The role of abiotic and coupled biotic/abiotic mineral controlled redox processes in nitrate reduction

Nitrogen is a major nutrient element which often limits productivity in natural and agroecosystems. Paradoxically, nitrogen is also readily lost through processes such as denitrification in soil environments. Historically, heterotrophic denitrification has been studied assuming soil organic carbon is the sole electron donor with little recognition for coupling with other elemental cycles. Recent discoveries have emerged which show a strong coupling between the iron and nitrogen cycles. Where soil iron(III) is utilized as a terminal electron acceptor during organic carbon respiration, the resulting iron(II) can serve as an electron donor during nitrate reduction. One example is the reduction of nitrate coupled to iron(II) oxidation by mixotrophic microorganisms in a process called nitrate-dependent, iron(II) oxidation. Nitrate-dependent, iron(II) oxidation is shown to occur in a moderately well-drained and a well-drained agricultural soil under anoxic conditions. The rates of nitrate reduction by heterotrophic bacteria are comparable to those by iron(II) oxidizers. Another example is the abiotic reduction of nitrate by iron(II) minerals which form in anoxic soil environments such as green rust. There is also the possibility of coupled biological-abiological processes where microbial Fe(III) reduction to Fe(II) is coupled to secondary chemical reactions involving nitrite and siderite, the latter represents an important Fe(II) mineral. These results underscore the complexity of the nitrogen cycle and will hopefully inspire future studies that will unravel the mechanisms behind soil Fe(II) oxidation by nitrate.