Complete ammonia oxidization in agricultural soils: High ammonia fertilizer loss but low N₂O production

Abstract

The contribution of agriculture to the sustainable development goals requires climate‐smart and profitable farm innovations. Increasing the ammonia fertilizer applications to meet the global food demands results in high agricultural costs, environmental quality deterioration, and global warming, without a significant increase in crop yield. Here, we reported that a third microbial ammonia oxidation process, complete ammonia oxidation (comammox), is contributing to a significant ammonia fertilizer loss (41.9 ± 4.8%) at the rate of 3.53 ± 0.55 mg N kg−1 day−1 in agricultural soils around the world. The contribution of comammox to ammonia fertilizer loss, occurring mainly in surface agricultural soil profiles (0–0.2 m), was equivalent to that of bacterial ammonia oxidation (48.6 ± 4.5%); both processes were significantly more important than archaeal ammonia oxidation (9.5 ± 3.6%). In contrast, comammox produced less N2O (0.98 ± 0.44 μg N kg−1 day−1, 11.7 ± 3.1%), comparable to that produced by archaeal ammonia oxidation (16.4 ± 4.4%) but significantly lower than that of bacterial ammonia oxidation (72.0 ± 5.1%). The efficiency of ammonia conversion to N2O by comammox (0.02 ± 0.01%) was evidently lower than that of bacterial (0.24 ± 0.06%) and archaeal (0.16 ± 0.04%) ammonia oxidation. The comammox rate increased with increasing soil pH values, which is the only physicochemical characteristic that significantly influenced both comammox bacterial abundance and rates. Ammonia fertilizer loss, dominated by comammox and bacterial ammonia oxidation, was more intense in soils with pH >6.5 than in soils with pH <6.5. Our results revealed that comammox plays a vital role in ammonia fertilizer loss and sustainable development in agroecosystems that have been previously overlooked for a long term.

AOB Nitrosospira cluster 3a.2 (D11) dominates N2O emissions in fertilised agricultural soils

“…Still, some AOA species with high ammonia (NH3) tolerance, such as those within the Candidatus Nitrosocosmicus (NS) clade (Lehtovirta-Morley et al, 2016), can contribute to N2O production to the same extent as AOB under high NH4 + inputs, as described for Candidatus Nitrosocosmicus agrestis (Liu et al, 2021;Jiang et al, 2023b). In addition, comammox Nitrospira clade A.2 and clade B also participate in nitrification in fertilised and unfertilised soils with NH4 + and manure addition (Li et al, 2019a;Wang et al, 2019;Xu et al, 2020;Lin et al, 2022), but there is rare empirical evidence of their contribution to N2O emissions in agricultural soils (Tan et al, 2022;Jiang et al, 2023a). Altogether, more knowledge of the key nitrifiers responsible for N2O emissions is required over a large spatial scale region, especially considering the high diversity of soil conditions and nitrifier taxa.…”