HR: 1340h
AN: B13B-0218    [Abstracts]
TI: Nitrogen Additions Affect Root Dynamics in a Boreal Forest Ecosystem
AU: * Turner, K M
EM: kmturner@uci.edu
AF: Department of Ecology and Evolutionary Biology, 341 Steinhaus Hall University of California, Irvine, Irvine, CA 92697 United States
AU: * Turner, K M
EM: kmturner@uci.edu
AF: Department of Earth System Science, University of California, Irvine, Irvine, CA 92697 United States
AU: Treseder, K K
EM: treseder@uci.edu
AF: Department of Ecology and Evolutionary Biology, 341 Steinhaus Hall University of California, Irvine, Irvine, CA 92697 United States
AU: Treseder, K K
EM: treseder@uci.edu
AF: Department of Earth System Science, University of California, Irvine, Irvine, CA 92697 United States
AB: As with many ecosystems, North American boreal forests are increasingly subjected to anthropogenic nitrogen deposition. To examine potential effects on plant growth, we created nitrogen fertilization plots in three sites along an Alaskan fire chronosequence composed of forests aged 5, 17, and 80 years. Each site had been exposed to two years of nitrogen fertilization, with four control plots and four nitrogen plots per site. General observations indicate that aboveground net primary productivity appears to be nitrogen limited in each site. We hypothesized that nitrogen fertilization would positively influence root dynamics as well, with nitrogen additions resulting in an increase in standing root biomass and length. To test our hypothesis, we used a minirhizotron camera to collect sequential images of roots in the top 10 cm of soil in both nitrogen fertilized and control plots in each site. Images were collected monthly during the growing season, with a total of five sampling times between May 2003 and May 2004. We then analyzed the images with WinRhizotron root measurement software. Nitrogen fertilization had varying effects on root biomass among the three sites, with a significant site by N interaction (P = 0.039). A decrease in root biomass was observed in the 5 and 80 year old sites, dropping from 207 g/m2 to 79 g/m2 and from 230 g/m2 to 129 g/m2 for the youngest and oldest sites, respectively. In contrast, root biomass increased from 52 g/m2 to 107 g/m2 in the 17 year old site. (Values are for the top 10 cm of soil only, and likely underestimate total root stocks.) Patterns in standing root lengths diverged from those of root biomass, with a 2.5-fold overall increase under nitrogen fertilization across all sites (P = 0.004). There were no significant differences among sites in nitrogen response. Standing root biomass and length differed from one another in their responses to nitrogen fertilization because nitrogen additions decreased specific root weight (as g per unit length) (P = 0.008). The divergent responses of root biomass and root length under nitrogen addition suggest a shift in root structure to finer roots. One possible mechanism is that phosphorus may have become limiting to plant growth following nitrogen additions. Fine-structured roots can better exploit soil phosphorus due to an increased surface area:volume ratio. Among the sites, standing root biomass was greatest in the oldest site (P = 0.014) but no significant difference occurred in standing root length. This pattern implies that roots become coarser with age, a possible consequence of plant succession from deciduous and annual species in the younger two sites to black spruce in the oldest. Altogether, forest fires could elicit a long-term reduction in carbon stocks in belowground plant biomass; nitrogen fertilization could reduce these stocks even further, but not necessarily at every successional stage.
DE: 0315 Biosphere/atmosphere interactions
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting