The Effects Of Nitrogen Deposition And Drought On Litter Decomposition In Two Typical Tree Species Of Subtropical Zone

Forest litter plays an important role in plant growth and development and nutrient cycling process of forest ecosystem.Climate factors are the most important influencing factors on litter decomposition,and of them,humidity is considered to be major one.In recent years,seasonal drought has become more and more frequent,which are seriously threatening the forest vegetation in southern China.Seasonal changes of precipitation will seriously affect the litter decomposition process of forest ecosystems and thus affect carbon cycle.In addition,nitrogen deposition also has become another important environmental factor that affects litter decomposition,due to the combustion of fossil fuels and use of nitrogen-containing fertilizers.However,at present the researches about the mechanism of the effects of environmental or climatic factors,such as drought or N deposition,on litter decomposition have been carried out only aming to single factor.Generally,it is believed that drought can inhibit litter decomposition,while nitrogen deposition can promote litter decomposition.However,when both factors work together in litter decomposition process,multi-factor comprehensive effect may be increase the complexity of the decomposition process.In recent years,Xiangxi region,belonging subtropical zone,has encountered serious nitrogen deposition and continuous seasonal drought.Therefore,Xiangxi region was selected as the study area,and two dominant species,Pinus massoniana and Cinnamomum camphora,in their plantation forests were selected as substrate materials,in order to analyze the interaction effect of drought and nitrogen deposition on the decomposition of P.massoniana and C.camphora litter in different habitats through a series of field simulation experiments including cross-transplantation experiments.The results showed that the combined effects of various factors on litter decomposition were different from a single factor.This study,to some degree,clarified the patterns of litter decomposition under multi-factors and multi-levels,and further uncovered their driving mechanism,and provided a theoretical basis for predicting carbon balance of regional forest ecosystems.The detailed results obtained in this study were shown as follows.?1?The effect of nitrogen deposition on the litter decomposition of P.massoniana and C.camphora was observed to be confused,via a reciprocal transplant experiment.The results showed that the mass loss of decomposition of P.massoniana litter was higher at home field during the entire decomposition period when nitrogen was not applied,although the statistical difference was not significant.However,the mass loss of C.camphora litter was significantly higher in other habitats than in the home field at the early stage of decomposition,but higher in the home habitat at the later stage of decomposition.Nitrogen application changed the performance of the mass loss of two litters in different habitat.For P.massoniana litter,there was no significant difference in the moss loss between the two forest types in the early stage of decomposition,but low nitrogen application resulted in a significantly higher mass loss in the home field at the later stage of decomposition.For C.camphora litter,nitrogen application increased the mass loss of litters in the C.camphora forest at the early stage of decomposition,but there was no significant difference between two forest types,and the lower nitrogen application rate resulted in higher mass loss in P.massoniana forest than in home habitat at the later stage of decomposition.The sensitive responses to different nitrogen concentration gradients were different from each other for both litters.The results showed that for P.massoniana litter,in the early stage of decomposition,the mass loss in both habitats had significant sensitivity to nitrogen gradients,but in the later stages of decomposition,the mass loss reached the maximum value in the midium nitrogen treament in the P.massoniana forest.For C.camphora litter,in the early stage of decomposition,the mass loss of C.camphora litter decreased with increasing nitrogen in P.massoniana forest,while increased in C.camphora forest.In the later stage of decomposition,the sensitivity responses of the mass loss of C.camphora litter in different habitats to nitrogen gradients were consistent to P.massoniana litter.In this study,nitrogen deposition had a complex effect on the microbial activity leves,which was not completely consistent to the mass loss.?2?Drought inhibited the decomposition of both litters.In the reciprocal transplant experiment,the mass loss of two litters were higher in their home habitat in the treatment without drought,especially for C.camphora litter.Drought had changed the decomposition performance of the mass loss of two litters in different habitats:for P.massoniana litter,medium and high drought resulted in higher mass loss in C.camphora forest than at home habitat;but for C.camphora litter,the mass loss was higher at home habitat.Drought inhibited the decomposition of P.massoniana and C.camphora,but the response patterns of two litters to drought were different from each other.For P.massoniana litter,the mass loss increased with the increasing of drought in the P.massoniana forest;while in the C.camphora forest the mass loss decreased.For C.camphora litter,medium drought and high drought significantly inhibited its decomposition in the P.massoniana forest,while in the C.camphora forest,the mass loss of the C.camphora litter showed a consistent trend with the P.massoniana litter.The effects of drought on the fungal biomass of two litter species showed the same results with the mass loss,but the results of CO₂ release were the opposite to the mass loss,while the effect on microbial enzyme activity showed a diverse inconsistency with the mass loss.?3?Significant interactions were observed between nitrogen deposition and drought treatments for the deomposition of P.massoniana and C.camphora litters.When no applying drought treatment,the effect of nitrogen deposition on the decomposition process of P.massoniana litter was not significant during the entire decomposition period.For C.camphora litter,nitrogen application resulted in a single-peak distribution of mass loss throughout the decomposition period,which reached a maximum in the medium nitrogen treatment.The introduction of drought changed the responses nitrogen sensitivity for the litters of P.massoniana and C.camphora.For P.massoniana litter,in the early stage of decomposition,the medium and high drought treaments resulted in the mass loss increases with the increase of nitrogen concentration gradient.However,in extreme droughts,the performance was appeared as a single-peak distribution with increasing nitrogen concentration gradient and although in most cases no significant difference.In the later stage of decomposition,medium drought treament caused the mass loss of decomposition of P.massoniana to be a single-peak distribution with increasing nitrogen but extreme drought increased.For C.camphora litter,in the early stage of decomposition,extreme drought changed the pattern of decomposition along nitrogen gradients,i.e.the mass loss of C.camphora litter increased with the increasing of nitrogen,although the statistical difference was not significant.At the later stage of decomposition,the medium drought resulted in the mass loss of the C.camphora litter increased with the increase of the nitrogen concentration gradient and the high drought treament resulted in a linear inhibition of the mass loss.When no applying nitrogen,the mass loss of P.massoniana litter reached the maximum under the medium drought treament at the early stage of decomposition,but the mass loss decreased with the drought strengthenin,although the statistical difference was not significant.At the later stage of decomposition,the drought inhibited the decomposition of the litter of P.massoniana,especially in severe drought.For the litter of C.camphora,in the early stage of decomposition,its mass loss showed a tendency of increasing first and then decreasing with increasing drought gradient.However,at the later stage of decomposition,the mass loss of C.camphora showed a single-peak distribution along drought gradient,and the mass loss was highest in severe drought.Nitrogen deposition changed the responses of two litters to drought.The results showed that for P.massoniana litter,in the early stage of decomposition,the moderate nitrogen application caused the mass loss to increase with increasing drought gradient and the severe nitrogen application caused to be a single-peak distribution with increasing nitrogen concentration gradient,although there was no statistically significant difference.At the later stage of decomposition,moderate nitrogen application resulted in the maximum mass loss during moderate drought.For C.camphora litter,in the early stage of decomposition,nitrogen deposition did not change the response of its decomposition to drought,and in the later stage of decomposition,nitrogen deposition caused severe decomposition of C.camphora litter during severe drought.?4?Two litters havd different decomposition sensitivity responses to nitrogen sources.The results showed that in the control group,the decomposition of P.massoniana litter was fastest when applying ammonium nitrate,while the fastest in the urea group for C.camphora litter.However,drought changed the sensitivity response of the two litters to nitrogen sources.The mass loss of the P.massoniana litter when applying drought was highest in the ammonium chloride group,although there was no statistically significant difference.While there was no change in the sensitive response of the C.camphora litter to nitrogen sources,i.e.the highest rate of mass loss was observed in the urea group.The sensitivity response of the fungal biomass and CO₂release of the two litters to the nitrogen source also showed a trend consistent with the mass loss,but inconsistent for enzyme activity.Drought inhibited the decomposition of two litters,but the response of two litter decomposition to drought under different nitrogen sources was different.For the litter of P.massoniana,the mass loss decreased along drought gradient under each nitrogen sources.The mass loss of the C.camphora litter caused by urea and ammonium chloride to increase with the increase of drought gradient,but decreased in the ammonium nitrate group,although there was no statistically significant difference.The effects of drought on the microbial activity of two litters showed more complex inconsistencies to mass loss.