Exchange Fluxes Of Volatile Organic Sulfur Compounds Between Land Surfaces And Atmosphere In Selected Typical Terrestrial Ecosystems In The Pearl River Delta

Sulfur cycle is an important component of global biogeochemical cycles. Volatile organic sulfur compounds (VOSCs) are principal precursors of sulfate aerosol and cloud condensation nuclei in troposphere and stratospheric, and thus have great impacts on global climate change and acid rain. The exchange fluxes of VOSCs between land surfaces and atmosphere differ significantly in different ecosystems, which play different roles in global change. In the present study typical ecosystems, such as forest, rice paddy field, city lawn and vegetable land in the Pearl River Delta in subtropic China were selected to measure COS, DMS, CS₂ and DMDS exchange fluxes using static enclosure chamber technique, and the influence of environmental factors on VOSCs fluxes were investigated for each ecosystem. Through soil incubation in laboratory, VOSCs fluxes under different soil water content and after adding different sulfur-containing chemicals were also studied. The results are as the follow:1. With or without litter, forest soils acted as COS sinks and DMS sources, but there existed no consistent direction of fluxes for CS₂ and DMDS. The highest COS uptake rates were observed in the mixed forest (MF) in medium successional stage. DMS emission rates in broad-leaf forest (BF) was significantly higher than those in the MF and the pine forest (PF). There were no significantly variations for CS₂ and DMDS fluxes between the forests. The differences in VOSCs fluxes between the forests might be attributed to the varing soil fertility and soil microorganisms. Average COS and DMS fluxes at plots with litter were higher than those at plots without litter, but litter had little effects on CS₂ and DMDS fluxes.Due to different source contributions and meterological factors, ambient VOSCs concentration varied in different months. Ambient COS concentrations in March were significantly higher than those in July, August, September and October, and DMS concentrations in July, August and September were significantly higher than those in October and March. COS fluxes in March were higher than those in other months; and DMS fluxes in July, August and September were higher than those in October and March. COS uptake rates in daytime were a little higher than those in nighttime, but DMS emission rates in daytime were significantly higher than those at nighttime. No significant differences were found between daytime and nighttime fluxes for CS2 and DMDS.Soil COS and DMS exchange rates increased positively with soil respiration rates. The correlation between VOSCs fluxes and environmental factors implied that VOSCs exchange rates were combinedly influenced by temperature, soil water content, soil microorganisms and ambient VOSCs concentration.2. Rice paddy field acted as COS sink and DMS source, but there existed no consistent flux direction for CS₂ and DMDS. Rice crops played great roles in VOSCs fluxes. COS was uptaken by rice-planted field, but it emitted from non rice-planted field. Paddy field acted as DMS sources whether it had rice crops or not, with significant higher emission rates in rice-planted fields than those in non rice-planted fields. This implied that rice crops could absorb COS, but was a emission source of DMS.VOSCs fluxes differed significantly in different growth stages of rice crops, with the maximum COS uptake rates and DMS emission rates in jointing-booting stage. No significant variation of CS₂ and DMDS fluxes were found between growth stages. VOSCs flux variations among rice-planted fields, no rice fields and fields rice-havested fields indicated that VOSCs fluxes were mainly controlled by rice plants and soil microorganisms.Soil water contents greatly influenced COS fluxes in the paddy ecosystem. COS were found to be absorbed in dry field and emitted in waterlogged field. But soil water content had little influence on DMS, CS₂ and DMDS fluxes. The difference in COS fluxes between dry and waterlogged fields might be attributed to different soil redox potential and soil microbial community.3. Whether with grass or not, city lawn acted as a COS sink and a DMS source, with higher exchange rates with grass than without grass. But grasses had no significant effect on CS₂ and DMDS fluxes. There were no consistent flux direction for CS₂ and DMDS, and the CS₂ and DMDS fluxes were significantly lower than those of COS and DMS. No significant variations of VOSCs fluxes were found for the lawn before and after mowing. Seasonal and diel variations of COS fluxes showed that temperature and ambient COS concentration were important factors influencing COS fluxes. Temperature also influenced DMS and CS₂ emission rates, but it had no effect on DMDS fluxes.4. VOSCs fluxes in vegetable ecosystem are quite different from those in forest, paddy field and city lawn ecosystems. COS, DMS, CS₂ and DMDS were emitted from vegetable soils when soils were planted with vegetables and DMDS accounted for about 75% of all the VOSCs fluxes.COS emission rates in soil without vegetable were significantly higher than those in soil with vegetable. CS₂ and DMDS emission rates increased with the growth of vegetable, but DMS emission rates increased sharply after the vegetables were harvested. Temperature played great roles in COS and CS₂ emission. Positive correlation were found between CO₂ and COS or DMS exchange rates, but CS₂ and DMDS emission rates decreased with the increase of CO₂ fluxes.5. The results from laboratory soil incubation showed that soils acted as COS sink and DMS source when soil water contents were low, and soils were emission sources for both COS and DMS when soil water contents were high. COS and DMS emission rates were significantly higher when soil water was saturated compared to the emissions when soil was in dry and humid condition. Soil water content had little influence on CS₂ and DMDS emission. Soil redox potentials were significantly different under different soil water content, which might be the main factor influencing COS and DMS emissions.The results from soil incubation after adding sulfur-containing chemicals to the soils showed that adding methionine increased the emission of DMDS and DMS, and adding cysteine increased the emission of CS₂ and DMS. DMDS uptake rates increased sharply with the addition of cystine and its emission rates increased significantly with the application of Na₂S to soil. Adding Na₂SO₄ to soils had little effect on VOSCs fluxes. This implied that the formation of VOSCs was controlled by different mechanisms or microorganisms, which respond differently to the micro-environment and substrate.