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1. Plant community re-organization and increased productivity due to multi-year nutrient enrichment of a coastal grassland

   https://doi.org/10.1371/journal.pone.0270798  

   Brown, Joseph K.; Moulton, Ashley; Zinnert, Julie C. (July 2022, PLOS ONE)

   Kim, Daehyun (Ed.)

   Nutrient enrichment alters plant community structure and function at a global scale. Coastal plant systems are expected to experience increased rates of nitrogen and phosphorus deposition by 2100, caused mostly by anthropogenic activity. Despite high density of studies investigating connections between plant community structure and ecosystem function in response to nutrient addition, inconsistencies in system response based on the ecosystem in question calls for more detailed analyses of nutrient impacts on community organization and resulting productivity response. Here, we focus on nutrient addition impacts on community structure and organization as well as productivity of different lifeforms in a coastal grassland. We established long-term nutrient enrichment plots in 2015 consisting of control (C), nitrogen (N), phosphorus (P), and nitrogen + phosphorus (NP) treatments. In 2017 we collected graminoid and forb productivity, root productivity, and community composition for each plot. We found no N x P interaction, but N enrichment was a significant main effect on productivity, highlighting N limitation in coastal systems. Importantly, nutrient enrichment treatments did not alter root productivity. However, all treatments caused significant differences in community composition. Using rank abundance curves, we determined that community composition differences were driven by increased dominance of nitrophilous graminoids, re-organization of subordinate species, and species absences in N and NP plots. Results of this study highlight how coastal grassland communities are impacted by nutrient enrichment. We show that community re-organization, increased dominance, and absence of critical species are all important mechanisms that reflect community-level impacts of nutrient enrichment in our coastal grassland site. 

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2. Long-Term Nutrient Enrichment of an Oligotroph-Dominated Wetland Increases Bacterial Diversity in Bulk Soils and Plant Rhizospheres

   https://doi.org/10.1128/mSphere.00035-20  

   Bledsoe, Regina B.; Goodwillie, Carol; Peralta, Ariane L. (June 2020, mSphere)

   Campbell, Barbara J. (Ed.)

   ABSTRACT In nutrient-limited conditions, plants rely on rhizosphere microbial members to facilitate nutrient acquisition, and in return, plants provide carbon resources to these root-associated microorganisms. However, atmospheric nutrient deposition can affect plant-microbe relationships by changing soil bacterial composition and by reducing cooperation between microbial taxa and plants. To examine how long-term nutrient addition shapes rhizosphere community composition, we compared traits associated with bacterial (fast-growing copiotrophs, slow-growing oligotrophs) and plant (C 3 forb, C 4 grass) communities residing in a nutrient-poor wetland ecosystem. Results revealed that oligotrophic taxa dominated soil bacterial communities and that fertilization increased the presence of oligotrophs in bulk and rhizosphere communities. Additionally, bacterial species diversity was greatest in fertilized soils, particularly in bulk soils. Nutrient enrichment (fertilized versus unfertilized) and plant association (bulk versus rhizosphere) determined bacterial community composition; bacterial community structure associated with plant functional group (grass versus forb) was similar within treatments but differed between fertilization treatments. The core forb microbiome consisted of 602 unique taxa, and the core grass microbiome consisted of 372 unique taxa. Forb rhizospheres were enriched in potentially disease-suppressive bacterial taxa, and grass rhizospheres were enriched in bacterial taxa associated with complex carbon decomposition. Results from this study demonstrate that fertilization serves as a strong environmental filter on the soil microbiome, which leads to distinct rhizosphere communities and can shift plant effects on the rhizosphere microbiome. These taxonomic shifts within plant rhizospheres could have implications for plant health and ecosystem functions associated with carbon and nitrogen cycling. IMPORTANCE Over the last century, humans have substantially altered nitrogen and phosphorus cycling. Use of synthetic fertilizer and burning of fossil fuels and biomass have increased nitrogen and phosphorus deposition, which results in unintended fertilization of historically low-nutrient ecosystems. With increased nutrient availability, plant biodiversity is expected to decline, and the abundance of copiotrophic taxa is anticipated to increase in bacterial communities. Here, we address how bacterial communities associated with different plant functional types (forb, grass) shift due to long-term nutrient enrichment. Unlike other studies, results revealed an increase in bacterial diversity, particularly of oligotrophic bacteria in fertilized plots. We observed that nutrient addition strongly determines forb and grass rhizosphere composition, which could indicate different metabolic preferences in the bacterial communities. This study highlights how long-term fertilization of oligotroph-dominated wetlands could alter diversity and metabolism of rhizosphere bacterial communities in unexpected ways. 

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3. Distinct microbial communities alter litter decomposition rates in a fertilized coastal plain wetland

   https://doi.org/10.1002/ecs2.3619  

   Koceja, Megan_E; Bledsoe, Regina_B; Goodwillie, Carol; Peralta, Ariane_L (June 2021, Ecosphere)

   Abstract Human activities have led to increased deposition of nitrogen (N) and phosphorus (P) into soils. Nutrient enrichment of soils is known to increase plant biomass and rates of microbial litter decomposition. However, interacting effects of hydrologic position and associated changes to soil moisture can constrain microbial activity and lead to unexpected nutrient feedbacks on microbial community structure–function relationships. Examining feedbacks of nutrient enrichment on decomposition rates is essential for predicting microbial contributions to carbon (C) cycling as atmospheric deposition of nutrients persists. This study explored how long‐term nutrient addition and contrasting litter chemical composition influenced soil bacterial community structure and function. We hypothesized that long‐term nutrient enrichment of low fertility soils alters bacterial community structure and leads to higher rates of litter decomposition especially for low C:N litter, but low‐nutrient and dry conditions limit microbial decomposition of high C:N ratio litter. We leveraged a long‐term fertilization experiment to test how nutrient enrichment and hydrologic manipulation (due to ditches) affected decomposition and soil bacterial community structure in a nutrient‐poor coastal plain wetland. We conducted a litter bag experiment and characterized litter‐associated and bulk soil microbiomes using 16S rRNA bacterial sequencing and quantified litter mass losses and soil physicochemical properties. Results revealed that distinct bacterial communities were involved in decomposing higher C:N ratio litter more quickly in fertilized compared to unfertilized soils especially under drier soil conditions, while decomposition rates of lower C:N ratio litter were similar between fertilized and unfertilized plots. Bacterial community structure in part explained litter decomposition rates, and long‐term fertilization and drier hydrologic status affected bacterial diversity and increased decomposition rates. However, community composition associated with high C:N litter was similar in wetter plots with available nitrate detected, regardless of fertilization treatment. This study provides insight into long‐term fertilization effects on soil bacterial diversity and composition, decomposition, and the increased potential for soil C loss as nutrient enrichment and hydrology interact to affect historically low‐nutrient ecosystems. 

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4. Change in functional trait diversity mediates the effects of nutrient addition on grassland stability

   https://doi.org/10.1111/1365-2745.14404  

   Chen, Qingqing; Wang, Shaopeng; Seabloom, Eric_W; Isbell, Forest; Borer, Elizabeth_T; Bakker, Jonathan_D; Bharath, Siddharth; Roscher, Christiane; Peri, Pablo_Luis; Power, Sally_A; et al (September 2024, Journal of Ecology)

   Abstract Nutrient enrichment impacts grassland plant diversity such as species richness, functional trait composition and diversity, but whether and how these changes affect ecosystem stability in the face of increasing climate extremes remains largely unknown.We quantified the direct and diversity‐mediated effects of nutrient addition (by nitrogen, phosphorus, and potassium) on the stability of above‐ground biomass production in 10 long‐term grassland experimental sites. We measured five facets of stability as the temporal invariability, resistance during and recovery after extreme dry and wet growing seasons.Leaf traits (leaf carbon, nitrogen, phosphorus, potassium, and specific leaf area) were measured under ambient and nutrient addition conditions in the field and were used to construct the leaf economic spectrum (LES). We calculated functional trait composition and diversity of LES and of single leaf traits. We quantified the contribution of intraspecific trait shifts and species replacement to change in functional trait composition as responses to nutrient addition and its implications for ecosystem stability.Nutrient addition decreased functional trait diversity and drove grassland communities to the faster end of the LES primarily through intraspecific trait shifts, suggesting that intraspecific trait shifts should be included for accurately predicting ecosystem stability. Moreover, the change in functional trait diversity of the LES in turn influenced different facets of stability. That said, these diversity‐mediated effects were overall weak and/or overwhelmed by the direct effects of nutrient addition on stability. As a result, nutrient addition did not strongly impact any of the stability facets. These results were generally consistent using individual leaf traits but the dominant pathways differed. Importantly, major influencing pathways differed using average trait values extracted from global trait databases (e.g. TRY).Synthesis. Investigating changes in multiple facets of plant diversity and their impacts on multidimensional stability under global changes such as nutrient enrichment can improve our understanding of the processes and mechanisms maintaining ecosystem stability. 

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5. 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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