The effect of different tree species on through fall and soil solution chemistry at Gisburn Forest
A study was carried out to investigate the effect of four tree species on the acidity and chemistry of rainwater as it passed down the forest profile. The four species, Norway spruce (Picea abies), Scots pine (Pinus sylvestris), oak (Quercus petraea) and alder (Alnus glutinosa) were all at the same site at Gisburn (Bowland Forest), north-west England. The water was collected as rain, beneath the canopy and the forest floor, at the boundary of the A and B horizons and within the B horizon. The water was collected and analysed for pH, PO43--P, NO3--N, NH4+-N, Cl, SO42--S, Na+, Ca2+ and Mg2+ every two weeks. There were occasional analyses for aluminium. Porous ceramic cups were used to collect water in the soil horizons and so volumes had to be estimated. A further correction had to be made to allow for the loss of water as runoff over the surface of the forest floor. There were consistent differences between the species. The species differed in both the processes responsible for and the location of the production of acidity. Under all species there was a great reduction in acidity in the deep soil horizons. The source and nature of the anion thought to be associated with cation leaching also differed under the species. Most of the species differences could be attributed to processes occurring in the forest floor. Of most importance were: the fixation of atmospheric nitrogen by alder and its subsequent release as NO3--N; the rate of decomposition of the various litters; and the horizon where most root uptake occurred. The differences observed in the throughfall were not reflected in the forest floor or soil horizons. There was no evidence of depletion of cations in the mineral soils after thirty years of tree growth but it was considered that the risk was greater under alder and pine than under spruce and least under oak.