Soil organic matter stabilization in buried paleosols of the Great Plains

Understanding the mechanisms that control soil organic matter (SOM) stabilization is important for understanding how soil carbon is sequestered over millennia, and for predicting how future disturbances may affect soil carbon stocks. We are studying the mechanisms controlling SOM stabilization in the Brady Soil, a buried paleosol in Holocene loess deposits spanning much of the central Great Plains of the United States. The Brady Soil developed 9,000-13,500 years ago during a time of warming and drying that resulted in a shift from C3 to C4 dominated plants. The Brady soil is unusual in that it has very dark coloring, although it contains less than <1 % organic C. Although the Brady Soil has low C concentrations, it contains significant carbon stocks due to its thickness (~1 m) and wide geographic extent. We sampled the modern surface A horizon and multiple buried paleosol horizons from two roadcuts near Wauneta in southwestern Nebraska. We are using isotopic, spectroscopic, and geochemical techniques to examine what plant and microbially-derived compounds are have been preserved in the Brady Soil. We used a combined physical density and particle size fractionation method to separate particulate organic matter associated with minerals from that within and outside of soil aggregates. We found the largest and darkest amounts of organic C in aggregate-protected SOM greater than 20 µm in diameter. Density and textural fractionation revealed that much of the SOM is bound within aggregates, indicating that protection within aggregates is a major contributor to SOM- stabilization in the Brady Soil. We are conducting a long-term lab soil incubation with soils collected from the modern A horizon and the Brady Soil to determine if the buried SOM becomes microbially available when exposed to the modern atmosphere. We are measuring potential rates of respiration and production of CH4 and N2O. Results so far show respiration rates at field moisture for both modern and buried horizons are limited by water, suggesting dry environmental conditions may have helped to preserve SOM in the Brady Soil. We are investigating the potential for chemical stabilization of the dark SOM preserved in the buried paleosol by characterizing C chemistry using solid-state 13C-NMR spectroscopy. Furthermore, we plan to use lipid analyses and pyrolysis GC/MS to determine likely sources for the SOM: microbial vs plant. Combining information on the physical location of SOM in the soil, its chemical composition, decomposability, and radiocarbon based mean residence time estimates will allow us to determine (a) the source of the dark coloration in the Brady soil, (b) the mechanisms that have contributed to its preservation for the last 10,000 years, and (c) the likelihood this large soil C stock will be lost to the atmosphere if exposed during disturbance.