| The freeze-thaw period in spring and autumn is the key period connecting growing season and non-growing season,and its strong temperature changes profoundly affect the soil ecological process.In order to understand the biochemical process of forest soil in Changbai Mountain during freeze-thaw period in spring and autumn,the activities of urease,catalase,invertase,nitrate reductase,nitrite reductase,soil microbial biomass carbon(MBC),nitrogen(MBN)contents and their responses to soil freezing and thawing in upper soil(0-10 cm)and lower soil(10-20 cm)of five forest-type soils in Changbai Mountain were studied by in situ culture and continuous sampling method and indoor simulated freeze-thaw method..The results show that:1.During the process of soil thawing in spring,the dynamics of urease activity in upper soil of secondary birch forest,Korean pine broad-leaved forest and Changbai pine forest are basically similar,while the changes of urease activity in lower soil are different.The changes of urease activity in upper and lower soil of hard broad-leaved forest and Mongolian oak forest keeps synchronization.The change of catalase activity was different in five forest types.The catalase activity in the lower layer of hard broad-leaved forest(10-20 cm)was contrary to that in the upper layer(0-10 cm).That is to say,the catalase activity in the lower layer of hard broad-leaved forest decreased with the increase of catalase activity in the upper layer of hard broad-leaved forest in most periods.The maximum invertase activity in the upper soil of five forest types occurred after the middle freezing-thawing stage,while the maximum invertase activity in the lower soil occurred before the middle freezing-thawing stage.Nitrate reductase activity of soil at all levels of five forest types increased significantly at the beginning of spring thawing,and then changed in different periods,all of which had obvious temporal and spatial characteristics.The activity of nitrite reductase in the upper soil of five forest types increased significantly in the early and middle stages of spring freezing-thawing,and the activity of nitrite reductase in the lower soil of each forest type increased significantly in the early stage of spring freezing-thawing,and showed a different change pattern from that in the upper soil.Soil urease,invertase,catalase and nitrite reductase activities decreased with the increase of soil depth,while soil nitrate reductase activity increased with the increase of soil depth.In the same soil layer at different thawing stages,soil urease and invertase activities in secondary birch forest,Mongolian oak forest and hard broad-leaved forest were relatively high,followed by Korean pine broad-leaved forest and Changbai pine forest.The activities of soil urease and invertase were broadleaved forest>coniferous-broadleaved mixed forest>pure coniferous forest.2.During the process of soil freezing in autumn,soil enzymes of five forest types could maintain high activity,and the freeze-thaw pattern significantly affected soil enzymes activities of all forest types.Except secondary birch forest,the dynamic changes of Urease Activities of the other four forest types were basically the same,and there were three obvious peaks during the freeze-thaw period.The dynamic changes of catalase of five forest types showed better performance during the whole study period.Similarity,at the same time,the upper and lower layers of soil also maintained consistency,reaching the maximum peak in the middle and late freezing period.The invertase activity of different layers of soil in five forest types varied in different periods,and experienced several explosive increases and then rapid decreases with the drastic change of temperature.Soil nitrate reductase activity in Changbai pine forest increased at the beginning of freezing and thawing in autumn,while the other four forest types showed a downward trend.At the same time,except for the spatial heterogeneity of soil nitrate reductase activity in Changbai pine forest,the other four forest types did not show this characteristic.Soil nitrite reductase activity in five forest types showed different characteristics,and the fluctuation range of upper soil was greater than that of lower soil.The activity of nitrite reductase in soil and other forest types did not show spatial heterogeneity.3.In the indoor simulated freeze-thaw test,urease of five forest-type soils decreased significantly in the first and ninth freeze-thaw periods,but increased significantly in the second freeze-thaw period with a peak value.The other freeze-thaw stages showed different performances.The catalase activity of five forest-type soils fluctuated slightly under freeze-thaw and changed steadily as a whole;the invertase activity of five forest-type soils showed significant differences among freeze-thaw frequencies,and decreased significantly and reached the minimum value at the eighth freeze-thaw.In the first five freeze-thaw cycles,the invertase activity of upper soil of secondary birch forest,Korean pine broad-leaved forest and hard broad-leaved forest decreased significantly.The activity of soil invertase was positively correlated with temperature,but negatively correlated with temperature in the next five freeze-thaw cycles.The activity of nitrate reductase in the lower layer of Quercus mongolica forest and hard broad-leaved forest was larger than that in the upper layer,and the activity of nitrate reductase in the lower layer of five forest types was more than that in the upper layer of five forest types.Soil nitrite reductase activity showed significant differences among freeze-thaw frequencies.During the whole freeze-thaw period,there were five peaks of nitrite reductase activity in the upper soil of secondary Betula platyphylla forest,Pinus Alba forest and Quercus mongolica forest,and the five peaks were obtained at low temperature.Under the same freeze-thaw frequencies,the nitrite reductase activity in the upper and lower soil of five forest types showed five peaks.The activity of nitrite reductase was significantly higher than that of the lower soil,showing a distinct vertical distribution.4.From the correlation between soil enzyme activity and soil microbial biomass carbon and nitrogen under freeze-thaw in spring,it can be seen that there is a significant negative correlation between soil microbial biomass carbon and soil nitrite reductase in the upper layer of Korean pine broad-leaved forest;soil microbial biomass nitrogen in the upper layer of hard broad-leaved forest and soil urease activity,soil microbial biomass nitrogen in the lower layer of Mongolian oak forest and soil invertase activity,and soil microbial biomass nitrogen in the lower layer of hard broad-leaved forest,respectively.Microbial biomass nitrogen was positively correlated with soil nitrite reductase activity,while microbial biomass nitrogen was negatively correlated with soil invertase activity in upper layer of hard broad-leaved forest.The three-factor variance analysis of the effects of forest type,soil layer and freeze-thaw period on soil enzyme activity showed that the changes of forest type,soil layer and freeze-thaw period and the interaction of the three factors combination significantly affected the changes of soil catalase,invertase,nitrate reductase and nitrite reductase during spring freeze-thaw period,and the interaction between soil layer and freeze-thaw period,forest type,soil layer and freeze-thaw period.The activity of urease was not significantly affected.5.From the correlation between soil enzyme activity and soil microbial biomass carbon and nitrogen under freeze-thaw in autumn,we can see that there is no significant correlation between soil microbial carbon and soil enzymes under freeze-thaw in autumn.Only soil microbial biomass nitrogen in the upper layer of Quercus mongolica forest was significantly correlated with soil nitrate reductase activity.The three-factor variance analysis of the effects of forest type,soil layer and freeze-thaw period on soil enzyme activity showed that the changes of forest type,soil layer and freeze-thaw period and the interaction of three factors combination significantly affected soil urease,catalase,nitrate reductase and nitrous oxide during autumn freeze-thaw period.For invertase,only the interaction among forest type,soil layer,forest type and soil layer significantly affected the activity of invertase. |
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