Variability in soil organic matter composition in a buried soil: differences along eroding and depositional transects

Abstract:

Paleosols that are formed when the topsoil gets buried by lateral distribution of soil can store large quantities of soil organic matter (SOM) that may persistent over millennial timescales due to its detachment from the disturbances at the surface. We studied buried SOM dynamics in the Brady paleosol, a deep loess (aeolian) deposit in Nebraska, USA where climate has historically driven varying rates of loess deposition during the late Pleistocene and Holocene, burying soils up to 50 m below the surface. Soils were sampled along the burial and erosional transects at the depths from 0.2 to 4.2 m to understand the variability in physical and chemical composition of the soils in buried vs modern surface. We determined SOM composition using Fourier Transformed Infrared Spectroscopy (FTIR) and conducted path analysis to provide a better understanding of the relationships between carbon (C) and nitrogen amounts, sources, and other soil physio-chemical properties with SOM composition along depth gradients from modern to buried soil layers in the brady paleosol. As soil enzyme activity, nutrient availability and SOM decomposition rate differ along with soil depth, we aimed to determine how SOM composition varies with across the site. Our results show a general decreasing trend of d¹³C and d¹⁵N values with depth, suggesting root input to soil C pools and presence of less decomposed SOM in the deep buried soil layers. FTIR data show that the buried soil organic matter tends to be enriched in aliphatic compound similar to the modern soils, and that association of C with minerals (in particular Ca⁺²) is likely a major mechanism for chemical stabilization of the buried SOM. Our study highlights that soil physical and chemical properties play important roles in controlling SOM stock and composition across the site, suggesting that buried SOM may be vulnerable to changes in precipitation that may directly or indirectly affect a range of soil physio-chemical properties.