Influence Of Simulated Nitrogen Deposition On Forest Residues Decomposition And Their Nutrient Dynamics

Nitrogen deposition is the major environmental problems in the world.China has become the third largest nitrogen deposition area followed Europe and the United States.The influence of nitrogen deposition on forest ecosystem is the current research hotspot and scientific frontier.Forest residue is an important carbon and nutrient reservoir for forest ecosystems and is essential to maintain the structure and functioning in forest ecosystems.The effect of nitrogen deposition on the decomposition rate and nutrient dynamics of forest residues can provide a scientific reference for the further study of nitrogen deposition on the cycle and energy flow of forest ecosystem.In this study,six different quality residues(C: N: P stoichiometric ratios)were selected in the subtropical forest.We used the laboratory microcosm experiment and the experiment was incubated for 720 days.The influences of nitrogen deposition on the decomposition rate and nutrient dynamics of different types of forest residues were studied.Meanwhile,the retention of simulated nitrogen deposition was measured by 99.99% 15 N isotope tracer technique.The Soyya-Cork microtiter plate fluorescence method was used to measure 5 kinds of enzyme activity.Our result try to reveal the effect of nitrogen deposition on the decomposition process of residues matter and the response mechanism of nitrogen sink itself and enzyme activity.The main results are:(1)The initial chemical composition and chemical stoichiometry of the six residues were significantly different.The decreasing order of the initial C/N ratio from is: Albizia julibrissin leaf litter(AJL)?Liquidambar formosana leaf litter(LQL)?mixed leaf litters of Cunninghamia lanceolata and L.formosana(CLL+LQL)?moss residual(BRL)?CLL and pine wood residual(PWL).The initial N,P concentration of CLL+LQL,LQL and AJL was higher than that of PWL,BRL and CLL.The decomposition rate of each residue was significantly correlated with two or more of the initial C,N,P concentrations and their stoichiometric ratios.(2)The N addition treatment significantly reduces the decomposition rate of each residue as a whole.After the 720 d decomposition test,the mass remaining percentage of each residue treated with no N addition was BRL(82.07%)and PWL(81.95%)> CLL(54.97%)> CLL+LQL(41.87%)and LQL(41.48%)>AJL(27.77%).The mass remaining percentage of each residue treated with N addition was PWL(86.03%)> BRL(84.23%)>CLL(78.62%)> CLL+LQL(70.38%)> LQL(62.00%)> AJL(33.51%).(3)Compared with the control,N addition treatment slowed down the carbon release of PWL,BRL,CLL and CLL+LQL,and promoted the carbon release of LQL and AJL.N addition treatment in addition to the decomposition process of AJL nitrogen net release oflittle effect,the other five kinds of residues of nitrogen net retention and release of the whole are significant.Compared with the control,the nitrogen deposition slowed down the P concentration of each residue,and the net release of PWL,BRL,CLL+LQL,LQL and AJL P were promoted to different degrees in different stages.(4)N addition treatment of the residue 15 N abundance trend is basically the same:early or mid-period showed an upward trend,medium or late decline;The retention abundance of PWL? CLL?BRL?CLL+LQL?LQL and AJL were 6.73 ‰,13.15 ‰,11.37 ‰,13.65 ‰,14.30 ‰ and 5.81 ‰ respectively,and 8.35 ‰,19.24 ‰,17.55 ‰,14.52 ‰,15.95 ‰ and 5.81 ‰ of the release abundance.It can be seen that different types of residues and release of the degree and the law is inconsistent,and their quality(C/N,C/P and N/P)are related.(5)The influecce of nitrogen deposition on the enzyme activities of 5 kinds of residues was different due to the different types of residues.However,the enzyme activity of Acid phosphatase and Leucine aminopeptidase were increased and the enzyme activity of ?-Glucosidase,Cellobioside and ?-N-acetylglucosaminidase were decreased.In summary,the effect of nitrogen deposition on the decomposition rate of residues and nutrient dynamics and the fate of exogenous N were related to the quality of residues,decomposition stage and element migration.The initial C/N and C/P ratios of the residues and their dynamic changed with the decomposition process are the key features to explain the effect of nitrogen deposition.Nitrogen deposition on the analysis of residue decomposition and nutrient return of subtropical forest ecosystems still needs to be simulated in situ.