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Critical loads and nitrogen availability under deposition and harvest scenarios for conifer forests in Ireland

Johnson, J., T. Cummins and J. Aherne . 2016. Science of The Total Environment

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 N ha-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.

Key Words

Sitka spruce; Soil; Nutrients; Forest floor; C:N; Harvest residue; Monitoring; Ammonia; Air pollution