| As the most direct carbon sink in cities,urban forest ecosystems play an integral role in reducing urban carbon emissions,mitigating the heat island effect and maintaining the global carbon balance.With the acceleration of urbanization,the social benefit attributes of urban forests have increased,and human interference has increased,and the forest gaps area has increased.As the main place for human flow activities in urban forests,forest gaps have large soil disturbance,which in turn affects soil respiration and microorganisms in urban forests.This study was carried out in urban forests in Changsha City.Through in situ fixed-point observation tests of forest gaps and soil respiration within forests,and studies on soil physicochemical properties and microbial community characteristics,we aimed to investigate the effects of forest gaps on soil respiration and microorganisms in urban forests in Changsha,and to reveal the interrelationships between them and environmental factors,so as to provide a scientific basis for the accurate assessment of urban forest carbon income and expenditure and the management of soil disturbance in urban forests.The main findings are as follows:(1)The average annual air temperature in the forest gaps was 19.61℃,0.04℃ higher than in the forest,and the relative air humidity was 78.73%,2.01%lower than in the forest.The soil temperature in the forest gaps was 20.01℃,0.68℃ higher than that in the forest,and the soil humidity was 19.37%,3.06%higher than that in the forest.The seasonal pattern of microclimate factors in the forest gaps was basically the same as that in the forest.The air temperature in the forest gaps was higher than that in the forest in spring and summer,and lower than that in the forest in autumn and winter,and the extreme range of air temperature in the forest gaps was larger than that in the forest;the relative humidity in the forest gaps was lower than that in the forest throughout the year,and the extreme range of relative humidity in the forest gaps was smaller than that in the forest.However,the effect of forest gaps on air and soil temperature and humidity in urban forests in Changsha did not reach a significant level(p>0.05).(2)Soil bulk density(g·cm-2),TN(0.11 g·kg-1),SOC(10.16 g·kg-1),EOOC(1.64 g·kg-1)and MBC(0.40 g·kg-1)were significantly lower(p<0.05)in the forest gaps of urban forest in Changsha compared to the forest interior;Soil porosity(42.31%),pH(6.51),DOC(0.31 g·kg-1)and cellobiose hydrolase(139.81 U·g-1)were significantly(p<0.05)higher in the forest gaps compared to the forest interior.There was no significant effect of forest gaps on soil water content,total phosphorus(TP),polyphenol oxidase(PPO),peroxidase(POD)and β-glucosidase(β-GC)content(p>0.05).(3)The daily mean values of soil respiration in the forest gaps were generally lower than those in the forest.The daily dynamics of soil respiration in the forest gaps showed a single-peaked curve,with greater variation during the spring and summer festivals,with soil respiration rates higher than in the forest at 11:00 and 14:00,and peaking at 14:00 with 2.54 μmol·m-2·s-2 and 5.13 μmol·m-2·s-2 respectively;the variation was smaller during the autumn and winter festivals,with the peak occurring at 11:00 in autumn(1.18 μmol·m-2·s-2)and the winter peak occurred at 14:00(0.46 μmol·m-2·s-2).There was a highly significant effect of the forest gaps on the daily dynamics of soil respiration during summer,autumn and winter(p<0.01).In addition,the annual mean soil respiration was 1.71 μmol·m-2·s-2 for the forest gaps and 2.62 μmol·m-2·s-2 for the forest interior,with the forest gaps significantly reducing total soil respiration(p<0.05),most in summer and least in winter.The variation in total soil respiration in the forest gaps varied significantly from season to season and month to month,with a single peak in total soil respiration throughout the year,the maximum value occurring in July at 4.95 μmol·m-2·s-2 and the minimum value occurring in January at 0.18 μmol·m-2·s-2,with the same trend as in the forest.(4)The relative abundance of Proteobacteria,Acidobacteri,Planctomycetes and WPS-2 significantly increased and the relative abundance of Gemmatimonadetes and Rokubacteria significantly decreased in the forest gaps soil bacterial community(p<0.05);the relative abundance of Ascomycota and Basidiomycota in the fungal community increased significantly(p<0.05).The diversity of soil bacterial communities increased significantly in the forest gaps,while the diversity of fungal communities decreased significantly(p<0.05).It is evident that the forest gaps had a significant effect on the composition and diversity of both soil bacterial and fungal communities.(5)There was a highly significant exponential regression relationship(p<0.01)between soil respiration and soil temperature in both the forest gaps and within the forest,and a highly significant quadratic polynomial regression relationship(p<0.01)between soil respiration and soil moisture in the forest only.Among the soil physicochemical properties,soil β-GC had the greatest influence on soil respiration with a contribution of 59.2%,followed by soil bulk density(15.1%)and soil DOC(8.2%).Among the soil bacterial communities,Actinobacteria had the greatest influence on soil respiration with a contribution of 78.1%,and among the fungal communities,Glomeromycota had the greatest influence on soil respiration with a contribution of 52.3%.The alpha diversity of soil bacterial microorganisms in the forest gaps and within the forest was significantly negatively correlated with SOC and TN,and positively correlated with soil cellobiose hydrolase(C1).There was a significant positive correlation between alpha diversity of soil fungal microorganisms SOC,TN in the forest gaps and within the forest. |
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