| A piece of Kandelia obovata forest which is beside the shrimp ponds and along the south coastline of Jiulongjiang,Fujian Province was regarded as the research area.Two polluted areas were chosen.It had been 7 years since it began to receive cleaning-effluents in area ? and 13 years in area ? Two control areas were determined respectively beside each of the polluted areas.By comparing the polluted area and the corresponding control area,the effects of direct discharge of cleaning-effluents from high-level shrimp pond on main processes of organic carbon in K.obovata mangrove ecosystem were investigated.Plants in polluted area ? were obviously higher than those in control area ?,plants in polluted area ? were not as good as those in control area ?.Biomass of all parts,aboveground/underground,total biomass and the annual increment in plants of each group of sample areas all showed that polluted area ?>control area ?,polluted area ?<control area ?,and branch,leave,root and total biomass in polluted area ? were significantly higher than those in control area ?.The annual increment of K.obovata plants' biomass in polluted area ? was negative in this year.Organic carbon accumulation of all parts,aboveground/underground,total accumulation and the annual increment in plants of each group of sample areas showed the similar rule with the biomass.This suggested that short-term direct discharge of cleaning-effluents from high-level shrimp pond had a positive impact on the carbon accumulation of K.obovata mangrove,and long-term direct discharge had a negative effect on the growth and organic carbon accumulation of K.obovate plants.Plants of K.obovata in each sample area all observed the normal phenology and the same seasonal change.Due to the super typhoon "cuckoo",the production of leaf litter and branch litter got the highest values in October.And because of the species biological characteristics,the production of flower litter and fruit litter got the highest values in August,September and April,May respectively.Litter production of all parts and total litter production in plants of each group of sample areas generally followed the rule that polluted area ?>control area ?,polluted area ?<control area ?,and the production of leaf litter and flower litter in polluted area I were significantly higher than that in control area ?,the production of leaf litter in polluted area II was significantly lower than that in control area ?.This indicated that short-term direct discharge of cleaning-effluents from high-level shrimp pond had a positive impact on the litter production of K.obovata plants,and long-term direct discharge had a negative effect on it,which was similar to the conclusion of K.obovata plants organic carbon stock.TOC,TN,TP content of litter fall in each group of sample areas did not show significant difference,maybe because control area sediment was polluted by external nutrition as well,the influences of direct discharge of cleaning-effluents from high-level shrimp pond on TOC,TN,TP content of each litter fall were masked.Fallen leaves decomposed so fast that the weight loss rates got to be 67.43%?83.40%in summer and autumn,and 39.58%?61.10%in winter and spring after 6 weeks' decomposition in each sample area.But the weight loss rates of fallen branches were just 26.10%?37.76%50 weeks later.Decomposition rate of fallen leaves in each group of sample areas showed that polluted area ?>control area ?,polluted area ?<control area ? as a whole.Mainly because that plants in polluted area ? were tall and luxuriant,the crown density was high,the sediment humidity was high,bacteria and microbial activity strengthened.However,long-term direct discharge made polluted area ? low crown density and low sediment humidity,which might seriously inhibit biological activity and reduce the decomposition rate.The decomposition dynamics of fallen branches in each group of sample areas were similar to the fallen leaves's.During the decomposition process of leaf litter in each season and each sample area,changes in TOC content were not regular,it might be higher or lower.TN content showed an upward trend on the whole.TP content decreased first and then increased on the whole.Influenced by TOC and TN content,C/N showed a downward trend on the whole.There was no significant difference between each group of sample areas in TOC,TN,TP content and C/N of leaf litter in each season.This indicated that direct discharge of cleaning-effluents from high-level shrimp pond had no obvious influence on nutrient release cycle of leaf litter.There was little difference in standing stocks of leaf litter in each season.Leaf consumption by crabs mainly happened in August and September.Standing stocks of leaf litter in each group of sample areas and each season showed that polluted area I>control area ?,polluted area ?>control area ? as a whole,and polluted area ?significantly higher than control area ? in November,December and January.Leaf consumption rates by crabs in each group of sample areas and each season showed that polluted area ?<control area ?,polluted area ?>control area ? as a whole,but there were no significant differences.In the 4 sample areas,ratios of crab feeding carbon and total carbon in leaf litter were 7.66%,11.95%,10.87%,2.56%respectively,and ratios of tidewater output carbon and total carbon in leaf litter were 38.3%,44.6%,17.7%,74.2%respectively.The ratio of tidewater output carbon and total carbon in leaf litter in polluted area ? was low,maybe because that plenty of rubbish on the beach hindered the flushing action of the tides.The ratio of tidewater output carbon and total carbon in leaf litter in control area ? was high,which was due to the low crab feeding carbon.On the whole,direct discharge of cleaning-effluents from high-level shrimp pond had no significant influence on the fate of leaf litter.Fluxes of N2O,CH4 and CO2 from soil to air were higher in summer and lower in autumn,winter and spring in each sample area.N2O,CH4 and CO2 fluxes of each group of sample areas generally showed that polluted area ?>control area ?,polluted area ?>control area ? in each season,and CO2 flux in polluted area ? was significantly higher than that in control area I in summer.The annual flux of the three greenhouse gases and total annual flux showed the similar regulation,and CO2 annual flux in polluted area ? was significantly higher than that in control area I.The TOC content and C/N in the soil in each sample area were higher in winter and spring and lower in summer and autumn,TN and TP content were slightly different in each season.In each season in each group of sample areas,the TOC content and C/N in the soil showed that polluted area ?<control area ?,polluted area ?<control area ?,TP content showed that polluted area ?>control area ?,polluted area ?>control area ?,TN content were slightly different.The correlation analysis between the three greenhouse gas fluxes and the soil nutrient content showed that N2O flux was significantly positively related with TOC content,TN content and C/N,CH4 flux and CO2 flux were significantly positively related with TOC content and C/N.According to carbon precomputation of K.obovata mangrove ecosystem,net carbon fixed in each group of sample areas showed that polluted area ?<control area?,polluted area ?<control area ?,which suggested that direct discharge of cleaning-effluents from high-level shrimp pond decreased the net fixed carbon.That is to say,direct discharge of cleaning-effluents from high-level shrimp pond had some impacts on the main organic carbon processes in K.obovata mangrove ecosystem,which might weaken the carbon sequestration capacity of mangrove ecosystem from a certain extent. |
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