The Differences Of Soil Nitrogen Cycle Between Different Forest Types In Subtropical China

In order to improve the soil fertility resulted from replanted Chinese fir forests,broadleaf tree species are increasing planted in subtropical China.Studying the differences of soil nitrogen cycle between different forest types is important for the tree species selection.In this study,I compared the nitrogen cycling processes among three forest types including a coniferous Chinese fir plantation,a broadleaf Castanopsis kawakamii plantation and a secondary natural forest.The objectives of this study were to assess:(1)the seasonal variation in soil dissolved organic nitrogen,(2)soil microbial biomass carbon and nitrogen,(3)the monthly dynamics of soil mineral N,(4)nitrogen transformation rates,and(5)nitrogen isotope(?15N)in foliage,litter and soils.The results showed that:(1)Annual litterfall biomass in the natural forest was 6.03 t·hm-2·a-1,1.3 and 2.2 times of those in Castanopsis kawakamii(4.36 t·hm-2·a-1)and Chinese fir(2.07 t·hm-2·a-1).The N storage in the natural forest was greater than that in the plantations,and the C/N is lowest in the natural forest.Differed significantly among forest types in soil microbial biomass carbon,and the content in nature forest is highest,Chinese fir is lowest.The content of soil microbial biomass in winter is significantly lower than other seasons in all forest types.(2)Soil nitrate nitrogen(NO3--N)was the dominant form in mineral N pool in all forest types.The content of NO3--N in nature forest were significantly higher than others.But no significant differences in soil ammonium nitrogen(NH4+-N)among different forest types.There were obviously seasonal variations in NO3--N and mineral N pool in all forests,and both were higher in dormant season(October to February)than that in growing season(March to September).(3)Soil net ammonification rate was the major process of soil net mineralization rate.The soil net mineralization rate in Chinese fir were significantly lower than others.There were obviously seasonal variations in soil net mineralization,the value in February and October were lower,in May to September or December were higher.(4)There were significant differences between forests in top soil dissolved organic nitrogen(DON),and the concentrations in natural forest were significantly higher than others.Those in deep soil were lower than top soil and no significant differences between forests.The concentrations of DON extracted by hot water were higher than those in KCl solution.There were obviously seasonal variations in the concentrations of DON in all forest types,and those pools extracted by KCl solution were highest in summer,lowest in antumn,spring and winter in middle.(5)?15N in foliage,litter and soil(0-20 cm depth)differed significantly among forest types and those were significantly higher in Chinese fir than others.The 815N in the 0-40 cm showed significantly increases with soil depth but no significant differences 40-100 cm.The 15N enrichment factor(EF)in foliage of Chinese fir tend to be closer to zero than the others.Overall,our results indicate that soil nitrogen retention in Chinese fir was significantly lower than natural forest and Castanopsis kawakamii.