3. SOIL

The soil was identified as a Ulloa gravelly silt loam, a Dystric Cryandept and thixotropic soil (Table I). These soils typically have thick O2(Oe), thin A2(E) horizons, thin dark,

reddish-brown to dusky red B21(BA) horizons over slightly acid to neutral dark, yellowish-brown to dark-brown B2(B) horizons. Ulloa soils occur on steep side-slopes

of glaciated valleys. The regolith is weathered colluvium derived from limestone and

metamorphosed marble. The climate is characteristically humid with annual rainfall of about 2540 mm. The average annual air temperature is about 7 °C and the average summer air temperature is 13 ° C. The average annual soil temperature at 50 cm is 5 o C, and the average summer soil temperature at 50 cm is 8 °C (Stephens et al., 1969).

3. Methodology

Water samples were collected as: (1) precipitation, (2) forest floor leachate, (3) mineral soil leachate (near the bottom of the rooting zone), and (4) groundwater from a spring.

Precipitation recorder inlets were covered with spun fiberglass to prevent sample con- tamination from windblown litterfall and detritus. Forest floor leachate O2(Oe) was

collected by ceramic lysimeter plates (1.3 x 20 cm) (Gessel and Cole, 1965), installed under the forest floor. A hanging column of water provided a tension of approximately 10 kPa and drained into 20 L carboys buried in the soil. Mineral soil leachates were collected by suction cup lysimeters (5 cm diameter) on 1 m tubes evacuated to 15 kPa and buried from 0.6 to 1.0 m deep. Forest floor and mineral soil leachate were each collected from four sites within a 10 m square area. Groundwater samples were collected from a spring 5 m downslope from the sample sites.

A pH meter with an automatic titrator measured alkalinity as bicarbonate (HCO3 - ) and pH (APHA, 1975) to an endpoint of pH 4.5. A Varian 1200 Atomic Absorption Emission Spectrophotometer was used to measure dissolved Ca, Mg, Na, K, Fe, Mn, and A1.

A Technicon II Autoanalyzer was used to measure N and P forms. A Cu-Cd reduc tor

column reduced nitrate to nitrite for measurement (Wood et al., 1967). Macro-Kjeldahl

266 J.D. STEDNICK

and perchloric acid digestions on untreated samples converted total-N and total-P to ammonia and ortho-phosphate (APHA, 1975) which were measured by the Berthelot reaction and molybdenum blue method, respectively (Technicon Industrial System, 1971; Murphy and Riley, 1962; and Gales et al., 1966). Sulfate-S was measured by the methylthymol blue technique (Gales et al., 1968) after leaching through an ion exchange column (APHA, 1975). The molybdate-stannous chloride procedure was used to measure Si (Goelterman, 1969). Chloride was measured by using the ferricyanide method (Brian, 1962).

4. Results

Ion concentrations were expressed as geq L - ~ and an ion deficit was calculated as the difference between total cations and total anions. Total cations were calculated from the geq L - 1 of Ca, Na, Mg, K, Si, H, Fe, A1, and Mn. Iron, A1, and Mn were often below detection limits. Total anions were calculated from the geq L - a of H C O 3 - , SO4-S, C1-, NO3-N, PO4-P, and CO3 = (when present) (Table II).

Total cation concentrations were lower than the total anion concentrations for all sampling points, except precipitation. Cation deficits have also been measured on other Spodosol soils in Southeast Alaska (Johnson, 1981 ; Stednick and Johnson, 1982). Soil solution chemistry often exhibits an anion deficit with the deficit being attributed to organic acids. Solution chemistry at all sampling points was higher in total cation and anion concentrations than other similar studies (Sollins et al., 1980; Johnson etal . , 1977; Feller, 1977; Johnson, 1981).

Hydratedpolyvalent cations such as A1 and Fe may exist in soil solution. The attraction of the central ion for the water ligands is strong and the cations charge tends to repel the H + or protons of the water molecules. As the solution is made more alkaline, more H + tends to dissociate from the solvation sheath (Bohn et al., 1979). Increased deprotonation of compounds would occur at higher pH's. It is not known what organic compounds are present in these soil solutions and if they would deprotonate.

A range of organic acids and compounds would probably occur in solution and can only be treated as large unknown polymers. The relatively cool temperatures, short growing seasons, and high precipitation levels result in an organic layer of 10 to 60 cm deep, thus humus decomposition to humates may not be complete before leaching occurs. Humates are recognized for their negative charge, other organics have not been investigated to date. Most intermediates in soil organic matter decomposition are probably quickly metabolized since the rate-limiting step is the breakdown of complex molecules of humus (Alexander, 1961). However, the effect of large volumes of water percolating through the soil profile and the subsequent leaching effect on the intermed- iate products is not known.

Electrical conductivity was calculated using conductivity factors (APHA, 1975) for each ion present in the average composition of each sampling point (Table III). Measur- ed electrical conductivities were volume weighted and averaged. Their distributions were assumed to be normal and 80~o confidence intervals calculated.

SOIL SOLUTION CHEMISTRY IN A SOUTHEAST ALASKA SPODOSOL 267

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268 J.D. STEDNICK

TABLE III

Calculated and measured electrical conductivity (with 80~o confidence inter- val) for the forest floor, mineral soil and spring solutions

Location Calculated Measured conductivity conductivity

dS m -1 Oe 64 87 +_ 22.2 BC 125 172 _+ 21.4 Groundwater 157 183 + 19.2

The average calculated electrical conductivities were outside the 80~o confidence estimate of the measured conductivities for all sampled soil solutions. This suggests that the solution analysis was not complete in determining the chemical composition or that the organic compounds present may carry an electrical charge and add to solution conductivity. The analysis appeared complete based on present thinking of podzoli- zation processes. Therefore, it is suggested that organic compounds may be positively charged in these northern spodosols.

Solution differences color did not correlate with the cation deficit. Albeit, the differ- ences between measured and calculated electrical conductivities were related to the calculated cation deficit. Thus the repeated cation deficit in soil solution, suggests that the missing ion or ions are positively charged and may be attributed to the organic compounds.

5. Conclusions

Soil solution chemistry in a Sitka spruce-western hemlock forest on spodosol soils exhibited a cation deficit. Solution electrical conductivity was calculated using conduc- tivity factors for each ion measured. Analysis included N, P, and S species and Ca, Mg, Na, K, A1, Fe, Mn, Si, C1, and HCO3. Calculated solution electrical conductivities were lower than measured electrical conductivities with differences greater than expected measurement errors. Solutions exhibiting the cation deficit were colored by organic compounds. It is suggested that organic acids or other organic compounds in northern spodosol soils may be positively charged. The cool temperatures and relatively short growing seasons in Southeast Alaska may not allow organic material to decompose completely to humates before leaching. Humates are recognized for their negative charge. Incomplete organic decomposition, or intermediate steps may result in a range of large and generally unknown polymers.

References

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SOIL SOLUTION CHEMISTRY IN A SOUTHEAST ALASKA SPODOSOL 269

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