| Non-methane hydrocarbons?NMHCs?play critical roles in atmospheric chemistry and have great effect on global carbon cycle.At present,the research on NMHCs in forest ecosystems mainly focuses on the canopy layer,but the forest floor as an important source of NMHCs still lacks attention.Due to the increasing in human activities such as the burning of fossil fuels,the use of nitrogenous fertilizers,and the deforestation and land-use changes,the reactive nitrogen entering the ecosystem far exceeds the natural nitrogen load.Continuing elevated nitrogen deposition will change forest soil ecological processes,which will ultimately affect NMHC fluxes from the forest floor.Therefore,studying the effect of nitrogen deposition on the flux of NMHCs in forest soil and litter has important ecological and environmental significance.In this study,forest soil and the leaf litters of Pinus massoniana and Cunninghamia lanceolata,two common subtropical tree species,were selected as research object.According to the natural nitrogen deposition of Fujian Province?30 kg N ha-1 a-1?,two different nitrogen levels treatments,namely CK?0 kg N ha-1 a-1,as ammonium nitrate?and NI?60 kg N ha-1 a-1?were set up.Three types of soil treatments were set up,i.e.,bare soil?BS?,Pinus massoniana litter plus soil?PS?,and Cunninghamia lanceolata litter plus soil?CS?,with 3 replicates for each treatment.Soils were maintained at 60%of its maximum water holding capacity and cultured at the temperature of 30? for the total period of 294 days.10 batches of gas samples were collected with dynamic chamber system and analyzed using preconcentrator-GC-MS.The main results of this work were as follows:1.The effect of nitrogen addition on NMHCs varied for different types of NMHCs.Overall,nitrogen addition promotes the release of NMHCs.For the accumulative emission amount of NMHCs,the different types of NMHCs fluxes for the same leaf litter treatment showed that:firstly,for both BS and CS treatment with similar results,there was no significant difference in the accumulative release among alkanes,alkenes,and aromatic hydrocarbons for CK treatment.For NI treatment,the accumulative release of aromatics was significantly higher than that of the alkane and alkenes,implying that nitrogen addition can promote the release of aromatic hydrocarbons from BS and CS.Secondly,the result of the PS treatment was different.In the CK treatment,the accumulated amount of alkenes was the most?2578.63 ± 363.12 ?mol m-2?,followed by alkanes?1059.51 ± 1462.84 ?mol m-2?and aromatic hydrocarbons?427.28 ± 604.68 ?mol m-2?.And the accumulated amount of alkenes was significantly higher than the others for NI treatment,reaching 5028.97±391.92 ?mol m-2 for alkenes in PS.In addition,there was significant difference of alkene accumulation between the two nitrogen levels?p<0.05?,suggesting that nitrogen addition could significantly promote the release of alkenes from leaf litter of Pinus massoniana.2.There were some difference in NMHCs emission between different litters.The difference among the different leaf litter treatment during the process of litter decomposition was not significant.But as for the NMHCs accumulation,In the CK treatment,the alkene flux in PS was significantly higher than that of the other two treatments?p<0.05?,with the value of 2578.63 ± 363.12 ?mol M-2 in PS.indicating that PS is the main source of alkenes in the absence of nitrogen.In the NI treatment,the order of alkene accumulation was:PS?5028.97± 391.92 ?mol m-2?>CS?385.79 ± 242.92 ?mol m-2?>BS?-373.30 ± 163.51 ?mol of m-2?.The alkene accumulation for the NI treatment was significantly higher than that for CK in PS,indicating that nitrogen addition significantly promoted the release of alkenes from the PS treatment.3.The flux of NMHCs in the early stage of leaf litter decomposition is greater than that in late stage.In general,the time when the fluxes of NMHCs reached maxmum or minimum appeared in the early stage of leaf litter decomposition.Firstly,alkane fluxes reached the maximum(535.95 ± 541.63 pmol m-2 s-1)and the minimum?-232.57 ± 127.93 pmol M-2 s-1?on the 20th day of litter decomposition in BS for both CK and NI treatment.In CS treatment,the alkane fluxes reached their minimum on the 38th day of leaf litter decomposition(-244.43 ± 98.04 pmol m-2 s-1)and on the 10th day?-132.80 ± 221.46 pmol M-2 s-1?for CK and NI.For PS,the alkane flux reached the lowest value(-161.14 ± 28.50 pmol m-2 s-1)and the maximum value(278.97 ± 245.47 pmol m-2 s-1)on the 10th day of litter decomposition.Secondly,For alkene flux,the maximum?72.07 ±82.32 pmol m-2 S-1?and the minimum(-51.75 ± 52.89 pmol m-2 s-1)values appeared on the 20th day of leaf litter decomposition in BS.Finally,in PS treatment,the minimum aromatic flux from CK(-93.05 ± 151.27 pmol m-2 s-1)and the maxmimum aromatic flux(317.94 ± 58.89 pmol m-2 s-1)from NI appeared on the 38th and 10th day respectively. |
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