PARENT SESSION
Oral Session # 98: Biogeochemistry V: Forests; Nitrogen.
Presiding: RL Sanford
Friday, August 8. 8:30 AM to 12:00 PM, SITCC Meeting Room 204.
Plant and Microbial Controls on Nitrogen Retention and Loss in Puerto Rican Forest Soils.
Templer, Pamela*,1, Firestone, Mary1, Silver, Whendee1, 1 University of California Berkeley, Berkeley, CA, USA
ABSTRACT- Understanding the mechanisms that lead to nitrogen (N) retention in tropical forests is important because these systems currently have high potential for N losses, including the greatest nitrous oxide (N2O) emissions globally for natural ecosystems, and large potential nitrate losses to groundwater. The majority of research on N biogeochemistry has been conducted in north temperate ecosystems where N limitation to net primary productivity (NPP) is common, and where anthropogenic N deposition is altering N cycling. Much less research has been conducted in the tropics, which are generally characterized by rapid rates of N cycling and the lack of strong N limitation to NPP. We conducted a field experiment using 15N tracers and root ingrowth cores to compare and contrast the relative importance of N loss pathways (denitrification, N2O production via nitrification, and N leaching) versus N retention mechanisms (dissimilatory reduction of nitrate to ammonium (DNRA) and assimilatory N uptake by plants and soil microbes) in a subtropical forest in Puerto Rico. DNRA rates were detectable and within a range comparable to gross mineralization and gross nitrification. Plant roots at all sites took up approximately five times more N as ammonium than nitrate and were a relatively strong sink for N as indicated by the relatively large amount of 15N that they took up. This supports our hypothesis that DNRA is a N conserving mechanism within tropical forests because the ammonium that is produced from this process is likely to be taken up by plants rather than re-nitrified and potentially leached or lost as N2O gas from the ecosystem.
Key words: Assimilatory Processes, Stable Isotopes, Tropics, Dissimilatory Processes