How are peat bogs affected by drainage?

Effect of drainage on peat properties (evaluation of the state before restoration)

The University of South Bohemia in České Budějovice (Department of Ecosystem Biology, Faculty of Science), as one of the four partners of the project, provides monitoring of soil processes in peat at selected localities.

Zuzana Urbanová a Tomáš Picek introduce the first results of monitoring activities carried out within the LIFE for MIRES project:

Our work mainly consists of soil sampling and evaluation of several physico-chemical properties that can show to what extent peat is degraded due to long-term drainage and how quickly it reacts to restoration (water level increase / re-wetting). These analyses should contribute to the evaluation of the success of the restoration measures which are being carried out. For these purposes, we have selected several indicators based on previous experience and these are: bulk density, peat moisture, pH, total content of carbon, nitrogen and phosphorus, biological parameters include microbial biomass, microbial production of carbon dioxide and methane and the decomposition rate of organic matter. In the following text we will focus on the first characteristic and that is the bulk density of peat.

One of the main indicators of peat “disturbance” is bulk density. This property tells us how heavy the peat is for a given volume in its natural state, i.e. how much the peat is decomposed, how many pores it has and how much water it retains. Undisturbed peat consists of undecomposed or only partially decomposed plant residues and is very fluffy, porous and retains a large amount of water. Up to 95 % of the peat volume can be water! When peat is exposed to a long-term draw down in the water level due to drainage, it is aerated and is “exposed” to aerobic microbial decomposition (decomposition with access to oxygen). Due to decomposition, it descends, loses porosity and thus the ability to bind such a large amount of water – its bulk density increases.

The results of monitoring before restoration clearly show a significant effect of long-term drainage on this physical property of peat. As part of our work, we took samples of peat from raised bogs (a type of peatland saturated only with rainwater), which were exposed to different intensity of drainage. We measured the properties of peat at undisturbed natural sites, which represent control plots and the state of “what it should look like”. We also sampled four bogs in different stages of degradation from slightly disturbed to very intensively drained.

The results clearly show the negative effect of drainage on the bulk density of peat. There was an increase in bulk density in moderately and heavily drained project sites. Peat has become more compact, it has lost a part of its pores, it is more decomposed and therefore it has lost part of its ability to retain water. Based on these data, we are able to say how much these sites are degraded, which may indicate how quickly and successfully their restoration will take place (e.g. changes in soil properties, vegetation, leaching of nutrients).

Changes in the structure of peat due to drainage are also evident when looking at the enclosed photographs, which show samples of peat from the surface to a depth of 50 cm from variously degraded bogs. The following picture shows a detail of peat from a depth of 10 to 30 cm from an undisturbed and intensively drained peat bog.

Detailed results, which show changes in the bulk density of peat due to drainage, are shown in the following graph. The individual colours represent different depths of peat collection (blue 0 – 10 cm; orange 10 – 30 cm; green 30 – 50 cm). The graph shows that the bulk density increased the most in the middle layer of peat 10 – 30 cm on moderately and intensively drained peatlands. The deeper layers of peat were affected by drainage only on intensively drained areas.