HR: 0800h
AN: B11A-1016    [Abstracts]
TI: Climate-Dependence of Plant-Soil 15N/14N Interactions Across Tropical Rainforests
AU: * Houlton, B Z
EM: houlton@stanford.edu
AF: Stanford University, Herrin Labs, Stanford, CA 94305 United States
AU: Sigman, D M
EM: sigman@princeton.edu
AF: Princeton University, Guyot Hall, Princeton, NJ 08544 United States
AU: Hedin, L O
EM: lhedin@princeton.edu
AF: Princeton University, Guyot Hall, Princeton, NJ 08544 United States
AB: In most areas of the world, the ¹⁵N/¹⁴N of bulk soils is higher than that of plant leaves, and the isotopic signatures of these two ecosystem N pools progressively diverge with increasing rainfall. However, both the cause for this isotopic trend and its implications for understanding interactions between climate and N cycles are largely unknown. We report ¹⁵N/¹⁴N measurements of nitrate, ammonium, and total dissolved N in soil extracts from a highly constrained rainfall sequence in Hawaii, across which this trend in ecosystem ¹⁵N/¹⁴N is captured, to examine the competing explanations for plant-soil ¹⁵N/¹⁴N uncouplings. While the isotopic influences of microbial transfers of N between nitrate and ammonium pools and plant-mycorrhizae interactions have been posited in plant-soil ¹⁵N/¹⁴N relationships, our data did not support an important role for either of these mechanisms. Instead, preferential regeneration of ¹⁴N during the breakdown of DON to ammonium explains why the ¹⁵N/¹⁴N of plants is lower than that of bulk soils. Fractionation at this step leads to two isotopically distinct N subcycles in each forest, a lower-¹⁵N/¹⁴N subcycle composed of ammonium, nitrate, and bulk plant biomass N that `spins' rapidly and a higher-¹⁵N/¹⁴N subcycle composed of bulk soil N and DON that is much less dynamic. The increased difference between soil and plant ¹⁵N/¹⁴N is due to changes in the impacts of nitrification and denitrification on the ¹⁵N/¹⁴N of ammonium and nitrate, coupled with a switch from nitrate to ammonium uptake by plants under the wettest conditions. For instance, the particularly large (~6 per mil) ¹⁵N/¹⁴N difference between plants and soils in the wettest sites is due to the lack of ¹⁵N-enrichment of ammonium by nitrification coupled with plant dependence on ammonium uptake only. Our results highlight the importance of interactions between DON breakdown, ecosystem N recycling, and gaseous N losses in the explaining the interactions between the ¹⁵N signatures of soils and plants across these tropical rainforests. We also show that these interactions are climate-dependent.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0439 Ecosystems, structure and dynamics (4815)
SC: Biogeosciences [B]
MN: Fall Meeting 2005