Sulfur cycling in two Dutch moorland pools

Due to atmospheric acid deposition, the chemistry of many moorland pools has changed, resulting in changes in their fauna and flora. Most moorland pools are sensitive to acid loading because underlying and surrounding soils are low in chemical buffering capacity. Biological processes in the sediment like denitrification and SO 42-reduction are of major importance to internal alkalinization necessary to neutralize the atmospheric input of potential acidifying N and S components. This study deals with the cycling of sulfur in moorland pools and was aimed at a better understanding of the factors influencing the S cycle in these freshwaters.Characterization and quantification of sulfur pools in sediment and overlying water column. Determination of spatial variability of chemical characteristics related to S-cycling.In highly organic moorland pool sediment (mean C content of top 10 cm = 9.8% dwt) about 46% of the total S is in organic S form, whereas in sandy sediment (mean C content = 1.4% dwt) the organic S fraction makes up about 75% of the total S pool. The sediments of the moorland pools Gerritsfles and Kliplo, have been compared statistically with respect to total S and S-related sediment contents (Chapter 6). The pools differ significantly in their horizontal and vertical distribution of S. Statistical analyses indicate that spatial variation of S in Kliplo is due to organically bound S. For Gerritsfles spatial interrelation between C, S or Fe could not be recognized. As evidenced by this study, a choice for a measurement unit like mass fraction (g g -1) or volumetric mass (Mg cm -3) is crucial for the interpretation of spatial variability. In spatial studies of sediment constituents, it is essential to express contents of these constituents in volumetric mass fractions, since spatial sediment patterns are often obscured by spatial variations in bulk density. Taking into account spatial variability is concise for a proper quantification of S-budgets of pools or lakes.Determination of SO 42-reduction rates and transformation rates of SO 42-into organic and inorganic S fractions. Estimation of S oxidation rates in sediments and overlying water column.In GerritsfIes S cycling near the sediment/water boundary was investigated by measuring (1) SO 42-reduction rates in the sediment, (2) depletion of SO 42-in the overlying water column and (3) release of 35S from the sediment into the water column (Chapter 2). Two locations differing in sediment type (highly organic and sandy) were compared with respect to reduction rates and depletion of SO 42-in the overlying water.Sulfate reduction rates, estimated by diagenetic modelling and whole core 35SO 42-injection, ranged from 0.27 to 11.2 mmol m -2d -1. Rates of SO 42-consumption in the overlying water were estimated by changes in SO 42-concentration over time in in situ enclosures and varied from -0.51 to 1.81 mmol m -2d -1. Maximum rates of oxidation to SO 42-in July 1990 estimated by combination of SO 42-reduction rates and rates of in situ SO 42-uptake in the enclosed water column were 10.3 and 10.5 mmol m -2d -1at an organic rich site and at a sandy site respectively.Experiments with 35S 2-and 35SO 42-tracer suggested (1) a rapid formation of organically bound S from dissimilatory reduced SO 42-and (2) transport of mainly non-SO 42--S, from the sediment into the overlying water.Sulfate reduction rates in sediments with higher volumetric mass fraction of organic matter did not significantly differ from those in sediments with a lower mass fraction of organic matter.The role of inorganic S, with emphasis on pyrite, in the S cycle of Gerritsfles and Kliplo.In Gerritsfles and Kliplo, pyrite is the most important iron sulfide phase (Chapter 3).The redox status appeared to be a main factor in determining whether pyrite or a monosulfide, defined by the content of acid volatile sulfur (AVS), is formed. Sedimentary FeS 2 -S/AVS-S molar ratios in sediments of Gerritsfles and Kliplo, which are overlain by a continuously oxygenated water column, were 32 and 55 respectively whereas in other lakes, where stratification caused anaerobic conditions in the hypolininion, FeS 2 /AVS ratios were <1.Framboidal pyrite, presumably formed slowly through sulfurization of iron sulfide precursors is thought to be an important pathway of pyrite formation in the freshwater sediments of Gerritsfles and Kliplo. The presence of single-crystal pyrite indicates that pyrite in both sediments may also form rapidly. The close association of pyrite framboids and organic matter, and the undersaturation of bulk porewaters with respect to amorphous FeS suggest that the framboidal pyrite is formed at microsites within organic matter. Alternating reduced and more oxic conditions in the sediment will supply Fe 2+and zerovalent sulfur respectively for the formation of pyrite. That a large fraction of pyrite is in (dead) plant cells may be explained by preferential oxidation of pyrite at locations where pyrite is not so protected against O 2 intrusion, in contrast to the pyrite located inside organic matter microsites.Calculation of a chemical S-budget in both freshwater systems using models and chemical data of pool- and rainwater.The chemical composition of surface waters of Gerritsfies and Kliplo and of incident precipitation, were monitored from 1982 to 1990. Sulfur and water budgets were calculated using a hydrochemical model developed for well-mixed nonstratifying lakes (Chapter 4). In Gerritsfies and Kliplo respectively 70 and80%of the incoming S is lost to the sediment primarily through reduction of SO 42-indicating that SO 42-reduction is an important mechanism to buffer the incoming acidic S compounds. Total atmospheric deposition of S has decreased significantly after 1986 at both locations. A model describing the sulfur budget in terms of input, output and reduction/oxidation processes predicted a fast decrease of poolwater SO 42-concentrations after a decrease of atmospheric input. However, SO 42-concentrations in the surface water were lowered only slightly or remained constant so there must be an extra source of SO 42-to the water column. Two possible mechanisms can explain this extra source of SO 42-: desorption of SO 42-from the sediment and the release of SO 42-through desiccation of a part of the pool bottom after dry summers. Further investigations would be needed to study the relative importance of these mechanisms.Mineralization of sedimentary organic S'Me mineralization rate of organic S compounds in Gerritsfles was estimated to reveal its importance in the overall S cycle (Chapter 5).The mineralization rate of organic S was estimated indirectly from diagenetic modelling of pore water NH 4+depth profiles and the ratio of organic N and S contents. Since the mineralization rates of organic N and S do not follow the stoichiometry of the contents of organic N and S, it was only possible to estimate a maximum rate of organic S mineralization. The maximum rate of organic S mineralization was 25 mmol m -22 y -1, only 1-2% of the SO 42-reduction in Gerritsfles sediments so, mineralization of organic S therefore, appears to play a minor role in the overall S-cycle.