UPDATE: This experiment is now published as a paper in the journal Biogeosciences. It is open access and can be downloaded free-of-charge here. Check it out!
Distinguishing biogeochemical processes in oxidizing and desiccating mud deposits from lake Markermeer
At the moment, the chemical composition of the muddy layer is unknown. Moreover, when the mud is collected, it will switch from an anoxic to an oxic state, which will change the chemistry and microbe composition in the mud drastically. It is important to monitor the chemical changes during this redox transition to confirm that no release of heavy metals occur to the porewater. Also, the chemical characteristics after the transition needs to be monitored as they are indirectly linked to the speed of soil formation. It is important to check whether seedlings can develop in this mud. This will not only depend on the compactness, cohesiveness or viscosity of the mud, but also on its chemistry. In turn, plants will also affect the (bio)geochemistry in the rhizosphere by oxygen and exudate release through their roots. These plant-soil interactions will also determine the speed of soil formation. Distinguishing these abiotic and biotic factors are necessary in order to predict the most important processes influencing the speed of soil formation and ecosystem development on a larger scale.
The aim of this study is to distinguish biogeochemical processes in porewater during oxidation and desiccation of mud deposits from this lake.
What have I done? To explore which processes are important, a small-scale pilot experiment was conducted where I tested many treatments and measured a lot of variables. Because the water chemistry of lake Markermeer is rather unique in terms of metals and nutrients compared to other lakes in the Netherlands, I tested a groundwater fed system (Markermeer water) and a rainwater fed system. I constructed two basins filled with these two types of water. In these basins, I placed pots filled with three types of sediment: 1) fluffy mud, 2) fluffy mud mixed with sand and 3) underlying Southern Sea deposit. The pots were perforated at the bottom so that water could penetrate into the sediment. Reed seedlings (Phragmites australis) were planted in half of these pots. At two different depths in the pot, I installed rhizons to extract porewater: 1 cm (oxic) and 11 cm (anoxic) depth. With this design I could monitor the effects of porewater-, soil- and plant quality through time and determine important biogeochemical processes. Especially when this mud is desiccating and oxidizing.
Some nice things to conclude from this “pilot” experiment
Especially in the first few months, the reed was growing pretty fast. It appears that reed is growing faster on fluffy mud than on the other sediment treatments. The growing speed of plants is important as it is indirectly linked to soil formation and consolidation on the future islands.
I did not observe any heavy metal release from the sediment to the porewater (like aluminum, copper or zinc). Oxidation and desiccation processes of this mud does not lead to toxic release of heavy metals.
Important elements that do change through time are sulfate (SO4), iron(III) (Fe), nitrate (NO) and phosphate (PO4). Biogeochemical processes affecting these elements are important for nutrient availability and plant growth. Due to oxidation of the sediment, SO42-concentrations in porewater rose from 100 ppm at the beginning of the experiment to well over 2000 ppm at the end of November. This is a significant finding because sulfate and iron are closely linked to phosphate mobilization in the sediment. Phosphate, in turn, is a vital nutrient for plant growth. But balance is crucial: too few phosphate and plant growth will be hampered, too much phosphate and the ecosystem state will change (negatively).
Phosphate, as always in ecology, is important. Normally a lot of phosphate is present in the soil. However, a large part is unavailable for plants (and also for moss and algae) as the phosphate is bound to, for example, iron or calcium. Measuring these so called “phosphate pools” is important to determine what part of the unavailable phosphate can becomes available when conditions change (due to oxidation, rewetting, acidification, etc.). That is why I applied a phosphate fractionation on the sediment to see where the phosphate is hiding. In five steps the following fractions are determined:
- Exchangeable phosphate
- Iron-bound phosphate
- Authigenic and Ca-bound phosphate
- Detrital phosphate
- Organic phosphate
A lot of labwork is involved in the extraction procedure but in the end, the only thing that matters is how blue the filtrate becomes after adding a reagent (see picture). What I found is that the largest amount of phosphate is attached to iron. This is a significant finding considering the high amount of SO4 present in the porewater.
So why is this important? In this experiment, the top 10 cm of the soil is desiccating and oxidizing at a constant rate. In reality, the high marsh zone of the wetland will regularly inundate through water level fluctuation (in summer, the water level of lake Markermeer is 20 cm higher than in winter), wave action and heavy rainfall. When these sediments become inundated, oxygen will be replaced by water. This results in the reduction of SO4 to S2. S2 in turn causes reduction of Iron(III)(hydr)oxides and iron bound phosphate leading to phosphate release. So, upon rewetting, it is expected that a lot of phosphate is mobilized. Because a lot of SO4 and iron bound phosphate is present in these sediments, it is expected that a lot of phosphate will be released. In follow-up experiments, tests will be carried out with dynamic water level fluctuations. The precise effect will then be known. Also its indirect effects on plant growth, soil formation and consolidation.
The results of this experiment demonstrate that it is useful to conduct a pilot experiment to explore what we’re dealing with and what kind of processes are worth investigating.