Soil respiration (Rs) releases 75–80 billion tons of C each year [Schlesinger, 1977; Raich and Potter, 1995; Raich et al., 2002]. This efflux is more than 11 times the recent rate of C produced from human combustion of fossil fuels [Marland and Boden, 1993]. So even a slight proportional change in global Rs could significantly alter atmospheric CO2 levels, and hence climate. Rs usually accounts for a large proportion of terrestrial ecosystem respiration [Lavigne et al., 1997; Janssens et al., 2001] and variation in Rs may determine whether an ecosystem is a net source or sink of CO2 [Valentini et al., 2000; Davidson et al., 2006]. Yet despite its clear importance for global C cycling and climate change, understanding of the processes controlling spatial and temporal variation in Rs is limited. This is largely because soil is a complex and spatially heterogeneous mixture of different compounds (e.g., ground surface organic litter, live roots, and soil organic matter pools). Understanding the individual responses of these compounds to environmental change and the net effect upon Rs remains a key objective for research into ecosystem C cycling and biosphere-atmosphere interactions.
 Rs is derived from autotrophic respiration by roots (Rr) and heterotrophic respiration by microorganisms that decompose ground surface organic litter (Rl) and soil organic matter or SOM (Rsom). In this study, Rsom also includes CO2 derived from microbial decomposition of root tissue and exudates, and contributions from mycorrhizal fungi. These different sources of soil CO2 may respond to environmental change in different ways, whilst estimates of the autotrophic component of Rs range between 12–93% depending upon the ecosystem studied and the method used to estimate Rr [Hanson et al., 2000]. Rl and Rsom are directly driven by microbial activity, which, in turn, is strongly affected by temperature [Davidson et al., 1998; Fang and Moncrieff, 2001] and available moisture [Davidson et al., 1998; Sotta et al., 2004]. This explains frequent observations, particularly in temperate and boreal regions where diurnal and seasonal fluctuations in temperature are greatest, that Rs rises as soil becomes warmer and wetter [e.g., Savage and Davidson, 2001]. However, both Rl and Rsom are also partly decoupled from local soil conditions because they are affected by the supply and quality of substrate from above-ground in the form of organic litter and root exudates [Melillo et al., 1982; Högberg et al., 2001]. Rr is also partly a product of the level of metabolic activity within root tissue, affected by factors such as soil temperature [see Atkin et al., 2000, and references therein], water availability [Bouma et al., 1997; Burton et al., 1998], N supply [Ryan et al., 1996; Zogg et al., 1996], and the supply of photosynthate from above-ground [Högberg et al., 2001; Nordgren et al., 2003], influenced by ecosystem GPP and plant allocation strategy. Thus, Rs and its component fluxes may display substantial spatial and temporal variability which is not readily attributable to changes in soil temperature and moisture. This variation reflects changes in both the total amount of respiring tissue (e.g., root mass) or available substrate, (e.g., surface litter mass) and the rate of respiration per unit mass of tissue or substrate (specific root respiration: SRR, specific litter respiration: SLR). Understanding the extent and causes of this variability represents an important step towards accurately modelling ecosystem C cycling, and up-scaling localized measurements across larger spatial scales for comparison with top-down measurement systems (e.g., satellites, flux towers). The overall objectives of this study, therefore, were to (1) partition Rs into Rl, Rr and Rsom over one full seasonal cycle at four rain forest sites with contrasting vegetation and soil types in the eastern Amazon; (2) investigate potential biotic (roots, ground surface litter) and abiotic (soil moisture, soil temperature) causes for observed differences in respiration within and between sites and seasons; and (3) quantify the contributions of component mass and respiration per unit mass to total Rr and Rl.
 We focused upon sites in the Amazon because the region plays an important role in global biogeochemical cycles [Houghton et al., 2001; IPCC, 2001], and displays a high degree of spatial heterogeneity in terms of many ecosystem properties [Williams et al., 2002], but may experience an increase in drought conditions over this century due to a possible increase in El Niño-Southern Oscillation events [Trenberth and Hoar, 1997; Schöngart et al., 2004] driven by global climate change, and reductions in rainfall caused by regional deforestation [Shukla et al., 1990] and fire [Andreae et al., 2004].