Pyrogenic Carbon Distribution And Controls In Soils Of The United States

Abstract

Given its slow turnover rates in soil, pyrogenic carbon (PyC) is considered an important carbon pool and relevant to global environmental change processes. Research on PyC has expanded greatly over recent years, but the core factors influencing its production, aggregation and dispersion still require elucidation. This dissertation contributes to this literature by examining PyC content in soils and its correlation with environmental predictors. Using mid-infrared spectroscopy (MIR) and partial least-squares (PLS) analysis in conjunction with ultraviolet photo-oxidation followed by nuclear magnetic resonance spectroscopy (UV-NMR) techniques, amounts of PyC were quantified for samples from soil profiles across different ecoregions in the United States. The first chapter focused on a soil catena in the Pacific Northwest. Expanding on a previously established method to predict soil properties with depth, equal-area quadratic splines were used to calculate PyC stocks within a soil profile. Presumably due to the pervasive combustion of grass and cereals, stock sizes were lowest at the agricultural sites (0.71 kg m-2; 10% of SOC). In contrast, the highest PyC stocks were found under cooler and moister conditions at a forested site dominated by Douglas Fir (5.66 kg m-2; 16% of SOC). The second chapter assessed PyC contents from topsoils at 165 field sites in the northeastern United States. Three spatial models under a newly developed Bayesian framework were applied to the data in order to relate critical environmental covariates to changes in spatial density of PyC over the landscape. Akaike Information Criterion (AIC) demonstrated that the Multivariate Linear Regression model performed best (r2=0.6; p<<0.0001), giving global mean density estimates for PyC of 25.8 g kg-1 (12.2 Gg km-2). Soil PyC correlated well with total soil sulfur (p<<0.001; n = 165), plant tissue lignin (p=0.003), and drainage class (p=0.008). In the third chapter, samples from soil catenae at five diverse Long Term Ecological Research (LTER) sites were examined for PyC content, which ranged from 9.8-56.4 mg g-1 between sites. Statistically, PyC was found to have a significant relationship with the environmental variables soil drainage (p<<0.0001), mean annual precipitation (p=0.007), mean annual temperature (p=0.038), vegetation (p=0.003) and silt-clay (p=0.086).