Effects On Nitrogen Addition On Extractable Nitrogen Fraction In Subtropical Forest Soil

Nitrogen plays an important role in chemical,physical,and biological processes in soil,it consists of organic and inorganic complicated fractions.Soil N is found that many differing forms and function participated in soil N mineralization and transformation.Most of these researches are focused on the N cycle of soil nitrogen fraction using different extaction method,the contribution of each of extractable nitrogen fraction to N mineralisation,and their mutual interaction,have not been quantified simultaneously.At the same time,To investigate the effects of nitrogen addition on the forest extractable nitro gen,the experiment was carried out in subtropical broad-leaved forest species(Cinnamomum chekiangense and Castanopsis fabri)and conifer forest(Cunninghamia lanceolata)with three nitrogen level applied as NH4NO3:control(0 kg·hm-2·a-1),low nitrogen(75 kg hm-2·a-1),high nitrogen(150 kg hm-2·a-1)and two litter treatments(removal of litter layer and retention of litter layer).To assess effects of increasing deposition on N cycling,we quantified the size of soil extractable organic N,and can study to the effects of nitrogen addition on soil extractable N.Then,indoor culture experiments were carried out with addition of three different nitrogen forms(protein,amino,inorganic nitrogen)incubation under 60%Water-holding capacity,to quantify the size of soil water and salt extraction nitrogen,and the N2O-N emission to exolore soil nitrogen pools on the response of the different nitrogen forms addition.The results showed as follows:(1)Soil extraction organic nitrogen and total nitrogen contents were higher at the Castanopsis fabri,the smaller content at Cinnamomum chekiangense.Exctrable nitrogen using distilled water and 0.5 mol·L-1K2SO4 in subtropical broad-leaved forest species was higher than conifer forest,as well as inorganic nitrogen contents.The different forest types did not have significant efferets on soil NH4+-N with distilled water and 0.5 mol·L-1 K2SO4.Therefore,soil NH4+-N of the Castanopsis fabri with distilled water was higher,but soil NH4+-N of the Castanopsis fabri with 0.5 mol-L-1K2SO4 were lowest.Soil NO3--N contents were higher in Cinnamomum chekiangense,the Castanopsis fabri was lowest Retention of litter layer could promote soil recalcitrant nitrogen transformation,also enhanced LPI-N,LPII-N and exctrable organic nitrogen contents.Hence,differences in the different forest types of extractable organic might be attributed to the quality and quantity of organic matter input,The result analysis showed that different extraction methods of organic nitrogen was significantly positively correlated with soil microbial biomass,it results that each soil extraction organic nitrogen represents source of soil microbial biomass nitrogen,but they had different efficiency.(2)To some degree,nitrogen addition has effects on exctratable organic nitrogen content of different forest types using distilled water,0.5 mol·L-1 K2SO4,2.5 mol·L-1H2SO4 and 13 mol·L-1H2SO4,nitrogen addition could reduce exctractable organic nitrogen of Castanopsis fabri using salt solution,and also reduce exctractable organic nitrogen of removal of litter layer treatment of Cunninghamia lanceolata using distilled water,while retention of litter layer treatment enhance.Meanwhile nitrogen deposition increased NH4+-N?NO3--N contents by water extraction,suggests that soil microbial biomass using low molecular compounds rapidly converted into inorganic nitrogen.The labile pool ?,? and recalcitrant was evaluated following the two-step H2SO4 the hydrolysis method.Then the effects of N addition on labile and recalcitrant N fractions and soil microbial biomass N(SMB-N)were investigated,results that high nitrogen treatment increased soil labile nitrogen pool ? of Cunninghamia lanceolata by 78.6%compared with control,Nitrogen addition significantly increased SMB-Nfrom 0%to 103.6%compared with control;Except for SMB-N/LPI-N ratio of Cunninghamia lanceolata decreasing with nitrogen addition,the utilization of soil labile N in the other forests by microorganism increased with nitrogen addition.The results indicated that the differences of soil labile and recalcitrant organic nitrogen pools between broad-leaved and conifer forest were related to the quality of litter,and the differences in litter decomposition might provide some evidence for the change of soil carbon and nitrogen pools.(3)Organic nitrogen addition promoted NH4+-N content using distilled water,the order contents were by step of CK+Casein>CK+AA,even CK+Casein treatment maintain the highest content up to 48 days.While water-soluble NO3--N increases with culture time continue to rise,it was related to exctractable organic nitrogen by salt solution decrese at the later experiment Recalcitrant nitrogen content was decrease at the late experiment,it may translate into LPI-N,consequently increase extractable organic nitrogen content,and promote to translate into NH4+-N which occupied more exchange sites,the result leaded to decrease extractable NO3--N content.Different nitrogen forms have impact on the subtropical forest soil nitrogen and increase N2O-N flux,the soil N2O-N-emission rates were step by step of CK+Casein>CK+AA>CK+N>CK at one week,it closely related with soil nitrogen forms transformation The result shows that inorganic nitrogen and amino acid addition have influence on soil nitrogen transformation,and procein can influent more long.Moreover,we should pay close attention to organic nitrogen which comes from the litter influence on ecosystem N cycling.