Soil respiration is derived from heterotrophic (decomposition of soil organic matter) and autotrophic (root/rhizosphere respiration) sources, but there is considerable uncertainty about what factors control variations in their relative contributions in space and time. We took advantage of a unique whole-ecosystem radiocarbon label in a temperate forest to partition soil respiration into three sources: (1) recently photosynthesized carbon (C), which dominates root and rhizosphere respiration; (2) leaf litter decomposition and (3) decomposition of root litter and soil organic matter >1-2 years old. Heterotrophic sources and specifically leaf litter decomposition were large contributors to total soil respiration during the growing season. Relative contributions from leaf litter decomposition ranged from a low of ~1 ±3% of total soil respiration (6 ±3 mg C m^-2 hr^-1) when leaf litter was extremely dry, to a high of 42 ±16% (96 ±38 mg C m^-2 hr^-1). Total soil respiration fluxes varied with the strength of the leaf litter decomposition source, indicating that moisture-dependent changes in litter decomposition drive variability in total soil respiration fluxes. Root/rhizosphere respiration accounted for 16 ±10% to 64 ±22% of total soil respiration, with highest relative contributions coinciding with low overall soil respiration fluxes. In contrast to leaf litter decomposition, root respiration fluxes did not exhibit marked temporal variation ranging from 34 ±14 to 40 ±16 mg C m^-2 hr^-1 at different times in the growing season with a single exception (88 ±35 mg C m^-2 hr^-1). Radiocarbon signatures of root respired CO₂ changed markedly between early and late spring (March vs. May), suggesting a switch from stored nonstructural carbohydrate sources to more recent photosynthetic products.

Stand-replacing crown fires in boreal spruce forests initiate a vegetation succession from forbs to deciduous trees to coniferous trees. Soils are warmest during the first decades and cool throughout the succession as shading by trees and cover with bryophytes and plant litter increase. It was postulated that the initially warmer soil temperatures enhance decomposition of soil organic matter (SOM) by microorganisms, and that decomposition would release similar amounts of CO₂ as combustion during fire [Auclair and Carter, 1993].

To evaluate the role of root respiration (Rr), we measured spatial and temporal variation of Rr. We measured root biomass, Rr and soil respiration (Rs) in temperature deciduous forest in central Japan. The size dependence of Rr was shown and Rr in fine root (< 2 mm) accounted more than half of total Rr per unit area. Moreover, we had measured continuously Rr and Rs using automated system. Rr responded exponentially to soil temperature. High soil moisture during and just after rainfall caused limiting factor in Rr. And the contribution of Rr to Rs changed seasonally.

The overarching goal of a long-term, multi-investigator, regional study of ecosystem-atmosphere carbon cycling in a mixed forest ecosystem in the upper Midwest of the USA is to observe ecosystem-atmosphere exchange of carbon dioxide at scales of relevance to the global carbon balance, while simultaneously understanding the mechanisms governing this exchange. This study, the Chequamegon Ecosystem-Atmosphere Study (ChEAS), brings together multiple approaches to observing carbon fluxes, including chamber flux, sap flux and biometric measurements at the plot scale (~1 m²), multiple stand-level (~1 km²) eddy-covariance flux towers, landscape-scale (~10-100 km²) eddy-covariance flux measurements from the WLEF tall tower, multiple regional (10³-10⁵ km²) atmospheric boundary layer (ABL) budget approaches using tall tower mixing ratio measurements, and a regional (~10⁵ km²) ABL budget using a network of CO₂ mixing ratio measurements on communications towers. Flux measurements have been up-scaled to the region using a variety of approaches, and compared to the regional ABL budget methods. Top-down and bottom-up methods fall within a range of values for growing-season flux estimates that suggests a level of precision for regional flux estimates of approximately 0.5 gC m^-2 d^-1. A multi-tower inverse study should increase the level of precision of the ABL budget flux estimates. Interpreting the mechanisms governing these fluxes requires plot- and stand-level data. These data show that variability in seasonal and annual fluxes among flux towers is large, refuting hypotheses that ecosystem-atmosphere exchange can be explained simply by climate, or that a sparse flux tower network can be used to map carbon fluxes over continental domains. Stand age and stand type (e.g. aspen, wetland, northern hardwood forest) explain a large fraction but not all of the observed variability among stands. More sophisticated land classification schemes may be needed to improve the precision of bottom-up methods. Multi-year records are used to examine interannual variability in the carbon balance of the region and show that interannual variability at WLEF is clearly correlated with climate variability. Limited multi-year records at the plot- and stand-level partly support the hypothesis that year-to-year variability in carbon fluxes are coherent across the region, and begin to describe the causes of the observed interannual variability. Further study is needed to evaluate the network design required to describe both the magnitude and mechanisms of interannual variability in the regional carbon balance.

Quantifying the regional scale (10-1000 km) exchange of carbon dioxide between terrestrial ecosystems and the atmosphere is vital for understanding the spatial and temporal variation in global CO₂ flux. Multiple investigations of top-down and bottom-up regional flux scaling are currently underway in the northern Great Lakes region, USA. Landscape and regional scale CO₂ fluxes from multiple line of evidence, including eddy covariance multi-tower aggregation, tall-tower flux footprint decomposition, ecosystem modeling, CO₂ mixing ratio boundary layer budgets and regional inversions reveal variations in CO₂ flux arising from variations in vegetation type, canopy structure and interannual climate variability. With careful calibration, encouraging consistency is seen from several independent regional flux estimates. Without parameter optimization and high resolution maps of land cover, global scale remote-sensing and ecosystem-model CO₂ flux estimates fail to accurately capture the local regional CO₂ flux. These results represent a first attempt to cross-compare multiple top-down and bottom-up regional flux estimates.

In terrestrial high-latitude regions, observations indicate recent changes in snow cover, permafrost, and soil freeze-thaw transitions due to climate change.  These modifications may result in temporal shifts in the growing season and the associated rates of terrestrial productivity. Changes in productivity will influence the ability of these ecosystems to sequester atmospheric CO₂. We use the Terrestrial Ecosystem Model (TEM), which simulates the soil thermal regime, in addition to terrestrial carbon, nitrogen and water dynamics, to explore these issues over the years 1960-2100 in extratropical regions (30Ëš-90Ëš N).  Our results reveal noteworthy changes in snow, permafrost, growing season length, productivity, and net carbon uptake, indicating that prediction of terrestrial carbon dynamics from one decade to the next will require that large-scale models adequately take into account the corresponding changes in soil thermal regimes.