Study On The Effect Of Rice Husk Biochar With Different Pyrolysis Temperatures On Greenhouse Gas Emissions From Green Roof Substrate

With the continuous acceleration of industrialization and urbanization,the degradation of urban ecological environment has led to the decline of the quality of human living environment and become one of the important factors restricting the sustainable development of cities.Urban green space system plays a very important role in improving the quality of urban human living environment.Green roof,as a new type of urban greening,has the functions of regulating rainwater runoff,reducing urban heat island effect and air pollution,and beautifying the environment.However,the special habitat conditions such as high wind speed,high temperature,drought,thin substrate layer and nutrient scarcity have restricted the application of green roofs.Biochar has been widely used as a soil amendment in agriculture and forestry production and ecological restoration,and has attracted much attention as a carbon sequestration technology with low application risk.Pyrolysis temperature is one of the key factors affecting the properties of biochar and thus its effectiveness in soil improvement and carbon sequestration and reduction.In this paper,we studied the physical and chemical properties and enzymatic activity characteristics of the substrate,greenhouse gas emissions and functional gene abundance of microorganisms after applying different pyrolysis temperatures（300℃,400℃,500℃）and different amounts（10%,20%,V/V）of biochar in the substrate,and explored the effect of biochar addition on the improvement of the substrate soil and greenhouse gas emissions.The optimal biochar pyrolysis temperature and addition amount were obtained to provide a reference basis for improving the quality of green roof substrate and giving full play to the ecological functions of green roofs in terms of energy saving,emission reduction,carbon sequestration and sink increase.The main results of the study are as follows:1.Effect of biochar on the characteristics of green roof substrate.The substrate bulk density decreased by 12.32%～24.11%after biochar addition;p H increased by 0.98～1.60 units.The organic matter content of the substrate increased by 1.64～3.71 times;the total nitrogen content increased by 5.26%～52.63%.Biochar addition enhanced NH₄⁺-N content in the substrate,but NO₃^--N content decreased by 74.02%～83.62%.The phosphorus and potassium contents in the substrate also increased to different degrees.The substrate cation exchange increased by 0.3～3.14cmol·kg^-1after biochar addition;the substrate microbiomass carbon increased by19.22%～57.34%and microbiomass nitrogen increased by 25.41～72.80 mg·kg^-1;its potential nitrification rates was higher than that of the control（CK）by 17.92%～155.59%.2.Effect of biochar on the enzyme activity of green roof substrate.Biochar addition increased the substrate urease and invertase activities compared to CK,which indicated that biochar application accelerated the rate of nutrient element cycling in the substrate.The substrate urease activity was increased by 11.65%～42.61%after biochar addition,and the most significant effect of biochar addition on urease activity was observed in the treatment with pyrolysis temperature of 500°C and 20%application rate.The biochar addition increased the invertase activity in the range of 6.12%～76.44%,and the substrate invertase activity was significantly increased by biochar pyrolysis temperature of 400°C and 500°C with higher application rate.3.Effect of biochar on greenhouse gas emissions from green roof substrates and related microbial functional gene abundance.The trends of CO₂emission fluxes from the substrate soils of the seven treatments were basically the same,and all of them showed emission peaks in summer when the temperature was higher;the CO₂emission rates were higher in the substrate soils without biochar addition compared to those with biochar addition.The CO₂emissions of the substrate were significantly reduced by the addition of biochar,ranging from 19.68%to 43.71%;CO₂emissions tended to decrease with the increase of biochar addition,and tended to increase with the increase of biochar pyrolysis temperature.Different pyrolysis temperatures and addition amounts had significant effects on substrate CO₂emissions.The trends of CH₄emission dynamics of substrate soils were similar among treatments.The cumulative matrix CH₄emissions were reduced by 2.40%～79.92%after biochar addition.Among them,biochar prepared at 300℃pyrolysis temperature had no significant effect on CH₄emissions,and biochar prepared at 500℃had a more obvious effect on matrix CH₄emission reduction than that prepared at400℃pyrolysis temperature,and the larger the addition amount the more obvious the CH₄emission reduction effect.Biochar addition reduced substrate soil CH₄emissions,probably caused by affecting the abundance of functional genes of substrate methanogenic bacteria（mcr A）and methane oxidizing bacteria（pmo A）.Biochar prepared at 300℃pyrolysis temperature had no significant effect on substrate mcr A and pmo A gene abundance after addition,and rice husk biochar at 400℃and 500℃significantly reduced mcr A gene abundance and increased pmo A gene abundance,and this trend was also more obvious with the increase of addition amount.After the addition of biochar,the strong and weak changes in substrate soil N₂O emissions did not reach a steady state in the short term,and the differences in substrate N₂O emissions did not reached significant levels at different rice husk biochar pyrolysis temperatures and addition amounts.The abundance of key functional genes related to the nitrification and denitrification processes associated with N₂O production was investigated,and it was found that biochar addition increased the abundance of functional genes for nitrifying ammonia-oxidizing bacteria（amo A-AOB）and decreased the abundance of functional genes for denitrification due to the altered substrate oxygen conditions.Among them,the abundance of nitrite reductase functional genes nir S and nir K was increased,which are involved in N₂O production,and the abundance of nitrous oxide reductase functional gene nos Z was decreased,which reduces N₂O.The results of the integrated greenhouse effect obtained from the uniform conversion of CO₂,CH₄and N₂O showed that biochar addition significantly reduced GWP by 18.87%～37.43%.4.Correlation analysis of green roof substrate characteristics and greenhouse gas emissions.The results of redundancy analysis showed that 11 soil factors,including nitrate nitrogen,cation exchange,total nitrogen,p H,available phosphorus,microbial quantity nitrogen,microbial quantity carbon,total potassium,ammonium nitrogen,available potassium,and total phosphorus in the substrate were the main factors affecting greenhouse gas emissions.Green roof substrate NO₃^--N was significantly and positively correlated with CO₂emissions and negatively correlated with soil factors such as p H,TN,TP,TK,and MBN（P<0.05）;substrate CH₄emissions were positively correlated with NO₃^--N and negatively correlated with p H,OM,NH₄⁺-N,AP,TP,TK,AK,CEC,MBC,MBN,urease activity,and sucrase activity were negatively correlated（P<0.05）;N₂O emissions from each treatment were significantly and positively correlated with OM,PCR,urease activity and MBC（P<0.05）.In summary,the application of rice husk biochar as a substrate amendment for green roofs improved the nutrient conditions and reduced greenhouse gas emissions of the substrate.Both biochar prepared at different pyrolysis temperatures and different biochar application amounts had significant effects on substrate improvement and greenhouse gas emission effects.The results of the study showed that the application of biochar prepared at 300℃pyrolysis temperature and the addition amount of 20%had the best effect on the combined greenhouse effect（GWP）reduction of substrate soil.