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1. Dissolved nitrogen form mediates phycocyanin content in cyanobacteria

   https://doi.org/10.1111/fwb.13892  

   Gladfelter, Matthew F.; Buley, Riley P.; Belfiore, Angelea P.; Fernandez‐Figueroa, Edna G.; Gerovac, Bridget L.; Baker, Nicole D.; Wilson, Alan E. (March 2022, Freshwater Biology)

   Abstract Chemically reduced nitrogen forms are increasing in aquatic systems and beginning to reach concentrations not previously measured. Despite this, little research has examined the potential of reduced nitrogen forms to encourage excess nitrogen storage and promote algal bloom longevity compared to oxidised forms.A 2‐week field, pulse‐application experiment was carried out using 1,100‐L plastic limnocorrals to examine cyanobacterial community response to three nitrogen forms, including nitrate, ammonium, and urea (added as 600 µg N/L). Cell pigments and counts were used to calculate cell‐specific pigment concentrations, and cell‐associated microcystin concentrations were also measured to examine toxin response to a shift in nitrogen source.Results showed that, upon nitrogen introduction, extracellular nitrogen quickly decreased in accordance with an increase in cellular phycocyanin 72 hr after fertilisation. Ammonium and urea treatments had more phycocyanin/cell than nitrate or control treatments at 72 hr. After 72 hr, phycocyanin content quickly decreased, consistent with the use of nitrogen from phycobiliproteins. Despite the decrease in light‐harvesting pigments, the total number of cyanobacterial cells increased in the ammonium and urea treatments after 2 weeks. Cyanobacterial particulate toxin (microcystin) quotas were not affected by nitrogen additions.Results show that reduced nitrogen forms encourage greater nitrogen storage as pigments and increase bloom longevity compared to oxidised forms.Findings support previous studies that suggest reduced nitrogen forms encourage greater cell density and algal bloom persistence. Results further point to excess nitrogen storage as another mechanism that allows cyanobacteria to dominate freshwater systems despite variable environmental conditions. 

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2. 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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3. Long-term nitrogen deposition reduces the diversity of nitrogen-fixing plants

   https://doi.org/10.1126/sciadv.adp7953  

   Moreno-García, Pablo; Montaño-Centellas, Flavia; Liu, Yu; Reyes-Mendez, Evelin Y; Jha, Rohit Raj; Guralnick, Robert P; Folk, Ryan; Waller, Donald M; Verheyen, Kris; Baeten, Lander; et al (October 2024, Science Advances)

   Biological nitrogen fixation is a fundamental part of ecosystem functioning. Anthropogenic nitrogen deposition and climate change may, however, limit the competitive advantage of nitrogen-fixing plants, leading to reduced relative diversity of nitrogen-fixing plants. Yet, assessments of changes of nitrogen-fixing plant long-term community diversity are rare. Here, we examine temporal trends in the diversity of nitrogen-fixing plants and their relationships with anthropogenic nitrogen deposition while accounting for changes in temperature and aridity. We used forest-floor vegetation resurveys of temperate forests in Europe and the United States spanning multiple decades. Nitrogen-fixer richness declined as nitrogen deposition increased over time but did not respond to changes in climate. Phylogenetic diversity also declined, as distinct lineages of N-fixers were lost between surveys, but the “winners” and “losers” among nitrogen-fixing lineages varied among study sites, suggesting that losses are context dependent. Anthropogenic nitrogen deposition reduces nitrogen-fixing plant diversity in ways that may strongly affect natural nitrogen fixation. 

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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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