HR: 15:10h
AN: B13B-07    [Abstracts]
TI: Insights into Belowground Processes from Carbon and Nitrogen Isotopes
AU: * Hobbie, E A
EM: erik.hobbie@unh.edu
AF: CSRC, University of New Hampshire, Morse Hall, Durham, NH 03824 United States
AB: ¹³C and ¹⁵N content generally increase with greater depth in the soil but no completely satisfactory explanation of this phenomenon has been advanced. Several explanations have been proposed, including discrimination against ¹³C and ¹⁵N during decomposition, preferential preservation of ¹³C-enriched compound classes, incorporation of soil CO₂ into microbial biomass by anaplerotic reactions, enrichment in ¹³C and ¹⁵N of belowground plant and mycorrhizal fungal components compared to aboveground components, and preferential loss of ¹⁵N during nitrification, amonification, and denitrification reactions. Here, I examine the theoretical, experimental, and field evidence for the differing proposed hypotheses. The ratio of enrichment in ¹⁵N and ¹³C with depth is between 2 and 3, suggesting that the movement of carboxyl groups (-CO₂) and amino groups (-NH₂) during decomposition may control soil isotope patterns. Another important factor is the incorporation of ¹³C-enriched root and mycorrhizal carbon into deeper soil layers versus the incorporation of ¹³C-depleted above-ground plant litter at the soil surface. Anaplerotic fixation of soil CO₂ appears unimportant as a contributing factor to ¹³C enrichment of soil profiles, based on the average efficiency of microbial metabolism (40%) and the relative proportion of microbial C attributed to anaplerotic reactions (3-5%). Because mycorrhizal fungi and plant roots appear generally enriched in ¹³C and ¹⁵N relative to above-ground plant components, I suggest that increasing ¹³C and ¹⁵N enrichment of soil relative to above-ground plant foliage should correlate with the relative carbon allocation belowground to roots and mycorrhizal fungi in many terrestrial systems.
DE: 0428 Carbon cycling (4806)
DE: 0439 Ecosystems, structure and dynamics (4815)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 0469 Nitrogen cycling
DE: 0486 Soils/pedology (1865)
SC: Biogeosciences [B]
MN: Fall Meeting 2005