Laboratory optical measurements to assess long-term changes in water-extractable soil organic matter, U.S. Great Plains

Carbon (C) and nitrogen (N) extracted from soil with water are associated with available plant nutrients and microbial activity, but information about how C and N have changed over time in the U.S. Great Plains is sparse. Optical measurements of absorbance and fluorescence were used to compare dissolved organic matter (DOM) extracted from historic (1947) and contemporary (2018) soil samples collected from Moccasin, MT, Akron, CO, and Big Spring, TX. Extracts were prepared at two temperatures (20 and 80 degrees Celsius) associated with different DOM pools to evaluate changes to soil DOM composition after 71 years of dryland farming practices. Together, water extracts and subsequent optical methods can produce insights into long term patterns of soil change while conserving limited and irreplaceable physical samples of archived soil.