Soil Organic Matter Stability And Its Interactions With Soil Physicochemical And Microbial Properties In Urban And Rural Forests Of Shanghai

With the development of urbanization,urban forest soil carbon pool plays an increasingly significant role in global carbon cycle.In the context of global climate change,to explore the possible impact mechanisms of urban forest soil organic matter(SOM)stability will provide scientific and effective guidance for urban forest management,contribute to the sustainable development of urban green space ecosystem.In this paper,in each of four study areas along the urban-rural gradient(ZX,JJ,YF,YQ)of Shanghai,three typical stand types(Cinnamomum camphora forest(CCF),Metasequaia glyptostroboides and Taxodium ascendens forest(MTF),mixed broadleaf forest(MBF))were selected.The soil physicochemical properties,soil enzyme activities,soil microbial community structure and chemical composition of SOM were determined and analyzed by combining field investigation with laboratory experiments.The possible influencing mechanisms of SOM stability of typical stand types in Shanghai were discussed.The main results are as follows:(1)Among the study areas,SOC,TN,C/N ratio,NH₄⁺-N,NO₃^--N,mineral nitrogen content of the three stand types were all significantly higher in the central urban area compared with the suburbs.In the central urban area,CCF stands had higher levels of SOC,TN content,C/N ratio than MTF ones.There were no significant differences among the stand types in the suburbs.The results showed that the SOM content of each forest in the central urban area was high,and the soil quality of CCF in the central urban area was better,which might provide better ecological function.(2)In camphor forests,contents of total PLFAs and major microbial groups were significantly higher in the central urban area than those in other study areas;while in the other two stands,the differences between the study areas were not significant.The microbial content showed significant difference among stand types only in the central urban area,being CCF>MBF>MTF.Excluding YF,urease and invertase activities were basically higher in the central urban area compared with the suburbs,while phosphatase activity showed a generally opposite trend.(3)Lignin and non-lignin aromatic compounds constituted the smallest and the largest proportions of SOM,respectively.Moreover,the relative content of lignin in camphor forests in central urban area was significantly higher than that in suburbs;the relative content of phenolic substances was higher in the central urban area than in suburbs;the relative contents of polysaccharide compounds in ZX and YF were significantly lower than that in other two regions.At the stand level,both ZX and YF areas showed that CCF stands had the highest relative abundances of lipid,followed by those in MBF stands and then those in MTF stands.Indicating the degree of SOM decomposition,the Lip/PS ratio was highest in YF area,followed by ZX,YQ and JJ areas,respectively.For comparison among stand types,the ratio was highest in CCF stands,followed by MBF and MTF stands,respectively,in two suburban areas.The results indicated a higher degree of soil decomposition in mixed broad-leaved forests of YF area.(4)The Lip/PS ratio was positively correlated with p H and the F/B ratio,negatively correlated with phosphatase.The results of RDA analysis showed that p H had the greatest effect on SOM composition.In conclusion,the stability of SOM along the urban-rural gradient in Shanghai is affected by complex soil intrinsic factors,and different stand types also provide external inputs of different quality for the soil.