Soil Carbon Storage And Efflux In Urban Greenlands

With the acceleration of urbanization, the rapid urban land expansion changed thetypes of green-land at the same time, and these changes make profound impact on soilphysicochemical properties. The early researches focused on soil fertility worldwide,preliminary exploring urban soil nutrient status and heavy metal pollution. Nowadays,with the exacerbating of global climate change, the urban soil carbon pool is given anew connotation, which is associated with global carbon cycling and was widelyconcerned. A great attention has been paid to the carbon content of urban soil and itstemporal variation, with ignoring systematic studies on the characteristics of soil carbonpool and its carbon fluxes. Therefore, comprehensive study on the carbon pool of urbansoil and its influencing factors have a positively meaning for understanding the changeof global carbon pool.This study conducted in Hefei, the soil carbon pool, soil C:N ratio, soil activeorganic carbon and nitrogen, soil respiration were studied based on the differentgreen-land types （campus green-land, park green-land, factory green-land, roadsidegreen-land, residential green-land and suburb forest park）, the different planting mode（shrub-lawn, arbor-lawn, lawn and arbor-shrub-lawn modes）, and the differentfunctional area （primary industrial estate, primary shopping center, the science, cultureand education center and suburban forest park）. The results are as follows.In0^-30cm soil, the mean pH （H₂O） of the urban soils was8.64, being stronglyalkaline, while in forest park the mean pH （H₂O） was6.48. The mean soil bulk density,EC, NH4+–N, NO₃^-–N and total P concentrations in urban soils were1.40g cm^-3,154.98μS cm^-1,9.65mg kg^-1,6.89mg kg^-1and493.74mg kg^-1, which were higher than thoseof forest park soils (the corresponding content1.25g cm^-3,83.69μS cm^-1,7.48mg kg^-1,6.76mg kg^-1and242.61mg kg^-1). The moisture content of the urban soil is22.70%,which is lower than soils in the forest park （30.35%）.Soil depth, greenland types and planting mode have significant influence on soilorganic carbon content （P<0.05）. The vertical variation of organic carbon contents inurban soil changed more complex than in forest park soil. Generally, it still decreasedwith increasing soil depth. With regards to0^-30cm soil layer, the average soil organiccarbon concentration in the different green-lands was ranked as forest park (17.95g kg^-1)> roadside green-land (9.91g kg^-1)> park green-land (7.00g kg^-1)> campusgreen-land (6.87g kg^-1)> residential green-land (5.70g kg^-1)> factory green-land (5.01 g kg^-1). Under the different planting modes, it ranked as arbor-lawn mode (15.43g kg^-1)> shrub-lawn mode (7.48g kg^-1)> arbor-shrub-lawn modes (7.32g kg^-1)>lawn (5.91g kg^-1).Area have significant influence on soil organic carbon content （P<0.05）, thecontent of soil organic carbon in primary industrial estate, primary shopping center, thescience, culture and education center and suburban forest park were,respectively,6.67,8.58,6.31and17.95g kg^-1, with obviously environmental gradient from the east to thewest of the city. Industry type had no significant influence on soil organic carboncontent （P>0.05）. The content in the four industry types was followed in the order ofthe textile industry (6.47g kg^-1)> manufacturing (5.15g kg^-1)> resident services (4.27g kg^-1)> chemical industry (4.01g kg^-1).Correlation analysis showed that green-land types had significant influence on soilorganic carbon density （SOCD）（P<0.05）, the SOCD in suburban forest park wassignificantly different with these green-lands （P<0.05）, and SOCD in the roadsidegreen-land had significant difference with other green-lands （P<0.05）. The SOCD inurban soils was ranged from0.20to4.27kg m^-2, with1.13kg m^-2in0^-10cm,0.93kg m^-2in10^-20cm and0.87kg m^-2in20^-30cm soil. The SOCD in forest park rangedfrom1.07to3.40kg m^-2, with respective2.40,2.33and2.02kg m^-2in the abovementioned three soil layers.The functional area had significant influence on SOCD （P<0.05）. In the0^-30cmsoil, the SOCD in forest park, primary shopping center, primary industrial estate, thescience, culture and education center were2.22,1.14,0.94and0.90kg m^-2,respectively. Correlation analysis showed that SOCD were significantly and positivelycorrelated with NO₃^-–N, total P, moisture content, and negatively correlated with pH（H₂O） and bulk density.Soil C:N ratio was recognized as indicators for the conversion and accumulation ofsoil nutrients, and the stability of productivity. In the0^-30cm soil, the C:N ratio inurban soils was10.6, which was close to the national mean C: N ratio （10.84） for alltypes of soil. C: N ratio in the different green-lands was ranked as park green-land（14.36）> campus green-land （10.18）> residential green-land （9.38）> factorygreen-land （9.20）. Correlation analysis showed that soil C:N ratio was significantly andpositively correlated with soil organic carbon, NO₃^--N, NH₄⁺N, and total P.Green-land types have significant influence on soil dissolved organic carbon （DOC）and microbial biomass carbon （MBC）. The mean concentrations of DOC and MBC were lower in urban soils than in forest park soils. The vertical variation of DOC andMBC in the urban soils changed more complex, and generally it still decreasing withincreasing soil depth. Soil DOC and MBC concentration showed a significant seasonalvariation and significant impact by stands.The average concentration of DOC were ranked in the order of suburban forestpark (44.28mg kg^-1)> campus green-land （29.84）> park green-land （28.39）>residential green-land （26.82）> roadside green-land （26.72）> factory green-land（22.88）. Under the different planting mode the DOC concentration ranked as arbor-lawnmode (43.13mg kg^-1)> shrub-lawn mode （28.24）> lawn （28.16）> arbor-shrub-lawnmodes （26.05）. The change of land use resulted in the variation of DOC concentration.In the different functional area, DOC was ranked as suburban forest park (44.28mg kg^-1)> the science, culture and education center （32.96）> primary industrial estate（27.36）> primary shopping center （25.27）. Soil DOC concentration was positivelycorrelated with dissolved organic nitrogen （DON）, moisture content, NH₄⁺N whilenegative correlated with EC, soil bulk density, pH （H₂O）, pH （KCl） and total P.The concentrations of MBC were, respectively,489.86mg kg^-1in the forest park,260.51mg kg^-1in park green-land,253.24mg kg^-1in roadside green-land,233.55mg kg^-1in campus green-land,229.90mg kg^-1in residential green-land and174.42mg kg^-1in factory green-land. The MBC concentration were440.71,244.72,241.00and180.15mg kg^-1for arbor-lawn mode, shrub-lawn mode, arbor-shrub-lawn modes andlawn,respectively. Functional area have significant influence on MBC concentration（P<0.05）, with489.86mg kg^-1in forest park,277.66mg kg^-1in primary shoppingcenter,215.14mg kg^-1in the science, culture and education center, and197.58mg kg^-1in primary industrial estate. The correlation analysis showed that MBC wassignificantly and positively correlated with MBN, DON, NO₃^--N, total P, moisturewhile negatively correlated with NH₄⁺N, EC, pH （H₂O）, pH （KCl） and bulk density.The mean values of DOC/SOC in0^-10cm,10^-20cm,20^-30cm soil layer of urbangreen-lands were0.48%、0.59%、0.56%, higher than corresponding layer of suburbanforest park （0.30%、0.34%、0.20%）. The value of DOC/SOC in different green-landswere ranked as residential green-land （0.63%）> campus green-land （0.59%）> parkgreen-land （0.56%）> factory green-land （0.53%）> roadside green-land （0.42%）>forest park green-land （0.28%）. The value of MBC/SOC in0^-10cm,10^-20cm,20^-30cm soil were3.84%,3.49%,3.28%, which higher than the corresponding soil layers ofthe suburban forest park （2.74%，3.11%，2.55%）. In the different green-land type, the value of MBC/SOC ranked as residential green-land （4.03%）> park green-land（3.83%）> factory green-land （3.66%）> campus green-land （3.36%）> roadsidegreen-land （2.98%）> forest park （2.80%）.Months had significant influences on soil respiration （P<0.05）. Soil respiration rateincreased toward June, while decreased from December. The largest values of soilrespiration rates were recorded from June to August of2011（wet season）. Soilrespiration rates were ranged from0.72to2.39μmol m^-2s^-1in the residential,1.73to4.10μmol m^-2s^-1in the factory,1.95to5.59μmol m^-2s^-1in the park and1.85to5.09μmol m^-2s^-1in the campus green-lands.During study period, there was a marked seasonal pattern in temperature at0-5cmsoil depth （P<0.05）. When all potentially rain–affected data were excluded, a significantrelationship between soil respiration and0-5cm soil temperature was observed duringstudy period （P<0.05）. When soil moisture was between18%and25%, statisticallysignificant relationship was found between soil respiration and moisture （P<0.05）. InJune and August2011, soil respiration was significantly and positively correlated withsoil moisture （P<0.01）, while not correlated with soil temperature. The results fromcorrelation analysis showed that in0^-10cm soil, soil respiration was significantly andpositively correlated with SOC, NO₃–N, P and fine root density （R<2mm）, whilenegatively correlated with DOC. In10^-20cm soil, soil respiration was significantly andpositively correlated with N and P, while negatively correlated with DOC. In20^-30cmsoil, soil respiration was only positively correlated with fine root density.