HR: 0800h
AN: B51A-0176    [Abstracts]
TI: The Effects of N Addition on Belowground C Dynamics in a Tundra Ecosystem
AU: * Nowinski, N S
EM: nnowinsk@uci.edu
AF: University of California-Irvine, Earth System Science Dept. 3200 Croul Hall, Irvine, CA 92697 United States
AU: Mack, M C
EM: mcmack@ufl.edu
AF: University of Florida, Botany Dept. 220 Bartram Hall , Gainesville, FL 32611 United States
AU: Schuur, E A
EM: tschuur@ufl.edu
AF: University of Florida, Botany Dept. 220 Bartram Hall , Gainesville, FL 32611 United States
AU: Trumbore, S E
EM: setrumbo@uci.edu
AF: University of California-Irvine, Earth System Science Dept. 3200 Croul Hall, Irvine, CA 92697 United States
AB: In a study of moist acidic tundra, Mack et al. (2004) found that found that N fertilization caused unexpected C losses in deep organic (>5cm) and mineral soil horizons that more than offset C gains in litter and upper organic (0-5cm) horizons. Here archived soil and fine root (<2mm) samples from that study were analyzed for radiocarbon to determine which C pools contributed to the observed changes. The fertilized plots, near Toolik Lake, Alaska, have received 100 kg N/ha/yr as NH4NO3 and 50 kg P/ha/yr as P2O5 since 1981. The 14C signature of the bulk soil organic C increased with C content (R2=0.85) and was consistent between treatments. When the soil profile was broken down into layers, there was a significant relationship between 14C and treatment (p=0.04, two-way ANOVA), with fertilized plots having higher 14C values in the litter and 0-5cm organic layers and lower 14C values in the deeper organic and mineral layers. The increased 14C values in the upper layers are consistent with accumulation of (bomb-derived) C over two decades. The decreased 14C values in the lower layers may be due to a decline in root litter inputs (young C) into these layers and/or the loss of younger, more labile C, leaving behind the older recalcitrant C. Mack et al. (2004) found that overall root biomass did not change, but a large portion of root biomass shifted from the lower layers to the upper layers in response to N addition. We found no change in root age as estimated from 14C content in the organic 0-5cm and mineral layers, but in the litter fertilized roots were (p=0.10) and in the deep organic soil fertilized roots were younger (p=0.04). This suggests that some of the observed changes in biomass distribution are due to changes in root mortality as production did not change (Nadelhoffer et al. 2002). Decreased root inputs alone can not explain the loss of C or the change in 14C signature in the lower soil horizons; therefore, there must have been a disproportionate loss of the younger C in the deep soil horizons.
DE: 0428 Carbon cycling (4806)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 0469 Nitrogen cycling
DE: 0475 Permafrost, cryosphere, and high-latitude processes (0702, 0716)
SC: Biogeosciences [B]
MN: Fall Meeting 2005