Nitrogen cycling of atmosphere-plant-soft system in the typical Calamagrostis angustifolia wetland in the Sanjiang Plain,Northeast China

The nitrogen （N） distribution and cycling of atmosphere-plant-soil system in the typical meadow Calamagrostis angustifolia wetland （TMCW） and marsh meadow Calamagrostis angustifolia wetland （MMCW） in the Sanjiang plain were studied by a compartment model. The results showed that the N wet deposition amount was 0.757 gN/（m^2·a）, and total inorganic N （TIN） was the main body （0.640 gN/（m^2·a））. The ammonia volatilization amounts of TMCW and MMCW soils in growing season were 0.635 and 0.687 gN/m^2, and the denitrification gaseous lost amounts were 0.617 and 0.405 gN/m^2, respectively. In plant subsystem, the N was mainly stored in root and litter. Soil organic N was the main N storage of the two plant-soil systems and the proportions of it were 93.98% and 92.16%, respectively. The calculation results of N turnovers among compartments of TMCW and MMCW showed that the uptake amounts of root were 23.02 and 28.18 gN/（m^2·a） and the values of aboveground were 11.31 and 6.08 gN/（m^2·a）, the re-translocation amounts from aboveground to root were 5.96 and 2.70 gN/（m^2·a）, the translocation amounts from aboveground living body to litter were 5.35 and 3.38 gN/（m^2·a）, the translocation amounts from litter to soil were larger than 1.55 and 3.01 gN/（m^2·a）, the translocation amounts from root to soil were 14.90 and 13.17 gN/（m^2·a）, and the soil （0-15 cm） N net mineralization amounts were 1.94 and 0.55 gN/（m^2·a）, respectively. The study of N balance indicated that the two plant-soil systems might be situated in the status of lacking N, and the status might induce the degradation of C. angustifolia wetland.

Effects of elevated atmospheric CO2 and nitrogen fertilization on nitrogen cycling in experimental riparian wetlands

Studies on the relationship between plant nitrogen content and soil nitrogen reduction under elevated CO2 conditions and with different nitrogen additions in wetland ecosystems are lacking. This study was meant to assess the effects of elevated CO2 concentrations and inorganic nitrogen additions on soil and plant nitrogen cycling. A cultured riparian wetland, alligator weeds, and two duplicated open top chambers （OTCs） with ambient （380μmol/mol） and elevated （700 μmol/mol） CO2 concentrations at low （4 mg/L） and high （6 mg/L） nitrogen fertilization levels were used. The total plant biomass increased by 30.77% and 31.37% at low and high nitrogen fertilization levels, respectively, under elevated CO2 conditions. Plant nitrogen content decreased by 6.54% and 8.86% at low and high nitrogen fertilization levels, respectively. The coefficient of determination （R2） of soil nitrogen contents ranged from 0.81 to 0.96. Under elevated CO2 conditions, plants utilized the assimilated inorganic nitrogen （from the soil） for growth and other internal physiological transformations, which might explain the reduction in plant nitrogen content. A reduction in soil dissolved inorganic nitrogen （DIN） under elevated CO2 conditions might have also caused the reduction in plant nitrogen content. Reduced plant and soil nitrogen contents are to be expected due to the potential exhaustive use of inorganic nitrogen by soil microorganisms even before it can be made available to the soil and plants. The results from this study provide important information to help policy makers make informed decisions on sustainable management of wetlands. Larger-scale field work is recommended in future research.

The role of biocrusts in nitrogen cycling on the tropical reef islands,South China Sea

Harboring polyextremotolerant microbial topsoil communities,biological soil crusts(biocrusts)occur across various climatic zones,and have been well studied in the terrestrial drylands.However,little is known about the functional metabolic potential of microbial communities involved in the biogeochemical processes during the early succession of biocrusts on the tropical reef islands.We collected 26 biocrusts and bare soil samples from the Xisha Islands and Nansha Islands,and applied a functional gene array(GeoChip 5.0)to reveal nitrogen(N)cycling processes involved in these samples.Both physicochemical measurement and enzyme activity assay were utilized to characterize the soil properties.Results revealed the composition of N-cycling functional genes in biocrusts was distinct from that in bare soil.Additionally,microorganisms in biocrusts showed lower functional potential related to ammonification,denitrification,N assimilation,nitrification,N fixation,and dissimilatory nitrate reduction to ammonium compared to bare soils.Although the abundance of nifH gene was lower in biocrusts,nitrogenase activity was significantly higher compared to that in bare soils.Precipitation,soil physicochemical properties(i.e.,soil available copper,soil ammonia N and pH)and soil biological properties(i.e.,β-glucosidase,fluorescein diacetate hydrolase,alkaline protease,urease,alkaline phosphatase,catalase and chlorophyll a)correlated to the N-cycling functional genes structure.Nitrate N and ammonia N were more abundant in biocrusts than bare soil,while pH value was higher in bare soil.Our results suggested biocrusts play an important role in N-cycling in coral sand soil,and will be helpful in understanding the development and ecological functions of biocrusts on tropical reef islands.

Description and Application of a Model for Simulating Regional Nitrogen Cycling and Calculating Nitrogen Flux

A regional nitrogen cycle model, named IAP-N, was designed for simulating regional nitrogen （N） cycling and calculating N fluxes flowing among cultivated soils, crops, and livestock, as well as human, atmospheric and other systems. The conceptual structure and calculation methods and procedures of this model are described in detail. All equations of the model are presented. In addition, definitions of all the involved variables and parameters are given. An application of the model in China at the national scale is presented. In this example, annual surpluses of consumed synthetic N fertilizer; emissions of nitrous oxide （N2O）, ammonia （NH3） and nitrogen oxide （NOx）; N loss from agricultural lands due to leaching and runoff; and sources and sinks of anthropogenic reactive N （Nr） were estimated for the period 1961-2004. The model estimates show that surpluses of N fertilizer started to occur in the mid 1990s and amounted to 5.7 Tg N yr^-1 in the early 2000s. N20 emissions related to agriculture were estimated as 0.69 Tg N yr^-1 in 2004, of which 58% was released directly from N added to agricultural soils. Total NH3 and NOx emissions in 2004 amounted to 4.7 and 4.9 Tg N yr^-1, respectively. About 3.9 Tg N yr^-1 of N was estimated to have flowed out of the cultivated soil layer in 2004, which accounted for 33% of applied synthetic N fertilizer. Anthropogenic Nr sources changed from 2.8 （1961） to 28.1 Tg N yr^-1 （2004）, while removal （sinks） changed from to 2.1 to 8.4 Tg N yr^-1. The ratio of anthropogenic Nr sources to sinks was only 1.4 in 1961 but 3.3 in 2004. Further development of the IAP-N model is suggested to focus upon： Ca） inter-comparison with other regional N models; （b） overcoming the limitations of the current model version, such as adaptation to other regions, high-resolution database, and so on; and （c） developing the capacity to estimate the safe threshold of anthropogenic Nr source to sink ratios.

Cadaverine and putrescine exposure influence carbon and nitrogen cycling genes in water and sediment of the Yellow River

The response patterns of microbial functional genes involved in biogeochemical cycles to cadaver decay is a central topic of recent environmental sciences.However,the response mechanisms and pathways of the functional genes associated with the carbon(C)and nitrogen(N)cycling to cadaveric substances such as cadaverine and putrescine remain unclear.This study explored the variation of functional genes associated with C fixation,C degradation and N cycling and their influencing factors under cadaverine,putrescine and mixed treatments.Our results showed only putrescine significantly increased the alpha diversity of C fixation genes,while reducing the alpha diversity of N cycling genes in sediment.For the C cycling,the mixed treatment significantly decreased the total abundance of reductive acetyl-CoA pathway genes(i.e.,acsB and acsE)and lig gene linked to lignin degradation in water,while only significantly increasing the hydroxypropionate-hydroxybutylate cycle(i.e.,accA)gene abundance in sediment.For the N cycling,mixed treatment significantly decreased the abundance of the nitrification(i.e.,amoB),denitrification(i.e.,nirS3)genes in water and the assimilation pathway gene(i.e.,gdhA)in sediment.Environmental factors(i.e.,total carbon and total nitrogen)were all negatively associated with the genes of C and N cycling.Therefore,cadaverine and putrescine exposure may inhibit the pathway in C fixation and N cycling,while promoting C degradation.These findings can offer some new insight for the management of amine pollution caused by animal cadavers.

Multiple isotopes reveal the driving mechanism of high NO_(3)^(-) level and key processes of nitrogen cycling in the lower reaches of Yellow River

The continuous increase of nitrate(NO_(3)^(-))level in rivers is a hot issue in the world.However,the driving mechanism of high NO_(3)^(-)level in large rivers is still lacking,which has limited the use of river water and increased the cost of water treatment.In this study,multiple isotopes and source resolution models are applied to identify the driving mechanism of high NO_(3)^(-)level and key processes of nitrogen cycling in the lower reaches of the Yellow River(LRYR).The major sources of NO_(3)^(-)were sewage and manure(SAM)in the low-flow season and soil nitrogen(SN)and chemical fertilizer(CF)in the high-flow season.Nitrification was the most key process of nitrogen cycling in the LRYR.However,in the biological removal processes,denitrification may not occur significantly.The temporal variation of contributions of NO_(3)^(-)sources were estimated by a source resolution model in the LRYR.The proportional contributions of SAM and CF to NO_(3)^(-)in the low-flow and high-flow season were 32.5%-52.3%,44.2%-46.2%and 36.0%-40.8%,54.9%-56.9%,respectively.The driving mechanisms of high NO_(3)^(-)level were unreasonable sewage discharge,intensity rainfall runoff,nitrification and lack of nitrate removal capacity.To control the NO_(3)^(-)concentration,targeted measures should be implemented to improve the capacity of sewage and wastewater treatment,increase the utilization efficiency of nitrogen fertilizer and construct ecological engineering.This study deepens the understanding of the driving mechanism of high nitrate level and provides a vital reference for nitrogen pollution control in rivers to other area of the world.

SOIL NITROGEN CYCLING AND ENVIRONMENTAL IMPACTS IN THE SUBTROPICAL HILLY REGION OF CHINA: EVIDENCE FROM MEASUREMENTS AND MODELING

The subtropical hilly region of China is a region with intensive crop and livestock production,which has resulted in serious N pollution in soil,water and air.This review summarizes the major soil N cycling processes and their influencing factors in rice paddies and uplands in the subtropical hilly region of China.The major N cycling processes include the N fertilizer application in croplands,atmospheric N deposition,biological N fixation,crop N uptake,ammonia volatilization,N_(2)O/NO emissions,nitrogen runoff and leaching losses.The catchment nutrients management model for N cycle modeling and its case studies in the subtropical hilly region were also introduced.Finally,N management practices for improving N use efficiency in cropland,as well as catchment scales are summarized.

Characterization of the genes involved in nitrogen cycling in wastewater treatment plants using DNA microarray and most probable number-PCR

To improve nitrogen removal performance of wastewater treatment plants （WWTPs）, it is essential to understand the behavior of nitrogen cycling communities, which comprise various microorganisms. This study characterized the quantity and diversity of nitrogen cycling genes in various processes of municipal WWTPs by employing two molecular-based methods：most probable number-polymerase chain reaction （MPN-PCR） and DNA microarray. MPN-PCR analysis revealed that gene quantities were not statistically different among processes, suggesting that conventional actwated sludge processes （CAS） are similar to nitrogen removal processes in their ability to retain an adequate population of nitrogen cycling microorganisms. Furthermore, most processes in the WWTPs that were researched shared a pattern：the nitS and the bacterial amoA genes were more abundant than the nirK and archaeal amoA genes, respectivelv. DNA microarray analysis revealed that several kinds of nitrification and denitrification genes were detected in both CAS and anaerobic-oxic processes （AO）, whereas limited genes were detected in nitrogen removal processes. Results of this study suggest that CAS maintains a diverse community of nitrogen cycling microorganisms; moreover, the microbial communities in nitrogen removal processes may be specific.

Refining the Factors Affecting N_(2)O Emissions from Upland Soils with and without Nitrogen Fertilizer Application at a Global Scale

Nitrous oxide(N_(2)O)is a long-lived greenhouse gas that mainly originates from agricultural soils.More and more studies have explored the sources,influencing factors and effective mitigation measures of N_(2)O in recent decades.However,the hierarchy of factors influencing N_(2)O emissions from agricultural soils at the global scale remains unclear.In this study,we carry out correlation and structural equation modeling analysis on a global N_(2)O emission dataset to explore the hierarchy of influencing factors affecting N_(2)O emissions from the nitrogen(N)and non-N fertilized upland farming systems,in terms of climatic factors,soil properties,and agricultural practices.Our results show that the average N_(2)O emission intensity in the N fertilized soils(17.83 g N ha^(-1)d^(-1))was significantly greater than that in the non-N fertilized soils(5.34 g N ha^(−1) d^(−1))(p<0.001).Climate factors and agricultural practices are the most important influencing factors on N_(2)O emission in non-N and N fertilized upland soils,respectively.For different climatic zones,without fertilizer,the primary influence factors on soil N_(2)O emissions are soil physical properties in subtropical monsoon zone,whereas climatic factors are key in the temperate zones.With fertilizer,the primary influence factors for subtropical monsoon and temperate continental zones are soil physical properties,while agricultural measures are the main factors in the temperate monsoon zone.Deploying enhanced agricultural practices,such as reduced N fertilizer rate combined with the addition of nitrification and urease inhibitors can potentially mitigate N_(2)O emissions by more than 60%in upland farming systems.

Tracing nitrate sources in one of the world's largest eutrophicated bays(Hangzhou Bay):insights from nitrogen and oxygen isotopes

Eutrophication caused by inputs of excess nitrogen(N) has become a serious environmental problem in Hangzhou Bay(China),but the sources of this nitrogen are not well understood.In this study,the August 2019 distributions of salinity,nutrients [nitrate(NO_(3)^(-)),nitrite,ammonium,and phosphate],and the stable isotopic composition of NO_(3)^(-)(δ^(15)N and δ^(18)O) were used to investigate sources of dissolved inorganic nitrogen(DIN) to Hangzhou B ay.Spatial distributions of nitrate,salinity,and nitrate δ^(18)O indicate that the Qiantang River,the Changjiang River,and nearshore coastal waters may all contribute nitrate to the bay.Based on the isotopic compositions of nitrate in these potential source waters and conservative mixing of nitrate in our study area,we suggest that the NO_(3)^(- )in Hangzhou B ay was likely derived mainly from soils,synthetic N fertilizer,and manure and sewage.End-member modeling indicates that in the upper half of the bay,the Qiantang River was a very important DIN source,possibly contributing more than 50% of DIN in the bay head area.In the lower half of the bay,DIN was sourced mainly from strongly intruding coastal water.DIN coming directly from the Changjiang River made a relatively small contribution to Hangzhou Bay DIN in August 2019.

Review of study on mineralization, saturation and cycle of Nitrogen in forest ecosystems

Nitrogen is one of the most important elements that can limit plant growth in forest ecosystems. Studies of nitrogen mineralization, nitrogen saturation and nitrogen cycle in forest ecosystems is very necessary for understanding the productivity of stand, nutrient cycle and turnover of nitrogen of forest ecosystems. Based on comparison and analysis of domestic and in-ternational academic references related to studies on nitrogen mineralization, nitrogen saturation and nitrogen cycle in recent 10 years, the current situation and development of the study on these aspects, and the problems existed in current researches were reviewed. At last, some advices were given for future researches.

Comparisons of the effects of different drying methods on soil nitrogen fractions:Insights into emissions of reactive nitrogen gases(HONO and NO)

Reactive nitrogen(Nr)emission from soils,e.g.,nitrous acid(HONO)and nitric oxide(NO),is a key process of the global nitrogen(N)cycle and has significant implications for atmospheric chemistry.To understand the underlying mechanisms of soil Nr emissions,air-dried or oven-dried soils are commonly used in the laboratory.To date,few studies have compared the effects of different drying methods on soil Nr gas fluxes and N fractions.Here,the authors studied soil water content,pH,(in)organic N content,and Nr gas fluxes of air-dried,freeze-dried,oven-dried,and fresh soils from different land-use types.The results showed that the soil pH of air-dried and oven-dried samples was significantly lower compared with fresh soil from farmland and grassland,but higher compared with forest soil.The difference in soil pH between freeze-dried and fresh soil(mean±standard deviation:0.52±0.31)was the lowest.In general,all drying methods increased the soil NH4+-N,NO3−-N,and dissolved organic N contents compared with fresh soil(P<0.05).The maximum HONO and NO flux and total emissions during a full wetting–drying cycle of fresh soil were also increased by air-drying and oven-drying(P<0.001),but comparable with freeze-dried soil(P>0.2).In conclusion,all drying methods should be considered for use in studies on the land–atmosphere interface and biogeochemical N cycling,whereas the freeze-drying method might be better for studies involving the measurement of soil Nr gas fluxes.

Effects of phenolic acids on soil nitrogen mineralization over successive rotations in Chinese fir plantations

Phenolic acids are secondary metabolites of plants that significantly affect nutrient cycling processes.To investigate such effects,the soil available nitrogen(N)content,phenolic acid content,and net N mineralization rate in three successive rotations of Chinese fir plantations in subtropical China were investigated.Net N mineralization and nitrification rates in soils treated with phenolic acids were measured in an ex situ experiment.Compared with first-rotation plantations(FCP),the contents of total soil nitrogen and nitrate in second(SCP)-and third-rotation plantations(TCP)decreased,and that of soil ammonium increased.Soil net N mineralization rates in the second-and third-rotation plantations also increased by 17.8%and 39.9%,respectively.In contrast,soil net nitrification rates decreased by 18.0%and 25.0%,respectively.The concentrations of total phenolic acids in the FCP soils(123.22±6.02 nmol g^-1)were 3.0%and 17.9%higher than in the SCP(119.68±11.69 nmol g^-1)and TCP(104.51±8.57 nmol g^-1,respectively).The total content of phenolic acids was significantly correlated with the rates of net soil N mineralization and net nitrification.The ex situ experiment showed that the net N mineralization rates in soils treated with high(HCPA,0.07 mg N kg^-1 day^-1)and low(LCPA,0.18 mg N kg^-1 day^-1)concentrations of phenolic acids significantly decreased by 78.6%and 42.6%,respectively,comparing with that in control(0.32 mg N kg^-1 day^-1).Soil net nitrification rates under HCPA and LCPA were significantly higher than that of the control.The results suggested that low contents of phenolic acids in soil over successive rotations increased soil net N mineralization rates and decreased net nitrification rates,leading to consequent reductions in the nitrate content and enhancement of the ammonium content,then resulting in enhancing the conservation of soil N of successive rotations in Chinese fir plantation.

Microbial activity related to N cycling in the rhizosphere of maize stressed by heavy metals

A greenhouse experiment was carried out to compare differences in potential activities of ammonification, nitrification and denitrification in rhizosphere and bulk soil in a heavy-metal-stressed system. Exchangeable fractions of Cd, Cu and Cr were all higher in the rhizosphere of maize than in bulk soil. Results showed that the mineralization of N in soil was stimulated by low concentration of Cd. Addition of Cd at low levels stimulated the ammonifying and nitrifying activity in soil, while inhibitory influences were shown at high levels. Nitrifying bacteria was proved to be the most sensitive one, whilst the effect on denitrifying bacteria was very limited. Comparing Cd, Cu and Cr(VI) at 20 mg/kg soil, Cd was the most effective inhibitor of ammonification and denitrification, while Cr(VI) had the strongest inhibitory influence on nitrifying activity. Root exudates played important roles on the different exchangeable metal fractions and bacterial activities between rhizosphere and non-rhizosphere. Nitrate was the main form of mineral N in soil, as well as the main form of N absorbed by plants, but the formation and relative absorption of ammonium were promoted in response to high Cd exposure.

Summary of Recent Climate Change Studies on the Carbon and Nitrogen Cycles in the Terrestrial Ecosystem and Ocean in China

This article reviews recent advances over the past and their relationship to climate change in China. The was 0.19-0.26 Pg C yr-1 for the 1980s and 1990s. 4 years in the study of the carbon-nitrogen cycling net carbon sink in the Chinese terrestrial ecosystem Both natural wetlands and the rice-paddy regions emitted 1.76 Tg and 6.62 Tg of CH4 per year for the periods 1995 2004 and 2005 2009, respectively. China emitted -1.1 Tg N20-N yr-1 to the atmosphere in 2004. Land soil contained -8.3 Pg N. The excess nitrogen stored in farmland of the Yangtze River basin reached 1.51 Tg N and 2.67 Tg N in 1980 and 1990, respectively. The outer Yangtze Estuary served as a moderate or significant sink of atmospheric CO2 except in autumn. Phytoplankton could take up carbon at a rate of 6.4 ×1011 kg yr-1 in the China Sea. The global ocean absorbed anthropogenic CO2 at the rates of 1.64 and 1.73 Pg C yr-1 for two sinmlations in the 1990s. Land net ecosystem production in China would increase until the mid-21st century then would decrease gradually under future climate change scenarios. This research should be strengthened in the future, including collection of more observation data, measurement of the soil organic carbon （SOC） loss and sequestration, evaluation of changes in SOC in deep soil layers, and the impacts of grassland management, carbon-nitrogen coupled effects, and development and improvement of various component models and of the coupled carbon cycle-climate model.

A process model for simulating net primary productivity (NPP) based on the interaction of water-heat process and nitrogen: a case study in Lantsang valley

Terrestrial carbon cycle and the global atmospheric CO2 budget are important foci in global climate change research. Simulating net primary productivity （NPP） of terrestrial ecosystems is important for carbon cycle research. In this study, a plant-atmosphere-soil continuum nitrogen （N） cycling model was developed and incorporated into the Boreal Ecosystem Productivity Simulator （BEPS） model. With the established database （leaf area index, land cover, daily meteorology data, vegetation and soil） at a 1 km resolution, daily maps of NPP for Lantsang valley in 2007 were produced, and the spatial-temporal patterns of NPP and mechanisms of its responses to soil N level were further explored. The total NPP and mean NPP of Lantsang valley in 2007 were 66.5 Tg C and 416 g？m-2？a-1 C, respectively. In addition, statistical analysis of NPP of different land cover types was conducted and investigated. Compared with BEPS model （without considering nitrogen effect）, it was inferred that the plant carbon fixing for the upstream of Lantsang valley was also limited by soil available nitrogen besides temperature and precipitation. However, nitrogen has no evident limitation to NPP accumulation of broadleaf forest, which mainly distributed in the downstream of Lantsang valley.

Responses of nutrient biogeochemistry and nitrogen cycle to seasonal upwelling in coastal waters of the eastern Hainan Island

The coastal upwelling has profound influence on the surrounding ecosystem by supplying the nutrient-replete water to the euphotic zone.Nutrient biogeochemistry was investigated in coastal waters of the eastern Hainan Island in summer 2015 and autumn 2016.From perspectives of nutrient dynamics and physical transport,the nutrient fluxes entered the upper 50 m water depth(between the mixed layer and the euphotic zone)arisen from the upwelling were estimated to be 2.5-5.4 mmol/(m^(2)·d),0.15-0.28 mmol/(m^(2)·d),and 2.2-7.2 mmol/(m^(2)·d)for dissolved inorganic nitrogen(DIN),phosphate(DIP),and dissolved silicate(DSi),respectively,which were around 6-to 12-fold those in the background area.The upwelled nutrients supported an additional plankton growth of(14.70±8.95)mg/m^(2)for chlorophyll a(Chl a).The distributions of nitrateδ^(15)N andδ^(18)O above the 300 m water depth(top of the North Pacific Intermediate Water)were different among the upwelling area,background area in summer,and the stations in autumn,and the difference of environmental and biogeochemical conditions between seasons should be the reason.The higher DIN/DIP concentration ratio,nitrate concentration anomaly,and lower nitrate isotope anomaly(Δ(15,18))in the upper ocean in summer than in autumn indicated the stronger nitrogen fixation and atmospheric deposition,and the following fixed nitrogen regeneration in summer.The higher values of Chl a and nitrateδ^(15)N andδ^(18)O within the euphotic zone in autumn than the background area in summer suggested the stronger nitrate assimilation in autumn.The differences in relatively strength of the assimilation,nitrogen fixation and atmospheric deposition,and the following remineralization and nitrification between the two seasons made the higherδ^(18)O:δ^(15)N and larger difference of enzymatic isotope fractionation factors^(15)εand^(18)εfor nitrate assimilation in summer than in autumn above the North Pacific Tropical Water.

Effects and underlying mechanisms of damming on carbon and nitrogen cycles and transport in rivers of Southwest China:project introduction

Southwest China is the primary area for damming rivers to produce hydroelectric energy and store water.River damming has changed hydrodynamic,chemical,and biological processes,which are related to sinks and sources of greenhouse gases and carbon and nitrogen fluxes of different interfaces.Here,I provide an introduction to a river damming-related foundation,the National Key R&D Program of China(2016YTA0601000).Supported by the foundation,we carried out research on multiprocesses/multi-interfaces of carbon and nitrogen biogeochemical cycles in a dammed river system and have produced important results,as presented in this issue of the journal.

Dynamics of Microbial Activity Related to N Cycling in Cd-Contaminated Soil During Growth of Soybean

The potential influences of cadmium (Cd) on the biochemical processes of the soil nitrogen (N) cycle, along with the dynamics of ammonification, nitrification, and denitrification processes in the rhizosphere and non-rhizosphere (bulk soil), respectively, were investigated in a Cd-stressed system during an entire soybean growing season. In terms of Cd pollution at the seedling stage, the ammonifying bacteria proved to be the most sensitive microorganisms, whereas the effects of Cd on denitrification were not obvious. Following the growth of soybeans, the influences of Cd on ammonification in the bulk soil were: toxic impacts at the seedling stage, stimulatory effects during the early flowering stage, and adaptation to the pollutant during the podding and ripening stages. Although nitrification and denitrification in the bulk soil decreased throughout the entire growth cycle, positive adaptation to Cd stress was observed during the ripening stage. Moreover, during the ripening stage, denitrification in the bulk soil under high Cd treatment (20 mg kg-1) was even higher than that in the control, indicating a probable change in the ecology of the denitrifying microbes in the Cd-stressed system. Changes in the activity of microbes in the rhizosphere following plant growth were similar to those in the non-rhizosphere in Cd treatments; however, the tendency of change in the rhizosphere seemed to be more moderate. This suggested that there was some mitigation of Cd stress in the rhizosphere.

The integration of nitrogen dynamics into a land surface model. Part 1: model description and site-scale validation

Nitrogen cycling has profound effects on carbon uptake in the terrestrial ecosystem and the response of the biosphere to climate changes.However,nutrient cycling is not taken into account in most land surface models for climate change.In this study,a nitrogen model,based on nitrogen transformation processes and nitrogen fluxes exchange between the atmosphere and terrestrial ecosystem,was incorporated into the Atmosphere–Vegetation Interaction Model(AVIM)to simulate the carbon cycle under nitrogen limitation.This new model,AVIM-CN,was evaluated against site-scale eddy covariance–based measurements of an alpine meadow located at Damxung station from the FLUXNET 2015 dataset.Results showed that the annual mean gross primary production simulated by AVIM-CN(0.7073 gC m^-2 d^-1)was in better agreement with the corresponding flux data(0.5407 gC m^-2 d^-1)than the original AVIM(1.1403 gC m^-2 d^-1)at Damxung station.Similarly,ecosystem respiration was also down-regulated,from 1.7695 gC m^-2 d^-1 to 1.0572 gC m^-2 d^-1,after the nitrogen processes were introduced,and the latter was closer to the observed vales(0.8034 gC m^-2 d^-1).Overall,the new results were more consistent with the daily time series of carbon and energy fluxes of observations compared to the former version without nitrogen dynamics.A model that does not incorporate the limitation effects of nitrogen nutrient availability will probably overestimate carbon fluxes by about 40%.