Evidence for Enhanced Biogeochemical Cycling at Soil Interfaces in the Vadose Zone.

Chemical dynamics in the vadose zone including biogeochemical cycling, mineral-water interactions and sorption are poorly understood. This study explores the effects of soil structure (i.e. layers, lenses, macropores, or fractures) on linked geochemical, hydrological, and microbiological processes.   Three laboratory soil columns were constructed: a homogenized medium-grained sand, homogenized organic-rich loam, and a sand-over-loam layered column. Both upward and downward infiltration of water was evaluated during experiments to simulate rising water table and rainfall events respectively. In-situ collocated probes measured soil water content, matric potential, and Eh. Water samples extracted by lysimeter were analyzed for major cations and anions, ammonium, organic acids, alkalinity, Fe(II), and total sulfide. Enhanced biogeochemical cycling was observed in the layered column over the texturally homogeneous soil columns.  For example, concentrations of the electron acceptor sulfate were two times greater in the layered column than in either of the homogeneous columns due to increased oxidation/reduction reactions. Redox reactions were also directly linked to hydrological conditions. Variability of recharge produced the greatest geochemical changes in the columns. In response to rainfall, denitrification increased through the addition of NO₃^- via NH₄⁺ oxidation in the loam and layered columns with greater production of NH₄⁺ in the layered column. Biogeochemical changes initiated by water infiltration also directly impacted the physical hydrologic flow properties of the system.  Infiltration coupled with microbial reduction caused mineral redistribution resulting in a decrease in overall hydraulic conductivity (K_sat).  Reduction of K_sat was greatest in the layered column and decreased by an order of magnitude. These results demonstrate soil structure, in this case soil layers, can produce considerably greater biogeochemical effects than homogenous soil systems suggesting that quantifying coupled hydrologic-biogeochemical processes occurring at small scale soil interfaces is critical to accurately describing and predicting chemical changes at the larger system scale.