『Abstract
This paper reviews the roles of soil respiration in determining
ecosystem carbon balance, and the conceptual basis for measuring
and modeling soil respiration. We developed it to provide background
and context for this special issue on soil respiration and to
synthesize the presentations and discussions at the workshop.
Soil respiration is the largest component of ecosystem respiration.
Because autotrophic and heterotrophic activity belowground is
controlled by substrate availability, soil respiration is strongly
linked to plant metabolism, photosynthesis and litterfall. This
link dominates both base rates and short-term fluctuations in
soil respiration and suggests many roles for soil respiration
as an indicator of ecosystem metabolism. However, the strong links
between above and belowground processes complicate using soil
respiration to understand changes in ecosystem carbon storage.
Root and associated mycorrhizal respiration produce roughly half
of soil respiration, with much of the remainder derived from decomposition
of recently produced root and leaf litter. Changes in the carbon
stored in the soil generally contribute little to soil respiration,
but these changes, together with shifts in plant carbon allocation,
determine ecosystem carbon storage belowground and its exchange
with the atmosphere. Identifying the small signal from changes
in large, slow carbon pools in flux dominated by decomposition
of recent material and autotrophic and mycorrhizal respiration
is a significant challenge. A mechanistic understanding of the
belowground carbon cycle and of the response of different components
to the environment will aid in identifying this signal. Our workshop
identified information needs to help build that understanding:
(1) the mechanisms that control the coupling of canopy and belowground
processes; (2) the responses of root and heterotrophic respiration
to environment; (3) plant carbon allocation pattern, particularly
in different forest developmental stages, and in response to treatments
(warming, CO2, nitrogen additions); and (4)
coupling measurements of soil respiration with aboveground processes
and changes in soil carbon. Multi-factor experiments need to be
sufficiently long to allow the systems to adjust to the treatments.
New technologies will be necessary to reduce uncertainty in estimates
of carbon allocation, soil carbon pool sizes, and different responses
of roots and microbes to environmental conditions.
Key words: Belowground carbon allocation; Carbon cycling; Carbon
dioxide; CO2; Infrared gas analyzers; Methods;
Soil carbon; Terrestrial ecosystems』
Introduction
Conceptual basis for measurements of soil respiration to understand
terrestrial ecosystem carbon cycling
How observations can be used to assess ecosystem carbon fluxes
Comparison with night measurements of NEE by eddy covariance
Partitioning sources of ecosystem fluxes
Phenomenological studies linked to the environment or treatments
Manipulations to separate autotrophic and heterotrophic contributions
Soil respiration used to estimate total belowground carbon allocation
(TBCA)
How biological processes control CO2 production
Overview
Autotrophic respiration
Heterotrophic respiration
How wind and rain can decouple measured fluxes from biological
production
Potential biases in measurement and sampling techniques
Technique
Sampling
Experiments to define the controls over soil respiration
Modeling soil respiration and information needs
Conclusions
Acknowledgements
Appendix
Protocols for sampling, measurement, and reporting soil CO2 fluxes (including associated variables)
References