Changes Of Soil Carbon-nitrogen Content And Aggregate Stability After Secondary Forest Transformed Into Pinus Koraiensis Plantation

The broad-leaved Pinus koraiensis forest with Pinus koraiensis as the main tree species plays a very important role in the maintenance of ecological environment in Northeast China.Due to historical reasons and unreasonable utilization,the primitive broad-leaved Pinus koraiensis forest with complex structure,powerful and stable function,high biomass and biodiversity has been exhausted.Although efforts have been made to restore Pinus koraiensis forest through large-scale afforestation,a series of problems caused by Pinus koraiensis Artificial Pure Forest,such as the decline of biodiversity and soil fertility,have affected the restoration process of broad-leaved Pinus koraiensis forest.This paper selects Juglans mandshurica-Pinus koraiensis forest,Fraxinus mandshurica-Pinus koraiensis forest,Phellodendron amurensis-Pinus koraiensis forest and Pinus koraiensis pure forest built in 1987 after narrow strip clear cutting of natural forest in jianlagou Forest Cultivation Experimental Station of Northeast Forestry University as the research object,and takes secondary forests as the control.Through the combination of field sampling and indoor analysis,the soil carbon and nitrogen content and aggregate stability of five forest types were measured,and the changes of soil carbon and nitrogen content and aggregate stability after the secondary forest was transformed into Pinus koraiensis plantation were analyzed.The results showed that:(1)The contents of soil total organic carbon and reconstituted organic carbon in secondary forest were higher than those in Pinus koraiensis plantation.The contents of total organic carbon in the soil of three Pinus koraiensis mixed forests and Pinus koraiensis pure forest decreased by 1.79% ～ 48.74%,and the decline degree of Juglans mandshurica-Pinus koraiensis forest was the lowest,which was 1.79% ～ 30.06%.Compared with the pure Pinus koraiensis forest,the total organic carbon content of the three mixed Pinus koraiensis forests increased by 6.30% ～ 42.39%,and the highest rate of Juglans mandshurica-Pinus koraiensis forest was 29.39% ～ 42.39%.The mixing of broad-leaved tree species can promote the accumulation of total organic carbon and recombinant organic carbon,improve the environment of Pinus koraiensis coniferous pure forest and accelerate the carbon cycle.After the secondary forest was transformed into Pinus koraiensis plantation,the degree of soil humification decreased,and the degree of soil humification in Juglans mandshurica-Pinus koraiensis forest and Fraxinus mandshurica-Pinus koraiensis forest was higher than that in Pinus koraiensis pure forest.The humus quality of secondary forest is better than that of Pinus koraiensis plantation,the humus quality of three broad-leaved Pinus koraiensis forests is higher than that of Pinus koraiensis pure forest,and the humus degree and humus quality of Juglans mandshurica-Pinus koraiensis forest are relatively good.(2)The soil total nitrogen content of four Pinus koraiensis plantations was lower than that of secondary forests,and the soil total nitrogen content of three Pinus koraiensis mixed forests and Pinus koraiensis pure forests decreased by 6.43% ～ 58.53%.Compared with pure Pinus koraiensis forest,the soil total nitrogen content of the three mixed Pinus koraiensis forests increased by 9.80% ～ 98.13%,and the highest increase rate of Juglans mandshurica-Pinus koraiensis forest was 23.14% ～ 98.13%.Planting Pinus koraiensis plantation leads to the decrease of soil total nitrogen content,but compared with single planting Pinus koraiensis pure forest,the mixing of broad-leaved trees improves the level of soil total nitrogen accumulation,and Juglans mandshurica mixed trees have more advantages in improving soil nitrogen content.The contents of soil amino acid nitrogen,ammonia nitrogen and acid hydrolysis unknown nitrogen in Juglans mandshurica-Pinus koraiensis forest and Fraxinus mandshurica-Pinus koraiensis forest are higher than those in Pinus koraiensis pure forest.The mixture of Juglans mandshurica,Fraxinus mandshurica and Pinus koraiensis is conducive to improve the accumulation level of soil organic nitrogen under Pinus koraiensis plantation and promote the stability of available nitrogen pool.(3)After strip clear cutting to build Pinus koraiensis plantation,the content of aggregates with particle size > 2 mm decreased,and the reduction degree of Juglans mandshurica-Pinus koraiensis forest was the lowest,which was 17.94%;The content of aggregates with particle size of < 0.053 mm increased,and the highest increase degree of Pinus koraiensis pure forest was 45.78%.Compared with the secondary forest,the average mass diameter and geometric average diameter of soil in Pinus koraiensis plantation decreased,and the stability of soil aggregates decreased.In 0 ～ 10 cm and 10 ～ 20 cm soil layers,the fractal dimension of soil aggregates in Pinus koraiensis pure forest is higher than that in secondary forest and three Pinus koraiensis mixed forests.The soil dispersion of Pinus koraiensis pure forest is high and the stability is relatively poor.The organic carbon content and total nitrogen content of soil aggregates were increased to a certain extent in the three Pinus koraiensis mixed forests,especially in Juglans mandshurica-Pinus koraiensis forest.Based on secondary forest into Pinus koraiensis plantation soil carbon-nitrogen content and the change of the aggregate stability,in order to improve soil quality,planting broad-leaved tree species Juglans mandshurica,Fraxinus mandshurica and Phellodendron amurense mixed with Pinus koraiensis can alleviate the decline of soil carbon-nitrogen content and soil aggregate stability caused by Pinus koraiensis pure forest,and improve humus quality and humification degree,Prevent the decline of soil fertility of Pinus koraiensis plantation and promote the improvement of forest soil quality.