Impacts of increased atmospheric nitrogen deposition on a Calluna vulgaris upland moor, North Wales
1. Long-term nitrogen (N) addition plots on an upland Cal/una vulgaris moor had been treated for
more than ten years with 0,40, 80 and 120 kg N ha-1 yr- 1
2. Sampling of the soil solution from under the mor and the mineral gley horizons over an annual
cycle revealed a high degree of retention of N, between 60 % and 80 % of ambient N inputs in
the control, and rising to 90 % in response to higher inputs of the N treatments.
3. After passing through the mineral gley horizon, N was further retained, by 85 % of ambient N
inputs in the control, and rising to 97 % in response to the highest N treatment.
4. There was some evidence of an inverse relation between ammonium and dissolved organic
nitrogen (DON) under all N treatments. Microbial immobilisation and conversion to DON was
compatible with the few significant effects on cation concentration, although calcium and
calcium/aluminium ratios and pH were decreased by N treatment in the mineral gley horizon.
5. A budget revealed that the added N of the treatments had been allocated mainly to the canopy
and litter at low inputs and increasingly to the mor, and to a lesser extent the gley horizon, with
increasing N inputs.
6. In the system as a whole, 51 % of the entire system N was contained in the mor horizon, and
with increasing inputs of added N, the whole system N accounted for 60 %, 80 % and 90 % of
the added N in the low, middle and high N treatments respectively.
7. Nitrogen/phosphorus ratios in green tissue were decreased in response to increasing N inputs to
levels indicative of N limitation, in spite of increases in phosphatase activity in the litter and mor
layers. N uptake rates and mycorrhizal colonisation were not affected.
8. A moor management bum volatilised 90 % of the canopy N (amounting to 6 % of the total
system N). Increases in the gain ofN in the gley horizon after the bum decreased with increasing
N treatment, an indication of increasing N saturation in this layer and confirmed by increases in
N leaching in response to higher N inputs. The same response was observed without significance
in the mor layer owing to greater variance in the data.
9. Rates of net mineralisation and DON production in the litter layer increased with N inputs in
both field and laboratory incubations, but net nitrification rate only showed N treatment-related
increases in the laboratory incubation. Threshold values of litter % N and C/N ratios determined
the onset of these processes, as well as N treatment-related increases in rates of potential
10. Bryophyte cover under the Calluna canopy, density of the canopy, light transmission through the
canopy and N inputs were all related, either negatively or positively (see chapter 5), but only in
mature/degenerate plants. In "new' plots containing younger, building-phase Calluna, the
addition of phosphorus raised the competitive ability of bryophytes above that of Calluna,
particularly at lower inputs of N.