Jeffery S. Riggs

Soil Respiration and Litter Decomposition

Emissions of CO2 from the organic and mineral soil horizons into the atmosphere are considered a good representation of soil respiration (R soil) when the diffusion-driven losses to the atmosphere are at equilibrium with the biological rates of CO2 production (Hanson et al. 2000). R soil is the net effect of the biological activity of autotrophic roots and associated rhizosphere organisms plus the mineral soil heterotrophic activity of bacteria, fungi, and soil fauna. Whereas the activity of soil heterotrophic organisms is proportionate to the decomposition of soil carbon, CO2 lost from root and rhizosphere activity is a function of the consumption of organic compounds supplied by aboveground organs of plants (Horwath et al. 1994). Notwithstanding the complex nature of the sources of CO2 contributing to it, R soil represents a good measure of the collective response of root and soil biological activity to environmental conditions (Edwards et al. 1970; Hanson et al. 2000). Because soil respiration is a very large fraction of gross primary productivity (Curtis et al., 2002), its quantification must be a high priority in any attempt to establish carbon budgets for ecosystems (see Chapter 22, this volume).

AIR FLOW AND HEAT TRANSFER IN A TEMPERATURE-CONTROLLED OPEN TOP ENCLOSURE

A large 12-meter-diameter open top enclosure (OTE) equipped with two unique belowground and above ground heating systems was built and intensively tested in Oak Ridge, TN, USA. The OTE is a prototype for use within an environmental change experiment, in which replica units will be built in Minnesota to assess the response of northern peatland ecosystems to increases in temperature and elevated atmospheric CO2. For several months, temperatures, energy, wind speed and relative humidity were monitored throughout the enclosure space to assess the enclosure performance and efficiency. In parallel, Computational Fluid Dynamics (CFD) simulations were performed with ANSYS-CFX to investigate the impacts of external wind, buoyancy, and OTE design on the temperatures achieved within the enclosure. The addition of a frustum that partially reduced the top opening was also investigated experimentally and numerically. The OTE is capable of achieving a temperature differential of at least +6°C for air using a combination of 8 electrical heaters. Differential temperatures were sustained for several months. The experimental data and the numerical results showed that the addition of a frustum dramatically decreases the operating cost of the OTE and leads to better control over the differential air temperature in the enclosure. Buoyancy forces and winds heavily impacted enclosure performance. It was also found that the heating efficiency of the OTE depends mainly on the wind speed, and that there exists a critical wind speed at which the heating efficiency is the highest.Copyright