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^–1a^–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, N₂fixing 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^–1a^–1from 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.Key words: clear-cut, disturbance, forest management, nitrate, nitrogen cycling, nitrogen saturation.

There is considerable evidence that biodiversity promotes multiple ecosystem functions (multifunctionality), thus ensuring the delivery of ecosystem services important for human well-being. However, the mechanisms underlying this relationship are poorly understood, especially in natural ecosystems. We develop a novel approach to partition biodiversity effects on multifunctionality into three mechanisms and apply this to European forest data. We show that throughout Europe, tree diversity is positively related with multifunctionality when moderate levels of functioning are required, but negatively when very high function levels are desired. For two well-known mechanisms, ‘complementarity’ and ‘selection’, we detect only minor effects on multifunctionality. Instead a third, so far overlooked mechanism, the ‘jack-of-all-trades’ effect, caused by the averaging of individual species effects on function, drives observed patterns. Simulations demonstrate that jack-of-all-trades effects occur whenever species effects on different functions are not perfectly correlated, meaning they may contribute to diversity–multifunctionality relationships in many of the world’s ecosystems.

Significance: Numerous studies have demonstrated the importance of biodiversity in maintaining multiple ecosystem functions and services (multifunctionality) at local spatial scales, but it is unknown whether similar relationships are found at larger spatial scales in real-world landscapes. Here, we show, for the first time to our knowledge, that biodiversity can also be important for multifunctionality at larger spatial scales in European forest landscapes. Both high local (α-) diversity and a high turnover in species composition between locations (high β-diversity) were found to be potentially important drivers of ecosystem multifunctionality. Our study provides evidence that it is important to conserve the landscape-scale biodiversity that is being eroded by biotic homogenization if ecosystem multifunctionality is to be maintained.