WP2.1 The Role of Microbial Decomposition in Peat Subsidence

Reed Peat De Onlanden, Groningen Peat Sampling Preserved reed
Deonlanden Deonlanden Reedpeat Deonlanden Preservedreed


Peatlands occupy approximately 7% of Dutch land which have been intensively used for agriculture and mining since 13th century. This led to substantial amount of peat loss and shift their carbon accumulator’s role to carbon emitters. Currently, peat loss and concomitant land subsidence threatens the ecosystem services of peatlands and the economy of the country (e.g., infrastructure damage, flood risk and salinization of groundwater) which requires an urgent, feasible and sustainable solutions. Peat loss results in land subsidence due to the known processes shrinkage, creep, consolidation and aerobic decomposition. The rates of these processes depend on groundwater level in the polders. Groundwater level determines the oxygen intrusion into the peat and thickness of the anoxic zone. Previous studies mostly focused on aerobic peat decomposition due to its relatively large contribution to land subsidence and greenhouse gas emissions. Nevertheless, some studies showed that microbial and physical processes that take place under anoxic conditions can also significantly contribute to land subsidence. Organic matter (OM) decomposition under anoxic environment might be slow and negligible for the emissions of greenhouse gasses but key for understanding land subsidence. Furthermore, interactions between physical land subsidence processes and OM decay have been neglected and might alter land subsidence predictions. Therefore, a deeper mechanistic understanding of land subsidence under oxic and anoxic conditions is necessary to have reliable subsidence models for future challenges.

My research focus in the LOSS project is to inspect the role of microbial decomposition in peat subsidence and its relationship with other subsidence processes (primary consolidation and creep). I will investigate exoenzyme activity and its interaction with phenolic compounds in the peat. Also, I will check how decomposition under oxic and anoxic conditions alters the physical characteristic of peat.

This sub-project is carried out by Duygu Tolunay (UU-BIO), and supervised by Dr. Mariet Hefting, Dr. Gilles Erkens and Prof. Dr. George Kowalchuk.

Peatlands and Land subsidence

Land subsidence is a sudden or gradual sink of an area of land. This is a natural process but the rate of subsidence can be facilitated with anthropogenic activities. Delta cities or countries like Netherlands are subject to many risks due to land subsidence such as flood risk. You can notice that if you cycle in the Netherlands sometimes you are cycling next to the water the suddenly you are higher or lower than water level. So if you check the Netherlands's surface elevation map, some places are 6.5 meter under mean sea level. When we compare this map with the soil map of the Netherlands, you will see peatlands are aligned with the elevation map. Where you have lower surface elevation you will find peatlands. The reason is that peatlands are intensively used for human activities.

Since peatlands are formed under water saturated conditions, many plant materials preserved which makes the peatlands high in organic carbon content which is also very suitable for energy use. However after 1850s most of the peat stores were used in the NL. Current peatlands are used for dairy farming, agricultural practices, building houses and roads and some of them are preserved as a natural area. Using peatlands for human practices requires controlling the water level in the peat. If we want to build houses or put heavy machines in the agricultural land, we need to lower the water table.Therefore, water levels in many peatlands in the NL fluctuates throughout the year. This creates oxic, temporary oxic and permanently anoxic zones in the peat profile. Intrusion of air triggers some land subsidence processes which are degradation of organic content which is decomposition, physical processes shrinkage, primary and secondary consolidation. And my research aims to reveal microbial decomposition mechanisms which boost the rate of land subsidence and how these mechanisms interacts with physical land subsidence processes. Based on these goals, I developed four main research questions:

  1. How does the inhibition role of phenolic compounds in botanically different peatlands change hydrolytic exoenzyme activities under oxic and anoxic conditions?
  2. What is the effect of sulfur and iron presence on the activity of oxidative exoenzymes under anoxic conditions?
  3. How does increase in effective stress alter the rate of microbial decomposition in botanically different peats?
  4. How does the microbial decomposition impact the physical characteristics of peat under water-saturated environment?

Phenolic compounds and microbial activity

As mentioned peatlands consist of dead plant materials. When these plants were alive, they produced phenolic compounds to protect themselves from herbivores and to continue their metabolic activities. Peat is also home for microorganisms. Some of these microorganisms can produce special units called exoenyzmes. These units are secreted to peat matrix and they play an important role on degradation of organic materials. Until now, there are many findings about the interaction of phenolic compounds and exoenzymes. There are two types of exoenzymes which are secreted by microorganisms in the peat. One is called hydrolytic exoenzymes. They are responsible from degrading the peat. However, phenolic compounds which are produced by plants can stop functioning of hydrolytic enzymes by binding to them. This mainly happens if the peat is water saturated (natural conditions). And the other type of exoenzymes oxidative enzymes can detach the phenolics from hydrolytic enzymes. So this is the basic dynamics in the peat. However there are so many unknown and contrasting results on how these interaction alters when we change the environment (temperature, water availability, oxygen availability and so on). In my first research question, I will investigate how different phenolic compounds might alter the activity of hydrolytic enzymes in a changing environment. To test this, I will collect fresh and degraded peat samples from 4 different field sites. I selected these sites based on the dominant plant species in the peat profile. Each plant family can have different phenolics compositions which might alter the inhibition effects. Especially what I expect is that peat type which is rich in complex phenolic compounds should play role in exoenzyme inhibition than the simple phenolic compounds.

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