title: Critical loads and nitrogen availability under deposition and harvest scenarios for conifer forests in Ireland.
abstract: In this study we calculated the critical load of nutrient nitrogen (N) for Irish forest plots (n=380) under two harvesting scenarios: conventional stem-only harvest (SOH) and stem plus branch harvest (SBH) and two deposition scenarios: current and with a 10% increase in reduced-N. In addition, current N status was assessed using the following data from forest monitoring plots: forest floor C:N, foliar N and plant root simulation (PRS™) probe N supply rate. Average critical loads were 15.3 kg N ha(-1)year(-1) under SOH and 19.5 kg N ha(-1)year(-1) under SBH. Average total (wet+dry) N deposition was 18 kg N ha(-1)year(-1), ranging from 8.6 to 26 kg Nha(-1)year(-1). As a result, critical loads were exceeded at 67% of sites under SOH and 40% of sites under SBH. However, there was little evidence of exceedance at monitored plots. Foliar and forest floor C:N data indicated that most of these sites had low to intermediate N status. There were considerable differences in N cycling between soil types. Plant root simulation (PRS™) probe data indicated that this was likely due to differences in net N-mineralization and nitrification. Our results indicate that many sites are currently N limited but critical load exceedance suggests that these systems will accumulate N over time. The findings have implications for forest management, allowing for the assessment of nutrient management under different harvest scenarios.
authors.affiliation: UCD School of Agriculture and Food Science, University College Dublin, Belfield, Dublin 4, Ireland. Electronic address: jim.johnson@ucd.ie.
authors.forename: James
authors.initials: J
authors.lastname: Johnson
authors.affiliation: UCD School of Agriculture and Food Science, University College Dublin, Belfield, Dublin 4, Ireland.
authors.forename: Thomas
authors.initials: T
authors.lastname: Cummins
authors.affiliation: Environmental Resource Studies, Trent University, Peterborough, ON K9J 7B8, Canada.
authors.forename: Julian
authors.initials: J
authors.lastname: Aherne
keywords: Air pollution; Ammonia; Forest floor C:N; Harvest residue; Monitoring; Nutrients; Sitka spruce; Soil