Reduction of nitrate in clayey subsoils controlled by geochemical and microbial barriers

Abstract

Vertical distribution of redox zones, concentrations of redox‐sensitive constituents, numbers of aerobic heterotrophic bacteria, and potential denitrification activity were studied in 1‐m cores taken at the transition between the oxidized and reduced layers in two Danish clayey subsoils. Based on the matrix soil colors, a redox sequence of oxidized, suboxic, and reduced zones was identified at both sites. The geochemical composition of the oxidized brown colored zone (to depths of 2.6 and 3.2 m) was characterized by high concentrations of NO3 − and low amounts of total organic carbon, exchangeable forms of NH4 +, Fe2+, and Mn2+, and structural Fe(II) in the clay minerals. In the underlying 20‐ to 30‐cm‐deep suboxic zone, decreasing NO3 − concentrations were observed together with increasing amount of exchangeable forms of Fe2+ and Mn2+, and structural Fe(II). Finally, in the reduced grey zone, NO3 − was no longer present and maximum concentrations of other redox sensitive constituents occurred. Throughout the subsoils, the distribution of exchangeable Fe2+ corresponded most closely to changes in the colors of redox zones. The low‐organic Havrebjerg site displayed geochemical profiles indicating that NO3 − was chemically reduced by structural Fe(II) in the clay minerals of the suboxic zone, and that the Fe(II) formed a geochemical barrier for the downward progression of NO3 −. Aerobic heterotrophic bacteria occurred only in low numbers at this site and potential denitrification activity was very low. In contrast, the Sparresholm site had a significant population of bacteria in the suboxic zone, which also contained a heterogeneous distribution of potential denitrification activity. Specific microsites with facilitated transport of soluble organic substrates are proposed to support the denitrification activity in a heterogeneous distribution, constituting a microbial barrier for downward progression of NO3 − in this subsoil.