Abstract
<p>Traditional combustion methods for assessing soil carbon (C) and nitrogen (N) stocks are time consuming and expensive; visible and near-infrared (VNIR) methods offer a quick and inexpensive alternative for establishing soil C and N concentrations. We compared combustion and spectral methods for quantifying soil carbon and nitrogen concentrations. We sampled organic and mineral soil horizons in managed and old-growth Douglas-fir (<em>Pseudotsuga menziesii</em>) forests in western Oregon. We applied combustion methods to determine total soil carbon and nitrogen concentrations of these samples. We then applied VNIR methods to derive a reference spectral library for analyzing Pacific Northwest (PNW) andesitic forest soils. Our spectral analysis confirmed that visible range spectra (especially in the 650-750 nm range) are the most useful for distinguishing differences in sample soil organic matter content. Our results provide a robust calibration model for applying spectral analysis combined with Partial Least Square Regression (PLSR) to quantify carbon and nitrogen stocks in PNW Douglas-fir forest soils. Model validation resulted in R<sup>2</sup> values ranging from 0.92 to 0.95 for C and from 0.73 to 0.84 for N.</p>
Highlights
Awareness of the rate at which anthropogenic activities alter global carbon (C) (Marland, Boden, Andres, Brenkert, & Johnston, 2002) and nitrogen (N) (Vitousek et al, 1997; Galloway & Cowling, 2002) dynamics has led to widespread interest in modeling and monitoring global carbon (e.g. Solomon et al, 2007; IPCC, 2014) and nitrogen (e.g. International Nitrogen Initiative, http://initrogen.org, founded 2003) cycles
Our results provide a robust calibration model for applying spectral analysis combined with Partial Least Square Regression (PLSR) to quantify carbon and nitrogen stocks in Pacific Northwest (PNW) Douglas-fir forest soils
Policy initiatives aimed at reducing net greenhouse gas emissions, such as California Assembly Bill 32, the Regional Greenhouse Gas Initiative (RGGI) and the Kyoto Protocol, promote interest in quantifying C stored in managed landscapes
Summary
Awareness of the rate at which anthropogenic activities alter global carbon (C) (Marland, Boden, Andres, Brenkert, & Johnston, 2002) and nitrogen (N) (Vitousek et al, 1997; Galloway & Cowling, 2002) dynamics has led to widespread interest in modeling and monitoring global carbon (e.g. Solomon et al, 2007; IPCC, 2014) and nitrogen (e.g. International Nitrogen Initiative, http://initrogen.org, founded 2003) cycles. Due to significant soil organic matter (SOM) stocks in forest soils and evidence of a seasonal carbon sink in the northern hemisphere (Fung, Tucker, & Prentice, 1987; Pacala et al, 2001; Goodale et al, 2002; Fahey et al, 2010), temperate and boreal forest ecosystems have received particular attention from carbon researchers. Tools for quantifying SOM stocks, including C and N content, can support policy initiatives aimed at increasing C storage, can provide data for calibrating and validating models of C and N cycling, and can facilitate the monitoring of soil fertility in managed forest ecosystems. Forests of the Pacific Northwest (PNW) demonstrate significant soil C storage potential. Hudiburg et al (2009) quantified the significant potential for C accumulation in forests of the PNW and Hutyra, Yoon, and Alberti (2011) report even urban C stocks in the PNW exceed average U.S forest carbon stocks
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