Controls on annual nitrogen cycling in the understory of a subarctic birch forest

Abstract

Characterization of the controls on annual nitrogen (N) cycling is critical to understanding the functioning of high-latitude ecosystems and to predicting their responses to perturbations. Here, we describe an experimental evaluation of the effects of season and vegetation on annual N cycling in the understory heath vegetation of a subarctic birch forest. Our approach was to follow the partitioning of an isotopically enriched 15NH4Cl addition between microbes, plants, and the soil solution at intervals through winter, and in the following summer. To investigate the direct influence of vegetation on ecosystem N cycling, the isotope was added to control plots and to plots from which plants had been removed early in the previous growing season. Our results indicate that the dynamics of both microbial carbon and N were similar in treatment and control plots, suggesting that the presence of intact plants had negligible influence on seasonal patterns of microbial growth or N accumulation in the soils of this ecosystem. Instead, ecosystem N cycling was dominated by a substantial turnover of recently acquired N from microbes during the winter-summer transition that corresponded to a significant increase in understory plant 15N uptake. Vascular plant 15N uptake and accumulation in belowground tissue was substantial at the onset of winter, but ceased once full winter conditions developed. By contrast, there was a rapid resumption of vascular plant N uptake and strong allocation to aboveground tissue at the onset of summer. Vascular plant types varied strongly in 15N enhancement, depending on physiological differences in N uptake capacity as well as differences in biomass. In particular, Vaccinium vitis-idaea, Vaccinium myrtillus, and the herbs exhibited strong 15N acquisition capacities, despite their relatively low biomass. We found no consistent evidence that the evergreen vs. deciduous leaf habit contributed to species differences in N uptake capacity. Species capacity for allocation of 15N to new shoot tissue was closely correlated with aboveground production per unit total N and tended to be greatest in herbs and V. myrtillus, suggesting that inherent physiological capacities for N uptake and tissue allocation are important determinants of species productivity. Our results suggest a hierarchy of controls on annual N cycling. The importance of wintertime influences on annual ecosystem N cycling was indicated by ongoing net mineralization by microbes beneath snow cover and a critical N release from microbes during the winter-summer transition. By contrast, plant N uptake was confined to the snow-free season. Thus, seasonal environmental changes caused a rapid microbial turnover of recently acquired N that then became available for plant uptake in the subsequent growing season. Although vascular plant species appeared to compete strongly with each other to acquire this N, neither their presence nor their total N uptake appreciably affected microbial N-cycling activity. Together, these results suggest that season exerts primary control on annual N cycling in this ecosystem, and that its effect on microbial N turnover is a major process supplying N to plants. By comparison, the effects of understory vegetation on N cycling were small within the time frame of this experiment.