Mechanism Of Soil Carbon Sequestration And Its Main Controlling Factors Of Urban Lawn

With the speeding up of global urbanization,urban lawn area is rapidly increasing,and has become an important part of city landscape.Studies on urban lawn ecosystem show that urban lawn is a huge carbon sink.Carbon storage of urban lawn soil is even higher than that of suburban forest.However,researches on the soil carbon cycle of lawn are still few,most studies report that urban lawn soil has a high carbon sequestration rate.The process of soil organic carbon accumulation of urban lawn,process of grass decomposition,formation and decomposition of soil organic carbon have attracted less attention.Under global climate and urban environment changes,effects of the changes of carbon input and environmental factors on organic matter decomposition are important in the prediction of urban lawn soil organic carbon dynamics.The study is based on the following issues:(1)What will happen when grass is planted and enter the soil,how soil organic carbon would change after grass planting over time?Is the new carbon mainly in the form of protective aggregate or non protective aggregate?How will microorganism change,which is important to the accumulation of soil organic carbon?(2)Does the grass input change the native soil organic carbon decomposition,i.e.priming effect if any,what are the direction and strength and whether the priming effect would be affected by environmental factors(such as temperature,moisture and nitrogen)?(3)whether the large carbon storage of lawn soil is related to root?What are the differences between grass root and grass leaf decompositions and between their contributions to the formation of soil organic carbon?Whether their input will cause the priming effect of soil organic matter decomposition,if any,what are the direction and strength?To answer question 1,using the method of space for time,we choose a chronosequence of lawns at the Minjiang Park in Fuzhou(26°03’N,119°15’E),viz.2,8,12 and 14 years old lawns,and a reserved 17 years old Psidium guajava forest as a control(CT),using isotope technology,to study the mechanism of the rapid accumulation of organic carbon sequestration of the lawn.To answer question 2 and 3,we design series incubation experiments,using the stability of the 13C isotope technology,to discuss differences of grass decomposition and soil organic carbon mineralization and the priming effect under different environmental conditions such as temperature,moisture and nitrogen.The results are as follows.1)Soil organic carbon(SOC)storages(0 to 40 cm)of different lawns ranges from 39.32 to 67.46 t·hm-2,increasing with the age of lawns.Soil organic carbon storage of 8-year-old lawn is higher than that of the control(CT).The SOC sequestration rates of all lawns are 0.61～1.12 t·hm-2·a-1 at 0-20 cm depth.SOC derived from turfgrass is increased with age of lawn.Turfgrass input is the main contribution to the increase of SOC.About 12.99%(2-year-old),31.85%(8-year-old),53.10%(12-year-old)and 55.30%(14-year-old)of SOC derived from turfgrass at 0-20 cm depth.The new carbon mainly enter into large aggregates,and the rest part entered into the micro aggregates increases with the age of lawns.Compared to the original C3 plant,turfgrass is conducive to the accumulation and stability of soil organic carbon.With the increase of lawn age,the content of dissolved organic carbon and its HIXem increase,while its SUVA of SOC decreases,indicating that the dissolved organic matter of lawns increased,and the increased organic matter has higher degree of humification.Soil microbial biomass carbon content and total phospholipid fatty acid(PLFAs)of lawns also increase with the lawn age.The microbial community structure of soil transformed from C3 to C4 has changed,with the significant increase in bacteria and the gram positive.2)Temperature affects the mineralization of SOC significantly.The input of exogenous carbon(Manila turfgrass)reduces the temperature sensitivity of soil organic carbon mineralization.From the beginning of the seventh day to the end of culture,exogenous carbon addition significantly reduced the soil temperature sensitivity index Q10 value.After 1 month,the Q10 value of the soil with the temperature at 15～25℃ was decreased from 1.98 to 1.26,and the Q10 value with the temperature at 25～35℃ was decreased from 1.92 to 1.36.The cumulative priming effects of 15℃ 25℃ and 35℃were 5254、4827 and 3176 mg·kg-1,respectively at the end of incubation,which accounted for 44%,47%and 37%of plant carbon residues in the soil,respectively.The priming effect decreased with the increase of the temperature.The results show that acceleration of global warming on SOM decomposition may be partly compensated by the decrease in PE with increasing temperature in subtropical lawn ecosystems.3)Soil moisture significantly affected soil organic carbon mineralization.Soil organic carbon mineralization increased with the increase of soil moisture.After 1 year of incubation,the cumulative mineralization of H(100%of water hold capacity)without grass addition was 86%higher than that of L(30%of water hold capacity)and 28%higher than that of M(60%of water hold capacity),the cumulative mineralization of M was 46%higher than that of L.However,with grass addition,the cumulative mineralization of H was 9%higher than that of L and was 5%higher than that of M,the cumulative mineralization of M was 4%higher than that of L.The addition of grass reduced the difference of soil organic carbon mineralization among different soil moisture regime.The effect of grass input on the priming effect of SOC was decreased with the increase of soil moisture,and the cumulative priming effect of L was 39%higher than that of M,and was 59%higher than that of H after 1 year incubation.The change of precipitation in the future will affect the priming effect of soil organic carbon.The change of the priming effect of soil organic carbon with soil moisture was mainly related to the gram positive bacteria and fungi,but also affected by the demand and supply of nitrogen of soil microorganism.4)Inorganic nitrogen(NH4NO3)addition only promoted the mineralization of soil organic carbon of soil without exogenous carbon amended.The cumulative mineralization of LN(50 mg·kg-1·a-1)addition soil was 30%higher than that of the control(CT).The cumulative mineralization of HN(100 mg·kg-1·a-1)addition soil was 16%higher than that of the control(CT).Therefore,with the trend of increasing nitrogen deposition in the future,the soil carbon mineralization will be accelerated without vegetation coverage in urban soil.Nitrogen addition does not significantly affect organic carbon mineralization of the soil with exogenous carbon addition.Nitrogen addition inhibitates the priming effect of SOC and low nitrogen addition was more obvious.The cumulative effects of LN and HN was 9%and 5%lower than that of the control at the end of incubation.It can be concluded that the increase of nitrogen deposition in the future will inhibitate the priming effect of exogenous carbon input on the decomposition of soil organic carbon.5)the quality and quantity of exogenous carbon is an important factor affecting the accumulation of soil organic carbon,and the effect of the quality of organic matter input on the mineralization of soil organic carbon is significantly different.In our study,adding leaves of Manila grass has a significantly greater effect on soil organic carbon mineralization and the priming effects than those of adding roots of Manila grass,indicating that the root was more conducive to soil organic carbon sequestration.R-microorganism has a significantly effect on the soil organic carbon mineralization of soil with the addition of root,while K-microorganism affects the soil with leaf addition.Urban lawn is characterized by large amounts of carbon(C)storage and faster carbon sesquestration rate.The percentage of carbon derived from turfgrass turned into protective micro-aggegate increases with the age of lawn,which indicated that carbon in the older lawn has a higher stability.Environmental factors,such as temperature,moisture and nitrogen deposition on soil SOC can regulate carbon sequestration,which is also affected by carbon input.Compared to leaves,roots of turfgrass has higher carbon sequestration.