Nutrient and carbon cycling along nitrogen deposition gradients in broadleaf and conifer forest stands in the east of England

Abstract

Forest fragmentation dominates the landscape in England and there is a concern about the associated forest edge effect especially in areas with high N inputs. This study quantifies this effect and the impact of high N deposition on forest and soil biogeochemical processes under 26 year old Corsican pine and 72 year old beech forest stands at Thetford forest, the lowest rainfall area of East Anglia, UK. Nutrient and C cycling were studied along 200 m N deposition gradients away from intensive pig and chicken rearing units, over five years. The site is underlain by argillic brown sand over chalk. Total N deposition at the Corsican pine was 46 kg N ha−1 at the edge reducing to 22 kg N ha−1 in the inner forest, composed of 70% dry deposition, with 50% NH4-N. Under the beech total N deposition was 36 kg N ha−1 at the edge, reducing to 22 kg N ha−1 at 200 m, of which 63% was dry N deposition at the edge, composed of one third NH4-N.The results indicate a strong edge effect with an N deposition gradient along which soil acidity and soil available N (as NO3-N and NH4-N) significantly increased under both pine and beech. Tree N uptake also increased with increasing N deposition, which reduced the uptake of P and K. Uptake of Ca and Mg was not affected. Nitrogen deposition stimulated the growth of both species and enhanced litter and fine root decomposition at the beech but inhibited decomposition at the pine site. This had a positive effect on soil C storage under pine, which was negative under beech. Tree physiological responses to N deposition included an increase of beech flowering and Corsican pine coning, but reduced beech seeding.The edge effect was detectable across 200 m in these stands. Coniferous and broadleaved species in this study responded differently to N addition, due to different N cycling processes both above and belowground. Coniferous systems are more prone to N saturation than broadleaved through scavenging by forest canopy and soils, but they could be the more effective tree type/species choice in N abatement natural strategies. Local effects of N deposition should be accounted for in forest design plans in order to minimise edge effect impacts on the forest environment.