Nitrate Reduction Coupled to Iron(II) Oxidation In An Agricultural Soil.

Nitrogen is a major component of the soil environment with denitrification being a major process under anaerobic conditions. Iron is also abundant in the soil with its cycling being largely attributed to microbial respiration. However, new evidence shows that iron (II) oxidation is strongly correlated with nitrate reduction under anaerobic conditions in freshwater sediments and agricultural soils. This study investigated abiotic nitrate-dependent iron (II) oxidation in an agricultural soil (Sadler silt loam) using the stirred-batch kinetic technique.  Soil slurries were preincubated in an anaerobic chamber to reduce all indigenous soil Fe(III) to Fe(II) prior to nitrate addition.  During this time there was a rise in pH and a decline in Eh. Acetate was the predominant water-soluble organic carbon species that emerged during the preincubation.  Nitrate was added (as sodium nitrate) under anoxic conditions and relevant nitrogen, iron, and carbon species were monitored with time.  At early times (5 min- 6 h), water and acid-extractable fractions of iron (II) decreased with concomitant nitrate reduction and there was little change in acetate concentrations.  Nitrous oxide accumulated during the first 6 h of the reaction.  After 24 h, nitrate was completely reduced and nitrous oxide decreased to negligible levels.  In addition, acetate was completely consumed after 24 h. These results have shown a strong coupling of iron (II) oxidation and nitrate reduction at early times in the reaction and a strong coupling of nitrate and acetate at later times. Future experiments will explore the impact of plant residue amendments and nitrogen fertilizer sources on nitrate-dependent, iron(II) oxidation.