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Effects of sulfate deposition on pore water dissolved organic carbon, nutrients, and microbial enzyme activities in a northern peatland
Citation:
Seifert-Monson, L., B. Hill, R. Kolke, T. Jicha, L. Lehto, AND C. Elonen. Effects of sulfate deposition on pore water dissolved organic carbon, nutrients, and microbial enzyme activities in a northern peatland. SOIL BIOLOGY AND BIOCHEMISTRY. Elsevier Science Ltd, New York, NY, 79:91-99, (2014).
Impact/Purpose:
Few studies have examined the influence of deposition chemistry on dissolved organic carbon production specifically in peatlands. Although dissolved organic carbon has been shown to decrease in conjunction with an increase in sulfate concentration during drought years, the decrease in dissolved organic carbon may be due to reduced solubility under conditions of increased ionic strength, a relationship that has been documented in past research. To further investigate the relationship between atmospheric deposition and dissolved organic carbon export in peatlands, we installed mesocosms to isolate small areas of a bog and amended them with sulfate. The pore water chemistry, peat stoichiometry and microbial enzyme activities of amended mesocosms were compared with those of unamended control mesocosms. We hypothesized that sulfate amendments would decrease dissolved organic carbon concentrations, a change that could drive a response of nitrogen (N) and/or phosphorus (P).We also measured microbial enzyme allocation, which has been shown to shift in conjunction with changes in dissolved organic carbon and nutrients. This shift allows the microbial community to compensate for a discrepancy between the community's stoichiometric ratio and that of its resources. This compensation occurs as the community produces fewer enzymes to acquire a relatively abundant nutrient in order to focus more energy to produce enzymes to acquire the more limiting nutrient. For example, previous studies have shown that the activity of phosphatase, an enzyme utilized for phosphorus acquisition, increases as the availability of phosphorus decreases, indicating that the enzyme was produced to compensate for a limitation of phosphorus. In addition to assessing the effects of current sulfate amendments on dissolved organic carbon concentration and enzyme activity, we also sampled peat that had received sulfate amendments 3-5 years prior to this study. The goal was to gauge how long recovery from sulfate enrichment might take by comparing the recovering peat to both control peat and peat currently amended with sulfate. We expected that if the system had recovered, dissolved organic carbon and nutrient concentrations and microbial enzyme allocation would resemble control peat. However, if recovery was ongoing, the concentration of dissolved organic carbon and nutrients and pattern of enzyme allocation would presumably fall somewhere between the controls and currently sulfate amended peat. While it is likely that there are multiple causes, both local and widespread, of increasing dissolved organic carbon export, this study shows that increased inputs of sulfate can lead to decreased concentrations of dissolved organic carbon, which holding other hydrological factors constant, translates to decreased dissolved organic carbon export. After a return to ambient sulfate deposition, dissolved organic carbon may increase to levels greater than those before sulfate enrichment. We observed a consistent response of total dissolved phosphorus with perhaps the most striking response occurring in recovering peat. The increase of total dissolved phosphorus in recovering peat was reflected in microbial enzyme activities and was large enough to significantly alter peat and pore water C:N:P ratios, indicating a possible shift from P limitation to N limitation. The system does not appear to be returning to its previous condition in a linear fashion and is following an unexpected trajectory, as shown in other studies of recovering ecosystems. While this study supports previous hypotheses regarding the response of dissolved organic carbon to increased sulfate deposition, it also raises new questions about the response of P, both during and following increased sulfate inputs.
Description:
Export of dissolved organic carbon from lakes and streams has increased throughout Europe and North America over the past several decades. One possible cause is altered deposition chemistry; specifically, decreasing sulfate inputs leading to changes in ionic strength and dissolved organic carbon solubility. To further investigate the relationship between deposition chemistry and dissolved organic carbon export in peatlands, a field experiment was conducted to compare the pore water chemistry and peat microbial enzyme activity of mesocosms receiving sulfate amendments to mesocosms receiving no additions. To consider how peatlands respond during recovery from increased inputs of sulfate, samples were also analyzed from an area of the same peatland that was previously amended with sulfate. Current additions of sulfate decreased dissolved organic carbon concentration and increased dissolved organic carbon aromaticity. Total dissolved phosphorus decreased in response to current sulfate amendments but was elevated in the area of the peatland recovering from sulfate amendment. The total dissolved phosphorus increase, which was reflected in microbial enzyme activity, may have shifted the system from P limitation to N limitation. This shift could have important consequences for ecosystem processes related to plant and microbial communities. It also suggests that the recovery from previous sulfate amendments may take longer than may be expected.
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Effects of sulfate deposition on pore water dissolved organic carbon, nutrients, and microbial enzyme activities in a northern peatland