| Soil gaseous nitrogen(N)loss is a rather important but poorly understood process in N cycling in forest ecosystem.Due to the rapid N cycling rate,tropical forests are the important source of gaseous N loss.Majority of previous researches about soil N gaseous loss were focused on nitrogen oxides(NO and N2O),but the emission of dinitrogen(N2)from soil and contributions of microbial processes to soil gaseous N loss were seldomly reported.Under the context of global change,the atmosphere N deposition increasing severely across the world,which may increase the availability of N in soil.As a consequence,soil gaseous N losses may be enhanced and the contributions of microbial processes to soil gaseous N loss may be altered.However,the responses of soil N2 emission and microbial proesses to increased N deposition increasing still remain unclear.In this study,the increased N deposition was manipulated by adding exogenous N(NH4NO3)in a field experiment,afterwards,we combined 15 N labelling method and acetylene inhibition technique(AIT)in a laborary experiment to study the soil gaseous N gaseous(N2O and N2)losses and its responses to increased N addition in the two tropical forests in Hainan Island.The main results are as follows:1).Gaseous nitrogen losses from tropical forest soils are controlled by many factors,such as: forest type,season,soil water content etc.It is common that soil N gas emission rates in secondary forest are higher than those in primary forest,and soil N gas emission rates in wet season were higher than those in dry season,but no significant difference were found among them.This may be because that in secondary forest and wet season,N cycles rapidly with high microbal activities,which result in increased soil gaseous N losses.In addition,in field water contents,soil N gas emission rates were relative low,and N2 O and N2 emission rates were-0.004-0.03 n mol N h-1g-1and 0-0.022 n mol N h-1g-1,respectively;after 2ml water addition,N2 O and N2 emission rates increased sharply by more than 70 and 40 times,respectively.This probably be that water addition enhance or shift the contributions of microbial processes to soil N gas emissions.2).During laboratory incubation under aerobic conditions,long-term increased N addition did not cause a significant increase in either N2 O or N2 emissions,or N2O/(N2O+N2).Under anaerobic conditions,however,the responses of gaseous N losses to N additions showed different trend between the two forests.In the secondary forest,N2 O emissions decreased and N2 emissions enhanced with increasing N addition,which may be attributed to substantially greater N2 O reduction to N2 during denitrification,which could be further supported by the decreased N2O/N2 ratio with increasing N addition.However,no such effects were observed in the primary forest.3).Under anaerobic condition,by using 15 N labelling method,we aparted the contributions of microbial processes to gaseous N productions and determined their responses to elevated N additions.We found that,in both forests,denitrification contributed 39-58% of N2 O production and this contribution decreased with N additions.Meanwhile,co-denitrification and heterotropical nitrification holded 42-61% of N2 O production and this contribution increased with N additions.Furthermore,denitrification is the predominant pathway to N2 production(98-100%)and was not affected by N additions.However,we found no significant changes in the abundance of functional genes associated with denitrification.Our results indicate that after 6 years of N addition the microbial processes responsible for N gases production were shifted.4).By comparing the results determined by two methods,we found that soil N2 emission rates measured by 15 N labelling method were 1.2 to 3.5 times as higher as the results determined by AIT.This may be because the drawbacks of 15 N labelling method and AIT,which may overestimate or underestimate soil N2 emissions,respectively.This indicates that,the defects of the two method should be considered when evaluating soil N gas loss... |
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