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The effects of increased anthropogenic inputs of reactive nitrogen (N) have been studied at the Harvard Forest Chronic N Experiment, where NH4NO3 has been applied experimentally since 1988 to increase atmospheric deposition rates ∼6- and ∼18-fold above ambient. This paper asks whether conditions favorable to primary production also resulted in plastic increases to flower, fruit, and seedling traits, and focuses primarily on the oaks that dominate the hardwood stands of the Harvard Forest experiment. Litterfall samples collected between 1996 and 2001 revealed that flowers and fruits were significantly more abundant in N-treated plots, and an analysis of oak tree abundance found significant variation both among and within plots. Acorn samples collected during 2003 and 2004 (a mast and a postmast year) were therefore analyzed usingancova models that included an estimate of oak tree abundance. This tree abundance estimate was the only significant driver of increased acorn production during the mast year, and in both years it was a significant factor on plots receiving the highest levels of N. In the postmast year, acorn production was increased in direct response to N-related factors other than tree abundance. Our comparisons of control and N-treated plots for acorn quality traits (e.g. rates of acorn damage, germination percentage, seedling growth) revealed negligible or only transient differences. Shifts in overall acorn abundance – particularly disproportionate N-mediated increases during nonmast years – could have a wide range of ecological consequences beyond the more frequently examined impacts of N deposition on primary production and carbon sequestration. [ABSTRACT FROM AUTHOR]

Response of soil respiration (CO2 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; Low-N, 5 g N m−2 yr−1; Medium-N, 10 g N m−2 yr−1; and High-N, 15 g N m−2 yr−1 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±30 g CO2-C m−2 yr−1 in the Controls. Annual mean soil respiration rate in the Control, Low-N and Medium-N treatments (69±3, 72±3 and 63±1 mg CO2-C m−2 h−1, respectively) did not differ significantly, whereas it was 14% lower in the High-N treatment (58±3 mg CO2-C m−2 h−1) compared with the Control treatment, also the temperature sensitivity of respiration, Q10 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. [ABSTRACT FROM AUTHOR]