Leaching of nitrate from temperate forests - effects of air pollution and forest management

We compiled regional and continental data on inorganic nitrogen (N) in seepage and surface water from temperate forests. Currently, N concentrations in forest waters are usually well below water quality standards. But elevated concentrations are frequently found in regions with chronic N input from deposition (> 8-10 kg ha(-1) a(-1)). We synthesized the current understanding of factors controlling N leaching in relation to three primary causes of N cycle disruption: (i) Increased N input (air pollution, fertilization, N-2 fixing plants). In European forests, elevated N deposition explains approximately half of the variability in N leaching, some of the remaining variability could be explained by differences in N availability or N status. For coniferous forests, needle N content above 1.4% and (or) forest floor C:N ratio lower than 25 were thresholds for elevated nitrate leaching. At adjacent sites conifer forests receive higher N deposition and exhibit higher nitrate loss than deciduous forests; an exception is alder that shows substantial nitrate leaching through N fixation input. Fertilization with N poses limited risk to water quality, when applied to N-limited forests. (ii) Reduced plant uptake (clear-cut, thinning, weed control). The N cycle responses to plant cover disturbance by clear-cut are well studied. Nitrate losses peak after 2-3 years and are back to pre-cut levels after 3-5 years. Nitrogen losses increase with deposition and are higher at N rich sites. The extent and duration of the nitrate response is especially connected to the recovery of the vegetation sink. Less intensive disturbances like thinning have only minor effects on N loss. (iii) Enhanced mineralization of soil N (liming, ditching, climate change). Responses in nitrate leaching after liming may increase with N deposition and in older stands. However data on these types of N cycle disruption are too sparse to allow general conclusions on controlling factors. Nitrate leaching occurs when N deposition (input) and net mineralization (N status) exceed plant demand. A combined N flux to the soil of 50 to 60 kg ha(-1) a(-1) from N deposition and litterfall may be a threshold for nitrate leaching in undisturbed forests. This threshold also indicates risk of increasing losses in case of a disturbance (e.g., clear-cut). We conclude by discussing forest management options for water quality protection. These options focus on decreasing input, increasing plant uptake, increasing biomass removal, and (re)establishing immobilization and denitrification processes at the catchment scale.