Hilde B. M. Tomassen

Glasshouse vs field experiments: do they yield ecologically similar results for assessing N impacts on peat mosses?

• Peat bogs have accumulated more atmospheric carbon (C) than any other terrestrial ecosystem today. Most of this C is associated with peat moss (Sphagnum) litter. Atmospheric nitrogen (N) deposition can decrease Sphagnum production, compromising the C sequestration capacity of peat bogs. The mechanisms underlying the reduced production are uncertain, necessitating multifactorial experiments. • We investigated whether glasshouse experiments are reliable proxies for field experiments for assessing interactions between N deposition and environment as controls on Sphagnum N concentration and production. We performed a meta-analysis over 115 glasshouse experiments and 107 field experiments. • We found that glasshouse and field experiments gave similar qualitative and quantitative estimates of changes in Sphagnum N concentration in response to N application. However, glasshouse-based estimates of changes in production--even qualitative assessments-- diverged from field experiments owing to a stronger N effect on production response in absence of vascular plants in the glasshouse, and a weaker N effect on production response in presence of vascular plants compared to field experiments. • Thus, although we need glasshouse experiments to study how interacting environmental factors affect the response of Sphagnum to increased N deposition, we need field experiments to properly quantify these effects.

Restoration of cut-over bogs by floating raft formation: An experimental feasibility study

Abstract Stimulation of floating peat by the introduction of poorly humified peat from four cut-over bogs in The Netherlands was studied in a one-year outdoor experiment. The pH of the various peat substrates was increased by adding different amounts of lime: 0–2 to 4–8 g-CaCO3.kg−1 fresh peat. Both peat type and lime addition were found to affect the buoyancy of the peat substrates. Low nutrient concentrations in the peat, together with a high bulk density, proved to be unfavourable for creating floating peat. Three of the peat types proved to be too acidic (pH < 4.5) to produce sufficient methane (ca. 400–600 µmol.L−1), and buoyancy was only achieved if lime was mixed in with the peat. The smallest amount of lime added (2 g.kg−1 fresh peat) was sufficient to maintain buoyancy for at least one year. Lime addition did not stimulate nutrient mobilization by mineralization, although P, N and K concentrations in the peat water were relatively high. It is concluded that floating peat can be initiated by the introduction of poorly humified peat. If the peat substrates are too acidic, lime can be added to stimulate buoyancy of the peat. Results are discussed in relation to restoration prospects. Nomenclature: van der Meijden (1996); Daniels & Eddy (1990). Abbreviations: BV = Bargerveen reserve; DW = dry weight; HV = Haaksbergerveen reserve; MP = Mariapeel reserve; TP =Tuspeel reserve.

Effects and Empirical Critical Loads of Nitrogen for Europe

Empirical critical loads of nitrogen (N) were first presented in a background document for a workshop in 1992 in Sweden. Since their first presentation, the critical loads of N have been updated at regular intervals and for a large number of habitats. This chapter presents a brief history of the empirical critical loads and explains the process of determination of empirical critical loads for nitrogen and their reliability. For European habitats (defined as EUNIS and Natura 2000 habitat classes), current empirical critical loads for nitrogen are presented. For each of these habitats, the main effects of enhanced nitrogen inputs are discussed that have formed the basis for the determination of the empirical critical loads. Factors other than nitrogen, that may affect ecosystem processes or ecosystem functioning, are discussed as these may modify the nitrogen critical load under specific conditions.

Restoration of Raised Bogs: Mechanisms and Case Studies from the Netherlands

This chapter discusses and explains various peat bog restoration strategies relating to peat quality, water chemistry and hydrology based on case studies from the Netherlands. Inundation of bog remnants can lead to a rapid redevelopment of (floating) Sphagnum vegetation, usually when poorly humified Sphagnum peat is still present. After inundation, the peat either swells up to the newly created water table or becomes buoyant, in both cases creating a favourable substrate for Sphagnum mosses. Methane production rate and peat chemistry play an important role in the buoyancy of floating peat. The presence of (slightly) calcareous groundwater in the peat base may enhance the development of floating peat by stimulating decomposition processes. Alternatively, the growth of submerged Sphagnum species can also lead to the development of floating rafts. This depends on the availability of light and carbon dioxide in the water layer. Many bog remnants, however, only have strongly humified peat, which does not favour the redevelopment of Sphagnum carpets after deep inundation. On the other hand, most Sphagnum species appear to do very well on surface-soaked strongly humified peat, which is why shallow inundation is to be preferred in such cases. An important prerequisite for the successful restoration of bog remnants is the development of a hydrologically self-regulating acrotelm. Key species involved in this development are Sphagnum magellanicum, Sphagnum papillosum and Sphagnum rubellum. Since these species are usually very slow colonisers, introduction of key species can accelerate the development of a functional acrotelm.