Variation of PM_2.5 concentration and its influencing factors in protective forests under different patterns

【Objective】 To explore the variations in（PM_2.5）concentrations in different protective forest patterns and their influencing factors，which could provide theoretical basis for optimizing plant arrangements in protective forests and for the development of livable urban environments. 【Method】 This study focused on 6 plant configuration patterns within protective forests，including mixed forests of Zelkova serrata and Prunus persica，pure stands of Taxodium distichum， mixed forests of Taxodium distichum and Photinia serratifolia，mixed forests of Ligustrum lucidum and Taxodium distichum，pure stands of Ligustrum lucidum，and mixed forests of Zelkova serrata and Prunus serrulata，all with an approximate stand age of 10 years. An open space served as the control（CK）. The 6 environmental factors（air temperature，air humidity，atmospheric pressure，wind direction，PM_2.5 concentration and light intensity）and 9 soil physicochemical properties were measured in both the protective forests and the control site. Correlations between climatic and soil factors across different protective forest patterns were analyzed. A random forest model was employed to assess the importance scores of different planting patterns. 【Result】 PM_2.5 concentrations under pure stands of Taxodium distichum，mixed forests of Taxodium distichum and Photinia serratifolia and mixed forests of Ligustrum lucidum and Taxodium distichum were lower than those in mixed forests of Zelkova serrata and Prunus persica，pure stands of Ligustrum lucidum，mixed forests of Zelkova serrata and Prunus serrulata，as well as the CK site. Soil pH at all 7 monitoring points was slightly alkaline. Organic matter content in mixed forests of Ligustrum lucidum and Taxodium distichum and pure stands of Ligustrum lucidum was significantly higher than in other protective forest patterns and the CK（P<0.05）. The highest available phosphorus content（14.53 mg/kg）was found in pure stands of Taxodium distichum. Correlation analysis indicated that PM_2.5 concentration was extremely significantly positively correlated with air humidity，soil microbial carbon content and soil microbial nitrogen content（P<0.01，the same below），and extremely significantly negatively correlated with air temperature. There was no significant correlation with ammonium nitrogen（P>0.05）. Additionally，the random forest model revealed that air temperature，air humidity，atmospheric pressure，wind direction，light intensity，soil microbial carbon， soil microbial nitrogen and moisture content had high importance scores in influencing PM_2.5 concentration. 【Conclusion】 There are differences in wind direction and PM_2.5 concentrations under various protective forest patterns. Air temperature， air humidity，soil microbial carbon，and soil microbial nitrogen content are important factors affecting atmospheric PM_2.5 concentration variations. The inclusion of Taxodium distichum in protective forest configurations can effectively reduce atmospheric PM_2.5 concentrations.