Carbon Sequestration Of Urban Forests And Lawns In Fuzhou City

Urban green space expanding rapidly due to urbanization could improve urban environment and has considerable carbon (C) sequestered ability. Recently, Chinese government has committed to dramatically reduce CO2emissions intensity. However, increasing C sick of traditional forest has been more difficult. Therefore, accurate assessing the C sink of urban green space will be meaningful for China to relieve the pressure in reducing emission of CO2and to be helpful for fulfilling Chinese obligations towards the Kyoto Protocol.This study was conducted at the Minjiang Park in Fu'zhou City, the capital city of Fujian province (26°03'N,119°15'E) and one of the fastest urbanized inshore city in southeast China. In this study, ecosystem C sequestration in three urban forests and lawns were quantified by inventory method and C budget method. Below ground C allocation and soil organic C (SOC) formation mechanism were also investigated. This study is not only essential for objective evaluating C sink of urban green spaces, but also helpful for perfecting C budget of China.The result showed that three urban forests had higher vegetation and ecosystem C storage than abut lawns. Lawns have relative higher seasonal maximum of soil respiration rate and lower minimum than each close-by urban forest. Moreover, the Q10values of soil respiration and its components in lawns were also higher each close-by urban forest. These results suggest soil respiration in lawns will be more sensitive to future climate change. Seasonal dynamics of liiterfall in urban forests showed binomial patterns with the first peak occurring from April to May and the second peak occurring during August and September. Lawn mowing conducted in May and October. Annual clipping residue of lawns (4.1-4.8t·C·hm-2·a-1) were higher than the annual aboveground litterfall in urban forests (2.5-4.0t·C·hm-2·a-1). The peak values of belowground litterfall in urban forests were not appeared in same month, while those in lawns usually appeared in October. There was no significant difference in annual belowground litterfall between each urban green space (P>0.05). Litter decomposition rate (k) of yellow flower of pagodatree (k=3.54) was fastest among these urban green spaces. Lawns came next with k values from2.90to2.98. The k values of araucaria and guava were lowest (1.09and1.22, respectively).Field C isotope analysis indicated, lawn-derived C contributed42.0%soil organic C (SOC) under10years old lawn. Indoor simulative experiment showed that during incubation experiment, cumulative CO2emissions of5%(CL) and10%(CH) clipping addition treatments (2.94mg C·g-1soil and3.61mg C·g-1soil, respectively) were significantly higher than that of control (CK,0%clipping addition). The cumulative CO2emissions in CL and CH consisted chiefly of C4-CO2which accounted for64.5%and80.2%, respectively, of cumulative CO2emissions in CL and CH. Moreover, clipping addition could obviously increase soil microbial biomass, potential mineralizable C and SOC decomposition rate. These results indicate C4was easier than C3for microbe utilizing and could enhance both microbial biomass and potential mineralizable C, therefore improve C4-SOC formation.Annual net primary productivity (NPP) in urban forests (10.3-14.0t·C·hm-2·a-1) and lawns (11.8-13.1t·C·hm-2·a-1) were similar. However, urban forests had higher net ecosystem productivity (NEP,4.20-6.47t·C·hm-2·a-1) than that of lawns (1.8-3.4t·C·hm-2·a-1). There are different NEP distribution between urban forests and lawns. Urban forest has higher vegetation C accumulation rate and lower soil C accumulation rate than lawn. This discrepancy may partly due to the different C allocation strategy in arboreal and herbaceous species. On the other hand, the discrepancy was also likely attribute to different managements in these two types of green spaces. Moreover, various destinies of aboveground litterfall removed from urban forests add the uncertainty of there C budget. SOC accumulations in lawns are time limited. SOC accumulate rate in 4-years-old and8-years-old lawns were1.4and1.3t·C·hm-2·a-1, respectively. However, the rate in8-years-old lawn rapidly dropped to0.4t·C·hm-2·a-1.In conclusion, above results suggest that although urban lawns have faster SOC accumulate rate, urban forests have larger ecosystem C sick than lawns. Therefore, urban forest deserves to give priority in urban green space construction, and suitable managements should be adopted to avoid C loss.