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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. [ABSTRACT FROM AUTHOR]

Trophic cascades in which predators regulate densities of organisms at lower trophic levels are important drivers of population dynamics, but effects of trophic cascades on ecosystem-level fluxes and processes, and the conditions under which top-down control is important, remain unresolved. We manipulated the structure of a food web in boreal feather mosses and found that moss-inhabiting microfauna exerted top-down control of N2-fixation by moss-associated cyanobacteria. However, the presence of higher trophic levels alleviated this top-down control, likely through feeding on bacterivorous microfauna. These effects of food-web structure on cyanobacterial N2-fixation were dependent on global change factors and strongly suppressed under N fertilisation. Our findings illustrate how food web interactions and trophic cascades can regulate N cycling in boreal ecosystems, where carbon uptake is generally strongly N-limited, and shifting trophic control of N cycling under global change is therefore likely to impact ecosystem functioning. [ABSTRACT FROM AUTHOR]

Productivity in boreal ecosystems is primarily limited by available soil nitrogen (N), and there is substantial interest in understanding whether deposition of anthropogenically derived reactive nitrogen (Nr) results in greater N availability to woody vegetation, which could result in greater carbon (C) sequestration. One factor that may limit the acquisition of Nr by woody plants is the presence of bryophytes, which are a significant C and N pool, and a location where associative cyanobacterial N-fixation occurs. Using a replicated stand-scale N-addition experiment (five levels: 0, 3, 6, 12, and 50 kgNha-1 yr-1; n56) in the boreal zone of northern Sweden, we tested the hypothesis that sequestration of Nr into bryophyte tissues, and downregulation of N-fixation would attenuate Nr inputs, and thereby limit anthropogenic Nr acquisition by woody plants. Our data showed that N-fixation per unit moss mass and per unit area sharply decreased with increasing N addition. Additionally, the tissue N concentrations of Pleuorzium schreberi increased and its biomass decreased with increasing N addition. This response to increasing N addition caused the P. schreberi N pool to be stable at all but the highest N addition rate, where it significantly decreased. The combined effects of changed N-fixation and P. schreberi biomass N accounted for 56.7% of cumulative Nr additions at the lowest Nr addition rate, but only a minor fraction for all other treatments. This 'bryophyte effect' can in part explain why soil inorganic N availability and acquisition by woody plants (indicated by their δ15N signatures) remained unchanged up to N addition rates of 12 kg ha-1 yr-1 or greater. Finally, we demonstrate that approximately 71.8% of the boreal forest experiences Nr deposition rates at or below 3 kg ha-1 yr-1, suggesting that bryophytes likely limit woody plant acquisition of ambient anthropogenic Nr inputs throughout a majority of the boreal forest. [ABSTRACT FROM AUTHOR]