The Responses Of VOCs Fluxes From Forest Floor And Plants To Simulated Nitrogen Deposition In Subtropical China

Volatile organic compounds (VOCs) are known as important compounds in the atmosphere. They can take part in atmospheric photochemical processes which may produce secondary atmospheric pollutants like ozone and organic aerosol and pose threats to human health and environment.Forest floor (including soil and litter) can act as both source and sink for VOCs. Elevated nitrogen deposition may influence soil ecological processes, which can lead to changes in forest floor VOCs fluxes. Terrestrial plants also make great contributions to VOCs. Nitrogen deposition may affect VOCs emissions from plants through affecting physiological and biochemical processes of plant. Although elevated nitrogen deposition has influenced the material circulation and has received much attention, the research on responses of VOCs exchanges between forest floor or plants and the atmosphere to elevated nitrogen deposition is still scarce. In this study, the influence of elevated nitrogen deposition on VOCs fluxes from the forest floor and and the plants were studied, the main contents and results were as follows:(1) The static-chamber coupled with preconcentrator-GC-MS techniques were employed, the responses of VOCs fluxes from the forest floor fluxes to simulated elevated nitrogen deposition were studied in two typical forests, namely, pine forest (PF) and monsoon evergreen broadleaf forest (BF) in Dinghushan. The results showed that larger fluxes were found for n-undecane, n-dodecane, a-pinene and BTEX (Benzene, toluene, ethyl benzene and xylene). PF floor acted as sinks for most VOCs in the controlled sites. The uptake rates decreased with low nitrogen additions and forest floor changed from "sink" to "source" with medium-N additions in PF. In BF, meanwhile, forest floor mainly acted as sources for VOCs in controlled sites. The emission rates decreased, or the forest floor changed from "source" to "sink" after nitrogen additions in BF, with the significant difference in low and high nitrogen addition sites from those in controlled sites. On the basis of measurements every 3 hours each day in controlled and high nitrogen addition sites in BF, no clear diurnal variations were found for VOCs fluxes. The highest emission rates appeared at 7:00 and 10:00 and the highest uptake rates appeared at 13:00 and 19:00. Forest floor CO2 emission rates in BF were significantly higher than those in PF in controlled sites, and the emission rates in all the nitrogen addition sites were higher than those in the controlled sites in both forests. Some VOCs fluxes showed significant correlations with soil temperature, soil water content as well as CO2 flux. PF litter and soil mainly acted as sources for VOCs; BF soil mainly acted as sinks while BF litter acted as both sources and sinks. VOCs emissions from litter and soil increased after low nitrogen and medium nitrogen additions while VOCs emissions from litter decreased or even BF litter changed the function from "source" to "sink" after high nitrogen additions.(2) With dynamic-chamber systems and PTR-TOF-MS, the impacts of nitrogen deposition on VOC emissions from Pinus massoniana L. (Pm) and Schima superba (Ss) under different nitrogen application methods, namely, soil nitrogen fertilization and foliage nitrogen fertilization were studied. The results showed that TVOCs (Total VOCs) emission rates ranged from 544.86-2056.92 nmol g-1 h-1 in the controlled sites, with higher TVOC emission rates from Ss than those from from Pm. OVOCs (Oxygenated VOCs) made the biggest contribution to TVOCs under both nitrogen applications, which accounted for 32.54%～64.48%. Among the identified VOCs, acetaldehyde, methanol and monoterpene exhibited higher emission rates. The effects of nitrogen additions on plant VOCs emissions depended on nitrogen levels, nitrogen application methods, plant types as well as compound types. After soil nitrogen fertilizations, VOCs emissions from Pm increased while VOCs emissions from Ss decreased. Most VOCs release rates from both plants decreased after foliar nitrogen fertilizations. With the simulated nitrogen deposition, OVOCs (e.g. formaldehyde, actone, pyroracemic acid, methyl chloride) emissions from Pm showed significantly positive correlations with air temperature, and significantly negative correlations with air relative humidity. For Ss, NMHCs (e.g. isoprene, cycloheptene, monoterpene and toluene) emission rates showed significantly negative correlations with air temperature, and positive correlations with air relative humidity. The simulated elevated nitrogen deposition had no obvious effects on soil moisture, total phosphorus, total nitrogen and available nitrogen. Soil pH decreased significantly after nitrogen additions, and soil pH with foliar nitrogen fertilization were higher than those with soil nitrogen fertilization. VOCs emissions from potting soil mainly declined after nitrogen additions, and, the effects for foliar nitrogen fertilizations were much more significant than those for soil nitrogen fertilizations.