Patterns of water-extractable soil organic matter in the US Great Plains: Insights from the Haas Soil Archive

Authors: Halvorson, Jonathan; HANSEN, ANGELA - Us Geological Survey (USGS); Stewart, Catherine; Liebig, Mark

Submitted to: Agrosystems, Geosciences & Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/25/2025
Publication Date: 3/31/2025
Citation: Halvorson, J.J., Hansen, A.M., Stewart, C.E., Liebig, M.A. 2025. Patterns of water-extractable soil organic matter in the US Great Plains: Insights from the Haas Soil Archive. Agrosystems, Geosciences & Environment. 8. Article e70060. https://doi.org/10.1002/agg2.70060.
DOI: https://doi.org/10.1002/agg2.70060

Interpretive Summary: Carbon (C) and nitrogen (N) extracted from soil with water are associated with available plant nutrients and microbial activity. But information about how they have changed over time in the U.S Great Plains is sparse. We used cool and hot water extracts from historic (1947) and current (2018) soil samples collected at Moccasin, MT, Akron, CO, and Big Spring, TX. The extracts were used to evaluate changes in extractable C and N and optical properties, how the samples interact with light, after 71 years of dryland cropping. Concentrations of C and N extracted with cool water decreased between 1947 and 2018 in surface samples from Moccasin and Big Spring but were unchanged at Akron. Conversely, net (hot – cool) extractable C did not change at Moccasin or Big Spring sites but increased at Akron. Net extractable N decreased at Moccasin but did not change elsewhere. Both sites and year were accurately classified using only three optical properties. Each optical property is associated with chemical complexity of soil organic matter. These results suggested the chemical complexity and resistance to decomposition of extractable soil organic matter had increased over time at Akron more than the other locations. Water extracts and optical methods can be useful for understanding long-term changes in labile soil organic matter. These methods can also reduce the amount of archived soil used for analysis, leaving more of the sample available for future use.

Technical Abstract: Carbon (C) and nitrogen (N) extracted from soil with water are associated with available plant nutrients and microbial activity but information about how they have changed over time in the U.S Great Plains is sparse. We used cool (20 oC) and hot (80 oC) water extracts from historic (1947) and current (2018) soil samples collected at Moccasin, MT, Akron, CO, and Big Spring, TX to evaluate changes to extractable C and N and optical properties after 71 years of dryland cropping. Concentrations of C and N extracted with cool water decreased between 1947 and 2018 in surface (0-15.2 cm) samples from Moccasin, by 51 and 34%, and Big Spring, by 36 and 31%, respectively, but remained unchanged at Akron. Conversely, net (hot – cool) extractable C did not change at Moccasin or Big Spring sites but increased at Akron by 27%. Net extractable N decreased at Moccasin by 20% but did not change elsewhere. Accurate classification of both sites and year required only three optical properties. Values of SUVA254 in extracts did not change at Moccasin between 1947 and 2018 but increased at Akron indicating increased aromaticity. Conversely SUVA254 decreased at Big Spring. Values for QSag350-400, inversely related to extract molecular weight and aromaticity, decreased at Moccasin but not elsewhere. The ratio of recalcitrant to labile compounds (C:T) increased in extracts from all sites but particularly Akron. Together, water extracts and optical methods can produce insights into long-term changes in labile soil organic matter while conserving physical samples of archived soil.