Characteristics Of Soil Dissolved Organic Carbon And Nitrogen And Soil Respiration In The Different Stands And Their Controls

Soil dissolved organic carbon （DOC） and soil dissolved organic nitrogen（DON）are important parts of soil carbon and nitrogen pools. Because of their activities andimportant role on the soil nutrients, DOC and DON have been the research focus onglobal carbon cycling. In addition, the contents of soil DOC are significantlycorrelated with soil CO₂flux. Soil organic pool releases CO₂to the atmospherethrough soil respiration. Thus soil respiration is the main way in carbon circulation. Inthis study, four kinds of the different stands were selected on a suburban forest park inHefei. The objectives of the study are （1） to analyse the dynamic change of soil DOCand DON, and the relationship between DOC, DON and soil nutrients,（2） todetermine the seasonal change in soil respiration, and （3） to reveal the characteristicsof carbon and nitrogen cycles in the urban forest ecosystem. The main researchfindings were summarized as follows.1. The soil DOC contents decreased with soil depth in the different stands andseasons. The soil DOC contents ranged from52.38to126.96mg·kg^-1in the heavilydamaged Pinus massoniana stand （P1）,45.02to111.29mg·kg^-1in the slightlydamaged Pinus massoniana stand （P2）,48.58to180.50mg·kg^-1in the maturedQuercus acutissima stand （P3） and53.22to173.71mg·kg^-1in the young Quercusacutissima stand （P4）, respectively. The soil DOC contents were higher in Quercusacutissima stands than in Pinus massoniana stands. This could be due to thesignificantly higher amounts of litter in the Quercus acutissima stands with greataccumulation of the soil organic matter.2. The soil DON contents decreased with soil depth in the different stands andseasons, which is consistent with the changes of soil DOC contents. The soil DONcontents ranged from3.68to25.81mg·kg^-1in pine stand P1,6.64to18.40mg·kg^-1inpine stand P2,8.76to23.34mg·kg^-1in oak stand P3, and2.12to18.09mg·kg^-1in oakstand P4, respectively. The soil DON contents were slightly higher in pine stands thanin the oak stands. This may be related to the type of vegetation.3. For the same stands either pine or oak ones, the order of average contents ofsoil DOC ranked as autumn> spring> winter> summer, while for soil DON, it waswinter> autumn> summer> spring. In the different soil layers, the maximum ratio ofDOC to DON was appeared in the pine stands and oak stand P4in spring, and theminimum was in winter. Howver, the maximum ratio was in autumn, and theminimum was in winter for the oak stand P3. 4. The pine wilt disease dsiturbnace had a significant impact on the soil nutrientsin Pinus massoniana stands. The annual average contents of NO₃^-N in soil wassignificantly higher in the seriously damaged stand （P1） than in the slightly damagedone （P2）（p﹤0.05）. However, the annual average contents of K in soil wassignificantly lower in stand P1than in stand P2（p﹤0.05）. The annual averagecontents of TP in soil decreased with soil depth in the different stands, withsignificantly lower TP in the0^-10cm soil in P1than in P2（p﹤0.05）.5. Soil total N contents declined with the soil depth in the oak stands, withsignificantly higher total P and NO₃^-N in P3than in P4（p﹤0.05）. The annual averagecontent of K was no signifcant differences in both stands, with slightly higher in thelower layer than in the upper layer. In the oak stands, the contents of DOC and DONhad a significant correlation with NH4+-N and NO₃^-N in both spring and winter.6. The diurnal variation of soil respiration appeared single-peak curve in thedifferent stands. The daily maximum values of soil respiration appeared from12:00to15:00. Based on the daily average, the soil respiration was the highest in the oak standP3from1.62to5.13μmol·m^-2·s^-1). Others were ranged from0.94to3.84μmol·m^-2·s^-1for pine stand P1,0.62to2.67μmol·m^-2·s^-1in pine stand P2and1.12to2.79μmol·m^-2·s^-1for oak stand P4, respectively. The seasonal change of soilrespiration was significantly different among the stands. The order of the seasonalvariation of soil respiration in the pine stand P2was summer> autumn> winter>spring, while for the other stands, they were in order as autumn> summer> spring>winter.7. The result from the regression analysis showed that soil respiration wassignificantly correlated with surface soil temperature in both oak and pine stands （R²=0.702–0.928）. In addition, the soil respiration was significantly correlated with soilDOC in both pine and oak stands. The significant correlations between soil respirationand DON were only found in the pine stands.