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.
Author: L.M. Cisneros-Dozal, S.E. Trumbore and P.J. Hanson
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