The causes and consequences of deeper rooting distributions under elevated [CO2]: Improved understanding of root-soil interactions from a Free-Air CO2 Enrichment experiment in a sweetgum plantation (Invited)
Abstract
Belowground processes are increasingly recognized as an important foundation for ecosystem responses to rising atmospheric [CO2]. Elevated [CO2] has been shown to increase the proportion of biomass in fine roots, and experimental evidence from a diverse set of forested ecosystems indicates that CO2-enrichment may lead to deeper rooting distributions. Deeper rooting distributions in CO2-enriched forests are likely a result of three interacting factors: (1) increased resource demand, (2) greater carbon (C) available for belowground allocation, and (3) increased competition for scarce resources in shallower soil. Increased production of fine roots at depth in the soil could drive changes in C cycling because fine roots turn over quickly in forests. However, the consequences of increased fine-root proliferation and turnover at depth are still poorly understood; this is in part because belowground research is often truncated at relatively shallow soil depths. We examined soil C dynamics after 12 years of CO2-enrichment and at soil depths to 90 cm in soil pits harvested at the conclusion of the Oak Ridge National Laboratory (ORNL) Free-Air CO2 Enrichment (FACE) located in a sweetgum plantation in eastern Tennessee, USA. We hypothesized that: (1) soil C content would increase in response to elevated [CO2], especially at deeper soil depths where large increases in root production and mortality were observed, and (2) greater C inputs under elevated [CO2] would lead to increased potential C mineralization in long-term laboratory incubations. As we hypothesized, total soil C content under elevated [CO2] was 20% greater throughout the soil profile to 90 cm depth. The CO2 effect was driven by an increase in the C content of the relatively labile particulate organic matter (POM) pool, which is likely derived primarily from fine roots. Contrary to what we hypothesized, we did not observe a significant increase in potential soil C mineralization under elevated [CO2]. While C mineralization rates were well-predicted by soil C content at shallower soil depths (i.e., 0 to 30 cm), this relationship did not hold at depths deeper than 30 cm. Therefore, root-derived C inputs to deeper soil may not decompose as quickly relative to inputs in shallower soil horizons, leading to increased ecosystem C storage under elevated [CO2]. ORNL FACE improved our understanding of belowground processes by allowing us to observe the feedbacks among changes in plant root dynamics and soil nutrient and C cycling in an intact soil system, with the added benefit of a depleted isotopic 13C signal that allowed the tracking and quantification of plant-derived inputs to the soil. These data have inspired the improvement of model process and structure, which will be critical in accurately projecting the sustainability of forest responses to rising atmospheric [CO2].
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMGC31C1069I
- Keywords:
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- 1630 GLOBAL CHANGE Impacts of global change;
- 1615 GLOBAL CHANGE Biogeochemical cycles;
- processes;
- and modeling