Buoyancy-driven flow in a peat moss layer as a mechanism for solute transport

Abstract

T he upper part of a living mire consists of a sponge-like layer of predominantly moss species, the acrotelm (1), with a porosity above 95%. The green and brownish plants near the surface ( Fig. 1) intercept light and fix CO 2 . Further down, the older plants turn yellow and start to decay. Aerobic decay in the acrotelm takes place relatively rapidly and makes nutrients available for recycling. Below the acrotelm, a denser layer, the catotelm, is present, where the hydraulic conductivity is much lower than in the acrotelm (2), and where the decay rate is several orders of magnitude smaller due to the anoxic conditions (3). It is the peat formation (4, 5) in the slowly growing catotelm that represents a sink of atmospheric CO 2 (5, 6).The production of organic matter at the surface largely depends on the recycling of nutrients originating from decomposing plant material. Because decomposition and photosynthesis take place at different depths, the transport of oxygen, carbon compounds, and nutrients forms an important element in the functioning of the mire ecosystem. This transport takes place both inside (7) and outside the plants by diffusion and fluid flow.In this paper, we investigate a mechanism for fluid flow in a water-saturated peat moss layer, which does not depend on capillarity or an external hydraulic pressure. During the night, the surface cools, leading to relatively cold water on top of warm water, and if the temperature drop is sufficiently large, the cold water sinks and the warm water rises. This type of flow is called buoyancy flow, and it implies convective transport of the heat and solutes carried with the water. Buoyancy flow often occurs as ''cells'' consisting of adjacent regions with upward and downward flow. We studied the phenomenon in a peat moss layer by means of a mathematical model, numerical simulation, and laboratory measurements. Model Equations and StabilityThe Mathematical Model. The model describes the heat flow in a water-saturated porous layer that undergoes periodic and sudden temperature changes at its surface.The imposed surface temperature involves the model parameters ⌬T, which is the temperature difference between ''day'' and ''night,'' and t 0 , which is the duration of each of the two periods. Four parameters describe geometrical and physical properties of the layer: the thickness H, the thermal expansion coefficient of the fluid ␣, the thermal diffusivity of the layer ‫ބ‬ eff , and the hydraulic conductivity K. Table 1 lists parameter values for the water-saturated peat moss layer used in the experiment.The equations for heat transport and fluid flow, together with the boundary conditions, are given in Appendix B. Here, we briefly discuss the physical content of the model equations in dimensionless form. Dimensionless temperatures T are expressed in units ⌬T and lie between 0 and ϩ1. Similarly, the time interval t 0 is used as the unit of time, and the distance ͌ ‫ބ‬ eff t 0 , as the unit of length. This length scale (Ϸ0.078 m) characterizes the distance over which a...

Field evidence for buoyancy-driven water flow in a Sphagnum dominated peat bog

“…We calculated the Rayleigh number for each layer according to Rappoldt et al (2003) by using the calculated hydraulic conductivity and a temperature difference of 8°C between day and night. All other parameter values for calculating the Rayleigh number were identical to those in Rappoldt et al (2003).…”

“…All other parameter values for calculating the Rayleigh number were identical to those in Rappoldt et al (2003).…”

“…However, Malmer (1993) considered upward transport of nitrogen and phosphorus trough capillary transport insignificant due to the low concentrations of these nutrients in the pore water. Rappoldt et al (2003) provided evidence for another external transport mechanism in a water-saturated peat moss layer. They showed that buoyancy-driven water flow could occur if the temperature difference between day and night is sufficiently large.…”

Heat transport in the Red Lake Bog, Glacial Lake Agassiz Peatlands

“…Chapman and Thurlow (1996) calculated for a bog in Scotland that an increase in surface temperature of 4Ð5°C might double CO 2 emissions and increase methane emissions by 60% based on observations of two peat sites. Rappoldt et al (2003) proposed that near-surface waters (less than 35 cm depth) in the peat column freely convect. In their study, water moved primarily because of daily density differences driven by diurnal temperature changes.…”

Cesium (137Cs and 133Cs), Potassium and Rubidium in Macromycete Fungi and Sphagnum Plants

“…The mechanism of radiocesium and alkali metal relocation within Sphagnum is probably the same active translocation as described for metabolites by Rydin & Clymo (1989). Although external buoyancy-driven transport (Rappoldt et al, 2003) could redistribute 137 Cs, field evidence suggests buoyancy creates a downward migration of K (Adema et al, 2006); thus, this mechanism appears unlikely. Likewise, a passive downwash and upwash (Clymo & Mackay, 1987) cannot explain accumulation towards the surface.…”