Effects Of Simulated Freezing-thawing Process On Available Carbon-nitrogen And Soil Enzyme Activities In Forest Soils Of Changbai Mountain

Soil freezing and thawing often occurs in late autumn and early spring in Changbai Mountains.The carbon and nitrogen cycles and soil enzyme activities of forest ecosystems in this area are obviously affected by freezing and thawing cycles.However,little research has been done on the effects of freeze-thaw process on microbial biomass,nitrogen mineralization and enzyme activity of forest soils in this area.Therefore,the soil of typical forest types(Hardwood broad-leafed forest,Korean pine broad-leaved forest,Secondary birch forest,Pinus sylvestriformis forest,Mongolian oak forest and Fir forest)in Changbai Mountains was taken as the research object.The nitrogen mineralization characteristics and the dynamic changes of soil enzyme activities in different forest types during freeze-thaw cycle were studied by using themethod of simulated freeze-thaw cycle(-15—5?).The results show that:1.In most cases,the soil microbial biomass carbon content in 0—10 cm soil layer of six forest types was higher than that of the control treatment(soil cultured at 5?),while the soil microbial biomass nitrogen content was lower after the 8th freeze-thaw cycle than that before the freeze-thaw cycle,compared with it decreased by 89.5%(Hardwood broad-leafed forest),41.9%(Secondary birch forest),74.8%(Korean pine broad-leaved forest),74.4%(Pinus sylvestriformis forest),22.0%(Mongolian oak forest)and 3.6%(Fir forest),respectively.Soil microbial biomass carbon and soil microbial biomass nitrogen in 0—10 cm soil layers of six forest types were significantly higher than those in 10—20 cm soil layers during freeze-thaw cycles,showing obvious spatial heterogeneity of soil microbial biomass carbon and soil microbial biomass nitrogen..2.Freeze-thaw process has a significant impact on the content of soluble organic carbon and soluble organic nitrogen in soil.Its dynamic changes are constrained by freeze-thaw temperature,freeze-thaw frequency,forest types and soil layers.The content of soluble organic carbon and soluble organic nitrogen in 0-10 cm layer soil of six forest types was lower than that before freeze-thaw cycle after the 8th freeze-thaw cycle.Under freeze-thaw treatment decreased by 55.5%(Hardwood broad-leafed forest),63.4%(Korean pine broad-leaved forest),3.8%(Secondary birch forest),4.9%(Pinus sylvestriformis forest),18.0%(Mongolian oak forest)and 37.8%(Fir forest)on the content of soluble organic carbon in soil,and under freeze-thaw treatment was 13.8%(Korean pine broad-leaved forest),25.9%(Secondary birch forest),28.4%(Pinus sylvestrifonnis forest),36.4%(Mongolian oak forest)and 35.0%(Fir forest)lower than that before freeze-thaw treatment on the content of soluble organic nitrogen in soil.During freezing and thawing,the vertical spatial distribution of soluble organic carbon and soluble organic nitrogen in different forest types was obvious.3.The effect of freeze-thaw process on soil NO3--N content was not obvious.There was no significant difference in soil NO3--N content between different forest types at the same level and different cycle frequencies,and there was no significant difference between 0—10 cm layer of soil during freeze-thaw process and its control treatment.The NH4+-N content in 0—10 cm layer soil of six forest types showed similar dynamic change trend with the increase of freeze-thaw frequency,and showed obvious NH4+-N release characteristics during the third,fourth and fifth freeze-thaw cycles,and During the fifth freeze-thaw cycle was the highest on the soil NH4+-N content(Hardwood broad-leafed forest:171.8±81.6 mg·kg-1,Korean pine broad-leaved forest:87,7±22.8 mg·kg-1,Secondary birch forest:102.6±37.5 mg·kg-1,Pinus sylvestriformis forest:87.2±52.0 mg·kg-1,Mongolian oak forest:87.1 ± 18.9 mg·kg-1 and Fir forest:313.4±2.5 mg·kg-1).After the second freeze-thaw cycle,inorganic nitrogen released from six forest-type soils.4.The response of six forest-type nitrogen minerals(nitrate)to freeze-thaw process is obvious.The changes of freeze-thaw temperature,freeze-thaw frequency and soil layer have significant effects on soil net nitrification rate and net nitrogen mineralization rate.Soil net nitrification fluctuated greatly under the six forest types.The maximum value of 0—10 cm layer appeared after the second freeze-thaw cycle,followed by the seventh freeze-thaw cycle.In 0—10 cm layer soil,the net nitrification rate and net nitrogen mineralization rate of the six forest-type soils reached their peak value after short-term freeze-thaw cycles,and then decreased in varying degrees.5.The dynamics of soil urease and invertase activities in six forest types varied significantly with freeze-thaw frequency,temperature,soil layer and forest type.The dynamics of soil urease in 0—10 cm layer of six forest types increased first,then decreased,and finally stabilized during freeze-thaw,while the soil invertase activity in 0—10 cm layer of six forest types increased during freeze-thaw,but in 0—10 cm layer of each forest type.The response of soil catalase to freeze-thaw process was not obvious.The synthetical expression of soil enzyme activity under freeze-thaw was:Abies forest was the strongest and Pinus Alba forest was the weakest.The activities of urease,invertase and catalase in 0—10 cm soil layer were significantly higher than those in 10—20 cm soil layer.