PS 12-113 - Plant community divergence, then convergence in response to chronic nitrogen and phosphorus enrichment over 20 years of grassland succession

Background/Question/Methods

How does chronic nutrient enrichment modulate plant secondary succession? We report results from a long-term fertilization experiment initiated on abandon farmland in eastern Kansas in 2001. For 20 years experimental plots were fertilized annually with four levels of nitrogen (0, 4, 8 and 16 g N/m²) and two levels of phosphorus (0 and 8 g P/m²) in a factorial design. We hypothesized that nutrient enrichment would constrain plant diversity and drive successional trajectories towards communities dominated by non-native, nutrient acquisitive species but diminished in native perennial tall grasses, forbs, and legumes. We also hypothesized that such changes associated with the addition of one resource (N or P) would be exacerbated by the addition of two resources (N and P), as would be predicted by the resource niche-dimensionality hypothesis. We report responses of species and functional guild abundance and species diversity assessed from 20 annual percent cover surveys conducted from 2002-2021.

Results/Conclusions

Functional guild composition varied over succession as modulated by N and P enrichment and their interaction. Composition diverged among nutrient treatments over the first 15 years of succession reflecting increased dominance by native perennial grasses and legumes at low enrichment to dominance by annual grasses at high enrichment. Annual grasses became especially dominant at the highest level of N where P was also added. This resulted in greater diversity loss compared to where N was added alone, consistent with loss of niche dimensionality. Although effects of enrichment remained significant beyond year 15, these effects subsequently weakened. This reversal is illustrated by a shift from trajectories of divergence in composition and diversity over the first 15 years to convergence over the subsequent five, an effect that was amplified in plots enriched with both N and P. The switch from divergence to convergence was coincident with changes in management to control shrub encroachment and subsequent decline in dominance by annual grasses at high N, suggesting a role of canopy disturbance in mediating coexistence. Although our findings support the niche dimensionality hypothesis, we cannot rule out other contributing factors such as changes in soil chemical conditions or alterations of the soil microbiome.