PARENT SESSION
Oral Session #35: Nitrogen cycling: Response to inputs, variation in time and space. Presiding: M. Fenn.
Tuesday, August 7, 2001. 1:00 PM to 5:00 PM. Hall of Ideas G.

Non-linear nitrogen retention in an unpolluted old-growth temperate forest receiving a geometric range of experimental ¹⁵N additions .

Perakis, Steven¹, Compton, Jana², Hedin, Lars¹, ^{1 2}

ABSTRACT- Anthropogenic nitrogen deposition has driven many previously nitrogen-limited temperate forests towards "nitrogen saturation," characterized by poor retention and enhanced losses of inorganic nitrogen. The development of ecosystem models to account for increased nitrogen requires new mechanistic information on the linearity vs. non-linearity of retention. Experimental nitrogen additions can provide insights into how anthropogenic nitrogen affects forests, but hidden causal factors (e.g., legacies of prior nitrogen deposition or land-use, species or successional variations) complicate data interpretation in many studies. Unpolluted, old-growth temperate forests of the Cordillera Piuchuè in southern Chile are minimally impacted by human activities, and can be useful for experiments that seek to isolate human effects on forests. To examine nitrogen retention in soils across a range of inputs, we added ¹⁵NH₄¹⁵NO₃ at geometrically increasing rates (0.2, 5, 10, 20, 40, 80, 160, 320, 640 kg N ha^-1) to 9 forested plots for one year. Lysimeter nitrate increased dramatically within 3 months at the two highest addition rates, indicating non-linear retention of inputs. Soil organic matter dominated as a sink for 15N, but retained a progressively smaller fraction as inputs increased. Retention in microbial biomass and fine roots was unrelated to inputs, yet ¹⁵N transfer to coarse roots increased with inputs. These data suggest that incorporation into slowly cycling plant and soil organic matter pools may control the transient, non-linear response of forests to new inputs of nitrogen.

KEY WORDS: nitrogen saturation, nutrient cycling, old growth temperate forest, Chile