The wet treatments consist of 44 plots, each with an area of 12.5 m2. Each treatment has 4 replicates, randomised in 4 blocks on the site. Treatments are supplied at three doses of N; 8, 24 and 56 kg N ha-1 y-1, on top of the ambient deposition of 8 kg N ha-1 yr-1, in both oxidised form as NaNO3 and reduced form as NH4Cl. The yearly input is estimated as the amount of the treatment plus the ambient deposition of 8 kg Nha-1 yr-1. Additional treatments at 8 and 56 kg N (oxidised and reduced) inputs are supplemented with P and K by adding K2HPO4 at 1:14 PN ratio.

Figure (1): Schematic diagram showing the wet treatment plots at Whim bog

To supply the wet treatments, rainfall is collected on site on a 178 m2 pitched surface and stored in a 1250 litre reservoir. The rainwater is pumped through a network of pipes, dosed with the appropriate N form and concentration, and sprayed onto the experimental plots. Spraying only occurs when the wind speed is below 5 m s-1. This results in realistic doses and frequencies, which allows us to study the cumulative effects of N deposition in its different forms. Wet treatments are closed down during freezing conditions and re-activated when night temperatures rise above 0oC.

Sprayers in action

Rain collector in action

Figures: (1) Sprayer in action; (2) Rain collector in action

Few direct effects from the wet N deposition treatments have been identified since the start of the experiment. Therefore it is likely that ecosystem and species damage are occurring on a longer time scale compared to the dry deposition treatment. In contrast, the combined N and PK treatments have had profound changes to the species composition within the plots with, most notably, decreases in Calluna vulgaris, Sphagnum sp. and Cladonia sp.; with invasion and expansion of nitrophiles (nitrogen-loving species), non-characteristic bog plants e.g. Epilobium angustifoliumEpilobium palustre and Juncus effuses (Sheppard et al., 2013). This may cause changes in plant cover, peat chemistry and decomposition rates.

Unimpacted control plotNO3 treatment plotImpacted N PK treatments

 Figures: (1) Control plot; (2) NO3 treatment plot; (3) Impacted N PK treatment plot

Moss expansion in N PK plotsPleurococcus expansion in N PK plots

 Figures: (1) Moss expansion in N PK plot; (2) Pleurococcus expansion in N PK plots