Moving beyond carbon bean counting: what information do we need to truly assess soil carbon sequestration?
Abstract
Measuring changes in bulk soil organic carbon alone will likely not give us an accurate picture of carbon sequestration in response to a land use or land management shift. For soil carbon sequestration to be an effective greenhouse gas mitigation measure, we must understand the decadal to century scale stability of newly sequestered soil carbon. This understanding will only come from studies that ask questions such as where and in what form is SOC accumulating. Here we present results from a study examining shifts in SOC when the subtropical C4 perennial grass, kikuyu (Pennisetum clandestinium), is sown into formerly C3 annual grass dominated pastures in contrasting soil types. The C3-C4 vegetation transition provided an excellent opportunity to trace changes in the stable carbon isotope composition of SOC and its fractions to better understand the dynamics of SOC in a system that is accumulating carbon. Using a paired-plot chronosequence approach spanning 33 years, we found that in the sandy soils of Western Australia, SOC accumulated at a rate of 0.90 Mg C ha-1 yr-1 over a 30 year period, but all of this increase was found to have occurred in the particulate organic carbon fraction. In finer textured soils in South Australia over a similar time span, sequestration rates were lower, averaging 0.26 Mg C ha-1 yr-1, but one third of the new SOC was found in a mineral-associated fraction. These findings raise some interesting questions that will be explored in this presentation. Why has there been so much accumulation of SOC in the sandy soil where protection mechanisms are minimal? Do these data fit with the our current understanding of SOC dynamics? Does the concept of carbon saturation hold in a soil with 2% clay? If there were a major perturbation (i.e. long-term drought or management shift away from pasture) to the system, would the newly sequestered SOC in the fine textured soil be more resistant to loss?
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.B12C..06S
- Keywords:
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- 0402 BIOGEOSCIENCES / Agricultural systems;
- 0414 BIOGEOSCIENCES / Biogeochemical cycles;
- processes;
- and modeling;
- 0428 BIOGEOSCIENCES / Carbon cycling;
- 0454 BIOGEOSCIENCES / Isotopic composition and chemistry