Conclusions
-
After 14 years of treatment, we detected significant reductions in cover in response to nitrogen in the plant species examined. The responses differed among species and nitrogen forms, but all species declined, and NH\({}_{3}\) produced the biggest change in cover per unit of nitrogen addition. The exception was the graminoid sedge Eriophorum vaginatum , which increased dramatically in the NH\({}_{3}\) treatment.
-
Multivariate analyses found significant responses to nitrogen, by combining information across all species. Principal response curves identified coherent, community-level effects of nitrogen deposition, which increased with nitrogen dose. The effects were similar across all nitrogen forms. The significant exception was when PK was added at intermediate levels of nitrogen, where community change took a quite different trajectory.
-
Partial least-squares regression identified high levels of NH\({}_{4}^{+}\) and NO\({}_{3}^{-}\) in the soil water as the chemical drivers of this change in the NH\({}_{3}\) treatment. A clear change in soil water chemistry was not found in the NH\({}_{4}^{+}\) and NO\({}_{3}^{-}\) treatments.
-
Phosphorus(and/or potassium) could radically alter the response to nitrogen addition, particularly at low-intermediate levels of nitrogen input, but effects were species-specific.
-
We surmised that the larger experimental response to nitrogen observed in the NH\({}_{3}\) treatment (compared with the NH\({}_{4}^{+}\) and NO\({}_{3}^{-}\) treatments) was because of the higher nitrogen concentrations at the vegetation surface produced by dry deposition. NH\({}_{3}\) is deposited as a gas, directly to the water on the leaf surface and through the stomata to the leaf apoplast. Because NH\({}_{3}\) deposits directly to the leaf, it stays contained within the small volume of water on and in the leaf, producing a high internal concentration of nitrogen ions. Much of NH\({}_{4}^{+}\) and NO\({}_{3}^{-}\) solution sprayed on will run off to the soil water, and be further diluted and subject to biological regulation of nitrogen uptake. So, the nitrogen ion concentrations produced at the leaf apoplast are inevitably lower, when comparing equivalent nitrogen deposition rates.
-
The same reasoning may explain why the observed experimental responses to nitrogen were smaller than in the closest comparable experiments; at Whim, the NH\({}_{4}^{+}\) and NO\({}_{3}^{-}\) were applied in many small doses, rather than in a few large monthly doses as in most other experiments. In this way, we avoid artificially high nitrogen concentrations at the vegetation surface, and maintain conditions closer to real-world nitrogen deposition.
-
Considering the actual deposition rates of NH\({}_{3}\), NH\({}_{4}^{+}\) and NO\({}_{3}^{-}\) on UK peat bogs, and the relative magnitude of their effects based on our experimental results, we estimate that NH\({}_{4}^{+}\) deposition has the largest impact on the keystone species Sphagnum capillifolium . Nitrate has a greater impact than NH\({}_{3}\), but this difference is within the confidence limits of our estimates.