PARENT SESSION
Contributed Oral Session 75: Biogeochemistry: Soil Chemical Flux
Wednesday, August 10, 8:00 AM - 11:30 AM, Meeting Room 513 E, Level 5, Palais des congrès de Montréal

Carbon, nitrogen, and phosphorus dynamics along a 3 million year volcanic soil age gradient in northern Arizona.

Selmants, Paul^{*,1, 2}, Hart, Stephen^{1, 2}, ¹ School of Forestry, Flagstaff, Arizona, USA² Merriam-Powell Center for Environmental Research, Flagstaff, Arizona, USA

ABSTRACT- We have established a soil-age gradient spanning 1 to 3,000 ky within the San Francisco Volcanic Field in northern Arizona, USA. The climate along this chronosequence is semiarid (MAAT = 10.8° C and MAP = 280 mm), and piñon (Pinus edulis) and one-seed juniper (Juniperus monosperma) trees dominate the open woodland overstory. All soils are derived from basaltic cinders. Chronosequence studies in more humid ecosystems suggest that carbon (C) and nitrogen (N) storage increase during early stages of soil formation, but a shift from N- to phosphorus (P)-limitation leads to a decline in C and N storage late in ecosystem development. We found that total C and N storage did not increase consistently across the chronosequence, but rather appeared to peak at the 750 ky site and decline thereafter. Experimental water, N, and P additions to the grass-dominated, intercanopy spaces has demonstrated a combined water and N limitation to understory plant growth at the younger sites (1 and 55 ky, respectively) and a P limitation to plant growth at the oldest (3,000 ky) site, based on total plant biomass per treatment. We expected that differences in soil C and N content between under tree canopies and in the intercanopy spaces would lessen with time, and the ¹⁵N signature of both soils and tree foliage would become more enriched with soil age. We found that total soil C and N was consistently higher under tree canopies than in the intercanopy spaces across the chronosequence, with the largest differences among canopy types at the youngest site, and that both total soil ¹⁵N and foliar ¹⁵N of the two dominant tree species became more enriched across the chronosequence, suggesting an increased rate of N loss with soil age. Overall, it appears that C, N, and P dynamics during ecosystem development in our semiarid climate behave in a remarkably similar fashion (albeit much more slowly) than in more humid climates.

Key words: Soil Chronosequence, Carbon, Nitrogen, Phosphorus