Nitrogen (N) cycling in organic tundra soil is characterised by pronounced seasonal dynamics and strong influence of the dominant plant functional types. Such patterns in soil N-cycling have mostly been investigated by the analysis of soil N-pools and net N mineralisation rates, which, however, yield little information on soil N-fluxes. In this study we investigated microbial gross N-transformations, as well as concentrations of plant available N-forms in soils under two dominant plant functional types in tundra heath, dwarf shrubs and mosses, in subarctic Northern Sweden. We collected organic soil under three dwarf shrub species of distinct growth form and three moss species in early and late growing season. Our results showed that moss sites were characterised by significantly higher microbial N-cycling rates and soil N-availability than shrub sites. Protein depolymerisation, the greatest soil N-flux, as well as gross nitrification rates generally did not vary significantly between early and late growing season, whereas gross N mineralisation rates and inorganic N availability markedly dropped in late summer at most sites. The magnitude of the seasonal changes in N-cycling, however, clearly differed among plant functional types, indicating interactive effects of seasonality and plant species on soil N-cycling. Our study highlights that the spatial variation and seasonal dynamics of microbial N transformations and soil N availability in tundra heath are intimately linked with the distinct influence of plant functional types on soil microbial activity and the plant species-specific patterns of nutrient uptake and carbon assimilation. This suggests potential strong impacts of future global change-induced shifts in plant community composition on soil N-cycling in tundra ecosystems.

有机苔原土壤中的氮 （N） 循环的特点是明显的季节性动态和主要植物功能类型的强烈影响。土壤 N 循环的这种模式主要通过分析土壤 N 池和净 N 矿化速率来研究，然而，关于土壤 N 通量的信息很少。在这项研究中，我们调查了瑞典北部亚北极地区苔原荒地、矮灌木和苔藓中两种主要植物功能类型下的微生物总 N 转化以及植物可用 N 形式的浓度。我们在生长季节的早期和晚期收集了 3 种生长形式的矮灌木和 3 种苔藓物种下的有机土壤。我们的结果表明，苔藓地点的特点是微生物氮循环速率和土壤氮有效性明显高于灌木地点。蛋白质解聚、最大的土壤氮通量以及总硝化速率在生长季节早期和晚期通常没有显著变化，而在大多数地点，总氮矿化速率和无机氮可用性在夏末显著下降。然而，氮循环季节性变化的幅度在植物功能类型之间明显不同，表明季节性和植物物种对土壤氮循环的交互影响。我们的研究强调，苔原荒地微生物氮转化的空间变化和季节动态以及土壤氮有效性与植物功能类型对土壤微生物活动以及植物物种特异性养分吸收和碳同化模式的不同影响密切相关。 这表明未来全球变化引起的植物群落组成变化对苔原生态系统中土壤氮循环的潜在强烈影响。