Atmospheric nitrogen deposition in Yangtze River Delta:insights gained from the nitrogen content and isotopic composition of the moss Haplocladium microphyllum

Simple and inexpensive estimation of the rates and sources of atmospheric nitrogen(N)deposition is critical for its effective mitigation in a region with different land-use types.In this study,the N content and N isotopic composition(δ15N)of moss(Haplocladium microphyllum)tissues and precipitation at six sites with three land-use types(urban,suburban,and rural)were measured in the Yangtze River Delta.A significant linear relationship between moss N content and wet N deposition,and a consistent decrease trend for moss N content and wet N deposition from urban to suburban to rural areas were observed.More negativeδ15N of suburban and rural mosses indicated N mainly released from agriculture and effluent,while the less negativeδ15N of urban mosses were mainly influenced by fossil fuel combustion and traffic emissions.Although the negative mossδ15N indicates that reduced N dominates wet N deposition,there was no significant correlation between mossδ15N and the ratio of ammonium to nitrate(NH4+/NO3−).These results reveal that the moss N content andδ15N can be used as a complementary tool for estimating the rates and sources of wet N deposition in a region with different land-use types.

Responses of soil CH_(4) fluxes to nitrogen addition in two tropical montane rainforests in southern China

Background:Atmospheric nitrogen(N)deposition is projected to increase in the next few decades,which may have a marked impact on soil-atmosphere CH_(4) fluxes.However,the impacts of increased atmospheric N depositions on soil CH_(4) flux in tropical rainforests are still poorly understood.From January 2015 to December 2018,a field experiment was conducted in a primary tropical montane rainforest(PTMR)and a secondary tropical montane rainforest(STMR)in southern China to quantify the impact of N additions at four levels(N0:0 kg N⋅ha^(-1)⋅year^(-1);N25:25 kg N⋅ha^(-1)⋅year^(-1);N50:50 kg N⋅ha^(-1)⋅year^(-1);N100:100 kg N⋅ha^(-1)⋅year^(-1)on soil CH_(4) flux.Results:Four years of measurements showed clear seasonal variations in CH_(4) flux in all treatment plots for both forest types(PTMR and STMR),with lower rates of soil CH_(4) uptake during the wet season and higher rates of soil CH_(4) uptake during the dry season.Soil CH_(4) uptake rates were significantly and negatively correlated with both soil temperature and soil moisture for both forest types.Annual CH_(4) uptake for the N0 plots from the PTMR and STMR soils were2.20 and1.98 kg N⋅ha^(-1)⋅year^(-1),respectively.At the PTMR site,mean CH_(4) uptake compared with the N0 treatment was reduced by 19%,29%,and 36%for the N25,N50,and N100 treatments,respectively.At the STMR site,mean CH_(4) uptake compared with the N0 treatment was reduced by 15%,18%,and 38%for the N25,N50,and N100 treatments,respectively.High level N addition had a stronger inhibitory impact on soil CH_(4) uptake than did the low level N addition.Conclusion:Our data suggest that soil CH_(4) uptake in tropical rainforests is sensitive to N deposition.If atmospheric N deposition continues to increase in the future,the soil CH_(4) sink strength of tropical rainforests may weaken further.

Atmospheric deposition of inorganic nitrogen in a semi-arid grassland of Inner Mongolia, China

Due to increasing global demand for crop production and energy use, more and more reactive nitrogen（Nr） has been generated and emitted to the environment. As a result, global atmospheric nitrogen（N） deposition has tripled since the industrial revolution and the ecological environment and human health have been harmed. In this study, we measured dry and wet/bulk N deposition from July 2013 to December 2015 in a semi-arid grassland of Duolun County, Inner Mongolia, China. The samples of dry and wet/bulk N deposition were collected monthly with a DELTA（DEnuder for Long Term Atmospheric sampling） system and with Gradko passive samplers and a precipitation gauge. The measured results show that the annual mean concentrations of NH_3, NO_2, HNO_3, particulate NH_4~＋（pNH_4~＋） and particulate NO_3^-（pNO_3^-） in atmosphere were 2.33, 1.90, 0.18, 1.42 and 0.42 μg N/m3, respectively, and that the annual mean volume-weighted concentrations of NH_4~＋-N and NO_3^--N in precipitation were 2.71 and 1.99 mg N/L, respectively. The concentrations of Nr components（including NH_3, NO_2, HNO_3, p NH_4~＋, pNO_3^-, NH_4~＋-N and NO_3^--N） exhibited different seasonal variations. Specifically, NO_2 and HNO_3 exhibited higher concentrations in autumn than in summer, while the other Nr components（NH_3, pNH_4~＋, pNO_3^-, NH_4~＋-N and NO_3^--N） showed the highest values in summer. Based on measured concentrations of Nr components and their deposition velocities estimated using the GEOS-Chem global atmospheric chemical transport model, the calculated annual mean dry deposition fluxes were 3.17, 1.13, 0.63, 0.91 and 0.36 kg N/（hm^2·a） for NH_3, NO_2, HNO_3, p NH_4~＋ and pNO_3^-, respectively, and the calculated annual mean wet/bulk deposition fluxes were 5.37 and 3.15 kg N/（hm^2·a） for NH_4~＋-N and NO_3^--N, respectively. The estimated annual N deposition（including dry N deposition and wet/bulk N deposition） reached 14.7 kg N/（hm^2·a） in grassland of Duolun County, approaching to the upper limit of the N critical load（10–15 kg N/（hm^2·a））. Dry and wet/bulk deposition fluxes of all Nr components（with an exception of HNO_3） showed similar seasonal variations with the maximum deposition flux in summer and the minimum in winter. Reduced Nr components（e.g., gaseous NH_3 and p NH_4~＋ in atmosphere and NH_4~＋-N in precipitation） dominated the total N deposition at the sampling site（accounted for 64% of the total N deposition）, suggesting that the deposited atmospheric Nr mainly originated from agricultural activities. Considering the projected future increases in crop and livestock production in Inner Mongolia, the ecological and human risks to the negative effects of increased N deposition could be increased if no mitigation measures are taken.

In situ reaction mechanism of MgAlON in Al–Al2O3–MgO composites at 1700°C under flowing N2

The Al–AlO–MgO composites with added aluminum contents of approximately 0wt%, 5wt%, and 10wt%, named as M, M, and M, respectively, were prepared at 1700°C for 5 h under a flowing Natmosphere using the reaction sintering method. After sintering, the Al–AlO–MgO composites were characterized and analyzed by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The results show that specimen Mwas composed of MgO and MgAlO. Compared with specimen M, specimens Mand Mpossessed MgAlON, and its production increased with increasing aluminum addition. Under an Natmosphere, MgO, AlO, and Al in the matrix of specimens Mand Mreacted to form MgAlON and AlN-polytypoids, which combined the particles and the matrix together and imparted the Al–AlO–MgO composites with a dense structure. The mechanism of MgAlON synthesis is described as follows. Under an Natmosphere, the partial pressure of oxygen is quite low; thus, when the Al–AlO–MgO composites were soaked at 580°C for an extended period, aluminum metal was transformed into AlN. With increasing temperature, AlOdiffused into AlN crystal lattices and formed AlN-polytypoids; however, MgO reacted with AlOto form MgAlO. When the temperature was greater than(1640 ± 10)°C, AlN diffused into AlOand formed spinel-structured AlON. In situ MgAlON was acquired through a solid-solution reaction between AlON and Mg AlOat high temperatures because of their similar spinel structures.

Trends in carbon sink along the Belt and Road in the future under high emission scenario

Over the past three decades,the drawdown of atmospheric CO_(2) in vegetation and soil has fueled net ecosystem production(NEP).Here,a global land-surface model(CABLE)is used to estimate the trend in NEP and its response to atmospheric CO_(2),climate change,biological nitrogen(N)fixation,and N deposition under future conditions from 2031 to 2100 in the Belt and Road region.The trend of NEP simulated by CABLE decreases from 0.015 Pg carbon(C)yr^(-2) under present conditions(1936–2005)to−0.023 Pg C yr^(-2) under future conditions.In contrast,the trend in NEP of the CMIP6 ensemble changes from 0.014 Pg C yr^(-2) under present conditions to−0.009 Pg C yr^(-2) under future conditions.This suggests that the trend in the C sink for the Belt and Road region will likely decline in the future.The significant difference in the NEP trend between present and future conditions is mainly caused by the difference in the impact of climate change on NEP.Considering the responses of soil respiration(RH)or net primary production(NPP)to surface air temperature,the trend in surface air temperature changes from 0.01℃ yr^(-1) under present conditions to 0.05℃ yr^(-1) under future conditions.CABLE simulates a greater response of RH to surface temperature than that of NPP under future conditions,which causes a decreasing trend in NEP.In addition,the greater decreasing trend in NEP under future conditions indicates that the C-climate-N interaction at the regional scale should be considered.It is important to estimate the direction and magnitude of C sinks under the C neutrality target.

Isotopic characteristics and source analysis of atmospheric ammonia during agricultural periods in the Xichuan area of the Danjiangkou Reservoir

Nitrogen deposition is an important means of exogenous nitrogen input in reservoir water.Agricultural activities around the reservoir lead to a sharp increase in the concentration of ammonia in the atmosphere,which poses a threat to the reservoir water body.Clarifying the contribution of agricultural ammonia release to atmospheric NH_(x)(gaseous NH_(3)and particulate NH_(4)^(+)),in the reservoir area can provide a theoretical foundation for local reactive nitrogen control.We collected atmospheric NH_(3)and NH_(4)^(+)samples during the agricultural periods and analyzed the isotopic characteristics of atmospheric NH_(x)and the contribution rates of different ammonia sources in the Xichuan area of the Danjiangkou Reservoir.The results showed that the initialδ~(15)N values of NH_(3)(-30.0‰to–7.2‰)and particulate NH_(4)^(+)(–33‰to+4.9‰for finer and coarser particles,respectively)are different,and their contribution ratios from dissimilar ammonia sources are also different,among which NH_(4)^(+)is more susceptible to meteorological factors.However,since the atmospheric NH_(x)in the Xichuan area is mainly gaseous NH_(3),the final sources of atmospheric ammonia nitrogen source depend on gaseous NH_(3).Agricultural sources(59%-74%)were the main NH_(3)sources in this area.Among them,the fertilizer use emission was dominant;it had the highest contribution rate in summer during the agricultural period and a more prominent impact in areas with less human interference.Reasonable regulation of the application of high-ammonia releasing fertilizer,especially during the agricultural period in summer,is an effective way to reduce the threat of atmospheric ammonia to water health.

NitroMAC：An instrument for the measurement of HONO and intercomparison with a long-path absorption photometer

Nitro MAC（French acronym for continuous atmospheric measurements of nitrogenous compounds） is an instrument which has been developed for the semi-continuous measurement of atmospheric nitrous acid（HONO）. This instrument relies on wet chemical sampling and detection using high performance liquid chromatography（HPLC）-visible absorption at540 nm. Sampling proceeds by dissolution of gaseous HONO in a phosphate buffer solution followed by derivatization with sulfanilamide/N-（1-naphthyl）-ethylenediamine. The performance of this instrument was found to be as follows： a detection limit of around 3 ppt with measurement uncertainty of 10% over an analysis time of 10 min. Intercomparison was made between the instrument and a long-path absorption photometer（LOPAP） during two experiments in different environments. First, air was sampled in a smog chamber with concentrations up to 18 ppb of nitrous acid. Nitro MAC and LOPAP measurements showed very good agreement. Then, in a second experiment, ambient air with HONO concentrations below250 ppt was sampled. While Nitro MAC showed its capability of measuring HONO in moderate and highly polluted environments, the intercomparison results in ambient air highlighted that corrections must be made for minor interferences when low concentrations are measured.