Interaction of water table management and mice infestation on greenhouse gas emissions from intensively used grasslands on Histosol

During the last century, drainage turned the majority of the bogs and fens in Germany into productive agricultural land, causing substantial emissions of greenhouse gases (GHG). The project ‘SWAMPS’ focuses both on maintaining the trafficability for conventional intensive grassland use and on the reduction of GHG emissions by managing the groundwater level by submerged drains and blocked ditches. Here, we aim to evaluate the interaction of water table management and a severe mice infestation on the emissions of carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄).

We set up two field sites on both fen and bog peat in North-Western Germany. Submerged drains were installed at a distance of 4 to 5 m and with a target ditch level of 45 to 50 cm below mean soil surface. On the parcels with blocked ditches, the target ditch level is adjusted at 30 to 35 cm. The control parcels are drained by ditches and/or drainage pipes. Since 2017, diurnal CO₂ flux measurement campaigns have been realised once every three to four weeks with transparent and opaque chambers and a portable gas analyser. CH₄ and N₂O samples are taken biweekly and additionally more frequently after fertilizer application.

However, our experimental design was disrupted when, after an extremely dry summer and a dry and mild winter, the mice population grew strongly in 2019. We monitored both the number of mouse holes and the damage by mice. At the bog site, nearly no grass was left at the control site at the end of the year, while at the fen site, less, but still significant damage was observed. In this year, this was typical for the situation in North-Western Germany, where around 150,000 ha of grassland were severely damaged by mice. The sites with water table management were less effected by mice, but as food became scarce, they started to move into these wetter areas as well.

Despite higher water levels, CO₂ emissions in 2019 were partially higher than in previous years, especially at those sites affected by mice. With this presentation, we would like to discuss the effects of mice damage on soil respiration and on possibilities to disentangle water management effects from this (experimental and agricultural) calamity.