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1. Climate drivers alter nitrogen availability in surface peat and decouple N₂ fixation from CH₄ oxidation in the Sphagnum moss microbiome

   https://doi.org/10.1111/gcb.16651  

   Petro, Caitlin; Carrell, Alyssa A.; Wilson, Rachel M.; Duchesneau, Katherine; Noble‐Kuchera, Sekou; Song, Tianze; Iversen, Colleen M.; Childs, Joanne; Schwaner, Geoff; Chanton, Jeffrey P.; et al (March 2023, Global Change Biology)

   Abstract Peat mosses (Sphagnumspp.) are keystone species in boreal peatlands, where they dominate net primary productivity and facilitate the accumulation of carbon in thick peat deposits.Sphagnummosses harbor a diverse assemblage of microbial partners, including N₂‐fixing (diazotrophic) and CH₄‐oxidizing (methanotrophic) taxa that support ecosystem function by regulating transformations of carbon and nitrogen. Here, we investigate the response of theSphagnumphytobiome (plant + constituent microbiome + environment) to a gradient of experimental warming (+0°C to +9°C) and elevated CO₂(+500 ppm) in an ombrotrophic peatland in northern Minnesota (USA). By tracking changes in carbon (CH₄, CO₂) and nitrogen (NH₄‐N) cycling from the belowground environment up toSphagnumand its associated microbiome, we identified a series of cascading impacts to theSphagnumphytobiome triggered by warming and elevated CO₂. Under ambient CO₂, warming increased plant‐available NH₄‐N in surface peat, excess N accumulated inSphagnumtissue, and N₂fixation activity decreased. Elevated CO₂offset the effects of warming, disrupting the accumulation of N in peat andSphagnumtissue. Methane concentrations in porewater increased with warming irrespective of CO₂treatment, resulting in a ~10× rise in methanotrophic activity withinSphagnumfrom the +9°C enclosures. Warming's divergent impacts on diazotrophy and methanotrophy caused these processes to become decoupled at warmer temperatures, as evidenced by declining rates of methane‐induced N₂fixation and significant losses of keystone microbial taxa. In addition to changes in theSphagnummicrobiome, we observed ~94% mortality ofSphagnumbetween the +0°C and +9°C treatments, possibly due to the interactive effects of warming on N‐availability and competition from vascular plant species. Collectively, these results highlight the vulnerability of theSphagnumphytobiome to rising temperatures and atmospheric CO₂concentrations, with significant implications for carbon and nitrogen cycling in boreal peatlands. 

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2. Phosphorus rather than nitrogen enhances CO ₂ emissions in tropical forest soils: Evidence from a laboratory incubation study

   https://doi.org/10.1111/ejss.12885  

   Hui, Dafeng; Porter, Wesley; Phillips, Jana_R; Aidar, Marcos_PM; Lebreux, Steven_J; Schadt, Christopher_W; Mayes, Melanie_A (January 2020, European Journal of Soil Science)

   Abstract Ecosystem functional responses such as soil CO₂emissions are constrained by microclimate, available carbon (C) substrates and their effects upon microbial activity. In tropical forests, phosphorus (P) is often considered as a limiting factor for plant growth, but it is still not clear whether P constrains microbial CO₂emissions from soils. In this study, we incubated seven tropical forest soils from Brazil and Puerto Rico with different nutrient addition treatments (no addition, Control; C, nitrogen (N) or P addition only; and combined C, N and P addition (CNP)). Cumulative soil CO₂emissions were fit with a Gompertz model to estimate potential maximum cumulative soil CO₂emission (C_m) and the rate of change of soil C decomposition (k). Quantitative polymerase chain reaction (qPCR) was conducted to quantify microbial biomass as bacteria and fungi. Results showed that P addition alone or in combination with C and N enhancedC_m, whereas N addition usually reducedC_m, and neither N nor P affected microbial biomass. Additions of CNP enhancedk, increased microbial abundances and altered fungal to bacterial ratios towards higher fungal abundance. Additions of CNP, however, tended to reduceC_mfor most soils when compared to C additions alone, suggesting that microbial growth associated with nutrient additions may have occurred at the expense of C decomposition. Overall, this study demonstrates that soil CO₂emission is more limited by P than N in tropical forest soils and those effects were stronger in soils low in P. HighlightsA laboratory incubation study was conducted with nitrogen, phosphorus or carbon addition to tropical forest soils. Soil CO₂emission was fitted with a Gompertz model and soil microbial abundance was quantified using qPCR. Phosphorus addition increased model parametersC_mand soil CO₂emission, particularly in the Puerto Rico soils. Soil CO₂emission was more limited by phosphorus than nitrogen in tropical forest soils. 

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3. Acclimation of Photosynthesis to CO ₂ Increases Ecosystem Carbon Storage due to Leaf Nitrogen Savings

   https://doi.org/10.1111/gcb.17558  

   Smith, Nicholas G; Zhu, Qing; Keenan, Trevor F; Riley, William J (November 2024, Global Change Biology)

   ABSTRACT Photosynthesis is the largest flux of carbon between the atmosphere and Earth's surface and is driven by enzymes that require nitrogen, namely, ribulose‐1,5‐bisphosphate (RuBisCO). Thus, photosynthesis is a key link between the terrestrial carbon and nitrogen cycle, and the representation of this link is critical for coupled carbon‐nitrogen land surface models. Models and observations suggest that soil nitrogen availability can limit plant productivity increases under elevated CO₂. Plants acclimate to elevated CO₂by downregulating RuBisCO and thus nitrogen in leaves, but this acclimation response is not currently included in land surface models. Acclimation of photosynthesis to CO₂can be simulated by the photosynthetic optimality theory in a way that matches observations. Here, we incorporated this theory into the land surface component of the Energy Exascale Earth System Model (ELM). We simulated land surface carbon and nitrogen processes under future elevated CO₂conditions to 2100 using the RCP8.5 high emission scenario. Our simulations showed that when photosynthetic acclimation is considered, photosynthesis increases under future conditions, but maximum RuBisCO carboxylation and thus photosynthetic nitrogen demand decline. We analyzed two simulations that differed as to whether the saved nitrogen could be used in other parts of the plant. The allocation of saved leaf nitrogen to other parts of the plant led to (1) a direct alleviation of plant nitrogen limitation through reduced leaf nitrogen requirements and (2) an indirect reduction in plant nitrogen limitation through an enhancement of root growth that led to increased plant nitrogen uptake. As a result, reallocation of saved leaf nitrogen increased ecosystem carbon stocks by 50.3% in 2100 as compared to a simulation without reallocation of saved leaf nitrogen. These results suggest that land surface models may overestimate future ecosystem nitrogen limitation if they do not incorporate leaf nitrogen savings resulting from photosynthetic acclimation to elevated CO₂. 

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4. Sensitivity of grassland carbon pools to plant diversity, elevated CO ₂ , and soil nitrogen addition over 19 years

   https://doi.org/10.1073/pnas.2016965118  

   Pastore, Melissa A.; Hobbie, Sarah E.; Reich, Peter B. (April 2021, Proceedings of the National Academy of Sciences)

   Whether the terrestrial biosphere will continue to act as a net carbon (C) sink in the face of multiple global changes is questionable. A key uncertainty is whether increases in plant C fixation under elevated carbon dioxide (CO₂) will translate into decades-long C storage and whether this depends on other concurrently changing factors. We investigated how manipulations of CO₂, soil nitrogen (N) supply, and plant species richness influenced total ecosystem (plant + soil to 60 cm) C storage over 19 y in a free-air CO₂enrichment grassland experiment (BioCON) in Minnesota. On average, after 19 y of treatments, increasing species richness from 1 to 4, 9, or 16 enhanced total ecosystem C storage by 22 to 32%, whereas N addition of 4 g N m⁻²⋅ y⁻¹and elevated CO₂of +180 ppm had only modest effects (increasing C stores by less than 5%). While all treatments increased net primary productivity, only increasing species richness enhanced net primary productivity sufficiently to more than offset enhanced C losses and substantially increase ecosystem C pools. Effects of the three global change treatments were generally additive, and we did not observe any interactions between CO₂and N. Overall, our results call into question whether elevated CO₂will increase the soil C sink in grassland ecosystems, helping to slow climate change, and suggest that losses of biodiversity may influence C storage as much as or more than increasing CO₂or high rates of N deposition in perennial grassland systems. 

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5. Suppressed p CO ₂ in the Southern Ocean Due to the Interaction Between Current and Wind

   https://doi.org/10.1029/2021JC017884  

   Kwak, Kyungmin; Song, Hajoon; Marshall, John; Seo, Hyodae; McGillicuddy, Jr, Dennis J. (November 2021, Journal of Geophysical Research: Oceans)

   Abstract The Southern Ocean, an important region for the uptake of anthropogenic carbon dioxide (CO₂), features strong surface currents due to substantial mesoscale meanders and eddies. These features interact with the wind and modify the momentum transfer from the atmosphere to the ocean. Although such interactions are known to reduce momentum transfer, their impact on air‐sea carbon exchange remains unclear. Using a 1/20° physical‐biogeochemical coupled ocean model, we examined the impact of the current‐wind interaction on the surface carbon concentration and the air‐sea carbon exchange in the Southern Ocean. The current‐wind interaction decreased winter partial pressure of CO₂(pCO₂) at the ocean surface mainly south of the northern subantarctic front. It also reducedpCO₂in summer, indicating enhanced uptake, but not to the same extent as the winter loss. Consequently, the net outgassing of CO₂was found to be reduced by approximately 17%when including current‐wind interaction. These changes stem from the combined effect of vertical mixing and Ekman divergence. A budget analysis of dissolved inorganic carbon (DIC) revealed that a weakening of vertical mixing by current‐wind interaction reduces the carbon supply from below, and particularly so in winter. The weaker wind stress additionally lowers the subsurface DIC concentration in summer, which can affect the vertical diffusive flux of carbon in winter. Our study suggests that ignoring current‐wind interactions in the Southern Ocean can overestimate winter CO₂outgassing. 

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