Spatial Distribution Characteristics And Impact Mechanism Of Forest Soil Organic Carbon Pool In Daxing'anling Forest Region

Forest is an important part of terrestrial ecosystem,an important source and sink of atmospheric greenhouse gases,and plays a key role in regional and global carbon cycles.Forest soil carbon pool is the largest carbon pool in terrestrial ecosystem.Forest soil carbon cycle,especially its source/sink function transformation,has become an important direction of global change research.It is of great significance to estimate regional carbon budget and formulate forest management measures and policies to deal with climate change.In order to accurately predict the variation trend and the potential of soil organic carbon pool in cold temperate zone under the background of global climate change,taking the Daxing'anling forest region as the research area and the Larix gmelinii forest as the main research object,the method of field sample plot investigation and indoor experimental analysis was used to analyze the organic carbon pool.The spatial distribution patterns of soil total organic carbon(TOC),organic carbon fractions and phytolith carbon(phyt OC)were discussed based on the protection mechanism of organic carbon fractions,and their relationships with soil physicochemical properties were analyzed.This paper systematically discussed the characteristics and influence mechanism of organic carbon in different environmental conditions,deeply revealed the stability and accumulation mechanism of different fractions of organic carbon,and evaluates the carbon sink potential of soil in Daxing'anling forest region.It can provide basic data for carbon cycle research of forest ecosystem and provide method reference for accurate and quantitative assessment of soil carbon pool in Daxing'anling forest region and cold temperate zone.The main results were as follows:(1)The chemical indexes of topsoil(0-20cm)in Daxing'anling forest region showed certain spatial characteristics.The p H value was relatively stable,and the spatial variations of NH₄⁺-N,AK and AP were greatly affected by structural factors such as climate,vegetation and soil;while TP and OP were affected by structural factors and random factors such as human activities.In general,the contents of all chemical indexes showed a certain east-west symmetrical distribution along the northeast-southwest trend.The TP and OP first increased and then decreased with the latitude decreasing,and other indexes showed patch distribution patterns.The soil physicochemical indexes of Larix gmelinii forest showed certain profile characteristics.With the increase of soil depth,the contents of NH₄⁺-N,AK,OP and other nutrient indexes and soil water content(SWC)decreased,while p H,bulk density(BD)and metal oxides contents(except for CaO)increased,and the surface content was significantly different from other soil layers.The soil physicochemical properties of Larix gmelinii forest with different ages and forest types were different.With the increase of stand age,SWC,BD and AK showed a"V"shape change trend,while OP showed an inverted"V"shape trend;p H increased and NH₄⁺-N,AP decreased with forest age increasing.(2)The mean TOC content of topsoil(0-20cm)in Daxing'anling forest region was48.60 g/kg with the high values distributed in the middle part of forest area.The mean content of TOC in Larix gmelinii forest was 53.35 g/kg,which decreased in a power function with the soil depth increasing.The surface aggregation characteristics were obvious and the average contribution rate of TOC in surface layer(0-10cm)was 71.83%.TOC content was significantly positively correlated with SWC,NH₄⁺-N,AK,TP and OP,and negatively correlated with BD,p H and metal oxides contents(except for CaO).During the growth process of Larix gmelinii forest,TOC showed a"V"shape change characteristics,and its content at different growing stages followed the order:young forest(64.49 g/kg)>mature and over-mature forest(55.34 g/kg)>middle aged forest(53.22 g/kg)>near-mature forest(46.81 g/kg).The effects of soil physicochemical indexes on TOC were different in different forest ages.The TOC content of Larix gmelinii forest with different forest types followed the order:Ledum palustre-Larix gmelinii forest(64.14 g/kg)>Rhododendron simsii-Larix gmelinii forest(63.48 g/kg)>grass-Larix gmelinii forest(39.99 g/kg).In the 0-10 cm soil layer,TOC content of the grass-Larix gmelinii forest was significantly smaller than that of Ledum palustre-Larix gmelinii and Rhododendron simsii-Larix gmelinii forest;and with the depth increasing,the differences among different forest types decreased.The dominant factors of TOC were different with different forest types.(3)The contents of organic carbon fractions in different protection mechanisms of Daxing'anling followed the order:uPOM(21.51 g/kg)>CPOM(10.00 g/kg)>pPOM(2.9g/kg)>pcPOM(2.72 g/kg).Four fractions showed high spatial autocorrelation and changed along the northeast-southwest direction along the Daxing'anling.The content of organic carbon fractions in different protection mechanisms of Larix gmelinii forest followed the order:uPOM(36.91 g/kg)>CPOM(6.90 g/kg)>pPOM(2.78 g/kg)>pcPOM(1.09 g/kg).Except for pcPOM,the contents of other fractions decreased gradually with the increase of soil depth,and the surface accumulation characteristics were obvious.Forest ages and forest types had certain effects on organic carbon fractions of Larix gmelinii forest.With the growth of Larix gmelinii,uPOM decreased first and then increased,pPOM and CPOM increased then decreased and then increased,the variation trend of pcPOM increased first and then decreased.The uPOM content in each forest type followed the order:Ledum palustre-Larix gmelinii forest(52.72 g/kg)>Rhododendron simsii-Larix gmelinii forest(35.33 g/kg)>grass-Larix gmelinii forest(28.61g/kg);the pPOM and CPOM content in each forest type followed the order:Ledum palustre-Larix gmelinii forest>grass-Larix gmelinii forest>Rhododendron simsii-Larix gmelinii forest;the pcPOM content in each forest type followed the order:grass-Larix gmelinii forest>Ledum palustre-Larix gmelinii forest>Rhododendron simsii-Larix gmelinii forest.There was a significant positive correlation between each fractions and TOC,and a close relationship among each fraction.The fractions had significantly positive correlations with SWC,significantly negative correlation with BD,and certain correlations with the nutrients and metal oxides.(4)The soil phytolith in Daxing'anling was mainly in the form of“smooth rod”with the mean content of 16.42 g/kg.The mean content of soil phytolith carbon(Sphyt OC)was274.09 mg/kg,accounting for 0.56%of TOC content.The contents of soil phytolith and Sphyt OC content showed certain zonal characteristics of longitude and latitude.All indexes contents increased with the increase of latitude,increased first and then decreased with the increase of longitude,and basically distributed symmetrically along the ridge.The contents of soil phytolith and Sphyt OC were affected by soil chemical properties and climatic factors.With the increase of precipitation,the content of soil phytolith and Sphyt OC increased gradually;and with the decrease of temperature,the content of Sphyt OC increased gradually.The mean content of soil phytolith and Sphyt OC in Larix gmelinii forest was 19.10 g/kg and588.57 mg/kg,respectively.With the increase of soil depth,the content of Sphyt OC decreased,and the surface accumulation effect was obvious.Forest types and forest ages had certain influences on Sphyt OC content of Larix gmelinii forest.The content of Sphyt OC in different forest ages of Larix gmelinii forest increased first and then decreased.The order of Sphyt OC content in different forest types of Larix gmelinii forest was Rhododendron simsii-Larix gmelinii forest>Ledum palustre-Larix gmelinii forest>grass-Larix gmelinii forest.(5)The TOC storage of topsoil(0-20cm)in Daxing'anling forest region was 99.32t/hm²,and the storage of each fractions(uPOM,pPOM,pcPOM and CPOM)was 44.69,6.35,5.88 and 21.75 t/hm²,respectively,accounting for 44.99%,6.39%,5.92%and 21.90%of the TOC storage,respectively;the Sphyt OC storage was 0.59 t/hm²,accounting for 0.6%of the TOC storage.The TOC storage of Larix gmelinii forest(0-40 cm)was 218.47 t/hm²,and each fraction storage(uPOM,pPOM,pcPOM and CPOM)was 155.95,11.35,3.90 and28.12 t/hm²,respectively,accounting for 71.38%,5.19%,1.79%and 12.87%of the TOC storage,respectively;the Sphyt OC storage was 2.13 t/hm²,accounting for 1.0%of the TOC storage.There were some differences in soil organic carbon storage among different forest ages and forest types.