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Response of soil respiration (CO 2 emission) to simulated nitrogen (N) deposition in a mature tropical forest in southern China was studied from October 2005 to September 2006. The objective was to test the hypothesis that N addition would reduce soil respiration in N saturated tropical forests. Static chamber and gas chromatography techniques were used to quantify the soil respiration, following four-levels of N treatments (Control, no N addition; 5 g N m À2 yr À1 ; Medium-N, 10 g N m À2 yr À1 ; and High-N, 15 g N m À2 yr À1 experimental inputs), which had been applied for 26 months before and continued throughout the respiration measurement period. Results showed that soil respiration exhibited a strong seasonal pattern, with the highest rates found in the warm and wet growing season (April-September) and the lowest rates in the dry dormant season (December-February). Soil respiration rates showed a significant positive exponential relationship with soil temperature, whereas soil moisture only affect soil respiration at dry conditions in the dormant season. Annual accumulative soil respiration was 601 AE 30 g CO 2 -C m À2 yr À1 in the Controls. Annual mean soil respiration rate in the Control, Low-N and Medium-N treatments (69 AE 3, 72 AE 3 and 63 AE 1 mg CO 2 -C m À2 h À1 , respectively) did not differ significantly, whereas it was 14% lower in the High-N treatment (58 AE 3 mg CO 2 -C m À2 h À1 ) compared with the Control treatment, also the temperature sensitivity of respiration, Q 10 was reduced from 2.6 in the Control with 2.2 in the High-N treatment. The decrease in soil respiration occurred in the warm and wet growing season and were correlated with a decrease in soil microbial activities and in fine root biomass in the N-treated plots. Our results suggest that response of soil respiration to atmospheric N deposition in tropical forests is a decline, but it may vary depending on the rate of N deposition.

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, N2 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.Key words: clear-cut, disturbance, forest management, nitrate, nitrogen cycling, nitrogen saturation.