Variation Of Glomalin-related Soil Protein Characteristics In Urban Forest In China

Rapid urbanization leads to the limited distribution space of green vegetation and the deterioration of soil environment,including many problems such as high soil compaction,lack of soil organic matter,low nutrients,and declining water holding capacity.Most reports suggested that glomalin-related soil protein?GRSP?is closely associated with metabolism of arbuscular mycorrhizal fungi in soils.GRSP accumulation can improve soil structure and quality effectively.According to the Bradford method,GRSP can be divided into easily extractable GRSP?EEG?and total GRSP?TG?.Most previous studies concentrated on GRSP in the natural ecosystem,however,there are few GRSP-related studies in the process of urbanization ecosystems and its correlation with urban forest characteristics.This knowledge gap will limit scientific improvement of degraded urban soil quality from the GRSP view.This study selected Harbin and Changchun city as subjects to explore GRSP changes in different urbanization gradients?ring road development gradient,urban settlement time gradient and impervious surface area gradient?and possible reasons of forest characteristics,soil physicochemical properties,and urbanization.The differences of GRSP amounts and compositions in 7 cities?Changchun,Wulanhaote,Chifeng,Shenzhen,Lanzhou,Luoyang,and Xuancheng?with different latitudes on a large scale were studied,and the possible reasons of climate,soil,and urbanization for the differences were also analyzed.Finally,we carried out a control experiment of urban forest management,and preliminarily explored the possibility of fertilization management for GRSP improvement and its association with soil organic carbon?SOC?,nitrogen?N?,and phosphorus?P?.The major conclusions are as follows:?1?Harbin:urbanization led to GRSP/SOC sharply decreased in Harbin city center.Moreover,urbanization induced significant increases in arbor richness,shrub richness,conifer proportion,soil pH,and building proportion,and a significant decrease in greenspace proportion.Change in soil physicochemical properties?pH,electric conductivity[EC],bulk density,and SOC?is one of the major reasons for the GRSP changes,followed by forest characteristics?tree size,tree density,richness,etc.?and urbanization factors?area proportions of road,building,greenspace,and water?.The future management in Harbin includes reducing snow melting salt in winter,loosing soil regularly,and decreasing conifer tree proportion to improve GRSP accumulation.?2?Results of Changchun is similar with Harbin,urbanization reduced the contribution of GRSP to SOC pool in Changchun city center.Redundancy analysis found that soil physicochemical properties,forest characteristics,and urbanization factors were closely related to GRSP changes in the process of urbanization.Soil physicochemical properties explained more in the GRSP changes,followed by forest characteristics and urbanization factors.In the future,from the perspective of improving GRSP accumulation,and put forward some suggestions for degraded soil quality improvement in Changchun:reducing snow melting salt in winter,decreasing shrub richness,and increasing greenspace area.Regarding changes of GRSP compositions in Changchun,we found that various changes in the ring road development gradients in different directions.Changes of similar GRSP characteristics?functional groups,diffraction peak,and fluorescence?showed a trend of directional consistency in the ring road development gradient.Through statistical analysis,the relative crystallinity of GRSP was positively correlated with tree growth parameters?diameter at breast height,tree height,crown size,and under-branch height?,Salicaceae proportion,EC,and SOC.Grain size of GRSP was positively correlated with broadleaf tree proportion,building area proportion,soil temperature,and pH,but negatively correlated with greenspace area proportion and Pinaceae?coniferous tree?proportion.Tryptophan-like and soluble microbial byproduct-like of GRSP were positively correlated with greenspace area proportion and Pinaceae?coniferous tree?proportion.Functional groups?IV is a mixture of symmetric COO–stretching of carboxylic acid salts and C–H bending of–CH₂–and–CH₃ groups;VII is O–H bending of structural OH in clay minerals and oxides?of GRSP were positively correlated with the Rosaceae proportion.?3?The difference of GRSP content between Harbin and Changchun?largest/lowest:1.2–1.7 fold?was larger than the difference among different forest types?largest/lowest:1.1–1.3 fold?.Moreover,GRSP/SOC in Harbin was 1.7-fold higher than the value in Changchun,while the difference among different forest types was slightly lower than the difference between Harbin and Changchun.Differently,the differences of soil physicochemical properties,urbanization factors,and part of forest characteristics?tree density,arbor richness,shrub richness,and under-branch height?among different forest types were usually larger than the differences between Harbin and Changchun.Soil physicochemical properties played key roles in the GRSP differences?40.3%?,followed by urbanization factors?33%?,and forest characteristics explained by 14.5%.The structural equation model showed that geographical location difference between Changchun and Harbin directly or indirectly affected the changes of GRSP content and GRSP/SOC via arbor richness,shrub richness,building area proportion,and greenspace area proportion.The changes of forest types indirectly affected the changes of TG content,EEG/TG,and EEG/SOC via soil pH,EC,building area proportion,and greenspace area proportion.?4?Differences of GRSP amounts and compositions among more cities were observed.Seven cities of different latitudes were selected in this study,including Shenzhen in the South,Wulanhaote in the North,Lanzhou in the West,and Changchun in the East.The EEG content showed that Lanzhou reached the maximum value?1.44g/kg?,significantly higher than that in Shenzhen?0.11 g/kg?and Xuancheng?0.58 g/kg??p<0.05?.The TG content also showed that Lanzhou reached a maximum?11.87 g/kg?,significantly higher than that in Shenzhen and Luoyang?p<0.05?.The EEG/TG showed that Shenzhen and Luoyang were 3.32-fold higher than that in Changchun?p<0.05?.But EEG/SOC showed no significant difference between different cities?p>0.05?.TG/SOC showed that Xuancheng reached the maximum value?0.70?,and it was significantly higher than that of Luoyang?0.31??p<0.05?.Significant differences were observed in GRSP characteristics,including functional groups?IV,V,VI,and VII?,fluorescent substances,and heavy metal?Cr,Cu,Zn,Cd,and Pb?contents among different cities.Fluorescent substances of tyrosine-like,tryptophan-like,fluvic acid-like,soluble microbial byproduct-like,and humic acid-like and heavy metals in Shenzhen were higher than those in other cities.The main reasons for the differences of GRSP amounts in different cities were the results of the interaction between soil physicochemical properties,natural conditions,and urbanization factors.The differences of GRSP compositions in different cities were affected by SOC,annual average temperature,annual average precipitation,built-up area,and gross domestic product.?5?Intensive management of urban forests can affect GRSP accumulation.Fertilization treatments in Poplar plantation showed that organic carbon fertilizer could significantly increase GRSP accumulation?p<0.05?;Bacterial fertilizer could significantly increase TG accumulation?p<0.05?;N and P fertilizer did not significantly increase GRSP accumulation?p>0.05?.Linear correlation and structural equation model analysis showed that significant positive correlations between GRSP content and SOC and P contents after fertilization when comparing with CK.To sum up,urbanization can significantly reduce the contribution of GRSP to SOC pool.There are significant differences of GRSP characteristics among different cities,mainly due to the influence of soil physicochemical properties.Urban forest management and impacts of urbanization on GRSP characteristics are also affected by soil physicochemical properties.