Soil biogeochemistry of the capillary fringe in laboratory mesocosms with contrasting soil textures

Abstract

AbstractThe capillary fringe is a poorly defined, tension‐saturated region with important influences on soil biogeochemistry. The goal of this research was to evaluate the effects of the capillary fringe moisture gradient on biogeochemical processes and to assess whether these processes are more similar to those of unsaturated or saturated zones. Effects were documented during 118 d using two soils with contrasting textures in laboratory mesocosms. A water table was maintained at a constant height, and the parameters measured included soil moisture, inorganic N, decomposition, and reduction changes. Moisture levels suggest that capillary rise occurred ≥22.5 cm above the water table in both soils. Although limited by experimental methods, including using homogenized and sieved soil materials, biogeochemical trends for all measured parameters were seen by depth and between soils and seem to be influenced by water content and O2 diffusion. Nitrate concentrations increased above the water table. Decomposition was lowest in and immediately above the saturated water table and greatest where moisture and O2 were balanced. Reducing conditions existed ≥10 cm above the water table in the sandy soil and ≥5 cm in the loamy soil, indicating that these regions share characteristics with the saturated zone. The sandy soil showed reduction higher above the water table than the loamy soil, possibly due to structural and O2 diffusion differences due to experimental methods. Specific heights of capillary fringe were not identified; instead, the data support a model of a continuous moisture content gradient in the capillary fringe between the water table and soil surface.