Heterotrophic ammonium oxidation is not active in acidic paddy soils

Abstract

In acidic paddy soils where crops with an ammonium (NH4+) preference are grown, ammonia (NH3) is largely transformed to NH4+, and organic nitrogen (N) is often too low to support organic nitrification, heterotrophic nitrifiers may have a preference for NH4+. If so, acidic paddy soils are likely a hotspot of heterotrophic NH4+ oxidation (OHNH4). To reveal if OHNH4 is active in acidic paddy soils, we investigated the existence of acetylene (C2H2)-resistant NH4+ oxidation at pH 5.4, 5.3, and 5.0 in paddy soils with 60% water holding capacity (WHC), using 15N tracing and nitrification inhibition techniques (1.0% C2H2). As expected, C2H2-resistant NH4+ oxidation (0.0088 ± 0.0012 mg N kg−1 soil d−1) was detected in soils with a pH of 5.0. In the presence of 1.0% C2H2, increased 15N in the NO3− pool under 15NH4NO3 labelling suggested the existence of C2H2-resistant NH4+ oxidizers. Through a concentration gradient experiment of C2H2 (0, 0.01%, 0.1%, and 1.0%) on the pH 5.0 soil, 0.01% C2H2 was found to be sufficient for complete inhibition of autotrophic nitrification, suggesting that NH4+ oxidation in samples receiving 0.1% or 1.0% C2H2 is unlikely driven by autotrophs. The trend toward higher rates when C2H2 increased from 0.01% to 0.1% suggested that, C2H2-resistant NH4+ oxidizers in paddy soils are likely carbon (C)-limited. The trend toward lower rates when C2H2 increased from 0.1% to 1.0% implied an inhibition in the high concentration. At 0.1% C2H2, the trend toward lower rates when soil moisture increased from 60%WHC to 100%WHC indicated that C2H2-resistant NH4+/NH3 oxidizers prefer drier conditions, and OHNH4 will be less active at flooded conditions. Since there is evidence of organic and inorganic N oxidation by heterotrophs are mutually exclusive, we thus used a 15N tracing model to separate organic nitrification from inorganic nitrification to clarify that low OHNH4 in samples was irrelevant to heterotrophic nitrification of organic N (OHORG). In summary, isotopic evidence has been provided to suggest that OHNH4, although with a low rate, has the potential to proceed in acidic paddy soils. In which, low OHNH4 is unrelated to OHORG, and an exogenous C source is needed to activate heterotrophic NH4+/NH3 oxidizers.