Introduction & Objectives

For the past seven years, Point Blue has been collecting soil carbon data across California’s rangelands using standardized monitoring methods as part of the Rangeland Monitoring Network (RMN; Porzig et al. 2018). This effort has produced a dataset that offers a unique opportunity to map soil carbon levels across California’s rangelands using digital soil mapping approaches. With increasing interest in managing soil carbon from policymakers and practitioners, we aimed to use these data to create predictive maps of soil carbon stocks across California’s rangelands.

The objectives of this project were to combine RMN soil data with publicly available geographic information service (GIS) data to predict soil carbon across California’s rangelands; to determine the accuracy of those predictions; and to produce maps that can be used to inform carbon stewardship of these rangeland soils. In addition to predicting static (“baseline”) carbon stocks, we aimed to predict changes in carbon stocks between 2015-2021 as well. As an additional way to identify sites that are ripe for management intervention, we also compared surface carbon stocks with deeper carbon stocks, with the assumption that sites with less surface carbon than expected based on deep carbon values have the potential for improvement. Our final objective was to assess how soil carbon changed over time at sites that had lower than expected surface carbon, which would provide additional insights to guide strategic management interventions.

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Key Findings

• Soil carbon stocks across our network fell within what we would expect for semi-arid rangelands of California.
• Soil carbon levels were generally highest in cooler, wetter sites like those of the Central Coast.
• The five counties with the largest soil stocks were, in descending order Monterey, San Luis Obispo, Modoc, Lassen and Kern.
• Climate change is likely to negatively affect rates of carbon storage in rangeland soils.
• There were lower carbon stocks in areas with more extreme seasonal temperatures and with higher rates of climatic water deficit.
• Actions to protect and restore soil carbon through management may be most important in higher carbon areas.
• Variables related to vegetation productivity were the best predictors of soil carbon stocks.

Future monitoring efforts that are designed to evaluate the predictions we produce in this report will serve an important role in continuing to improve our understanding of carbon dynamics of California’s rangelands across space and through time.