Abstract Mycorrhizal associations are key drivers of soil biogeochemistry, but previous studies have focused almost exclusively on ectomycorrhizal (EcM) and arbuscular mycorrhizal (AM) associations. Ericoid mycorrhizal (ErM) shrubs frequently occur in forest understories and are expanding in response to disturbance, but are rarely considered in biogeochemical frameworks. We investigated the relationships of understory ErM shrubs and overstory trees on carbon (C) and nitrogen (N) in soil organic matter fractions in a southern Appalachian temperate forest. We sampled the 0–10 cm mineral soil layer from 43 plots at the Coweeta Hydrologic Laboratory, across gradients in overstory EcM dominance and understory ErM shrub biomass. Soil C:N ratios increased with both increasing EcM dominance and increasing ErM shrub biomass. However, total particulate organic matter (POM) C, and the proportion of C and N held in POM increased with increasing ErM shrub biomass, but not with increasing EcM dominance. In contrast, mineral-associated organic matter (MAOM) C and N were negatively associated with EcM dominance, but were not related to ErM shrubs. Our findings suggest that ErM shrubs facilitate POM formation while AM trees promote MAOM formation. Because ErM shrub biomass represents a small fraction of total forest biomass, our work provides evidence that ErM shrubs have an outsized effect on soil organic matter, which advocates for their inclusion in mechanistic studies and biogeochemical frameworks.

Abstract In northern biomes, growth is nitrogen (N) limited, but bryophytes are abundant. These bryophytes often host N 2 -fixing microorganisms (diazotrophs) that play a crucial role in the N cycle of these ecosystems. Despite their importance, how the bryophyte-associated N 2 -fixation varies across species and seasons (summer, autumn, winter, and spring) remains poorly understood. We measured N 2 -fixation rates for 10 bryophyte species in situ throughout the entire year in the Arctic with additional incubations to verify the method. We measured positive N 2 -fixation during most of the year, except for the coldest period (February). The species growing in the wettest conditions ( Sphagnum spp.) had the highest N 2 -fixation rates in summer, while bryophytes in drier conditions peaked in N 2 -fixation rates in spring and autumn. The seasonal variation in N 2 -fixation activity was pronounced, but similar patterns were found among different species. This study reveals that bryophyte-associated N 2 -fixation in northern ecosystems is larger than previously assumed, as it occurs over a more extended part of the year than previously inferred. Furthermore, the importance of bryophyte-associated diazotrophs cannot be quantified without considering both the diversity of bryophytes and their variation in N 2 -fixing seasonal activity patterns. Both future changes in climatic conditions and biodiversity of bryophytes can thus have large implications for the N cycle in arctic regions.

Abstract Soil represents the most important terrestrial carbon sink on Earth. Using data from 9 forest sites located across an elevational gradient (range: 167–2928 m a.s.l.) in northwestern Colombia, which spans a wide range of climate, forest, and soil types, we aim to answer the following research questions: (i) How do SOC stocks change along the elevational gradient or between lowlands and highlands? (ii) What are the main drivers of SOC stocks along elevational gradients? Using structural equation modeling (SEM), we found that climate and soil fertility explained 67% of the variation in SOC stocks across the entire dataset, with SOC stocks declining under warmer and wetter conditions, but increasing with higher soil N:P ratios. Because soil mineralogy was closely correlated with elevation (all lowland were kaolinites while all highland sites were allophanes), SEM models for lowland and highland forests were also run separately. Lowland forests were dominated by trees associated with arbuscular mycorrhizas and nitrogen-fixing symbiont root associations, which could also increase decomposition rates, and thus, reduce the SOC stocks. This finding suggests that greater soil P availability stimulated microbial activity and decreased SOC. In highland forests, which had a wider range of climatic conditions and a greater proportion of trees with ectomycorrhizal associations, decreased temperatures as well as ectomycorrhizal modification of soil N:P ratios slowed soil C cycling, resulting in a greater accumulation of SOC. In conclusion, the increase in SOC stocks across either lowlands or highlands in the Northern Andes was driven by different combinations of abiotic and biotic factors. Since increases in temperature are expected to modify forest functioning and composition along elevational gradients, which in turn depends on soil conditions, improving our understanding on the likely fate of the large amount of C stored in soils should be seen as a priority in tropical montane ecosystems.

Abstract Herbivory may offset climate change-driven treeline expansion into the tundra, but the strength of this effect is rarely quantified. This study leverages a unique semi-natural experiment involving Malla Strict Nature Reserve in northernmost Finland, where the reindeer herding regime shifted from being nearly ungrazed for several decades to being heavily grazed for the past two decades. This is contrasted by low grazing pressure in the adjacent herding district in Norway, which is separated by the border fence preventing free reindeer movement between the two countries. We aimed to quantify the effects of reindeer browsing and grazing on mountain birch treeline position and structure on both sides. We measured seedling numbers and the allometry of trees, vegetation composition, nutrient concentrations in soils and birch leaves, and radial tree growth. We found higher numbers of seedlings and saplings in the area with lower reindeer density, indicating that the treeline may be responding to climatic forcing by expanding into the tundra. Contrastingly, we observed almost no recruitment and treeline expansion in the area with high reindeer density. Furthermore, while birch leaves showed signs of nitrogen enrichment under high reindeer density, we found no differences in soil chemical composition or birch tree growth rates. Our results suggest that the high density of reindeer in Malla Strict Nature Reserve keeps the treeline in a browsing trap, thereby preventing climate change-driven forest expansion. These results are highly relevant for land management decisions that aim to preserve mountain tundra.

Abstract Understanding food-web structure is crucial to determine the functioning of ecosystems and sustainably manage resources. The Scotia Sea is an important area for Antarctic krill and toothfish fisheries, and one of the regions most impacted by climate change in the Southern Ocean. Whilst the pelagic Antarctic krill-centric food web has been investigated in reasonable detail, the structure of deep-sea food webs associated with toothfish fisheries remain largely unknown. Utilising stable isotopes and fatty acids as trophic proxies, we studied the deep-sea food-web structure in three locations of the Scotia Sea, from South Georgia (SG) to the South Sandwich Islands (SSI; divided into north and south). Our analyses indicate that all food webs were similar, presenting high trophic redundancy and similar vertical structure. All food webs had five trophic levels, with the 5th and 4th trophic levels mainly constituted of fish and the 3rd trophic level of cephalopods and crustaceans. However, some differences existed with the SG food web presenting larger diversity of producers and the bigeye grenadier Macrourus holotrachys in the highest trophic position, while Patagonian toothfish Dissostichus eleginoides and both Patagonian and Antarctic toothfish D. mawsoni were the top predators at SSI-North and SSI-South, respectively. Compared to coastal and pelagic food webs in the Southern Ocean, our results suggest that deep-sea food webs, including the benthic/demersal components, have a longer food-chain length. This study provides essential knowledge of the ecological variability of Southern Ocean deep-sea food webs while contributing to the management of resources within the SG and SSI Marine Protected Area.