Rapid dissimilatory nitrate reduction to ammonium conserves bioavailable nitrogen in organic deficient soils

Large amounts of nitrogen fertilized to food production are lost via denitrification and leaching. Dissimilatory nitrate reduction to ammonium (DNRA) is a bioprocess competing with denitrification, and it conserves bioavailable nitrogen in soil ecosystems. The carbon to nitrogen (C/N) ratio has long been believed essential to balance the competition between DNRA and denitrification, but increasing evidence indicates that DNRA may occur unrelated to C/N ratio via a largely unknown mechanism. Taking Geobacter as representative bacteria, we found that the kinetic of nitrite bio-reduction to ammonium was the key to enhancing DNRA and demonstrated it in pure and mixed cultures and soil ecosystems. Compared to other two strains of DNRA bacteria, Geobacter sulfurreducens was confirmed for the first time to conduct a 3-fold more rapid DNRA (NO2−→NH4+) when it grew with the kinetically matched nitrite accumulator Delftia tsuruhatensis (NO3−→NO2−). They assisted and restricted each other to bring their population ratio close to the theoretical yield ratio of 2.7:1 while performing cross-feeding with nitrite and ammonium. This rapid DNRA was further demonstrated to conserve 2–8 times more reactive nitrogen in organic deficient soils, and up to 40% of 15NO3− was reduced to ammonium. Our results found a sustainable way of nitrogen retention in soils, and have broader implications for understanding nitrogen turnover in ecosystems.