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1. Tree symbioses sustain nitrogen fixation despite excess nitrogen supply

   https://doi.org/10.1002/ecm.1562  

   Menge, Duncan N. L.; Wolf, Amelia A.; Funk, Jennifer L.; Perakis, Steven S.; Akana, Palani R.; Arkebauer, Rachel; Bytnerowicz, Thomas A.; Carreras Pereira, K. A.; Huddell, Alexandra M.; Kou‐Giesbrecht, Sian; et al (January 2023, Ecological Monographs)

   Abstract Symbiotic nitrogen fixation (SNF) is a key ecological process whose impact depends on the strategy of SNF regulation—the degree to which rates of SNF change in response to limitation by N versus other resources. SNF that is obligate or exhibits incomplete downregulation can result in excess N fixation, whereas a facultative SNF strategy does not. We hypothesized that tree‐based SNF strategies differed by latitude (tropical vs. temperate) and symbiotic type (actinorhizal vs. rhizobial). Specifically, we expected tropical rhizobial symbioses to display strongly facultative SNF as an explanation of their success in low‐latitude forests. In this study we used¹⁵N isotope dilution field experiments in New York, Oregon, and Hawaii to determine SNF strategies in six N‐fixing tree symbioses. Nitrogen fertilization with +10 and +15 g N m⁻² year⁻¹for 4–5 years alleviated N limitation in all taxa, paving the way to determine SNF strategies. Contrary to our hypothesis, all six of the symbioses we studied sustained SNF even at high N.Robinia pseudoacacia(temperate rhizobial) fixed 91% of its N (%N_dfa) in controls, compared to 64% and 59% in the +10 and +15 g N m⁻² year⁻¹treatments. ForAlnus rubra(temperate actinorhizal), %N_dfawas 95%, 70%, and 60%. For the tropical species, %N_dfawas 86%, 80%, and 82% forGliricidia sepium(rhizobial); 79%, 69%, and 67% forCasuarina equisetifolia(actinorhizal); 91%, 42%, and 67% forAcacia koa(rhizobial); and 60%, 51%, and 19% forMorella faya(actinorhizal). Fertilization with phosphorus did not stimulate tree growth or SNF. These results suggest that the latitudinal abundance distribution of N‐fixing trees is not caused by a shift in SNF strategy. They also help explain the excess N in many forests where N fixers are common. 

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2. Structure of Molecular Nitrogen Nanoclusters Containing Stabilized Nitrogen Atoms

   https://doi.org/10.1007/s10909-024-03225-8  

   Wetzel, C K; Lee, D M; Khmelenko, V V (October 2024, Journal of Low Temperature Physics)

   Impurity-helium condensates (IHCs) formed by injecting the discharge products of gaseous mixtures of helium atoms and nitrogen molecules into bulk superfluid 4He at temperature 1.5 K, were studied by X-band electron spin resonance. IHCs consists of collections of N2 nanoclusters which form aerogel-like structure inside bulk HeII. It was found that N2 nanoclusters have a two shell structure, an outer shell which contains high concentration of stabilized N atoms and an interior shell with lower concentrations of N atoms. In this paper, we have studied the dependence of the shell structure of the N2 nanoclusters which compose the IHCs by varying the ratio of nitrogen to helium in the prepared gas mixture from 0.06 to 1%. The highest local concentration of N atoms in nanoclusters (1.2 ⋅ 1021 cm−3 ) was observed in the sample prepared from the gas mixture containing the lowest nitrogen admixture (0.06%). Additionally, the evolution of nanocluster structure was studied as the samples were drained of liquid helium (T ≤ 3.5 K) and warmed beyond the point of explosive recombination (3.5 K ≤ T ≤ 6.5 K). 

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3. Soil Nitrogen Supply Exerts Largest Influence on Leaf Nitrogen in Environments with the Greatest Leaf Nitrogen Demand

   https://doi.org/10.1111/ele.70015  

   Cheaib, Alissar; Waring, Elizabeth_F; McNellis, Risa; Perkowski, Evan_A; Martina, Jason_P; Seabloom, Eric_W; Borer, Elizabeth_T; Wilfahrt, Peter_A; Dong, Ning; Prentice, Iain_Colin; et al (January 2025, Ecology Letters)

   ABSTRACT Accurately representing the relationships between nitrogen supply and photosynthesis is crucial for reliably predicting carbon–nitrogen cycle coupling in Earth System Models (ESMs). Most ESMs assume positive correlations amongst soil nitrogen supply, leaf nitrogen content, and photosynthetic capacity. However, leaf photosynthetic nitrogen demand may influence the leaf nitrogen response to soil nitrogen supply; thus, responses to nitrogen supply are expected to be the largest in environments where demand is the greatest. Using a nutrient addition experiment replicated across 26 sites spanning four continents, we demonstrated that climate variables were stronger predictors of leaf nitrogen content than soil nutrient supply. Leaf nitrogen increased more strongly with soil nitrogen supply in regions with the highest theoretical leaf nitrogen demand, increasing more in colder and drier environments than warmer and wetter environments. Thus, leaf nitrogen responses to nitrogen supply are primarily influenced by climatic gradients in photosynthetic nitrogen demand, an insight that could improve ESM predictions. 

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4. Soil nitrogen fertilization reduces relative leaf nitrogen allocation to photosynthesis

   https://doi.org/10.1093/jxb/erad195  

   Waring, Elizabeth_F; Perkowski, Evan_A; Smith, Nicholas_G; Rogers, ed., Alistair (May 2023, Journal of Experimental Botany)

   Abstract The connection between soil nitrogen availability, leaf nitrogen, and photosynthetic capacity is not perfectly understood. Because these three components tend to be positively related over large spatial scales, some posit that soil nitrogen positively drives leaf nitrogen, which positively drives photosynthetic capacity. Alternatively, others posit that photosynthetic capacity is primarily driven by above-ground conditions. Here, we examined the physiological responses of a non-nitrogen-fixing plant (Gossypium hirsutum) and a nitrogen-fixing plant (Glycine max) in a fully factorial combination of light by soil nitrogen availability to help reconcile these competing hypotheses. Soil nitrogen stimulated leaf nitrogen in both species, but the relative proportion of leaf nitrogen used for photosynthetic processes was reduced under elevated soil nitrogen in all light availability treatments due to greater increases in leaf nitrogen content than chlorophyll and leaf biochemical process rates. Leaf nitrogen content and biochemical process rates in G. hirsutum were more responsive to changes in soil nitrogen than those in G. max, probably due to strong G. max investments in root nodulation under low soil nitrogen. Nonetheless, whole-plant growth was significantly enhanced by increased soil nitrogen in both species. Light availability consistently increased relative leaf nitrogen allocation to leaf photosynthesis and whole-plant growth, a pattern that was similar between species. These results suggest that the leaf nitrogen–photosynthesis relationship varies under different soil nitrogen levels and that these species preferentially allocated more nitrogen to plant growth and non-photosynthetic leaf processes, rather than photosynthesis, as soil nitrogen increased. 

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5. Symbiotic nitrogen fixation reduces belowground biomass carbon costs of nitrogen acquisition under low, but not high, nitrogen availability

   https://doi.org/10.1093/aobpla/plae051  

   Perkowski, Evan A; Terrones, Joseph; German, Hannah L; Smith, Nicholas G (October 2024, AoB PLANTS)

   Kalcsits, Lee (Ed.)

   Abstract Many plant species form symbiotic associations with nitrogen-fixing bacteria. Through this symbiosis, plants allocate photosynthate belowground to the bacteria in exchange for nitrogen fixed from the atmosphere. This symbiosis forms an important link between carbon and nitrogen cycles in many ecosystems. However, the economics of this relationship under soil nitrogen availability gradients is not well understood, as plant investment toward symbiotic nitrogen fixation tends to decrease with increasing soil nitrogen availability. Here, we used a manipulation experiment to examine how costs of nitrogen acquisition vary under a factorial combination of soil nitrogen availability and inoculation with Bradyrhizobium japonicum in Glycine max L. (Merr.). We found that inoculation decreased belowground biomass carbon costs to acquire nitrogen and increased total leaf area and total biomass, but these patterns were only observed under low fertilization and were the result of increased plant nitrogen uptake and no change in belowground carbon allocation. These results suggest that symbioses with nitrogen-fixing bacteria reduce carbon costs of nitrogen acquisition by increasing plant nitrogen uptake, but only when soil nitrogen is low, allowing individuals to increase nitrogen allocation to structures that support aboveground growth. This pattern may help explain the prevalence of plants capable of forming these associations in less fertile soils and provides useful insight into understanding the role of nutrient acquisition strategy on plant nitrogen uptake across nitrogen availability gradients. 

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