The methodology, implementation and analysis of the isotopic composition of soil respired carbon dioxide in forest ecological research .

摘要

Soils are the largest terrestrial pool of carbon, therefore it is critical to understand what controls soil carbon efflux to the atmosphere in light of current climate uncertainty. The primary efflux of carbon from soil is soil respiration which is typically categorized into autotrophic and heterotrophic respiration. These two components have different responses to changes in the environment, thus necessitating a means to quantify the contributions of each. Natural abundance 13C can identify autotrophic and heterotrophic sources of respiration, but there is a paucity of research concerning the soil isotope methodology and the subsequent analysis. This dissertation documents my contributions to the advancement of understanding carbon metabolism in forest ecosystems of the Pacific Northwest through the use of the natural abundance carbon isotopic signature of soil respiration.;The results of this research represent significant progress in the use of 13C in forest ecology. I show in a laboratory setting that a change in the isotopic signature of soil gas can take at least 48 hours to reach equilibrium. A change in the isotopic source of respiration is one mechanism behind non steady-state conditions while another mechanism is dynamic gas transport. I explored the impact of a negative pressure potential across the soil surface by inducing advection and found the isotopic signature of respiration to be 1‰ less than the theoretical steady-state value. I performed a source partitioning experiment in which I identified a highly depleted source of carbon contributing to respiration. I also considered the impacts of the potential errors associated with collecting and measuring isotopic samples on mixing-models currently used to identify the isotopic signature of respiration. I found that the effect of CO2 and delta 13C measurement error on large CO2 concentration regime to be substantially different than small concentration regimes, necessitating a unique mixing-model and regression-model combination for estimating the isotopic signal of respiration. Finally, I built upon the progress made in the previous experiments and analyze almost two years of soil respiration and its isotopic signature to determine potential environmental and biological drivers. I found that: transpiration was highly correlated with both respiration and the carbon isotopic signature; soil moisture primarily influenced tree processes related to respiration; and I found evidence of soil respiration under isotopic non steady-state conditions.