Study On The Characteristics Of Soil Aggregate Nutrients And Enzyme Activities Of Larch Plantations With Different Forest Ages

In order to understand the characteristics of soil aggregate stability,aggregate nutrient content and enzyme activity after vegetation restoration in severely burned areas of Greater Khingan Mountains,three larch plantations planted in 1989,2000 and 2010 and natural secondary forest were selected.The composition of soil aggregates,the content of organic carbon,total nitrogen,total phosphorus,alkali-hydrolyzed nitrogen,available phosphorus,available potassium and the activity of sucrase,urease and acid phosphatase in each particle size aggregate were determined.The distribution characteristics of soil aggregate nutrients and enzyme activities in larch plantations of different ages were comparied.It would provide a scientific basis for vegetation restoration in severely burned areas of Greater Khingan Mountains.The results showed that:（1）The soil mechanically stable aggregates and water-stable aggregates of three larch plantation and natural secondary forest were mainly composed of>0.5 mm aggregates,which were between 77.30～83.36%and 51.74～76.46%respectively.With the increase of forest age,the content of>2 mm aggregates decreased significantly,the content of 2～0.25 mm aggregates increased significantly.The content of 2～1 mm aggregates increased gradually and became the dominant aggregates.With the increase of the age of larch plantation,the MWD and GMD of soil water stable aggregates in 0-10 cm layer decreased gradually,WSA_0.25 increased gradually,and PAD_0.25 decreased gradually.The value of WSA_0.25 in natural secondary forest was significantly lower than that in larch plantation of 32 years,and PAD_0.25 showed the opposite characteristics.（2）In three forest age larch plantations and natural secondary forests,the contents of organic carbon,total nitrogen,total phosphorus,alkali-hydrolyzable nitrogen and available potassium in soil aggregates increased gradually with the decrease of particle size,and the content of<0.5mm was the largest.The nutrient content of soil aggregates in 32 a larch plantation was significantly higher than that in 11 a larch plantation.The contents of organic carbon,total nitrogen,total phosphorus,alkali-hydrolyzed nitrogen and available phosphorus in soil aggregates were increased by 77.35～130.24%,77.61～143.36%,105.84～147.98%,94.58～155.96%and 206.98～537.09%compared with 11 a larch plantation.With the growth of larch plantation,the nutrient contribution rate of soil aggregates from high to low was（2～1mm）>（5～2 mm）>（1～0.5 mm）>（>5 mm）>（0.5～0.25 mm）.2～1 mm size fraction gradually became the dominant size fraction of aggregate nutrient contribution rate,accounting for 20.58～31.75%.There was strong correlation between organic carbon and total nitrogen and alkali-hydrolyzable nitrogen of soil aggregates.The content of soil organic carbon and alkali-hydrolyzable nitrogen was the strongest correlation with the content of<0.25 mm fraction.The correlation between total nitrogen,total phosphorus,available phosphorus,and available potassium with>0.25 mm fractions was strongest.The nutrient of soil aggregates in larch plantation showed surface aggregation.（3）The activities of sucrase,urease and acid phosphatase in<0.5 mm aggregates of larch plantations and natural secondary forests were the highest.Compared with the 11 a larch plantation,three enzyme activities and the geometric mean of enzyme activity in 32 a larch plantation increased by 16.84～86.61%,67.95～264.63%,92.48～288.22%and 69.36～131.64%respectively.The contribution rate of soil aggregate enzyme activity from high to low was（2～1 mm）>（5～2 mm）>（1～0.5 mm）>（>5 mm）>（0.5～0.25 mm）.2～1 mm size fraction gradually became the dominant size fraction of aggregate nutrient contribution rate,accounting for 19.93～30.12%.The enzyme activity of soil aggregates in larch plantation showed surface aggregation.（4）The content of soil aggregates organic carbon,total nitrogen,total phosphorus,available phosphorus,urease,acid phosphatase and geometric mean of enzyme activity in 32 a larch plantation were higher than that in natural secondary forest.More than 30 years after restoration of severely burned areas,32 a larch plantation significantly increased the nutrient content and enzyme activity of soil aggregates.