Studies On Soil Nitrogen Transformations And The Functional Microorganisms In Natural Secondary Forests With Different Ages

The leaching and loss of nitrogen not only became an influencing factor for limiting forest productivity,but also posed a great threat to the safety of ecosystems.The experimental site was located at Shanghang Baisha Stated-owned Forest Farm(25°04??25°15?N,116°30??116°38?E)in Longyan City,Fujian Province.Six natural secondary forests with different ages(4,8,18,28,40,?100)were selected as objects,and 3?4 20 m×30 m plots were set in each natural secondary forest.To explore the effects of forest age on soil nitrogen transformation and microbial functional gene and their driving factors by measuring physicochemical properties,microbial biomass,nitrogen transformation rate and microbial functional gene of 0?10 cm soil in natural secondary forests with different ages.The main conclusions coming from the experiment were as follows:(1)From 4 to 40 year old in the natural secondary forest,the age of natural secondary forest has a significant effect on soil ammonium nitrogen(NH₄⁺-N)content only.But the physicochemical properties of?100 years natural secondary forest were significantly different from others,its soil pH was significantly lower than that of others,and the water content,total carbon(TC),total nitrogen(TN),ammonium nitrogen(NH₄⁺-N),nitrate nitrogen(NO₃^--N),mineral nitrogen(Mineral N),dissolved organic carbon(DOC),dissolved organic nitrogen(DON)content were the highest among the six natural secondary forests.(2)From 4 to 40 year old in the natural secondary forest,soil microbial mass(including MBC and MBN)showed a downward trend,and?100 years natural secondary forest showed a rising trend,and showed a significant positive correlation with indicators of soil nutrients(TC?TN?DOC?DON).These research results showed that with the development of stand,the changes and differences of soil microbial biomass were mainly determined by soil nutrients.Among the four hydrolases in natural secondary forests with different ages,?100 years natural secondary forest always maintained a high level.Soil pH significantly affected soil?-glucosidase and acid phosphatase,and indicators of soil nutrients(TC?TN?DOC?DON)significantly affected the activities of three hydrolases other than N-acetyl-?-d-glucosaminidase.(3)The soil net nitrogen mineralization rate of natural secondary forests with different ages had a significant negative correlation with soil pH.As the age of natural secondary forest increases,soil pH decreases,and the net nitrogen mineralization rate of soil showed an upward trend;the soil net nitrogen nitrification rate remained the same.Soil potential nitrification rate was significantly positively related to soil pH,and significantly negatively related to dissolved organic carbon(DOC).The change trend of soil potential nitrification rate and net nitrogen mineralization rate showed an opposite trend,indicating that with the age increases of natural secondary forests,the ability to produce nitrate in the soil would decrease,which could help soil nitrogen retention.Soil potential nitrification rate was significantly negatively related to soil pH,and the accumulation of carton and nitrogen in soil would also promote soil denitrification.(4)Soil pH affected the distribution and activity of ammonia oxidizing microorganisms.In the study of natural secondary forests with different ages,the AOA amoA abundance was significantly higher than AOB amoA.The soil pH of?100 years natural secondary forest was significantly lower than that of others,and it had the highest AOA amoA abundance;ammonium nitrogen as a reaction substrate was significantly positively related to the AOA amoA abundance;The carbon and nitrogen sources in the soil provided energy for the reaction of ammonia oxidizing microorganisms,and also affected the AOA amoA abundance.The functional gene abundance of soil denitrifying microorganisms was closely related to the carbon and nitrogen sources in the soil,but had no correlation with soil pH and nitrate nitrogen content.At the same time,it was found that in the natural secondary forest ecosystem,the abundance of nosZ was higher than that of nirK and nirS,which helped to reduce the emission of N₂O to the atmosphere.