PARENT SESSION
Oral Session #21: Nutrient cycling: Nitrogen.
Presiding: S. Hart
Monday, August 5. 1:00 PM to 3:45 PM. Grand Ballroom Central, Radisson.
Nitrogen distribution and 
15N in a 115 year chronosequence of old-field white pine.
Compton, Jana*,1, Hooker, Toby2, Perakis, Steven3, McKane, Robert1, 1 US EPA, Western Ecology Division, Corvallis, OR2 Utah State University, Logan, UT3 USGS, Forest Range and Ecosystem Science Center, Corvallis, OR
ABSTRACT- Regrowing forests of the northeastern US are expected to serve as an important sink for carbon and nitrogen as they recover from extensive clearing and agriculture. We used a chronosequence approach to examine the changes in ecosystem N pools during the first 115 years after agricultural abandonment in Rhode Island, USA. The sites differed in time since abandonment, but had similar soils, climate, land-use history, and overstory white pine (Pinus strobus). Total ecosystem N did not change over time. Instead, N accumulated in the forest floor (11.6 kg N ha-1 yr-1) and plant biomass (4.1 kg N ha-1 yr-1), and these increases were roughly balanced by a significant decrease in mineral soil N content (-12.4 kg N ha-1 yr-1). In young white pine stands, the 15N of foliage was very similar to the mineral soil (+2 to +3 per mil), while as the stands aged, foliar 15N decreased, and became isotopically similar to the developing O horizon (-1 to -2 per mil). The 15N of plants and mineral soil diverged over time, with the soil becoming more enriched in 15N. Our findings indicate that soil N cycling and availability to regrowing forests changes dramatically over time, and that O horizon development may play a key role in these changes.
KEY WORDS: nitrogen cycling, old field white pine, agricultural abandonment, stable isotopes