REDUCTION OF NITRATE IN CLAYEY SUBSOILS CONTROLLED BY GEOCHEMICAL ANDMICROBIAL BARRIERS

Vertical distribution of redox zones, concentrations of redox-sensitiv e constituents, numbers of aerobic heterotrophic bacteria, and potenti al denitrification activity were studied in I-m cores taken at the tra nsition between the oxidized and reduced layers in two Danish clayey s ubsoils. Based on the matrix soil colors, a redox sequence of oxidized , suboxic, and reduced zones was identified at both sites. The geochem ical composition of the oxidized brown colored zone (ro depths of 2.6 and 3.2 m) was characterized by high concentrations of NO3- and low am ounts of total organic carbon, exchangeable forms of NH4+, Fe2+, and M n2+, and structural Fe(II) in the clay minerals. In the underlying 20- to 30-cm-deep suboxic zone, decreasing NO3- concentrations were obser ved together with increasing amount of exchangeable forms of Fe2+ and Mn2+, and structural Fe(ll). Finally, in the reduced grey zone, NO3- w as no longer present and maximum concentrations of other redox sensiti ve 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( ll) in the clay minerals of the suboxic zone, and that the Fe(II)forme d a geochemical barrier for the downward progression of NO3-. Aerobic heterotrophic bacteria occurred only in low numbers at this site and p otential denitrification activity was very low. In contrast, the Sparr esholm site had a significant population of bacteria in the suboxic zo ne, which also contained a heterogeneous distribution of potential den itrification activity. Specific microsites with facilitated transport of soluble organic substrates are proposed to support the denitrificat ion activity in a heterogeneous distribution; constituting a,microbial barrier for downward progression of NO3- in this subsoil.