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1. Physiological and Growth Responses of Tropical Dry Forest Tree Seedlings to Water and Nutrient Additions: Comparisons Between Nitrogen Fixers and Non‐Fixers

   https://doi.org/10.1111/btp.70075  

   Calderón‐Morales, Erick; Werden, Leland K; Smith‐Martin, Chris M; Waring, Bonnie G; Cordero‐Solorzano, Roberto; Powers, Jennifer S (September 2025, Biotropica)

   Harrison, Rhett (Ed.)

   ABSTRACT Belowground resources are key determinants of seedling growth and survival in tropical forests. Nutrients and light may limit plant growth the most in tropical wet forests, whereas water may limit plant growth more in tropical dry forests. Nitrogen (N)‐fixing species play an important role in the nitrogen and carbon cycles across tropical dry forests. However, studies investigating the joint effects of water and nutrients on the physiology and performance of N‐fixing species are scarce. We implemented a full factorial shade house experiment that manipulated water and nutrients (NPK 20:20:20 and complete micronutrients) using eight tree species representing N‐fixing and non‐fixing tree species in the tropical dry forest of Costa Rica to determine: (1) How plant responses to water and nutrient availability vary between N‐fixing and non‐fixing tree species?; and (2) How nutrient and/or water availability influences seedling water‐ and nutrient‐use traits? We found that growth and physiological responses to water and nutrient addition depended directly on the capacity of species to fix atmospheric N₂. N‐fixing species responded more strongly to nutrient addition, accumulating 67% more total biomass on average (approximately double that of non‐fixing taxa) and increasing average height growth rate by 41%. N‐fixing species accumulated more biomass without compromising water‐use efficiency, taking full advantage of the increased nutrient availability. Interestingly, results from our experiment show that increased water availability rarely influenced tropical dry forest seedling performance, whereas nutrient availability had a strong effect on biomass and growth. Overall, our results highlight the ability of N‐fixing seedlings to take advantage of local soil resource heterogeneity, which may help to explain the dominance of N‐fixing trees in tropical dry forests. 

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2. Floral Color and Family Drive Contrasting Plant–Pollinator Responses to Nutrient Enrichment

   https://doi.org/10.1002/ece3.72153  

   Nelson, Rebecca A; Borer, Elizabeth T; Seabloom, Eric W (September 2025, Ecology and Evolution)

   ABSTRACT Nutrient enrichment has decreased the diversity and abundance of wildflower species, raising questions about whether nutrient enrichment can further decrease the diversity and abundance of pollinators that rely on wildflowers. Whether the effects of nutrient enrichment on plant–pollinator interactions differ by nutrient type remains an open question. Moreover, plant family and flower color, two core axes of pollination niches, may further mediate how wildflowers and their pollinators respond to nutrient enrichment. We tested these questions using a nutrient addition experiment replicated at three grasslands in California, a global plant diversity hotspot. We found that adding nitrogen increased the floral abundance of Asteraceae, while decreasing that of Fabaceae, Geraniaceae, Iridaceae, and Euphorbiaceae. Adding phosphorus and potassium in the absence of nitrogen produced the opposite effects. Pollinator abundance and composition varied strongly by floral family, suggesting that these differing responses to nutrient addition by floral family may alter pollinator community composition. Nitrogen addition decreased the abundance of native blue, native green, and exotic pink flowers, while increasing the abundance of native and exotic yellow and exotic purple flowers. Consequently, nitrogen addition increased pollinator abundance on purple flowers, while decreasing pollinator abundance on pink flowers. Purple and yellow Asteraceae species, which increased under nitrogen enrichment, acted as core hubs in structuring the plant–pollinator network.Synthesis:Our findings suggest that the type of nutrient, plant family, and flower color modulate how plant–pollinator interactions respond to eutrophication. 

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3. Phosphorus deprivation affects composition and spatial distribution of membrane lipids in legume nodules.

   https://doi.org/doi.org/10.1093/plphys/kiaa115  

   Dokwal, D; Romsdahl, TB; Kunz, DA; Alonso, AP; Dickstein, R (January 2021, Plant physiology)

   null (Ed.)

   In legumes, symbiotic nitrogen (N) fixation (SNF) occurs in specialized organs called nodules after successful interactions between legume hosts and rhizobia. In a nodule, N-fixing rhizobia are surrounded by symbiosome membranes, through which the exchange of nutrients and ammonium occurs between bacteria and the host legume. Phosphorus (P) is an essential macronutrient, and N2-fixing legumes have a higher requirement for P than legumes grown on mineral N. As in the previous studies, in P deficiency, barrel medic (Medicago truncatula) plants had impaired SNF activity, reduced growth, and accumulated less phosphate in leaves, roots, and nodules compared with the plants grown in P sufficient conditions. Membrane lipids in M. truncatula tissues were assessed using electrospray ionization–mass spectrometry. Galactolipids were found to increase in P deficiency, with declines in phospholipids (PL), especially in leaves. Lower PL losses were found in roots and nodules. Subsequently, matrix-assisted laser desorption/ionization–mass spectrometry imaging was used to spatially map the distribution of the positively charged phosphatidylcholine (PC) species in nodules in both P-replete and P-deficient conditions. Our results reveal heterogeneous distribution of several PC species in nodules, with homogeneous distribution of other PC classes. In P poor conditions, some PC species distributions were observed to change. The results suggest that specific PC species may be differentially important in diverse nodule zones and cell types, and that membrane lipid remodeling during P stress is not uniform across the nodule. 

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4. Interactions among nutrients govern the global grassland biomass–precipitation relationship

   https://doi.org/10.1073/pnas.2410748122  

   Fay, Philip A; Gherardi, Laureano A; Yahdjian, Laura; Adler, Peter B; Bakker, Jonathan D; Bharath, Siddharth; Borer, Elizabeth T; Harpole, W Stanley; Hersch-Green, Erika; Huxman, Travis E; et al (April 2025, Proceedings of the National Academy of Sciences)

   Ecosystems are experiencing changing global patterns of mean annual precipitation (MAP) and enrichment with multiple nutrients that potentially colimit plant biomass production. In grasslands, mean aboveground plant biomass is closely related to MAP, but how this relationship changes after enrichment with multiple nutrients remains unclear. We hypothesized the global biomass–MAP relationship becomes steeper with an increasing number of added nutrients, with increases in steepness corresponding to the form of interaction among added nutrients and with increased mediation by changes in plant community diversity. We measured aboveground plant biomass production and species diversity in 71 grasslands on six continents representing the global span of grassland MAP, diversity, management, and soils. We fertilized all sites with nitrogen, phosphorus, and potassium with micronutrients in all combinations to identify which nutrients limited biomass at each site. As hypothesized, fertilizing with one, two, or three nutrients progressively steepened the global biomass–MAP relationship. The magnitude of the increase in steepness corresponded to whether sites were not limited by nitrogen or phosphorus, were limited by either one, or were colimited by both in additive, or synergistic forms. Unexpectedly, we found only weak evidence for mediation of biomass–MAP relationships by plant community diversity because relationships of species richness, evenness, and beta diversity to MAP and to biomass were weak or opposing. Site-level properties including baseline biomass production, soils, and management explained little variation in biomass–MAP relationships. These findings reveal multiple nutrient colimitation as a defining feature of the global grassland biomass–MAP relationship. 

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5. Evidence for the Coupling of Plant Functional Diversity and Soil Biogeochemistry Under Climatic Stress in Pacific Northwest Grasslands

   https://doi.org/10.1029/2024JG008126  

   Bomfim, Barbara; Dawson, Hilary_R; Reed, Paul_B; Shek, Katherine_L; Bohannan, Brendan_J_M; Bridgham, Scott_D; Silva, Lucas_C_R (May 2025, Journal of Geophysical Research: Biogeosciences)

   Abstract Increasing warming and drought severity are projected for the Pacific Northwest (PNW) and are expected to negatively impact species composition and ecosystem function. In this study, we test the hypothesis that the impact of climatic stress (i.e., experimental warming and drought) on PNW grasslands are mediated by interactions between plant functional diversity and soil biogeochemical processes, including symbiotic nitrogen (N) fixation in legumes and free‐living asymbiotic nitrogen fixation (ANF) by soil microorganisms. To test this hypothesis, we measured the response of plants and soils to three years of warming (+2.5°C) and drought (−40% precipitation) in field experiments replicated at three different sites across a ∼520‐km latitudinal gradient. We observed interactive effects of warming and drought on functional diversity and soil biogeochemical properties, including both positive and negative changes in ANF. Although direct measurements of symbiotic nitrogen fixation (SNF) rates were not conducted, the observed variations in ANF, in conjunction with changes in legume cover, suggest a compensatory mechanism that may offset reductions in SNF. Generally, high ANF rates coincided with low legume cover, suggesting a connection between shifts in species composition and N cycling. Our ANF estimates were performed using isotopically labeled dinitrogen (¹⁵N₂) in tandem with soil carbon (C), phosphorus (P) and iron (Fe), pH, and moisture content. Along the latitudinal drought severity gradient, ANF rates were correlated with changes in species composition and soil N, P, moisture, and pH levels. These results highlight the importance of soil‐plant‐atmosphere interactions in understanding the impacts of climatic stress on ecosystem composition and function. 

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