SummaryAmmonia-oxidizers play a key role in nitrification, which is important for nitrogen cycling and soil function. However, little is known about how vegetation successions and agricultural practices caused by human activities impact the ammonia-oxidizers and nitrification process. Putative ammonia-oxidizing bacteria (AOB) and archaea (AOA) communities under different land utilization patterns of restoration (forest), degradation (pasture), cropland and pine plantation were analysed in an acidic red soil based on bacterial and archaeal amoA genes together with archaeal 16S rRNA gene. Real-time PCR, terminal restriction fragment length polymorphism (T-RFLP) and sequencing of clone libraries were conducted to study their abundance and community structure. Land utilization pattern showed significant effects on the copy numbers of all these genes, but only the bacterial amoA gene correlated significantly with potential nitrification rates (PNR). The cropland plot possessed the highest bacterial amoA gene copies and PNR, while the degradation plot was opposite to that. There were no significant variations in the bacterial amoA gene structure, which was dominated by Clusters 10 and 11 in Nitrosospira. However, archaeal amoA gene structure varied among different land utilization patterns especially for the cropland. The degradation plot was dominated by Crenarchaea 1.1c-related groups from which the amoA gene could not been amplified in this study, while other plots were dominated by Crenarchaea 1.1a/b group based on archaeal 16S rRNA gene analysis. These results indicated significant effects of land utilization patterns on putative ammonia oxidizers, which were especially obvious in the degradation and cropland plots where frequent human disturbance occurred.

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 (NH 4 þ and NO 3 À) 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 NH 4 þ on AOB and the negative effect of NO 3 À 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 NO 3 À addition on AOA abundance, which may be a key factor limiting contributions of AOA to nitrification in many soils.

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