Warming influences carbon and nitrogen assimilation between a widespread Ericaceous shrub and root-associated fungi.

High-latitude ecosystems are warming faster than other biomes and are often dominated by a ground layer of Ericaceous shrubs, which can respond positively to warming. The carbon-for-nitrogen (C-for-N) exchange between Ericaceous shrubs and root-associated fungi may underlie shrub responses to warming, but has been understudied. In a glasshouse setting, we examined the effects of warming on the C-for-N exchange between the Ericaceous shrub Empetrum nigrum ssp. hermaphroditum and its root-associated fungi. We applied different 13 C and 15 N isotope labels, including a simple organic N form (glycine) and a complex organic N form (moss litter) and quantified their assimilation into soil, plant biomass, and root fungal biomass pools. We found that warming lowered the amount of 13 C partitioned to root-associated fungi per unit of glycine 15 N assimilated by E. nigrum, but only in the short term. By contrast, warming increased the amount of 13 C partitioned to root-associated fungi per unit of moss 15 N assimilated by E. nigrum. Our study suggests that climate warming affects the short-term exchange of C and N between a widespread Ericaceous shrub and root-associated fungi. Furthermore, while most isotope tracing studies use labile N sources, we demonstrate that a ubiquitous recalcitrant N source may produce contrasting results.

The biological controls of soil carbon accumulation following wildfire and harvest in boreal forests: A review.

Boreal forests are frequently subjected to disturbances, including wildfire and clear-cutting. While these disturbances can cause soil carbon (C) losses, the long-term accumulation dynamics of soil C stocks during subsequent stand development is controlled by biological processes related to the balance of net primary production (NPP) and outputs via heterotrophic respiration and leaching, many of which remain poorly understood. We review the biological processes suggested to influence soil C accumulation in boreal forests. Our review indicates that median C accumulation rates following wildfire and clear-cutting are similar (0.15 and 0.20 Mg ha-1 year-1, respectively), however, variation between studies is extremely high. Further, while many individual studies show linear increases in soil C stocks through time after disturbance, there are indications that C stock recovery is fastest early to mid-succession (e.g. 15-80 years) and then slows as forests mature (e.g. >100 years). We indicate that the rapid build-up of soil C in younger stands appears not only driven by higher plant production, but also by a high rate of mycorrhizal hyphal production, and mycorrhizal suppression of saprotrophs. As stands mature, the balance between reductions in plant and mycorrhizal production, increasing plant litter recalcitrance, and ectomycorrhizal decomposers and saprotrophs have been highlighted as key controls on soil C accumulation rates. While some of these controls appear well understood (e.g. temporal patterns in NPP, changes in aboveground litter quality), many others remain research frontiers. Notably, very little data exists describing and comparing successional patterns of root production, mycorrhizal functional traits, mycorrhizal-saprotroph interactions, or C outputs via heterotrophic respiration and dissolved organic C following different disturbances. We argue that these less frequently described controls require attention, as they will be key not only for understanding ecosystem C balances, but also for representing these dynamics more accurately in soil organic C and Earth system models.

Predictors of taxonomic and functional composition of black spruce seedling ectomycorrhizal fungal communities along peatland drainage gradients.

Many trees depend on symbiotic ectomycorrhizal fungi for nutrients in exchange for photosynthetically derived carbohydrates. Trees growing in peatlands, which cover 3% of the earth's terrestrial surface area yet hold approximately one-third of organic soil carbon stocks, may benefit from ectomycorrhizal fungi that can efficiently forage for nutrients and degrade organic matter using oxidative enzymes such as class II peroxidases. However, such traits may place a higher carbon cost on both the fungi and host tree. To investigate these trade-offs that might structure peatland ectomycorrhizal fungal communities, we sampled black spruce (Picea mariana (Mill.)) seedlings along 100-year-old peatland drainage gradients in Minnesota, USA, that had resulted in higher soil nitrogen and canopy density. Structural equation models revealed that the relative abundance of the dominant ectomycorrhizal fungal genus, Cortinarius, which is known for relatively high fungal biomass coupled with elevated class II peroxidase potential, was negatively linked to site fertility but more positively affected by recent host stem radial growth, suggesting carbon limitation. In contrast, Cenococcum, known for comparatively lower fungal biomass and less class II peroxidase potential, was negatively linked to host stem radial growth and unrelated to site fertility. Like Cortinarius, the estimated relative abundance of class II peroxidase genes in the ectomycorrhizal community was more related to host stem radial growth than site fertility. Our findings indicate a trade-off between symbiont foraging traits and associated carbon costs that consequently structure seedling ectomycorrhizal fungal communities in peatlands.

Empirical and Earth system model estimates of boreal nitrogen fixation often differ: A pathway toward reconciliation.

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.