N. R. Urban

Sulfur cycling in a forested Sphagnum bog in northern Minnesota

The mass balance and internal cycle of sulfur within a small forested,Sphagnum bog in northern Minnesota are presented here based on a 4-year record of hydrologic inputs and outputs (precipitation, throughfall, streamflow, upland runoff) and a 3-year measurement of plant growth and sulfur uptake. Concentrations and accumulation rates of inorganic and organic sulfur species were measured in porewater. The bog is a large sink for sulfur, retaining 37% of the total sulfur input. Because of the relatively large export of organic S (21% of inputs), retention efficiency for total-S (organic S + SO4=; 37%) is less than that for SO4= (58%). There is a dynamic cycle of oxidation and reduction within the bog. Annual oxidation and recycling of S is equal to total inputs in the center of the bog. Plants receive 47% of their uptake requirement from atmospheric deposition, 5% from retranslocation from foliage, and the remainder from sulfur remineralized from peat. Mineralization is most intense in the aerobic zone above the water table. Inorganic sulfur species comprise <5% of the total sulfur burden within the peat.

Proton Cycling in Bogs: Geographic Variation in Northeastern North America

A detailed hydrogen ion budget has been constructed for the Marcell bog in north-central Minnesota based on a 5-year, intensive study of element cycles. Major features of the acidity balance for this site include the following: (1) production of organic acids (263 meg.m−2.y−1) is the dominant source of acidity and serves to buffer the bog water at pH 4; (2) seguestering of elements in peat is also a significant source of acidity (42.9 meg.m−2.y−1); (3) weathering of dustfall inputs is an important source of alkalinity (<76 meg.m−2.y−1) at this site which is situated near the major agricultural area of the plains; (4) nitrate and sulphate reduction contribute little alkalinity (<39.2 meg.m−2.y−1) because inputs (NO3 and SO4) to this bog are low. Analysis of peat and surface water from bogs across northeastern North America (Manitoba to Newfoundland) reveals the following: (1) production of organic acids across this region varies between 104 and 263 meg.m−2.y−1; (2) acidity-generation associated with net biological uptake (NBU, excluding nitrogen = 20–117 meg.m−2.y−1) varies in proportion to the rate of peat accumulation; (3) NBU-acidity exhibits high values in maritime bogs and lower values in mid-continental bogs; (4) bogs have a large capacity for sulphate reduction, and sulphate reduction becomes an increasingly important source of alkalinity as rates of sulphate deposition increase. From 60 to 93% of annual sulphate loadings are retained as reduced sulphur in bogs across eastern North America.

Aluminum geochemistry in peatland waters

The chemical speciation of aluminum was examined in surface water samples from Sphagnum peatlands in north-central Minnesota, from peatlands along the Canadian east coast, and from bogs in the Pennine Mountain area of England. In highly organic ([DOC]≈ 50 mg L−1 ), low pH waters, 80–90% of total dissolved Al was complexed with organic matter (OM), while in waters with low DOC ([DOC] ≈ 5 mg L−1) 54–86% of total dissolved Al existed as Al+3 or other inorganic Al species. Batch titrations of OM with Al revealed a high Al binding capacity, 1.4–2.8 μmol (mg DOC)−1, that generally was unsaturated with Al. Titrations of OM with Al in conjunction with a continuous distribution model were used to determine Al-OM conditional stability constants. Binding capacity (μmol Al (mg DOC)−1) and strength (formation constant) increased from pH 3 to 5 but decreased above pH 5 due to formation of AI-hydroxy species including A1(OH)3 (s). The high binding capacity of OM in bog waters facilitates metal mobility, especially in low pH (< 5) wetlands where metal solubility is high and OM concentrations are highest. Results showed that the relative degree of organic matter saturation with metal ions was important in modeling AI speciation in bog waters.

Sulfur cycling in the water column of Little Rock Lake, Wisconsin

The S cycle in the water column of a small, soft-water lake was studied for 9 years as part of an experimental study of the effects of acid rain on lakes. The two basins of the lake were artificially separated, and one basin was experimentally acidified with sulfuric acid while the other served as a reference or control. Spatial and seasonal patterns of sulfate uptake by plankton (53–70 mmol m−2 yr−1), deposition of sulfur to sediments in settling seston (53 mmol m−2 yr−1), and sulfate diffusion (0–39 mmol m−2 yr−1) into sediments were examined. Measurements of inputs (12–108 mmol m−2 yr−1) and outputs (5.5–25 mmol m−2 yr−1) allowed construction of a mass balance that was then compared with rates of S accumulation in sediments cores (10–28 mmol m−2 yr−1) and measured fluxes of S into the sediments. Because of the low SO42− concentrations (µmole L−1) in the lake, annual uptake by plankton (53–70 mmol m−2 yr−1) represented a large fraction (>50%) of the SO42− inventory in the lake. Despite this large flux through the plankton, only small seasonal fluctuations in SO42− concentrations (µmole L−1) were observed; rapid mineralization of organic matter (half-life <3 months) prevented sulfate depletion in the water column. The turnover time for sulfate in the water column is only 1.4 yr; much less than the 11-yr turnover time of a conservative ion in this seepage lake. Sulfate diffusion into and reduction in the sediments (0–160 µmole m−2 d−1) caused SO42− depletion in the hypolimnion. Modeling of seasonal changes in lake-water SO42− concentrations indicated that only 30–50% of the diffusive flux of sulfate to the sediments was permanently incorporated in solid phases, and about 15% of sulfur in settling seston was buried in the sediments. The utility of sulfur mass balances for seepage lakes would be enhanced if uncertainty about the deposition velocity for both sulfate aerosols and SO2, uncertainty in calculation of a lake-wide rate of S accumulation in sediments, and uncertainty in the measured diffusive fluxes could be further constrained.