Characteristics Of Soil Carbon Change During Forest Succession In Daxing'an Mountains

The forest ecosystem has a great influence on the change of forest soil carbon with the change of stand age and forest structure in succession process.Due to the complexity of tree species characteristics and influencing factors in forest ecosystems at different succession stages and the differences of each influencing factor's dominant positions,the monthly dynamic fluctuation of various morphology soil carbon also show heterogeneity.There is the only cold temperature coniferous forest in the Daxing'an Mountains,and it is also the main distribution zone of primeval forest in our country.It has a great significance for discovering the variation characteristics of soil carbon in forest succession process in the Daxing'an Mountains and the effect of forest vegetation on soil carbon pool that to do a quantitative study about content characteristic and their influence factor of each components of soil carbon in forest ecosystem succession process.This study adopts the method of space instead of time,at Mohe forest ecosystem positioning observation station,Heilongjiang province,we selected some typical forest type,the Be tula platyphylla forest(early succession),B.platyphylla-Larix gmelinii mixed forest(middle succession)and L gmelinii forest(later succession),to our experiment objects to reveal the soil carbon changes with the change of forest.The research results show that:(1)The content of total organic carbon(TOC),readily oxidized organic carbon(ROOC),particulate organic carbon(POC)and light fraction organic carbon(LFOC)and heavy fraction organic carbon(HFOC)in each soil layer showed a significant downward trend from May to October in the early and middle stages of succession.The ranges of variation were 9.63 g kg-1?58.72 g·kg-1,1.63 g·kg-1?17.69 g·kg-1 0.22 g·kg-1?16.00 g·kg-1,2.09 g kg-1?27.06 g·kg-1 and 6.30 g·kg-1-43.94 g·kg-1,respectively.In the later stage of succession,the declining trend is mainly manifested in the soil layer of 0?5 cm,but the decreasing range is small.The maximum content decrease was 8.43 g kg-1,and the differences were small in the lower layer.At the same time,the content of TOC in surface layer and LFOC in surface and middle layer soil showed an upward trend,and the contents increases are 15.35 g kg-1,15.67 g kg-1 and 1.64 g kg-1,respectively.It indicates that soil had an accumulation effect on these carbons with the progression of succession.(2)With the development of the succession process,the soil microbial biomass carbon(MBC)content in the soil layer of 0?5 cm and 5?10 cm increased gradually,it reached the peak in the mixed forest,and it has different degrees of decrease in L gmelinii forest,and the soil layer of 10?20 cm has the lest fluctuation.The monthly dynamics of soil MBC content in different soil layers of different years old B.platyphylla forest decrease at different levels.By the increase of forest age,the fluctuation ranges of MBC were 18.65 mg·kg-1?92.98 mg·kg-1,45.86 mg·kg-1?183.61 mg·kg-1 and 107.95 mg·kg-1-282.97 mg·kg-1,respectively.In the middle succession stage,the soil MBC content in the range of 0-5 cm and 5?10 cm showed fluctuated increasing trend,while the subsoil showed a decreasing trend.In the late succession,all soil layers showed a fluctuating rising trend,with a fluctuating range of 32.63 mg·kg-1?354.80 mg·kg-1.It indicates that MBC was accumulation with the advance succession.(3)With the advance of succession,except for B.platyphylla young forest,the differences of soil dissolvable organic carbon(DOC)content in different soil layers is small.The particulate organic carbon(POC)in the 0-5 cm soil layer increased gradually with the advance of succession,and the peak is shown in L gmelinii forest(12.19 g·kg-1).And it showed the highest value in B.platyphylla old forest in other soil layers.The highest content of TOC,ROOC,MBC,LFOC and HFOC were showed in the middle stage of succession stage which is the B.platyphylla-L.gmelinii mixed forest,their peaks were 58.79 g·kg-1,11.36 g·kg-1,483.62 g·kg-1,24.58 g·kg-1 and 34.21 g·kg-1,respectively.(4)With the advance of succession,the allocation proportion of DOC and ROOC increases with the deepened of soil layers;the allocation proportion of POC and LFOC decreased with the deepened of soil layers.The minimum values of MBC allocation proportion were found in the subsoil.The allocation proportion of EOOC has little change with the deepened of soil layers.In the advance of succession,the allocation proportions of EOOC,DOC and LFOC decreased slightly,but the allocation proportions of POC,MBC and ROOC had increased trends.(5)In the correlation analysis of soil carbon,except for DOC,other carbon showed extremely significant positive correlation(P<0.01).Nearly all of other carbon had significant positive correlation with soil total nitrogen content,carbon nitrogen ratio and water content(P<0.05),and significant negative correlation with soil pH.Therefore,the increase of soil total nitrogen content,carbon nitrogen ratio and water content are beneficial to the accumulation of other 6 kinds of soil carbon,while the increase of pH inhibits their accumulation.