Soil Nitrogen Content And Absorption Characteristics Of Chinese Fir Plantation At Different Forest Stand Ages

Chinese fir?Cunninghamia lanceolate?Lamb.?Hook.?is the main afforestation tree species in the subtropical region of south China,the plantation area ranked first among all Chinese plantations,and thus plays an important role in China's forest stock and timber yield.Nitrogen?N?is an important component of plant chloroplasts,proteins,nucleic acids and other life materials,as well as an essential nutrient element for plant growth and development.In this study,four Chinese fir plantations at different ages?7,15,24 and 34 years?were selected to measure the N content in different forms.The results showed that:?1?The total soil N content of Chinese fir plantation was the highest in stand age 15,and increased at first and then decreased with increasing stand age.There was no significant difference in the?¹⁵N value of rhizosphere soil between different forest ages,while the?¹⁵N value of non-rhizosphere soil decreased with the increase of forest stand age.There were significant differences between rhizosphere and non-rhizosphere soil in stand age 24 and 34?P<0.05?.?2?The content of total amino acid and microbial biomass in soil generally decreased with the increase of forest stand age.The content of ammonium N decreased at first and then increased,and that of nitrate N increased at first and then decreased with the increase of forest stand age.The contents of aspartic acid of Chinese fir plantation in stand age 7 were significantly different between rhizosphere and non-rhizosphere soil?P<0.05?,and the contents of glutamate,arginine,lysine,aspartic acid and alanine in stand age 34 were significantly different between rhizosphere and non-rhizosphere soil?P<0.05?.The results showed that the contents of amino acids in rhizosphere and non-rhizosphere soil were significantly different in young and mature forests.?3?The net nitrification rate of rhizosphere soil in Chinese fir plantation generally increased at first and then decreased with the increase of forest stand age,while non-rhizosphere soil showed the opposite trend.The net N mineralization rate of the non-rhizosphere soil of Chinese fir plantation in stand age 7 was significantly higher than that of other three forest ages?P<0.05?.The net mineralization rates of rhizosphere and non-rhizosphere soil were significantly different in stand age 15 and 24,and the net nitrification rates of rhizosphere and non-rhizosphere soil were significantly different in stand age 7 and 15.The soil enzyme activity of Chinese fir plantation was higher in summer than in winter,and decreased with the increase of forest stand age.?4?The total N content of roots and leaves of Chinese fir increased at first and then decreased with the increase of forest stand age.The?¹⁵N value of Chinese fir roots decreased with the increasing forest stand age,and there was no significant difference among different forest stand ages?P>0.05?.The?¹⁵N value of leaves increased at first and then decreased with the increase of forest stand age.?5?Chinese fir had the highest absorption rate of ammonium nitrogen,and had the same absorption rate of nitrate nitrogen and glycine.Chinese fir prefers to absorb ammonium nitrogen,the absorption rate of ammonium nitrogen decreased with the increase of forest age.Organic nitrogen is also an important nitrogen source of Chinese fir and it should be added appropriately in future fertilization measures.