Abstract
Hydroxylamine (NH(2)OH) undergoes biotic and abiotic transformation processes in soil, producing nitrous oxide gas (N(2)O(g)). Little is known about the magnitude of the abiotic chemical processes in the global N cycle, and the role of abiotic nitrification is still neglected in most current nitrogen trace gas studies. The abiotic fate of NH(2)OH in soil systems is often focused on transition metals including manganese (Mn) and iron (Fe), and empirical correlations of nitrogen residual species including nitrite (NO(2)(-)), nitrate (NO(3)(-)), and N(2)O(g). In this study, abiotic NH(2)OH nitrification by well-characterized manganese (Mn)- and iron (Fe)-bearing minerals (pyrolusite, amorphous MnO(2)(s), goethite, amorphous FeOOH(s)) was investigated. A nitrogen mass balance analysis involving NH(2)OH, and the abiotic nitrification residuals, N(2)O(g), N(2)O(aq), NO(2)(-), NO(3)(-), was used, and specific reactions and mechanisms were investigated. Rapid and complete NH(2)OH nitrification occurred (4-5 h) in the presence of pyrolusite and amorphous MnO(2)(s), achieving a 95-96% mass balance of N byproducts. Conversely, NH(2)OH nitrification was considerably slower by amorphous FeOOH(s) (14.5%) and goethite (1.1%). Direct reactions between the Mn- and Fe-bearing mineral species and NO(2)(-) and NO(3)(-) were not detected. Brunauer-Emmett-Teller surface area and energy dispersive X-ray measurements for elemental composition were used to determine the specific concentrations of Mn and Fe. Despite similar specific concentrations of Mn and Fe in crystalline and amorphous minerals, the rate of NH(2)OH nitrification was much greater in the Mn-bearing minerals. Results underscore the intrinsically faster NH(2)OH nitrification by Mn minerals than Fe minerals.