资源环境科技发展态势分析平台

The impacts of global environmental change on productivity in northern latitudes will be contingent on nitrogen (N) availability. In circumpolar boreal ecosystems, nonvascular plants (i.e., bryophytes) and associated N₂-fixing diazotrophs provide one of the largest known N inputs but are rarely accounted for in Earth system models. Instead, most models link N₂-fixation with the functioning of vascular plants. Neglecting nonvascular N₂-fixation may be contributing toward high uncertainty that currently hinders model predictions in northern latitudes, where nonvascular N₂-fixing plants are more common. Adequately accounting for nonvascular N₂-fixation and its drivers could subsequently improve predictions of future N availability and ultimately, productivity, in northern latitudes. Here, we review empirical evidence of boreal nonvascular N₂-fixation responses to global change factors (elevated CO₂, N deposition, warming, precipitation, and shading by vascular plants), and compare empirical findings with model predictions of N₂-fixation using nine Earth system models. The majority of empirical studies found positive effects of CO₂, warming, precipitation, or light on nonvascular N₂-fixation, but N deposition strongly downregulated N₂-fixation in most empirical studies. Furthermore, we found that the responses of N₂-fixation to elevated CO₂ were generally consistent between models and very limited empirical data. In contrast, empirical-model comparisons suggest that all models we assessed, and particularly those that scale N₂-fixation with net primary productivity or evapotranspiration, may be overestimating N₂-fixation under increasing N deposition. Overestimations could generate erroneous predictions of future N stocks in boreal ecosystems unless models adequately account for the drivers of nonvascular N₂-fixation. Based on our comparisons, we recommend that models explicitly treat nonvascular N₂-fixation and that field studies include more targeted measurements to improve model structures and parameterization.