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

Abstract

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.

Fig 1. Cross-section schematic of barrier island.

Fig 2. Graminoid and forb biomass production across nutrient enrichment plots.

A, B Effect of nitrogen and phosphorus on productivity of graminoid and forb lifeforms in coastal mesic grassland (mean ± SE). Brackets and asterisks indicate significant main effect of N treatments on productivity (‘*’ = p-value < 0.05, ‘***’ = p-value < 0.0001). C, D Effect each nutrient treatment had on productivity of graminoids and forbs (mean ± SE). Hollow points represent productivity values of individual replicates in each treatment. Treatments include control (N0 P0), phosphorus (N0 P10), nitrogen (N10 P0), and nitrogen + phosphorus (N10 P10). Nitrogen treatments are represented on the x-axis while P treatments are represented by color. Scale of the productivity axes are adjusted to improve visualization of variation in the data and are not directly comparable across lifeform type.

Fig 3. Non-metric multidimensional scaling ordination of species composition across nutrient enrichment treatments.

Non-metric multidimensional scaling (NMDS) ordination of species composition grouped into convex hulls by nutrient treatment. Points represent within treatment replicates with stars indicating centroids of each treatment group. Centroids can be viewed as a representation of mean community composition. Colors and symbols are matched to nutrient treatment (C = control, P = phosphorus, N = nitrogen, NP = nitrogen + phosphorus).

Fig 4. Rank abundance curves of species across nutrient enrichment treatments.

Rank abundance curves (RACs) for all species in each treatment group. A, B) RACs for species in control and phosphorus plots, respectively. C, D) RACs for species in nitrogen and nitrogen + phosphorus plots, respectively. Species that were found to significantly correlate with the spread on plots in the NMDS analysis (Fig 2) are highlighted with a colored halo.

Fig 5. Abundance differences of species between control and each nutrient enrichment treatment.

Abundance difference of each species present in A) control and phosphorus (P) plots; B) control and nitrogen (N) plots; and C) control and nitrogen + phosphorus (NP) plots. Control plots are used as a reference to the natural coastal grassland community. Negative values (yellow bars) indicate species that are less abundant in nutrient treated plots compared to control plots. Positive values (colors corresponding to nutrient treatment) indicate species that are more abundant in nutrient treated plots compared to control plots.