Abstract
Peat is formed in wet and acidic conditions, where net primary production exceeds the decomposition of organic matter. Peatlands cover a small part of the earth surface but hold vast amounts of carbon. Land use change and climate change can turn this large carbon sink into a carbon source, thereby
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generating a positive feedback for climate change. The peatlands in the Netherlands have a long history of drainage to facilitate agriculture. The downside of these practices is the oxidation of the organic soil, which leads to subsidence and associated desiccation of nature reserves, emission of greenhouse gasses, deterioration of surface water quality, increasing costs for water management and infrastructural maintenance, damage to building foundations, and, in the end, loss of the characteristic landscape. In this thesis, the effects of climate change on the decomposition of peat soils in the Netherlands are explored, focussing on the effects of summer drought and salinisation on peat decomposition. Throughout this research, the distinction is made between peat that was formed in minerotrophic versus oligotrophic conditions (fen and bog peat) and between two types of land use (agriculture and nature management). Because of climate change, it is expected that dry summers will occur more often. We showed that oxygenation of deep peat layers that had not previously been exposed to air led to acceleration of decomposition; moreover, this effect is still measurable in the period after such a dry summer. During dry summers, surface water originating from rivers or lakes is supplied to peat areas to prevent drying out of the soils and limit decomposition. However, during prolonged summer droughts, the river water has a poor quality and may become slightly brackish because of saltwater intrusion and evaporation. We showed that salinisation reduced aerobic decomposition rates and water quality deteriorated. Unexpectedly, no uniform effect of land use or peat origin on respiration rates was found. Nevertheless, substantial differences in microbial dynamics were found between land uses, whereas enzyme activities differed mostly between peat types. According to the enzymic latch theory, phenolic compounds and the enzyme phenol oxidase are the main determinants of decomposition rates. However, our results do not affirm the importance of this theory in Dutch peatlands. Subsidence rates were modelled for various case study areas and adaptations measured were explored with local stakeholders, resulting in several options for adaptive peatland management. For instance, freshwater storage and subsurface drainage could reduce the effects of summer droughts. Ploughing and the cultivation of deeply rooting crops should be minimised. Furthermore, it was recommended to stop adapting groundwater levels to the subsiding soil surface level in order to make a gradual transition to peatland regions with virtually no subsidence. In the meantime, research is needed on alternative crops that can grow in wetter conditions and farmers should be trained and guided in the process of implementing changes into their operational management. Research indicates that such a transition could result in profitable agricultural businesses with lower societal costs, less greenhouse gas emissions and better water quality.
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