Mining has led to dramatic ecosystem degradation, the destruction of vegetation and irreversible damage to soil structure and nutrient cycling; additionally, heavy metal (HM) contamination has affected soil nitrogen (N) cycle-associated microorganisms and disrupted soil aggregate structure. To explore the mechanism of soil N recovery in mining areas, we investigated the effects of two N fertilizers (urea (U) and ammonium chloride (AC)) and nine different fertilization patterns on the nitrification process and ammonia oxidizers in soil aggregates via incubation experiments. The results showed that different N treatments had different influences on the distribution of AOA and AOB amoA gene abundance and microbial community structure in soil aggregates. The AOB amoA gene abundance was significantly greater than the AOA amoA gene abundance in aggregates. The dominant species of AOA and AOB were Nitrososphaera and Nitrosospira , respectively, which were mainly found in microaggregates and accounted for 10.3 % to 25.0 % and 31.5 % to 60.1 %, respectively, of the microaggregates. Dissolved organic nitrogen (DON) can be used as an important variable to explain variations in AOA communities, and microbial nitrogen (MBN) content, tartaric acid content, cellulase activity and AOB amoA gene abundance can be used as important variables to explain variations in AOB communities. N fertilizer addition resulted in potential ammonia oxidation (PAO) values ranging from 0.079 to 0.236, 0.100 to 0.5953 and 0.146 to 0.905 mu g.NO2--N d(-1) g(-1) in mega-, macro- and microaggregates, respectively, which suggested that PAO values increased with decreasing aggregate size. In addition, the total nitrification potential (TNP) in macroaggregates was greater than that in mega- and microaggregates, which was the main reason for the increase in the NO3 content in macroaggregates. AOB amoA gene abundance was significantly positively correlated with TNP, and AOB amoA gene abundance was more significantly positively correlated with PAO values than was AOA gene abundance, which suggests that AOB dominated ammonia oxidation and nitrification processes in aggregates. Our research contributes to an understanding of the mechanisms underlying the effects of different types of N fertilizers on nitrification processes and ammonia oxidizers in soil aggregates and provides insights into N management in contaminated soils in mining areas.

Substantial amounts of topsoil organic matter (OM) in Arctic Cryosols have been translocated by the process of cryoturbation into deeper soil horizons (cryoOM), reducing its decomposition. Recent Arctic warming deepens the Cryosols' active layer, making more topsoil and cryoOM carbon accessible for microbial transformation. To quantify bacteria, archaea and selected microbial groups (methanogens - mcrA gene and diazotrophs - nifH gene) and to investigate bacterial and archaeal diversity, we collected 83 soil samples from four different soil horizons of three distinct tundra types located in Qikiqtaruk (Hershel Island, Western Canada). In general, the abundance of bacteria and diazotrophs decreased from topsoil to permafrost, but not for cryoOM. No such difference was observed for archaea and methanogens. CryoOM was enriched with oligotrophic (slow-growing microorganism) taxa capable of recalcitrant OM degradation. We found distinct microbial patterns in each tundra type: topsoil from wet-polygonal tundra had the lowest abundance of bacteria and diazotrophs, but the highest abundance of methanogens. Wet-polygonal tundra, therefore, represented a hotspot for methanogenesis. Oligotrophic and copiotrophic (fast-growing microorganism) genera of methanogens and diazotrophs were distinctly distributed in topsoil and cryoOM, resulting in different rates of nitrogen flux into these horizons affecting OM vulnerability and potential CO2 and CH4 release.