Effects Of Nitrogen Deposition On Soil Active Carbon And Nitrogen In Urban-Rural Forests

Soil active organic carbon(C)and nitrogen(N),as the important indicators of soil fertility,are considered to be the key components of forest C and N nutrient cycle,which have great potential in regulating biogeochemical processes and soil microenvironment.The process of urbanization deeply affects the forest ecosystem in or near the city,which changes the regeneration ability and soil quality of the forest ecosystem,especially for the soil active C and N with high instability.Moreover,the phenomenon of high N deposition caused by urban load is very serious for environmental interference.At present,it is not clear the net effects of N deposition on the reserves of soil activated C and N in urban forests.The environmental gradient method from the urban center to the rural has become a perfect research paradigm in the field of the ecology.However,due to the lack of simulated N deposition manipulation experiments on the soil C and N characteristics of urban-rural gradient forests,it is particularly urgent to study the soil activated C and N along the urban-rural environmental gradient.In order to investigate the effects of N deposition on forest soil activated C and N under urban-rural environmental gradient,a typical urban-rural gradient forest was established in Hefei City(31°52'N,117°17'E)along the direction of linear radiation from urban center.Field experiments were conducted with different N deposition levels(0,50 and 100 kg N·ha^-1·yr^-1).At the same time,soil temperature and soil moisture content,soil physical and chemical properties and microbial community characteristics were measured under N addition to explore how these key soil environmental factors could control the response mechanism of the soil activated C and N to N deposition in urban-rural forests.The results are as follows:(1)The dissolved organic carbon(DOC)and dissolved organic nitrogen(DON)in urban-rural forest soil generally showed a spatial distribution pattern of the urban hot spots.The soil DOC and DON in urban-rural forests showed significant seasonal distribution characteristics.In urban and suburban forests,the contents of the DOC and DON in summer were significantly higher than those in winter and spring.In the rural forest,the contents of the soil DOC and DON were higher in winter than in summer.In vertical distribution,the contents of soil DOC and DON in urban-rural forests decreased gradually with the deepening of soil layer.N deposition inhibited the contents of soil DOC and DON in the urban forest and suburban forest,but significantly promoted the contents of soil DOC and DON in the rural forest.(2)In terms of spatial distribution,the soil microbial biomass carbon(MBC)of the urban forest was the highest,while that of the suburban forest was the lowest.The contents of soil microbial biomass nitrogen(MBN)in the rural forest were higher than that in urban and suburban forests.In terms of seasonal dynamics,the soil MBC and MBN were higher in summer than in winter.In terms of vertical distribution pattern,the soil MBC and MBN contents in urban-rural forests also decreased with the increase of the soil depth.We observed that the soil MBC/MBN decreased gradually along the urban-rural gradient.Under N deposition,soil MBC and MBN contents in the urban forest and rural forest showed increasing trends,while in the suburban forest,soil MBC and MBN contents generally showed decreasing trends.(3)The soil temperature in urban-rural forests showed a decreasing trend with the increase of urban distance.Soil temperature and water content had strong correlation with soil active C and N under the simulated N deposition,and they were important factors affecting soil active C and N in urban-rural forests.(4)Different locations,months and soil layers had significant effects on the soil physical and chemical properties.Simulated N deposition generally increased the contents of TDN,TN,TC,NH4⁺-N and NO3^–-N in urban-rural forests,but decreased the contents of NO3^–-N in the suburban forest.Under the N deposition,most of the soil physical and chemical properties were significantly correlated with soil active C and nitrogen,and the dynamic changes of these physical and chemical properties in locations,months,and soil layers were consistent with soil active C and N.(5)Except for the Simpson index(D)and the Mc Intosh index(U)in the urban forest and the C source metabolism types of polyphenols and polyamines in the suburban forest,the effects of the simulated N deposition on AWCD values,Shannon index(H),Simpson index,Mc Intosh index and C source utilization type index of other soil microbial communities in urban-rural forests were not significant.In general,the effects of different locations and months on the soil microbial community indexs in the urban forests were significantly stronger than that of N addition treatment on soil microbial community indexs.The correlation between soil microbial community indexs and soil MBC and MBN were better than that with soil DOC and DON.