Subsurface variation of soil physicochemical properties and
redox-sensitive elements under different land covers in Gulf Coastal
Plains
Abstract
The redox potential of soils is critical in understanding the structure
and function of ecosystems. Soil redox state strongly governs the
speciation, bioavailability, and solubility of limiting nutrients like
nitrogen and phosphorous in soil. It also drives the reactivity,
mobility, and toxicity of redox-sensitive elements such as anthropogenic
contaminants, affecting soil and groundwater quality by altering or
retaining undesirable metals. Although these factors are highly variable
among different landcovers and soil depths, limited studies try to link
the redox-sensitive elements with soil physical and chemical properties
in various depths and with different landcovers. 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 C, N, and P
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 properties were determined
from varying soil depths (land surface up 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 in all land covers. However, the phosphorous content was
higher at 15-30 cm soil depth, because of the co-existing high iron and
aluminum oxides concentrations that provide high surface area for
phosphorous adsorption. C, N, P, and other redox-sensitive elements were
positively correlated to clay content at various depths. 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 C, N, P, and other redox-sensitive elements are more
profound in the salt marsh and forest covers with higher annual mean
temperature and precipitation as these factors stimulate microbial
activity and thus influence redox processes.