Effect of Soil Physical and Chemical Heterogeneity on the Distribution of Carbon, Nitrogen, and Phosphorous in Subsurface Soils

Redox biogeochemistry is a critical factor in understanding the structure and function of ecosystems. Although these factors are highly variable among different landcover and soil depths, there are limited studies that try to link the redox-sensetive elements with soil physical and chemical properties in various depths and with different landcover. With designed experiments in Brazos River corridor in Texas, we (1) evaluate the effect of different land use and land covers on the concentrations of electron acceptors (O₂, NO₃, and SO₄), reduced products (Mn(II), Fe(II)), and carbon, nitrogen, and phosphorous pools in the surface and deep soils, (2) determine effects of climatic gradient on redox biogeochemistry in deep soils, and (3) investigate the effects of soil physical and hydraulic properties on redox biogeochemistry. Soil physical and chemical analyses were performed on soils from varying soil depths (surface to 15 m) in different landcovers (grassland, forest, and salt marsh). Higher carbon and nitrogen content were observed in the surface soils due to carbon mineralization. However, the phosphorous content was higher at 15 -30 cm soil depth, because of the high iron and aluminum oxides concentrations that provide high surface area for phosphorous adsorption. Carbon, nitrogen, phosphorus, and other redox-sensitive elements were positively correlated to clay content. The biogeochemical properties, including ammonium, ferric iron and sulfate, were disproportionally higher at the interface of soil layers where soil texture and hydraulic properties change. This finding reflects the role of soil layers as hot spots of biogeochemical processes in the subsurface. With the climate gradient across the study river basin, our data indicates carbon, nitrogen, phosphorous, and other redox-sensitive elements are more profound in the fields (e.g. salt marsh and forest) with higher annual mean temperature and precipitation as these factors stimulate microbial activity and thus influence redox processes. Additionally, C and N concentrations are higher in forest sites relative to remnant grasslands as a consequence of the higher above- and below-ground productivity.