Contrasting effects of nitrogen forms and soil pH on ammonia oxidizing microorganisms and their responses to long-term nitrogen fertilization in a typical steppe ecosystem

- A previous study showed that N fertilization affected AOA and AOB abundance and soil pH in a steppe.
- The field experiment showed that responses of AOA and AOB to N fertilization were not due to soil pH.
- The microcosm study showed that NO3− negatively affects AOA and NH4+ positively affects AOB.
- N rather than soil pH explains the effects of long-term N fertilization on AOA and AOB abundance.

Ammonia-oxidizing microorganisms, i.e. ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA), are the primary agents responsible for soil nitrification. Few studies, however, have evaluated how AOA and AOB responses to long-term N fertilization are affected by soil pH and different forms of nitrogen (N). We examined the effects of soil acidification and different forms of N (NH4+ and NO3−) on abundances and community structure of AOA and AOB based on a field acid addition experiment and a short-term microcosm N addition experiment. The field acid addition experiment demonstrated that, with decreasing soil pH, AOB abundance decreased while AOA abundance mostly increased except for an extremely low pH treatment. Relationships between soil pH and abundance of ammonia oxidizers in the acid addition experiment conflict with those in the long-term N fertilization experiment, indicating a predominant role of N rather than soil pH on responses of AOB and AOA under N inputs. The short-term N addition experiment confirmed the general positive effect of NH4+ on AOB and the negative effect of NO3−on AOA which help explain the responses of AOB and AOA abundance to long-term N fertilization. Community structure of ammonia oxidizers, in contrast, showed little response to acid addition and short-term N additions. The positive responses of AOB and not AOA to various N additions revealed a dominant role of AOB in nitrification with broad ranges of soil pH and N availability. We found a negative effect of NO3− addition on AOA abundance, which may be a key factor limiting contributions of AOA to nitrification in many soils.