Using Soil Testing Data to Examine Organic Carbon Changes During the Past 27 Years in Maine Agricultural Soils

Agricultural soils may act as a source or a sink for carbon (C) in the global C cycle. With rising atmospheric CO2 levels, C sequestration in soils may play an important role in climate change mitigation. Soil organic carbon (SOC) also contributes to key aspects of soil health and fertility, such as aggregation, water-holding capacity, microbial biomass, and nutrient mineralization. Although SOC may be calculated from estimates of soil organic matter (SOM) obtained by loss-on-ignition (LOI), factors such as sample clay concentration and combustion temperature introduce error into estimates of SOM. We explored the potential for an extensive collection of LOI values determined by the Maine Soil Testing Service from a variety of agricultural cropping systems over the last 27 years to provide insights into trends in SOM, and therefore SOC, between crop groups and over time. We evaluated furnace temperatures from 375 to 950 °C, to determine the temperature that provides the strongest correlation between SOM estimated by LOI and SOC measured instrumentally by a LECO CN Analyzer. Next, we evaluated whether including a soil texture term in a regression between SOM estimated by LOI and SOC measured instrumentally by a LECO CN Analyzer improved the ability to predict SOC. Finally, we identified trends in the SOC in Maine agricultural soils over the past 27 years based on standard soil samples received by the Maine Soil Testing Service from 1995 to 2021. On a sample set of 48 representative agricultural soils, the standard LOI temperature used by the Maine Soil Testing Service, 375 °C, correlated as well with SOC measured instrumentally as each of three higher temperatures we studied. All temperatures produced a Pearson’s correlation coefficient between 0.97 and 0.99, although the estimated concentration of SOM increased with increasing furnace temperature. Including a texture term in the regressions between SOC and SOM as estimated by LOI did not significantly impact or improve the estimation of SOC, which we found to be approximately 50% of SOM among our sample set of Maine agricultural soils. Using historical data since 1995, the SOC concentration estimated by LOI for Maine agricultural soils has increased by 23% from 1995 to 2021. SOC concentration of soils for grain, hay, and small-scale conventional vegetable and organic vegetable production increased by 24%, 15%, 23%, and 18%, respectively. SOC remained steady for corn and large-scale conventional vegetable and organic vegetable production, while trends for blueberries and apples were difficult to interpret. Among crop groups, only potato soils showed a decline in SOC since 1995. These results show that LOI with a combustion temperature of 375 °C, without the need for clay correction, can reliably estimate SOC of Maine agricultural soils. Further, our results show the value of historical data archived by the Maine Soil Testing Service to examine changes in Maine’s agricultural soils over time