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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. [ABSTRACT FROM AUTHOR]

Summary: 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 13C and 15N 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 13C partitioned to root‐associated fungi per unit of glycine 15N assimilated by E. nigrum, but only in the short term. By contrast, warming increased the amount of 13C partitioned to root‐associated fungi per unit of moss 15N 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. [ABSTRACT FROM AUTHOR]

Nitrogen (N) deposition can enhance carbon (C) capture and storage in northern coniferous forests but it may also enhance the demand for phosphorus (P). While it is well established that long‐term N enrichment can decrease decomposition and enhance the accumulation of C in soils, it remains uncertain if a higher demand and acquisition of P influence soil C.We studied microbial phosphorus mobilization and growth within a long‐term N enrichment experiment in a Norway spruce forest, where N deposition was simulated by adding 0, 12.5 or 50 kg N ha−1 year−1 for 21 years (n = 12), by incubating microbial ingrowth cores with needles and humus with low and high P content, and with sand with and without mineral apatite P.Long‐term N enrichment had no effect on microbial P mobilization in needles and humus and did not enhance the positive effect that apatite had on fungal growth. However, it consistently strengthened the retention of C in the soil by decreasing decomposition of needle and humus, both with low and high P content, and by increasing fungal growth in sand‐filled ingrowth cores. Furthermore, we did not find any evidence that higher microbial P mobilization in response to N enrichment affected soil C storage.These results show that long‐term N enrichment in relatively young soils dominated by coniferous trees and ectomycorrhizal fungi can have relatively small impact on microbial P mobilization from organic sources and on the potential to mobilize P from minerals, and subsequently that elevated P demand due to N enrichment is unlikely to lead to a reduction in the soil C accumulation rate. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

Nitrogen (N) deposition can change the carbon (C) sink of northern coniferous forests by changing the balance between net primary production and soil respiration. We used a field experiment in an N poor Pinus sylvestris forest where five levels of N (0, 3, 6, 12, and 50 kg N ha−1 yr−1, n = 6) had been added annually for 12–13 years to investigate how litter C inputs and soil respiration, divided into its autotrophic and heterotrophic sources, respond to different rates of N input, and its subsequent effect on soil C storage. The highest N addition rate (50 kg N ha−1 yr−1) stimulated soil C accumulation in the organic layer by 22.3 kg C kg−1 N added, increased litter inputs by 46%, and decreased soil respiration per mass unit of soil C by 31.2%, mainly by decreasing autotrophic respiration. Lower N addition rates (≤ 12 kg N ha−1 yr−1) had no effect on litter inputs or soil respiration. These results support previous studies reporting on increased litter inputs coupled to impeded soil C mineralization, contributing to enhancing the soil C sink when N is supplied at high rates, but add observations for lower N addition rates more realistic for N deposition. In doing so, we show that litter production in N poor northern coniferous forests can be relatively unresponsive to low N deposition levels, that stimulation of microbial activity at low N additions is unlikely to reduce the soil C sink, and that high levels of N deposition enhance the soil C sink by increasing litter inputs and decreasing soil respiration. [ABSTRACT FROM AUTHOR]

There is evidence that anthropogenic nitrogen (N) deposition enhances carbon (C) sequestration in boreal forest soils. However, it is unclear how free‐living saprotrophs (bacteria and fungi, SAP) and ectomycorrhizal (EM) fungi responses to N addition impact soil C dynamics. Our aim was to investigate how SAP and EM communities are impacted by N enrichment and to estimate whether these changes influence decay of litter and humus. We conducted a long‐term experiment in northern Sweden, maintained since 2004, consisting of ambient, low N additions (0, 3, 6, and 12 kg N ha−1 year−1) simulating current N deposition rates in the boreal region, as well as a high N addition (50 kg N ha−1 year−1). Our data showed that long‐term N enrichment impeded mass loss of litter, but not of humus, and only in response to the highest N addition treatment. Furthermore, our data showed that EM fungi reduced the mass of N and P in both substrates during the incubation period compared to when only SAP organisms were present. Low N additions had no effect on microbial community structure, while the high N addition decreased fungal and bacterial biomasses and altered EM fungi and SAP community composition. Actinomycetes were the only bacterial SAP to show increased biomass in response to the highest N addition. These results provide a mechanistic understanding of how anthropogenic N enrichment can influence soil C accumulation rates and suggest that current N deposition rates in the boreal region (≤12 kg N ha−1 year−1) are likely to have a minor impact on the soil microbial community and the decomposition of humus and litter. [ABSTRACT FROM AUTHOR]

Decades of fire suppression following extensive timber harvesting have left much of the forest in the intermountain western United States exceedingly dense, and forest restoration techniques (i.e., thinning and prescribed fire) are increasingly being used in an attempt to mitigate the effects of severe wildfire, to enhance tree growth and regeneration, and to stimulate soil nutrient cycling. While many of the short-term effects of forest restoration have been established, the long-term effects on soil biogeochemical and ecosystem processes are largely unknown. We assessed the effects of commonly used forest restoration treatments (thinning, burning, and thinning + burning) on nutrient cycling and other ecosystem processes 11 yr after restoration treatments were implemented in a ponderosa pine ( Pinus ponderosa var. scopulorum)/Douglas fir ( Pseudotsuga menziesii var. glauca) forest at the Lubrecht Fire and Fire Surrogates Study ( FFS) site in western Montana, USA. Despite short-term (<3 yr) increases in soil inorganic nitrogen (N) pools and N cycling rates following prescribed fire, long-term soil N pools and N mineralization rates showed only subtle differences from untreated control plots. Similarly, despite a persistent positive correlation between fuels consumed in prescribed burns and several metrics of N cycling, variability in inorganic N pools decreased significantly since treatments were implemented, indicating a decline in N spatial heterogeneity through time. However, rates of net nitrification remain significantly higher in a thin + burn treatment relative to other treatments. Short-term declines in forest floor carbon (C) pools have persisted in the thin + burn treatment, but there were no significant long-term differences among treatments in extractable soil phosphorus (P). Finally, despite some short-term differences, long-term foliar nutrient concentrations, litter decomposition rates, and rates of free-living N fixation in the experimental plots were not different from control plots, suggesting nutrient cycles and ecosystem processes in temperate coniferous forests are resilient to disturbance following long periods of fire suppression. Overall, this study provides forest managers and policymakers valuable information showing that the effects of these commonly used restoration prescriptions on soil nutrient cycling are ephemeral and that use of repeated treatments (i.e., frequent fire) will be necessary to ensure continued restoration success. [ABSTRACT FROM AUTHOR]

Fire and land-use change are two major types of disturbances that strongly affect the structure and function of forest ecosystems around the world, although their impacts can be difficult to quantify due to concomitant changes in climate or other land-use change factors. In this study we examined how fire and subsequent land-use conversion impacted soil properties (i.e. organic matter (OM), total available pools of carbon (C), nitrogen (N), and phosphorous (P)), and processes (i.e. N cycling inferred through δ 15 N) in each of six different land cover types, including old- and second-growth native Nothofagus pumilio , ∼50 year old exotic conifer plantations, and grassland pastures. We selected six land cover types, including unburned old-growth (MF), and post-fire second-growth (SG) forests of N. pumilio , post-fire afforestations of Pinus contorta (PC), P. ponderosa (PP) and P. sylvestris (PS), and post-fire grassland (GR), in three watersheds in the Aysén Region, Chilean Patagonia. In one growing season, at each of 5–7 sampling locations within each site, two 10 cm deep soil cores were removed using a 12.4 cm diameter PVC soil-corer. From each soil core, organic matter content, total C and N concentrations, availability of NH 4 + , NO 3 − , and PO 4 − , and δ 15 N were determined. Additionally, we collected foliage of Osmorhiza chilensis , a forb that was present in every tree-cover condition, for δ 15 N determination. Unburned old-growth Nothofagus forests showed significantly higher stocks of OM, C, N and P than the P. contorta and P. sylvestris afforestations but not higher than Nothofagus second-growth forests and P. ponderosa afforestations. Conifer afforestations showed significantly lower NH 4 + values than unburned Nothofagus forests, whereas no differences in NO 3 − were found among the land cover types. Contrary to expectations, conifer afforestations showed significantly higher plant and soil δ 15 N values than the unburned Nothofagus forests. Although most land cover types resulted in significant alteration of soil properties and processes relative to the mature, unburned N. pumilio forests, we highlight that P. ponderosa afforestations generated the most similar characteristics, suggesting a utility of this species to restore some ecosystem properties. [ABSTRACT FROM AUTHOR]