Ecosystem modeling to understand global change effects to terrestrial and fresh water systems

Abstract

Concurrent changes in climate, atmospheric nitrogen (N) and sulfur (S) deposition, and increasing levels of atmospheric carbon dioxide (CO2) affect ecosystems in complex ways. Atmospheric deposition of S and N species have the potential to acidify terrestrial and aquatic ecosystems, but nitrate and ammonium are also critical nutrients for plant and microbial productivity and are a potential cause of eutrophication. Climate change and CO2 fertilization, with or without changes in N deposition, may affect rates of plant growth, water availability, soil organic matter decomposition rates, and net greenhouse gas flux. I developed a non-spatial biogeochemical model to simulate soil and surface water chemistry by linking the daily version of the CENTURY ecosystem model (DayCent) with a low temperature aqueous geochemical model, PHREEQC. The coupled model, DayCent-Chem, simulates the daily dynamics of plant production, soil organic matter, cation exchange, mineral weathering, elution, stream discharge, and solute concentrations in soil water and stream flow. The model was first validated against a rich data set for an alpine watershed in Rocky Mountain National Park, then for seven other forested montane and alpine watersheds in the United States. I modeled how much nitrogen deposition it takes to acidify an alpine watershed, and whether the rate at which deposition increases matters. I also used the model to investigate the combined effects of N deposition, warming, and increasing CO2 over the period 1980-2075 at seven forested montane and two alpine watersheds by looking at changes to net ecosystem production, soil organic C, soil nitrous oxide (N2O) emissions, and stream nitrate. I found that N was the main driver of change to net ecosystem greenhouse gas flux with warming and CO2 fertilization playing lesser roles. Overall, simulations with DayCent-Chem suggest individual site characteristics and historical patterns of N deposition are important determinants of forest or alpine ecosystem responses to global change. Both the ecological response and the hydrochemical response to these human-caused drivers of global change are of interest to scientists as well as regulatory and land management agencies. This model is appropriate for accurately describing the ecosystem and surface water chemical response of small watersheds to atmospheric deposition and climate change.