Using Soil Testing Data to Monitor Changes in Maine Agricultural Soil Organic Carbon from 1995 to 2021

Estimates of soil organic matter (SOM) are commonly part of soil testing, and typically use the loss-on-ignition (LOI) method. To explore the utility of existing soil testing databases as a resource for estimating soil organic carbon (SOC) concentrations over time, we determined the relationship between SOM estimated by LOI and SOC measured using IR detection of CO2 for Maine soils. We then examined trends in SOC concentrations in Maine agricultural soils over the past 27 years using data from 97,713 standard soil samples received by the Maine Soil Testing Service. For 91 agricultural soil samples representative of Maine cropping systems, we found that the standard LOI temperature used by the Maine Soil Testing Service, 375 degrees C, produced LOI values that correlated more strongly with SOC measured by IR detection of CO2 than the three higher temperatures we studied, although all temperatures produced data that was well correlated (R2 values from 0.95 to 0.98). Soil texture did not influence the correlation. Our analysis of soil testing data from 1995 to 2021 suggested increases in SOC concentrations in soils planted to grain and hay, and possibly corn, broadly corresponding with changing agricultural practices occurring over this time frame. Of our four focal crop categories, only potato soils decreased in SOC concentration from 1995 to 2021, underscoring the unique challenges associated with potato soil management relative to other cropping systems in Maine. Our study confirms the potential utility of soil testing databases in assessing changes in SOC concentrations in agricultural soils over decades.