The impact of anthropogenic nitrogen deposition on global forests: Negative impacts far exceed the carbon benefits

Humans have drastically altered the nitrogen (N) cycle during the past century, enriching ecosystems from the tropics to the tundra with unpresented inputs of novel nitrogen. The study by Schulte-Uebbing et al. (2021) quantified the impact of atmospheric N deposition on C uptake by forests globally, and weighed this climate benefit against the global warming impact of N2O emissions. A major conclusion was that the C benefits of atmospheric deposition in global forests are smaller than previously estimated (only 41 Tg C year−1), accounting for only 2% of the net annual forest C uptake

Linking vegetation change, carbon sequestration and biodiversity

1. Despite recent interest in linkages between above- and belowground communities and their consequences for ecosystem processes, much remains unknown about their responses to long-term ecosystem change. We synthesize multiple lines of evidence from a long-term ‘natural experiment’ to illustrate how ecosystem retrogression (the decline in ecosystem processes due to long-term absence of major disturbance) drives vegetation change, and thus aboveground and belowground carbon (C) sequestration, and communities of consumer biota. 2. Our study system involves 30 islands in Swedish boreal forest that form a 5000 year fire-driven retrogressive chronosequence. Here, retrogression leads to lower plant productivity and slower decomposition, and a community shift from plants with traits associated with resource acquisition to those linked with resource conservation. 3. We present consistent evidence that aboveground ecosystem C sequestration declines, while belowground and total C storage increases linearly for at least 5000 years following fire absence. This increase is driven primarily by changes in vegetation characteristics, impairment of decomposer organisms and absence of humus combustion. 4. Data from contrasting trophic groups show that during retrogression, biomass or abundance of plants and decomposer biota decreases, while that of aboveground invertebrates and birds increases, due to different organisms accessing resources via distinct energy channels. Meanwhile, diversity measures of vascular plants and aboveground (but not belowground) consumers respond positively to retrogression. 5. We show that taxonomic richness of plants and aboveground consumers are positively correlated with total ecosystem C storage, suggesting that conserving old growth forests simultaneously maximizes biodiversity and C sequestration. However, we find little observational or experimental evidence that plant diversity is a major driver of ecosystem C storage on the islands relative to other biotic and abiotic factors. 6. Synthesis. Our study reveals that across contrasting islands differing in exposure to a key extrinsic driver (historical disturbance regime and resulting retrogression), there are coordinated responses of soil fertility, vegetation, consumer communities, and ecosystem C sequestration, which all feed back to one another. It also highlights the value of well replicated natural experiments for tackling questions about aboveground-belowground linkages over temporal and spatial scales that are otherwise unachievable

Soil amendment with biochar and manure alters wood stake decomposition and fungal community composition

Biochar and manure can be used for sustainable land management. However, little is known about how soil amendments might affect surface and belowground microbial processes and subsequent wood decomposition. In a split-split-split plot design, we amended soil with two rates of manure (whole plot; 0 and 9 Mg ha(-1)) and biochar (split plot; 0 and 10 Mg ha(-1)). Wood stakes of three species (hybrid poplar, triploid Populus tomentosa Carr.; aspen, Populus tremuloides Michx.; and pine, Pinus taeda L.) were placed in two positions (horizontally on the soil surface, and inserted vertically in the mineral soil), which served as a substrate for fungal growth. In 3 years, the decomposition rate (density loss), moisture content, and fungal community (via high-throughput sequencing methods) of stakes were evaluated. Results indicated that biochar and/or manure increased the wood stake decomposition rates, moisture content, and operational taxonomic unit abundance. However, the richness and diversity of fungi were dependent on wood stake position (surface > mineral), species (pine > the two Populus), and sample dates. This study highlights that soil amendment with biochar and/or manure can alter the fungal community, which in turn can enhance an important soil process (i.e., decomposition)

Trait coordination in boreal mosses reveals a bryophyte economics spectrum

The study of plant trait spectra and their association with trade-offs in resource use strategy has greatly advanced our understanding of vascular plant function, yet trait spectra remain poorly studied in bryophytes, particularly outside of the Sphagnum genus. Here, we measured 25 traits related to carbon, nutrient and water conservation in 60 moss canopies (each dominated by one of 15 moss species) across diverse boreal forest habitats and used bi-variate correlations and multi-variate analyses to assess trait coordination and trait spectra. We found substantial trait coordination along a main principal components axis driven by trade-offs in carbon, nutrient and water conservation strategies. Along this trait spectrum, traits varied from resource-acquisitive at one end (e.g. high maximum photosynthetic capacity, high tissue nitrogen content, low water-holding capacity) to resource-conservative at the other end, in line with resource economics theory. Traits related to carbon turnover (photosynthesis and respiration rates, litter decomposability) were positively related to nitrogen content and to desiccation rates, in line with global trait spectra in vascular plants. However, architectural traits of the moss shoots and of the moss canopy were generally unrelated to the main axis of trait variation and formed a secondary axis of trait variation, contrary to what is observed for vascular plants. Resource-conservative trait spectra dominated in moss canopies from open and wet habitats (i.e. mires), indicating that high irradiance and possibly high moisture fluctuation induce a resource-conservative trait strategy in mosses. Synthesis. Our work suggests that trait relationships that are well established for vascular plants can be extended for bryophytes as well. Bryophyte trait spectra can be powerful tools to improve our understanding of ecosystem processes in moss-dominated ecosystems, such as boreal or arctic environments, where bryophyte communities exert strong control on nutrient and carbon cycling

Shifts in microbial community composition and metabolism correspond with rapid soil carbon accumulation in response to 20 years of simulated nitrogen deposition

Anthropogenic nitrogen (N) deposition and fertilization in boreal forests frequently reduces decomposition and soil respiration and enhances C storage in the topsoil. This enhancement of the C sink can be as strong as the aboveground biomass response to N additions and has implications for the global C cycle, but the mechanisms remain elusive. We hypothesized that this effect would be associated with a shift in the microbial community and its activity, and particularly by fungal taxa reported to be capable of lignin degradation and organic N acquisition. We sampled the organic layer below the intact litter of a Norway spruce (Picea abies (L.) Karst) forest in northern Sweden after 20 years of annual N additions at low (12.5 kg N ha−1 yr−1) and high (50 kg N ha−1 yr−1) rates. We measured microbial biomass using phospholipid fatty-acid analysis (PLFA) and ergosterol measurements and used ITS metagenomics to profile the fungal community of soil and fine-roots. We probed the metabolic activity of the soil community by measuring the activity of extracellular enzymes and evaluated its relationships with the most N responsive soil fungal species. Nitrogen addition decreased the abundance of fungal PLFA markers and changed the fungal community in humus and fine-roots. Specifically, the humus community changed in part due to a shift from Oidiodendron pilicola, Cenococcum geophilum, and Cortinarius caperatus to Tylospora fibrillosa and Russula griseascens. These microbial community changes were associated with decreased activity of Mn-peroxidase and peptidase, and an increase in the activity of C acquiring enzymes. Our results show that the rapid accumulation of C in the humus layer frequently observed in areas with high N deposition is consistent with a shift in microbial metabolism, where decomposition associated with organic N acquisition is downregulated when inorganic N forms are readily available

Long‐term nitrogen enrichment does not increase microbial phosphorus mobilization in a northern coniferous forest

* 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

Root trait variation along a sub-arctic tundra elevational gradient

Elevational gradients are useful for predicting how plant communities respond to global warming, because communities at lower elevations experience warmer temperatures. Fine root traits and root trait variation could play an important role in determining plant community responses to warming in cold-climate ecosystems where a large proportion of plant biomass is allocated belowground. Here, we investigated the effects of elevation-associated temperature change on twelve chemical and morphological fine root traits of plant species and plant communities in a Swedish subarctic tundra. We also assessed the relative contributions of plant species turnover and intraspecific variation to the total amount of community-level root trait variation explained by elevation. Several root traits, both at the species and whole community levels, had significant linear or quadratic relationships with elevation, but the direction and strength of these relationships varied among traits and plant species. Further, we found no support for a unidirectional change from more acquisitive root trait values at the lower elevations towards trait values associated with greater nutrient conservation at the higher elevations, either at the species or community level. On the other hand, root trait coefficients of variation at the community level increased with elevation for several root traits. Further, for a large proportion of the community-level traits we found that intraspecific variation was relatively more important than species turnover, meaning that trait plasticity is important for driving community-level trait responses to environmental factors in this tundra system. Our findings indicate that with progressing global warming, intraspecific trait variation may drive plant community composition but this may not necessarily lead to shifts in root resource-acquisition strategy for all species

European Aspen with High Compared to Low Constitutive Tannin Defenses Grow Taller in Response to Anthropogenic Nitrogen Enrichment

Boreal forests receive nitrogen-(N)-enrichment via atmospheric deposition and industrial fertilization. While it is known that N-enrichment can intensify interactions with natural antagonists, it remains poorly understood how genetic variability in plant defense chemistry can affect biotic interactions and height growth in N-enriched environments. We grew replicates of five low- and high-tannin Populus tremula genotypes, respectively, under three N-treatments (ambient, 15, and 150 kg N ha−1 yr−1). We assessed shoot blight occurrence (i.e. symptoms caused by Venturia fungi) during four growing seasons, and tree height growth during the same period. Damage by Venturia spp. increased with N-addition during all years, likely due to enhanced foliar quality. Low–tannin plants showed higher incidences of Venturia infection than high-tannin plants, regardless of the N-input-level. Height responded to an N-by-tannin-group interaction, which occurred because high-tannin plants grew taller than low-tannin plants at the high N-treatment, but not under the other N-levels. This pattern indicates that innate resource investment into tannin production yields a positive effect on growth under N-enriched conditions. Given that N-deposition is increasing globally, our research suggests that further studies are needed to investigate how N-enrichment interacts with plant defense traits globally. Moreover, our research suggests that N-deposition may provide an advantage for well-defended, high-tannin plants; and further, that genetic diversity in plant defense may be a key mechanism by which plant populations respond to this change

Nitrogen Spatial Heterogeneity Influences Diversity Following Restoration in a Ponderosa Pine Forest, Montana

The resource heterogeneity hypothesis (RHH) is frequently cited in the ecological literature as an important mechanism for maintaining species diversity. The RHH has rarely been evaluated in the context of restoration ecology in which a commonly cited goal is to restore diversity. In this study we focused oil the spatial heterogeneity of total inorganic nitrogen (TIN) following restoration treatments in a ponderosa pine (Pinus ponderosa)/Douglas-fir (Pseudotsuga inenziesii) forest in western Montana, USA. Our objective was to evaluate relationships between understory species richness and TIN heterogeneity following mechanical thinning (thin-only), prescribed burning (burn-only), and mechanical thinning with prescribed burning (thin/burn) to discern the ecological and management implications of these restoration approaches. We employed a randomized block design, with three 9-ha replicates of each treatment and an untreated control. Within each treatment, we randomly established a 20 X 50 in (1000 m(2)) Plot in which we measured species richness across the entire plot and in 12 I-m 2 quadrats randomly placed within each larger plot. Additionally, we measured TIN from a grid consisting of 112 soil samples (0-5 cm) in each plot and computed standard deviations as a measure of heterogeneity. We found a correlation between the net increase in species richness and the TIN standard deviations one and two years following restoration treatments, supporting RHH. Using nonmetric multidimensional scaling ordination and chi-squared analysis, we found that high and low TIN quadrats contained different understory communities in 2003 and 2004, further supporting RHH. A comparison of restoration treatments demonstrated that thin/burn and burn-only treatments created higher N heterogeneity relative to the control. We also found that within prescribed burn treatments, TIN heterogeneity was positively correlated with fine-fuel consumption, a variable reflecting burn severity. These findings may lead to more informed restoration decisions that consider treatment effects on understory diversity in ponderosa pine/Douglas-fir ecosystems