Christine Alewell

The biogeochemistry of sulfur at Hubbard Brook

A synthesis of the biogeochemistry of S was done during 34 yr(1964–1965 to 1997–1998) in reference and human-manipulated forestecosystems of the Hubbard Brook Experimental Forest (HBEF), NH. There have beensignificant declines in concentration (−0.44µmol/liter-yr) and input (−5.44mol/ha-yr)of SO42− in atmospheric bulk wet deposition, and inconcentration(−0.64 µmol/liter-yr) an d output (−3.74mol/ha-yr) of SO42− in stream water ofthe HBEF since 1964. These changes arestrongly correlated with concurrent decreases in emissions of SO2from the source area for the HBEF. The concentration and input ofSO42− in bulk deposition ranged from a low of 13.1µmol/liter (1983–1984) and 211 mol/ha-yr(1997–1998) to a high of 34.7 µmol/liter(1965–1966) and 479 mol/ha-yr (1967–1968), with along-term mean of 23.9 µmol/liter and 336mol/ha-yr during 1964–1965 to 1997–1998. Despiterecentdeclines in concentrations, SO42− is the dominantanion in both bulk deposition and streamwater at HBEF. Dry deposition is difficult to measure, especially inmountainousterrain, but was estimated at 21% of bulk deposition. Thus, average totalatmospheric deposition was 491 and 323 mol/ha-yr during1964–1969 and 1993–1998, respectively. Based on the long-termδ34S pattern associated with anthropogenic emissions,SO42− deposition at HBEF is influenced by numerousSO2sources, but biogenic sources appear to be small. Annual throughfall plusstemflow in 1993–1994 was estimated at 346 molSO42−/ha. Aboveground litterfall, for thewatershed-ecosystemaveraged about 180 mol S/ha-yr, with highest inputs (190 molS/ha-yr) in the lower elevation, more deciduous forest zone. Weatheringrelease was calculated at a maximum of 50 mol S/ha-yr. Theconcentration and output of SO42− in stream waterranged from a low of 42.3µmol/liter (1996–1997) and 309 mol/ha-yr(1964–1965), to a high of 66.1 µmol/liter(1970–1971) and 849 mol/ha-yr (1973–1974), with along-term mean of 55.5 µmol/liter and 496mol/ha-yr during the 34 yrs of study. Gross outputs ofSO42− in stream water consistently exceeded inputsin bulkdeposition and were positively and significantly related to annualprecipitationand streamflow. The relation between gross SO42−output and annual streamflow changed with time asatmospheric inputs declined. In contrast to the pattern for bulk depositionconcentration, there was no seasonal pattern for streamSO42− concentration. Nevertheless, stream outputs ofSO42− were highly seasonal, peaking during springsnowmelt, andproducing a monthly cross-over pattern where net hydrologic flux (NHF) ispositive during summer and negative during the remainder of the year. Nosignificant elevational pattern in streamwaterSO42− concentration was observed. Mean annual,volume-weightedsoil water SO42− concentrations were relativelyuniform by soil horizon andacross landscape position. Based upon isotopic evidence, much of theSO42− entering HBEF in atmospheric depositioncycles throughvegetation and microbial biomass before being released to the soil solution andstream water. Gaseous emissions of S from watershed-ecosystems at HBEF areunquantified, but estimated to be very small. Organic S (carbon bonded andestersulfates) represents some 89% of the total S in soil at HBEF. Some 6% exists asphosphate extractable SO42− (PSO4).About 73% of the total S in the soilprofile at HBEF occurs in the Bs2 horizon, and some 9% occurs in the forestfloor. The residence time for S in the soil was calculated to be ∼9 yr, butonly a small portion of the total organic soil pool turns over relativelyquickly. The S content of above- and belowground biomass is about 2885mol/ha, of which some 3–5% is in standing dead trees. Yellowbirch, American beech and sugar maple accounted for 89% of the S in trees, with31% in branches, 27% in roots and 25% in the lightwood of boles. The pool of Sin living biomass increased from 1965 to 1982 due to biomass accretion, andremained relatively constant thereafter. Of current inputs to the availablenutrient compartment of the forest ecosystem, 50% is from atmospheric bulkdeposition, 24% from net soil release, 11% from dry deposition, 11% from rootexudates and 4% is from canopy leaching. Comparing ecosystem processes for Sfrom 1964–1969 to 1993–1998, atmospheric bulk deposition decreasedby 34%, stream output decreased by 10%, net annual biomass storage decreased by92%, and net soil release increased by 184% compared to the 1964–1969values. These changes are correlated with decreased emissions of SO2from the source area for the HBEF. Average, annual bulk deposition inputsexceeded streamwater outputs by 160.0 ± 75.3 SD molS/ha-yr,but average annual net ecosystem fluxes (NEF) were much smaller, mostlynegativeand highly variable during the 34 yr period (−54.3 ± 72.9 SDmol S/ha-yr; NEF range, +86.8 to −229.5). While severalmechanisms may explain this small discrepancy, the most likely are netdesorption of S and net mineralization of organic S largely associated with theforest floor. Our best estimates indicate that additional S from dry depositionand weathering release is probably small and that desorption accounts for about37% of the NEF imbalance and net mineralization probably accounts for theremainder (∼60%). Additional inputs from dry deposition would result fromunmeasured inputs of gaseous and particulate deposition directly to the forestfloor. The source of any unmeasured S input has important implications for therecovery of soils and streams in response to decreases in inputs of acidicdeposition. Sulfate is a dominant contributor to acid deposition at HBEF,seriously degrading aquatic and terrestrial ecosystems. Because of the strongrelation between SO2 emissions and concentrations ofSO42− in both atmospheric deposition and streamwater at HBEF,further reductions in SO2 emissions will be required to allowsignificant ecosystem recovery from the effects of acidic deposition. Thedestruction or removal of vegetation on experimental watershed-ecosystems atHBEF resulted in increased rates of organic matter decomposition andnitrification, a lowering of soil and streamwater pH, enhancedSO42− adsorption on mineral soil and smallerconcentrations andlosses of SO42− in stream water. With vegetationregrowth, this adsorbedSO42− is released from the soil, increasingconcentrations andfluxes of SO42− in drainage water. Streamwaterconcentration ofSO42− and gross annual output ofSO42−/ha are essentially the same throughout theHubbard BrookValley in watersheds varying in size by about 4 orders of magnitude, from 3 to3000 ha.

Is acidification still an ecological threat

There has been a significant reduction in anthropogenic acid deposition in Europe and North America, and now we need to gauge the rate and extent of ecosystem recovery. Stoddard et al. have reported a widespread aquatic recovery from acidification in European ecosystems in response to a fall in sulphate deposition. But many sites in central Europe are showing a significant delay in aquatic recovery from acidification, or even no recovery at all, and only some of them show biological recovery of waters or a recovery from soil acidification. Ecosystem management still needs to consider the consequences of acidification.

Sources of stream sulfate at the Hubbard Brook Experimental Forest: Long-term analyses using stable isotopes

Sulfur deposition in the northeastern U.S. has been decreasing since the 1970s and there has been a concomitant decrease in the SO42− lost from drainage waters from forest catchments of this region. It has been established previously that the SO42− lost from drainage waters exceeds SO42− inputs in bulk precipitation, but the cause for this imbalance has not been resolved. The use of stable S isotopes and the availability of archived bulk precipitation and stream water samples at the Hubbard Brook Experimental Forest (HBEF) in New Hampshire provided a unique opportunity to evaluate potential sources and sinks of S by analyzing the long-term patterns (1966–1994) of the δ34S values of SO42−. In bulk precipitation adjacent to the Ecosystem Laboratory and near Watershed 6 the δ34S values were greater (mean: 4.5 and 4.2l, respectively) and showed more variation (variance: 0.49 and 0.30) than stream samples from Watersheds 5 (W5) and 6 (W6) (mean: 3.2 and 3.7j; variance: 0.09 and 0.08, respectively). These results are consistent with other studies in forest catchments that have combined results for mass balances with stable S isotopes. These results indicate that for those sites, including the HBEF, where atmospheric inputs are ≤10 kg S ha−1 yr−1, most of the deposited SO42− cycles through the biomass before it is released to stream water. Results from W5, which had a whole-tree harvest in 1983–1984 showed that adsorption/desorption processes play an important role in regulating net SO42− retention for this watershed-ecosystem. Although the isotopic results suggest the importance of S mineralization, conclusive evidence that there is net mineralization has not yet been shown. However, S mass balances and the isotopic result are consistent with the mineralization of organic S being a major contributor to the SO42− in stream waters at the HBEF.

Effects of reduced atmospheric deposition on soil solution chemistry and elemental contents of spruce needles in NE—Bavaria, Germany

The decrease in anthropogenic deposition, namely SO42- and SO2, in European forest ecosystems during the last 20 years has raised questions concerning the recovery of forest ecosystems. The aim of this study was to evaluate if the long term data of element concentrations at the Fichtelgebirge (NE-Bavaria, Germany) monitoring site indicates a relationship between the nutrient content of needles and the state of soil solution acidity. The soil at the site is very acidic and has relatively small pools of exchangeable Ca and Mg. The trees show medium to severe nutrient deficiency symptoms such as needle loss and needle yellowing. The Ca and Mg concentrations in throughfall decreased significantly during the last 12 years parallel to the significant decline in the throughfall of H+ and SO42- concentrations. Soil solution concentrations of SO42-, Ca and Mg generally decreased while the pH value remained stable. Aluminum concentrations decreased slightly, but only at a depth of 90 cm. Simultaneously a decrease in the molar Ca/Al and Mg/Al ratios in the soil solution was observed. Ca and Mg contents in the spruce needles decreased, emphasizing the relevance of soil solution changes for tree nutrition. The reasons for the delay in ecosystem recovery are due to a combination of the following two factors: (1) the continued high concentrations of NO3— and SO42— in the soil solution leading to high Al concentrations and low pH values and, (2) the decreased rates of Ca and Mg deposition cause a correlated decrease in the concentration of Ca and Mg in the soil solution, since little Ca and Mg is present in the soils exchangeable cation pools. It is our conclusion that detrimental soil conditions with respect to Mg and Ca nutrition as well as to Al stress are not easily reversed by the decreasing deposition of H+ and SO42—. Thus, forest management is still confronted with the necessity of frequent liming to counteract the nutrient depletion in soils and subsequent nutrient deficiencies in trees.

Use of objective criteria for the assessment of biogeochemical ecosystem models

Ecosystem modeling is confronted with complex biological systems and changing environmental conditions. A model which describes ecosystem behavior under all conditions has not been found yet and there does not exist one `true model for a specific ecosystem. Often ecosystem models describe the measured data more or less well, but most judging criteria for model performance are rather subjective. Furthermore, from a mathematical view point the calibrations of ecosystem models are hardly ever unique. The aim of this study was to develop and use criteria which permit an objective comparison of different models to the observed field data and to each other. A given model which describes a specific system significantly better will be declared the `valid model while the other will be rejected. The term `valid is used here in a sense that any model that could not be proven invalid would be a valid model for the system. We used the biogeochemical soil models MAGIC (Cosby, B.J., Hornberger, G.M., Wright, R.F., 1985. Modelling the effects of acid deposition: assessment of a lumped-parameter model of soil water and stream water chemistry. Water Resour. Res. 21, 51–63) and the SO-Model (derived from the Batch Equilibrium Model (BEM; Prenzel, J., 1991. Introduction to BEM (Batch Equilibrium Model), vol 28. Berichte des Forschungszentrums Waldokosysteme/Waldsterben, Gottingen, 51 pp.). The data set used was the soil solution chemistry in a forest ecosystem of the Solling area (North-West Germany). To test the performance of the models four criteria were used: the efficiency (Martinec, J., Rango, A., 1989. Merits of statistical criteria for the performance of hydrologic models. Water Resour. Bull. 25 (2), 421–432; Hinzman, L.D., Kane, D.L., 1991. Snow hydrology of a headwater artic basin; 2. Conceptual analysis and computer modelling. Water Resour. Res. 27, 1111–1121), the Normalized Mean Absolute Error (NMAE, given by Janssen, P.H.M., Heuberger, P.S.C., 1995. Calibration of process orientated models. In: van Grinvsen, J.J.M. (Ed.), Modelling Water, Carbon and Nutrient Cycles in Forests: Application of 16 Simulation Models to a Spruce Stand at Solling, Germany. Ecological Modelling, vol. 83, pp. 55–66), the confidence interval test (CIT, developed in this study) and the model rejection criteria (Sun, N.Z., 1994. Inverse Problems in Groundwater Modelling. Dordrecht, 337 pp.). Whereas the efficiency and NMAE are related to the averaged data, the CIT and the model rejection criteria include the spatial heterogeneity at every time step. When evaluated visually, both model results might be accepted. From the application of the model performance criteria we selected the MAGIC model as the `valid model for our system.

Use of stable isotope ratios for evaluating sulfur sources and losses at the Hubbard Brook Experimental Forest

Anthropogenic S emissions have been declining in eastern North America since the early 1970s. Declines in atmospheric S deposition have resulted in decreases in concentrations and fluxes of SO42− in precipitation and drainage waters. Recent S mass balance studies have shown that the outflow of SO42− in drainage waters greatly exceeds current S inputs from atmospheric deposition. Identifying the S source(s) which contribute(s) to the discrepancy in watershed S budgets is a major concern to scientists and policy makers because of the need to better understand the rate and spatial extent of recovery from acidic deposition. Results from S mass balances combined with model calculations and isotopic analyses of SO42− in precipitation and drainage waters at the Hubbard Brook Experimental Forest (HBEF) suggest that this discrepancy cannot be explained by either underestimates of dry deposited S or desorption of previously stored SO42−. Isotopic results suggest that the excess S may be at least partially derived from net mineralization of organic S as well as the weathering of S-bearing minerals.

Spotting zones of dissimilatory sulfate reduction in a forested catchment: the 34S-35S approach

The localization of sulfate reducing sites in forested catchments is of major importance, because dissimilatory sulfate reduction can be a considerable sink for deposited sulfate. To localize dissimilatory sulfate reduction sites in a forested catchment (northeastern Bavaria, Germany), three sites within the catchment (upland site, intermittent seep, fen) were investigated for delta 34S depth profiles of soil sulfur and potential sulfate reduction rates were measured with 35S radiolabeling. Stable sulfur isotopes indicate that aerobic metabolism is the dominant process on the upland site and the intermittent seep (delta 34S of soil sulfur between +1.6 and +9.0@1000) and dissimilatory reduction is not a significant sink for sulfate. However, results of the 35S radiolabeling indicated for the upland site that the soil has potentially high sulfate reduction rates under laboratory conditions. Soil sulfur of the fen was markedly depleted in 34S (delta 34S between -6 and +2.6@1000). Both, 34S and 35S data indicated that dissimilatory sulfate reduction is an ongoing process on this site. The 34S and 35S approaches are complementary. While measurements using 35S can show momentary potential for dissimilatory bacterial sulfate reduction, delta 34S data reflect long-term predominance of either assimilatory or dissimilatory S metabolism at a particular site.

Predicting Reversibility of Acidification: The European Sulfur Story

Because of the deleterious effects of acid rain and the need to predict reversibility of acidification, various scientific tools such as modeling, stable isotopes and flux/budget calculations have been used in biogeochemical sulfur (S) research. The aim of this study was to evaluate consistencies and discrepancies between these different tools. While modeling has been seemingly successful in predicting S dynamics in soil solution and stream water by considering inorganic sulfate sorption and desorption only, stable S isotopes indicate that biological S turnover plays a crucial role for the sulfate released to soil solution and stream water. A comparison of budget calculations with soil S pools reveals that inorganic sulfate sorption and desorption are the controlling processes as long as deposition is high (> 15 kg S ha−1yr−1) and soils have a high sulfate sorption capacity. This explains the successful model predictions of the last two decades. However, for soils with low sulfate sorption capacity and under low sulfate deposition, organic S seems to be a significant source for stream water sulfate and has to be considered in future modeling.

Patterns of stable S isotopes in a forested catchment as indicators for biological S turnover

Despite intensive biogeochemical research during the last thirty years, the relative importance of biological S turnover for the overall SO42− budget of forested catchments remains uncertain. The objective of the present study was (i) to gain new insight into the S cycle of theLehstenbach catchment (Northeastern Bavaria, Germany) through the analysis of stable isotopes of S and (ii) to differentiate between sites which are ‘hot spots’ for SO42− reduction and sites where mineralization and adsorption/desorption processes are more important. The δ34S values and SO42− concentrations of soil solutions, throughfall and groundwater at four different sites as well as runoff of the catchment were measured. The relatively low variability of δ34S in throughfall and bulk precipitation was in contrast to the high temporal and spatial variability of δ34S in the soil solution. Sulfate in the soil solution of upland sites was slightly depleted in34S compared to input values. This was most likely due to S mineralization. Sulfate in the soil solution from wetland soils was clearly enriched in34S, indicating dissimilatory SO42− reduction. The observed spatial and temporal patterns of34S turnover and SO42− concentrations might explain the overall balanced S budget of the catchment. At a time of decreasing anthropogenic deposition SO42− is currently released from upland soils. Furthermore, mineralization of organic S may contribute to SO42− release. Wetland soils in the catchment represent a sink for SO42− due to dissimilatory SO42− reduction.

Trends in deposition and canopy leaching of mineral elements as indicated by bulk deposition and throughfall measurements

In the past three decades, numerous studies on the biogeochemistry of forested ecosystems in Europe and North America have shown that the deposition of mineral elements from the atmosphere strongly influences their functioning. Acidification of soils, surface- and groundwaters, N saturation and forest decline are key processes that change with rates of deposition of mineral elements (Ulrich 1994; Fenn et al. 1998; Evans et al. 2001). As an example of ecosystem functioning, the losses of elements from the ecosystem by seepage and runoff can be considered. On a European-wide, scale the deposition of S and N was shown to determine the Al losses from seepage and runoff in acid forest soils (Dise et al. 2001), the N deposition to determine the NO3 losses (MacDonald et al. 2002) and the Mg deposition to largely determine the Mg losses (Armbruster et al. 2002).