Visible and near infrared reflectance spectroscopy (VisNIr) has been used across a number of spatial scales to predict soil organic carbon (OC) content. The rapid Carbon Assessment Project (raCA) is a nationwide project that collected 144,000+ soil samples from across the conterminous United States for C stock mapping using VisNIr. The objective of this study was to calibrate and validate the VisNIr soil OC and total C (TC) models with ~20,000 samples from raCA. Models were developed with either partial least squares regression (PlSr) or the Artificial Neural Network (ANN) model. Four auxiliary variables-raCA region, land Use land Cover, Master Horizon, and Textural Class-were tested to stratify the dataset and to develop local models. The results showed that OC and TC models calibrated with ANN (R 2 > 0.94; rPD > 4.0) outperformed those of PlSr (R 2 = 0.83; rPD = 2.4). For PlSr, local models developed with all four auxiliary variables exhibited improvement in prediction accuracy, and the improvement was only marginal for ANN models. Master Horizon and Textural Class appeared to be more effective in stratifying samples into homogeneous groups because they gave an overall lower root mean squared error of prediction (rMSE P ) for the validation samples. For the majority of the local Textural Class models, the rMSE P of OC prediction ranged from 0.5 to 1.5%. To maximize the applicability of the raCA spectral library on external soil samples, PlSr local models developed from Master Horizon or Textural Class appeared to be more favorable.Abbreviations: ANN, Artificial Neural Network; IC, inorganic carbon; LULC, land use land cover classes; OC, organic carbon; PLSR, partial least squares regression; RaCA, Rapid Carbon Assessment Project; RPD, ratio of performance to deviation; RPIQ, ratio of performance to interquartile range; TC, total carbon; VisNIR, visible and near infrared reflectance spectroscopy. S oil organic carbon (OC) is a key soil property that plays many critical roles, from agriculture production to biogeochemical cycling to ecosystems functioning (Adhikari and Hartemink, 2016;Chen et al., 2000;Lal, 2004; van Wesemael et al., 2010). The capability of soils to sequester C and therefore regulate atmospheric CO 2 concentration and mitigate climate change is widely recognized and has been an active area of research (Grunwald et al., 2011;Lal, 2004;West and Post, 2002). Up-to-date, baseline soil C stock maps across different scales are a very useful tool for researchers, stakeholders, and policymakers for a wide variety of applications, ranging from best land management practices to natural resource conservation to C auditing (de Gruijter et al., 2016;Minasny et al., 2011).Producing large-scale (national or continental) soil C maps (e.g., for the purpose of C inventory and understanding of soil C sourcing and sinking) is highly challenging. Many previous studies have used legacy soil data (Aitkenhead and Coull, 2016;Minasny et al., 2013;Mulder et al., 2016
Core Ideas• We modeled ~20,000 samples in raCA from...