Abstract
AbstractSoil 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 h−1) when leaf litter was extremely dry, to a high of 42±16% (96± 38 mg C m−2 h−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. In the surface mineral soil layer, decomposition of C fixed in the original labeling event (3–5 years earlier) dominated the isotopic signature of heterotrophic respiration.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 h−1 at different times in the growing season with a single exception (88±35 mg C m−2 h−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.
Highlights
A key area of uncertainty in the terrestrial C cycle concerns the processes, referred to collectively as respiration, that control the vast majority of C lost annually
We report the uncertainty in from autotrophic respiration (FRR) by treatment calculated using the standard deviations of total soil respiration, root and heterotrophic respiration according to Phillips & Gregg, (2001)
In September 2003, we have no data for soil moisture content, but leaf litter at Walker Branch was extremely dry and crunchy, while soil respiration measurements at Tennessee Valley Authority (TVA) were carried out the day after a heavy rain event
Summary
A key area of uncertainty in the terrestrial C cycle concerns the processes, referred to collectively as respiration, that control the vast majority of C lost annually. Soil respiration is derived from autotrophic and heterotrophic sources. Autotrophic respiration includes CO2 derived from root metabolism and the activity of microorganisms in the rhizosphere. Heterotrophic respiration includes CO2 released during microbial r 2006 Blackwell Publishing Ltd decomposition of soil organic matter. While autotrophic respiration is linked to the supply of photosynthetic products from plants, heterotrophic respiration is derived from several different kinds of dead plant material (leaf litter, root detritus, soil organic matter) available to decomposers. Knowledge of the relative contributions of the two sources is relevant to studies of C cycling as each source returns C to the atmosphere on different time scales (years or less through root/rhizosphere respiration vs decades to centuries through soil organic matter decomposition). Important questions include: (1) how much of total soil respiration comes from autotrophic vs heterotrophic sources? Important questions include: (1) how much of total soil respiration comes from autotrophic vs. heterotrophic sources? (2) what portion of heterotrophic respiration comes from decomposition of different substrate types? and (3) how do changes in phenology, soil moisture and temperature, affect each of the sources of soil respiration?
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