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NON-LINEAR NITROGEN RETENTION IN AN UNPOLLUTED OLD-GROWTH TEMPERATE FOREST RECEIVING A GEOMETRIC RANGE OF EXPERIMENTAL 15N ADDITIONS
Citation:
Perakis, S. S., J E. Compton, AND L. O. Hedin. NON-LINEAR NITROGEN RETENTION IN AN UNPOLLUTED OLD-GROWTH TEMPERATE FOREST RECEIVING A GEOMETRIC RANGE OF EXPERIMENTAL 15N ADDITIONS. Presented at Ecological Society of America 2001 meeting, Madison, WI, August 5-10, 2001.
Description:
Over the past century, human activities have increased the rate and extent of atmospheric nitrogen (N) deposition over large regions of Earth. These novel N inputs have driven many previously N-limited temperate forests towards a condition of "N saturation," characterized by poor retention and enhanced losses of inorganic forms of N. The development of ecosystem models to account for changing deposition regimes requires new information on the linearity vs. non-linearity of retention across a wide range of N inputs, and the mechanisms which underlie such patterns. Experimental additions of N provide a method to examine how anthropogenic deposition may affect forests, but hidden causal factors (e.g., historical legacies of prior N deposition or land-use, variations in species composition or successional state) may complicate data interpretation in many studies. Old-growth temperate forests of the Cordillera Piuchue Ecosystem Study in southern Chile are only minimally impacted by air pollution and other human activities, and can serve as useful experimental templates for manipulations that seek to isolate the causal nature of human impacts on biogeochemical cycles in forests. To examine how N retention and loss respond to a wide range of N inputs, we applied isotopically labeled 15NH415NO3 at geometrically increasing addition rates (0.2, 5, 10, 20, 40, 80, 160, 320, 640 kg N ha-1to 9 small plots of mixed broadleaf-conifer forest for a period of one year. Lysimeter losses of nitrate increased dramatically within only 2-3 months at the two highest addition rates, indicating a non-linear response of N retention and loss to inputs. Soil organic matter dominated as a sink for 15N across the range of inputs to soils, but retained a progressively smaller fraction of 15N as inputs increased. In contrast, retention in live and dead coarse roots increased linearly with N inputs, whereas retention microbial biomass and fine roots was unrelated to inputs. These data suggest that the kinetics and capacity of N incorporation into slowly cycling plant and soil organic matter pools may control the transient, non-linear response of forests to novel inputs of N.