| Forest soil organic matter with organic carbon as the main structure is not only the core of soil fertility,which directly determines the growth of trees and the stability and improvement of forest productivity;it is also one of the most important forms of carbon storage in the terrestrial ecosystem,and its change is related to the concentration of greenhouse gases such as atmospheric carbon dioxide,thus affecting the comprehensive function of forest to cope with climate change.However,about 90%of the organic carbon in forest soil is stored in soil agglomerates.Soil aggregates have a physical protective effect on organic carbon.At the same time,organic carbon components such as humus are necessary components for the formation of soil aggregates.Seasonal freeze-thaw cycle is a common natural phenomenon in high mountains and mid high latitudes around the world.It has a significant impact on the laws and structures of soil water and heat movement.It may destroy or recombine the soil aggregate structure,affect the storage of soil organic carbon,and be regulated by the characteristics of above ground vegetation.Influenced by frequent natural disasters and human activities,and the alpine forest ecosystem in western Sichuan on the eastern edge of the Tibetan Plateau is sensitive and fragile,forming a colorful leaf forest with high landscape aesthetic value,important ecological service functions and various types,such as deciduous broad-leaved forest,coniferous broad-leaved mixed forest and deciduous pine forest.In order to fully understand the mechanism of soil organic carbon fixation and preservation of different types of colorful leaf forest,the soil organic carbon reserves of seven typical colorful leaf forest and their distribution rules in different particle size aggregates were dynamically monitored before and after the freeze-thaw season and in the growth season in 2018 and2019,and the soil organic carbon and its reserves,the particle size distribution and stability of soil aggregates in different seasonal key periods of freeze-thaw were analyzed Qualitative impact characteristics.The main results are as follows:(1)The soil organic carbon reserves of different colorful leaf forest in the subalpine of Western Sichuan are as follows:Quercus Baroni forest 813.22 t/hm~2,Betula platyphylla-Picea asperata-Acer caudatumvar.prattii forest 370.05 t/hm~2,Bare land363.02 t/hm~2,Betula platyphylla-Abies faxoniana forest 305.21 t/hm~2,Quercus baroni-Sophora davidii-Cotinus coggygria forest 300.82 t/hm~2,Larix gmelinii forest259.85 t/hm~2,Betula luminifera-Rhus Potanini forest 121.46 t/hm~2.The time,the type of colorful leaf forest and the interaction effect of soil organic carbon storage between the two showed a very significant difference(P<0.01).Seasonal freeze-thaw significantly reduced the content of organic carbon in different soil layers of all types of colorful leaf forest.Among them,the organic carbon content of mineral layer is more affected than that of organic layer.Compared with other types of colorful leaf forest,the soil organic carbon content of Quercus baronii forest and Quercus baronii-Sophora davidii-Cotinus coggygria forest is most affected by seasonal freeze-thaw.(2)The overall stability of soil aggregate was as follows:coniferous broad-leaved mixed forest>deciduous pine forest>Bare land>deciduous broad-leaved forest>deciduous shrub.The large agglomerate content(R 0.25),the aggregate average weight diameter(MWD)and the aggregate geometric mean diameter(GMD)in different types of colorful leaf forest soil layers are as follows:the soil organic layer of coniferous broad-leaved mixed forest and deciduous broad-leaved forest is less than the mineral layer,while the soil organic layer of deciduous pine forest and Bare land is greater than the mineral layer,and D is just the opposite.Seasonal freeze-thaw significantly changed the particle size distribution of aggregates,but on the whole,it reduced the stability of soil aggregates in the higher altitude colorful leaf forest,and improved the stability of soil aggregates in the lower altitude colorful leaf forest,which had a greater impact on the soil mineral layer than the soil organic layer.(3)There were significant differences in different types of colorful leaf forest in different size aggregates in subalpine of Western Sichuan(P<0.01).The organic carbon content of soil aggregates in different types of colorful leaf forest is mainly the organic carbon content of its dominant aggregates.Except for Betula luminifera-Rhus potaninii forest the main organic carbon content of the aggregates with grain size<0.106 mm,the other types of colorful leaf forest is the main organic carbon content of aggregates with grain size>2 mm.There was a significant negative correlation between soil organic carbon content and R0.25,and there was a significant positive correlation between the total organic carbon content and the organic carbon content of agglomerates with particle size<0.106 mm.Before and after seasonal freeze-thaw,soil organic carbon storage was negatively correlated with R0.25,MWD and GMG,and positively correlated with D.In conclusion,seasonal freeze-thaw significantly changed the particle size distribution of soil aggregates,and improved the stability of soil aggregation in low altitude stands,but it also changed the migration and transformation of soil organic carbon.On the one hand,the smaller the particle size of soil aggregate,the larger its specific surface area,the more organic matter it adsorbs,resulting in the higher organic carbon content of micro-aggregate;on the other hand,freeze-thaw not only increased the release of nutrient elements in the soil,but also promoted its loss,resulting in the reduction of soil organic carbon content to a certain extent after freeze-thaw.Because of the high altitude area,the freeze-thaw effect is stronger than that of the low altitude area,and the increase of the thick snow content freeze-thaw cycle will lead to the decrease of the stability of soil aggregates;however,the appropriate soil moisture content and the number of freeze-thaw cycles in the low altitude area will help to enhance the stability of soil aggregates.Compared with other colorful leaf forest,the content of macro-aggregate in Quercus baronii forest and Quercus baronii-Sophora davidii-Cotinus coggygria forest increased more after seasonal freeze-thaw,the stability of soil aggregate and its organic carbon pool were more sensitive to seasonal freeze-thaw cycle,while the response of coniferous and broad-leaved mixed forest and larch forest to seasonal freeze-thaw was relatively stable.These results are helpful to understand the impact of seasonal freezing and thawing on soil texture and fertility under the climate change scenario,and provide some basic data and theoretical basis for the management and sustainable management of subalpine forest. |
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