模拟SO42-沉降对闽江口淡水感潮野慈姑湿地甲烷排放通量的影响

2015年12月—2016年10月,每月小潮日原位定期向闽江口塔礁洲淡水感潮野慈姑(Sagittaria trifolia L.)湿地施加剂量为60、120 kg S hm~(-2)a~(-1)的K_2SO_4溶液(分别记做S-60和S-120),探讨模拟硫酸根(SO_4~(2-))沉降对河口淡水感潮湿地甲烷(CH4)排放通量及间隙水SO_4~(2-)浓度的影响。对照、S-60和S-120处理组CH_4排放通量年均值分别为(7.88±1.00)mg h~(-1)m~(-2)、(6.55±0.97)mg h~(-1)m~(-2)和(6.66±1.49)mg h~(-1)m~(-2)。在年尺度上,两个高强度模拟SO_4~(2-)沉降处理组均未显著降低闽江口淡水感潮野慈姑湿地CH_4排放通量(P0.05),即高强度SO_4~(2-)沉降不会对河口淡水感潮湿地CH_4排放通量产生类似于其对泥炭湿地和水稻田的显著抑制效应。在年尺度以及秋、冬季,两个施加K_2SO_4溶液处理显著增加了野慈姑湿地10 cm深度土壤间隙水SO_4~(2-)浓度。对于各个处理组,温度较高的夏、秋季CH_4排放通量均显著高于温度相对较低的冬、春季(P0.05)。不同处理组CH_4排放通量均与土壤温度呈显著正相关关系,温度仍然是影响亚热带河口淡水感潮湿地CH_4排放通量的重要环境因子。     

Biotic controls on CO2 and CH4 exchange in wetlands – a closed environment study

Wetlands are significant sources of the important greenhouse gas CH₄. Here we explore the use of an experimental system developed for the determination of continuous fluxes of CO₂ and CH₄ in closed ecosystem monoliths including the capture of ¹⁴CO₂ and ¹⁴CH₄ following pulse labelling with ¹⁴CO₂. We show that, in the ecosystem studied, ebullition (bubble emission) may account for 18 to 50% of the total CH₄ emission, representing fluxes that have been difficult to estimate accurately in the past. Furthermore, using plant removal and ¹⁴C labelling techniques, we use the system to detail the direct influence of vascular plants on CH₄ emission. This influence is observed to be dependent on the amount of vascular plants present. The results that may be produced using the presented experimental set-up have implications for an improved understanding of wetland ecosystem/atmosphere interactions, including possible feedback effects on climate change. In recent years much attention has been devoted to ascertaining and subsequently using the relationship between net ecosystem productivity and CH₄ emission as a basis for extrapolation of fluxes across large areas. The experimental system presented may be used to study the complex relationship between vascular plants and CH₄ emission and here we show examples of how this may vary considerably in nature between and even within ecosystems.     

温带针阔混交林土壤碳氮气体通量的主控因子与耦合关系

中高纬度森林地区由于气候条件变化剧烈,土壤温室气体排放量的估算存在很大的不确定性,并且不同碳氮气体通量的主控因子与耦合关系尚不明确。以长白山温带针阔混交林为研究对象,采用静态箱-气相色谱法连续4a(2005—2009年)测定土壤二氧化碳(CO2)、甲烷(CH4)和氧化亚氮(N2O)净交换通量以及温度、水分等相关环境因子。研究结果表明:温带针阔混交林土壤整体上表现为CO2和N2O的排放源和CH4的吸收汇。土壤CH4、CO2和N2O通量的年均值分别为-1.3 kg CH4hm-2a-1、15102.2 kg CO2hm-2a-1和6.13 kg N2O hm-2a-1。土壤CO2通量呈现明显的季节性规律,主要受土壤温度的影响,水分次之;土壤CH4通量的季节变化不明显,与土壤水分显著正相关;土壤N2O通量季节变化与土壤CO2通量相似,与土壤水分、温度显著正相关。土壤CO2通量和CH4通量不存在任何类型的耦合关系,与N2O通量也不存在耦合关系;土壤CH4和N2O通量之间表现为消长型耦合关系。这项研究显示温带针阔混交林土壤碳氮气体通量主要受环境因子驱动,不同气体通量产生与消耗之间存在复杂的耦合关系,下一步研究需要深入探讨环境变化对其耦合关系的影响以及内在的生物驱动机制。     

张掖湿地甲烷通量动态特征及其影响因子

于2012年7月—2014年6月对地处干旱区的张掖湿地甲烷(CH_4)通量进行观测,分析其CH_4通量的变化特征及其影响因子。结果表明:CH_4通量的日变化趋势总体表现为白天大于夜间;不同季节CH_4通量排放特征差异明显,夏季最大,春秋次之,冬季最小;CH_4通量日总量与空气温度、土壤温度之间指数相关关系显著,其中4 cm处土壤温度与之相关性最强;1—6月摩擦风速(U*)与CH_4通量显著正相关;结合CO_2通量观测数据,研究时段张掖湿地净碳吸收量为495.92 g C m~(-2)a~(-1),为明显碳汇。     

Methane emissions in Danish riparian wetlands: Ecosystem comparison and pursuit of vegetation indexes as predictive tools

The present study was conducted to (i) investigate parameters influencing the fluxes of the greenhouse gas methane (CH₄) in Danish riparian wetlands with contrasting vegetation characteristics and (ii) develop models relating CH₄ emissions to soil and/or vegetation parameters integrating the spatial and temporal variability in the fluxes. Fluxes of CH₄ were monitored in 12 wetland plots over a year using static chambers, yielding a dataset with more than 800 measured fluxes of CH₄. Yearly emissions of CH₄ ranged from −0.2 to 38.3 g CH₄-C m⁻² year⁻¹, and significant effects of groundwater level, soil temperature (10 cm depth), peat depth, sulfate, nitrate, and soil carbon content were found. Two methods based on easily available environmental parameters to estimate yearly CH₄ emissions from riparian wetlands are presented. The first uses a generalized linear model (GLM) to predict yearly CH₄ emissions based on the humidity preference of vegetation (Ellenberg-F), peat depth and degree of humification of the peat (von Post index). The second method relies solely on plant species composition and uses weighted-average regression and calibration to link the vegetation assemblage to yearly CH₄ emission. Both models gave reliable predictions of the yearly CH₄ fluxes in riparian wetlands (modeling efficiency > 0.35). Our findings support the use of vegetation, possibly in combination with some soil parameters such as peat depth, as indicator of CH₄ emission in wetlands.     

不同干扰因素对森林和湿地温室气体通量影响的研究进展

森林和湿地是CO2、CH4和N2O等温室气体重要的源、汇和转换器,在全球气候变化过程中起着重要作用。森林和湿地温室气体通量受到诸多因子的作用,其中干扰便是一个重要的因素。不同干扰因素对于森林和湿地生态系统温室气体通量的影响,国际上已经开展了相应的研究。基于人为和自然两大类干扰方式,分别从采伐、施肥、垦殖等人为干扰因素和火烧、台风(飓风)等自然干扰因素综述了干扰对于森林和湿地生态系统CO2、CH4和N2O通量的影响。根据目前研究中存在的不足,提出了今后应需加强的领域,以期更好地揭示干扰对于森林和湿地生态系统温室气体通量的影响及作用机制,为今后深入开展相关研究提供一定的参考价值。     

湿地微生物介导的甲烷排放机制

湿地生态系统是陆地上巨大的有机碳库,同时也是大气中甲烷(CH_4)的主要排放源。由于CH_4对全球的增温潜能是CO2的34倍,因此关于湿地CH_4排放在全球气候变化中有关碳汇、碳源的研究具有极其重要的意义。全球80%–90%的CH_4排放离不开微生物活动,湿地生态系统中产CH_4菌和CH_4氧化菌的种类组成、数量及功能与CH_4通量密切相关,但基于湿地生态系统中介导CH_4循环的功能微生物对甲烷排放通量的影响及作用机制研究相对比较分散。为更好地认识微生物介导的CH_4排放过程的微生物调控机制,本文综述了湿地生态系统中参与CH_4循环的功能微生物,对介导CH_4循环相关微生物活性的影响因子进行了回顾,重点总结了湿地生态系统微生物介导的CH_4排放机制,并对未来的相关研究方向进行了展望。由于湿地微生物介导的碳循环过程也可能决定了湿地生态系统对全球气候变暖的反馈,因此本文也能为全球气候变化研究提供微生物方面的参考。     

Soil properties and sediment accretion modulate methane fluxes from restored wetlands

Wetlands are the largest source of methane (CH₄) globally, yet our understanding of how process‐level controls scale to ecosystem fluxes remains limited. It is particularly uncertain how variable soil properties influence ecosystem CH₄ emissions on annual time scales. We measured ecosystem carbon dioxide (CO₂) and CH₄ fluxes by eddy covariance from two wetlands recently restored on peat and alluvium soils within the Sacramento–San Joaquin Delta of California. Annual CH₄ fluxes from the alluvium wetland were significantly lower than the peat site for multiple years following restoration, but these differences were not explained by variation in dominant climate drivers or productivity across wetlands. Soil iron (Fe) concentrations were significantly higher in alluvium soils, and alluvium CH₄ fluxes were decoupled from plant processes compared with the peat site, as expected when Fe reduction inhibits CH₄ production in the rhizosphere. Soil carbon content and CO₂ uptake rates did not vary across wetlands and, thus, could also be ruled out as drivers of initial CH₄ flux differences. Differences in wetland CH₄ fluxes across soil types were transient; alluvium wetland fluxes were similar to peat wetland fluxes 3 years after restoration. Changing alluvium CH₄ emissions with time could not be explained by an empirical model based on dominant CH₄ flux biophysical drivers, suggesting that other factors, not measured by our eddy covariance towers, were responsible for these changes. Recently accreted alluvium soils were less acidic and contained more reduced Fe compared with the pre‐restoration parent soils, suggesting that CH₄ emissions increased as conditions became more favorable to methanogenesis within wetland sediments. This study suggests that alluvium soil properties, likely Fe content, are capable of inhibiting ecosystem‐scale wetland CH₄ flux, but these effects appear to be transient without continued input of alluvium to wetland sediments.     

大气氮沉降对森林土壤甲烷吸收和氧化亚氮排放的影响及其微生物学机制

水分非饱和的森林土壤是大气甲烷(CH4)汇和氧化亚氮(N2O)源,大气氮沉降增加是导致森林土壤碳氮气体通量不平衡的主要原因之一。土壤CH4吸收和N2O排放之间存在协同、消长和随机等复杂的耦合关系,关于氮素对两者产生过程的调节作用以及内在的微生物学机制至今尚不完全清楚。综述了森林土壤CH4吸收和N2O排放耦合过程的理论基础,土壤CH4和N2O的产生与消耗过程对增氮响应的生物化学和微生物学机制,指出各研究领域的不足和未来的研究重点。总体而言,低氮倾向于促进贫氮森林土壤CH4吸收,不改变土壤N2O的排放,而高氮显著抑制富氮森林土壤CH4吸收以及促进N2O排放。外源性氮素通过竞争抑制和毒性抑制来调控森林土壤CH4的吸收,而通过促进土壤硝化和反硝化过程来增加N2O的排放。然而,由于全球氮沉降控制试验网络分布的不均匀性、土壤碳氮通量产生过程的复杂性以及微生物分子生态学方法的局限性等原因,导致氮素对森林土壤碳氮通量的调控机制研究一直进展缓慢,未能将微生物功能群落动态与土壤碳氮通量真正地联系起来。未来研究应该从流域、生态系统和分子尺度上深入探讨土壤碳氮通量耦合作用的环境驱动机制,氮素对土壤CH4氧化和N2O产生过程的调控作用,以及增氮对土壤甲烷氧化菌和N2O产生菌活性和群落组成的影响。     

藏香猪放牧对滇西北高原湿地土壤CO2通量的影响

高原湿地是生态系统中重要的碳汇。土壤CO_2通量作为高原湿地生态系统碳收支的重要组成部分,碳的释放对区域碳平衡发挥着重要的作用。藏香猪放牧是我国高海拔藏区一种特有的放牧方式,是导致高原湿地土壤退化的重要干扰因素之一,并影响着土壤CO_2通量的变化。采用土壤CO_2通量自动测量系统(LI-8100A,LI-COR,USA),分别在不同季节对滇西北布伦、哈木谷、伊拉草原上藏香猪干扰和对照(非干扰土壤)CO_2通量变化进行监测,研究发现,藏香猪干扰型放牧降低了土壤CO_2排放通量,且表现出明显的日波动变化特征。相比旱季,雨季不同放牧方式影响下的土壤CO_2通量差异性更为明显,其中布伦、哈木谷、伊拉草原较对照分别降低了70.4%、87.5%、60.7%。CO_2排放通量与土壤理化性状及植物生物量的回归分析表明,对照样地的土壤容重、孔隙度、pH、总活性碳、植物生物量与土壤CO_2通量具有显著的相关性(P0.01)。通过植物-土壤指数(plant-soil index,PSI)分析了藏香猪干扰型放牧对高原湿地的影响,总体来看,对照样地中土壤CO_2通量与PSI之间具有较好的线性关系,可以用来很好的预测未来高原湿地土壤CO_2通量的变化。该研究结果不仅有效估算了强干扰放牧影响下的高原湿地土壤碳排放量,而且为加强藏香猪放牧的科学管理,高原湿地生态系统的保护、恢复及重建提供了理论支持。     

Drought‐induced saltwater incursion leads to increased wetland nitrogen export

Coastal wetlands have the capacity to retain and denitrify large quantities of reactive nitrogen (N), making them important in attenuating increased anthropogenic N flux to coastal ecosystems. The ability of coastal wetlands to retain and transform N is being reduced by wetland losses resulting from land development. Nitrogen retention in coastal wetlands is further threatened by the increasing frequency and spatial extent of saltwater inundation in historically freshwater ecosystems, due to the combined effects of dredging, declining river discharge to coastal areas due to human water use, increased drought frequency, and accelerating sea‐level rise. Because saltwater incursion may affect N cycling through multiple mechanisms, the impacts of salinization on coastal freshwater wetland N retention and transformation are not well understood. Here, we show that repeated annual saltwater incursion during late summer droughts in the coastal plain of North Carolina changed N export from organic to inorganic forms and led to a doubling of annual NH₄⁺ export from a 440 hectare former agricultural field undergoing wetland restoration. Soil solution NH₄⁺ concentrations in two mature wetlands also increased with salinization, but the magnitude of increase was smaller than that in the former agricultural field. Long‐term saltwater exposure experiments with intact soil columns demonstrated that much of the increase in reactive N released could be explained by exchange of salt cations with sediment NH₄⁺. Using these findings together with the predicted flooding of 1661 km² of wetlands along the NC coast by 2100, we estimate that saltwater incursion into these coastal areas could release up to 18 077 Mg N, or approximately half the annual NH₄⁺ flux of the Mississippi River. Our results suggest that saltwater incursion into coastal freshwater wetlands globally could lead to increased N loading to sensitive coastal waters.     

基于过程模型的青藏高原湿地甲烷排放格局评估

甲烷(CH_4)是大气中最丰富的碳氢化合物,是仅次于二氧化碳(CO_2)的温室气体。湿地是甲烷的重要来源,在全球碳循环中发挥着重要作用,其排放的甲烷占所有天然甲烷排放源的70%,占全球甲烷排放总量的24.8%。青藏高原平均海拔4000 m以上,占有中国约三分之一的湿地。近几十年来,由于全球气候变暖和降水增加,该地区甲烷排放率和湿地面积都发生着巨大变化,因此,青藏高原湿地CH_4排放的长期变化在很大程度上仍存在较大的不确定性。利用TRIPLEX-GHG模型模拟了青藏高原湿地1978—2008年CH_4排放的动态特征,研究结果表明:(1)1978—2008年青藏高原湿地CH_4排放速率呈逐渐增加趋势。(2)青藏高原大多数湿地区域CH_4排放速率为0—6.13 g CH_4 m~(-2 )a~(-1);东北部分湿地区域CH_4排放速率为6.14—20.19 g CH_4 m~(-2 )a~(-1);较高的CH_4排放速率分布于青藏高原南部湿地区域,为56.14—74.97 g CH_4 m~(-2 )a~(-1)。(3)青藏高原湿地CH_4排放量在1978、1990、2000年和2008年分别为0.21、0.23、0.27和0.32 Tg CH_4 a~(-1)。在1978—1990年,尽管CH_4排放速率增加,但湿地面积减少,因此这一时期青藏高原湿地CH_4排放量并未发生明显变化。随后由于降水增加和冰川融化,使得湿地面积逐渐增加,青藏高原湿地CH_4排放量也呈现增加趋势。     

Regional-scale measurements of CH4 exchange from a tall tower over a mixed temperate/boreal lowland and wetland forest

The biosphere–atmosphere exchange of methane (CH₄) was estimated for a temperate/boreal lowland and wetland forest ecosystem in northern Wisconsin for 1997–1999 using the modified Bowen ratio (MBR) method. Gradients of CH₄ and CO₂ and CO₂ flux were measured on the 447‐m WLEF‐TV tower as part of the Chequamegon Ecosystem–Atmosphere Study (ChEAS). No systematic diurnal variability was observed in regional CH₄ fluxes measured using the MBR method. In all 3 years, regional CH₄ emissions reached maximum values during June–August (24±14.4 mg m^?2 day^?1), coinciding with periods of maximum soil temperatures. In 1997 and 1998, the onset in CH₄ emission was coincident with increases in ground temperatures following the melting of the snow cover. The onset of emission in 1999 lagged 100 days behind the 1997 and 1998 onsets, and was likely related to postdrought recovery of the regional water table to typical levels. The net regional emissions were 3.0, 3.1, and 2.1 g CH₄ m^?2 for 1997, 1998, and 1999, respectively. Annual emissions for wetland regions within the source area (28% of the land area) were 13.2, 13.8, and 10.3 g CH₄ m^?2 assuming moderate rates of oxidation of CH₄ in upland regions in 1997, 1998, and 1999, respectively. Scaling these measurements to the Chequamegon Ecosystem (CNNF) and comparing with average wetland emissions between 40°N and 50°N suggests that wetlands in the CNNF emit approximately 40% less than average wetlands at this latitude. Differences in mean monthly air temperatures did not affect the magnitude of CH₄ emissions; however, reduced precipitation and water table levels suppressed CH₄ emission during 1999, suggesting that long‐term climatic changes that reduce the water table will likely transform this landscape to a reduced source or possibly a sink for atmospheric CH₄.     

放牧对若尔盖高原湿地CH4排放的影响

湿地是大气甲烷(CH_4)的主要排放源,而有关放牧对湿地CH_4排放的影响特征仍未得到足够的报道。因此,通过静态箱法,研究了放牧对四川省若尔盖高原湿地CH_4排放的影响,CH_4气体通过快速温室气体分析仪测量。结果表明:放牧样地和围栏内样地生长季CH_4排放量为(31.32±19.57)g/m~2和(30.31±23.46)g/m~2,它们之间无差异显著;但是集中放牧期间(7—9月),放牧样地(21.01±12.35)g/m~2较围栏内样地显著增加了CH_4排放量为54.3%。2014年生长季期间通过刈割植物模拟放牧表明两种刈割强度CH_4排放量为(5.01±5.37)g/m~2和(4.69±5.99)g/m~2,较未刈割样地(1.15±1.89)g/m~2增加了335.9%和308.0%,其原因可能是放牧或者刈割减少地表植物生物量,降低植物高度,缩短了CH_4排放的路径距离。该结果可为我国高原湿地保护与管理决策提供基础数据支撑。     

Nitrous oxide emissions from agricultural fields: Assessment, measurement and mitigation

In this paper we discuss three topics concerning N₂O emissions from agricultural systems. First, we present an appraisal of N₂O emissions from agricultural soils (Assessment). Secondly, we discuss some recent efforts to improve N₂O flux estimates in agricultural fields (Measurement), and finally, we relate recent studies which use nitrification inhibitors to decrease N₂O emissions from N-fertilized fields (Mitigation).To assess the global emission of N₂O from agricultural soils, the total flux should represent N₂O from all possible sources; native soil N, N from recent atmospheric deposition, past years fertilization, N from crop residues, N₂O from subsurface aquifers below the study area, and current N fertilization. Of these N sources only synthetic fertilizer and animal manures and the area of fields cropped with legumes have sufficient global data to estimate their input for N₂O production. The assessment of direct and indirect N₂O emissions we present was made by multiplying the amount of fertilizer N applied to agricultural lands by 2% and the area of land cropped to legumes by 4 kg N₂O-N ha^-1. No regard to method of N application, type of N, crop, climate or soil was given in these calculations, because the data are not available to include these variables in large scale assessments. Improved assessments should include these variables and should be used to drive process models for field, area, region and global scales.Several N₂O flux measurement techniques have been used in recent field studies which utilize small and ultralarge chambers and micrometeorological along with new analytical techniques to measure N₂O fluxes. These studies reveal that it is not the measurement technique that is providing much of the uncertainty in N₂O flux values found in the literature but rather the diverse combinations of physical and biological factors which control gas fluxes. A careful comparison of published literature narrows the range of observed fluxes as noted in the section on assessment. An array of careful field studies which compare a series of crops, fertilizer sources, and management techniques in controlled parallel experiments throughout the calendar year are needed to improve flux estimates and decrease uncertainty in prediction capability.There are a variety of management techniques which should conserve N and decrease the amount of N application needed to grow crops and to limit N₂O emissions. Using nitrification inhibitors is an option for decreasing fertilizer N use and additionally directly mitigating N₂O emissions. Case studies are presented which demonstrate the potential for using nitrification inhibitors to limit N₂O emissions from agricultural soils. Inhibitors may be selected for climatic conditions and type of cropping system as well as the type of nitrogen (solid mineral N, mineral N in solution, or organic waste materials) and applied with the fertilizers.     

Species-specific Effects of Vascular Plants on Carbon Turnover and Methane Emissions from Wetlands

Species composition affects the carbon turnover and the formation and emission of the greenhouse gas methane (CH₄) in wetlands. Here we investigate the individual effects of vascular plant species on the carbon cycling in a wetland ecosystem. We used a novel combination of laboratory methods and controlled environment facilities and studied three different vascular plant species (Eriophorum vaginatum, Carex rostrata and Juncus effusus) collected from the same wetland in southern Sweden. We found distinct differences in the functioning of these wetland sedges in terms of their effects on carbon dioxide (CO₂) and CH₄ fluxes, bubble emission of CH₄, decomposition of ¹⁴C-labelled acetate into ¹⁴CH₄ and ¹⁴CO₂, rhizospheric oxidation of CH₄ to CO₂ and stimulation of methanogenesis through root exudation of substrate (e.g., acetate). The results show that the emission of CH₄ from peat–plant monoliths was highest when the vegetation was dominated by Carex (6.76 mg CH₄ m⁻² h⁻¹) than when it was dominated by Eriophorum (2.38 mg CH₄ m⁻² h⁻¹) or Juncus (2.68 mg CH₄ m⁻² h⁻¹). Furthermore, the CH₄ emission seemed controlled primarily by the degree of rhizospheric CH₄ oxidation which was between 20 and 40% for Carex but >90% for both the other species. Our results point toward a direct and very important linkage between the plant species composition and the functioning of wetland ecosystems and indicate that changes in the species composition may alter important processes relating to controls of and interactions between greenhouse gas fluxes with significant implications for feedback mechanisms in a changing climate as a result.     

Observational constraints reduce model spread but not uncertainty in global wetland methane emission estimates

The recent rise in atmospheric methane (CH₄) concentrations accelerates climate change and offsets mitigation efforts. Although wetlands are the largest natural CH₄ source, estimates of global wetland CH₄ emissions vary widely among approaches taken by bottom-up (BU) process-based biogeochemical models and top-down (TD) atmospheric inversion methods. Here, we integrate in situ measurements, multi-model ensembles, and a machine learning upscaling product into the International Land Model Benchmarking system to examine the relationship between wetland CH₄ emission estimates and model performance. We find that using better-performing models identified by observational constraints reduces the spread of wetland CH₄ emission estimates by 62% and 39% for BU- and TD-based approaches, respectively. However, global BU and TD CH₄ emission estimate discrepancies increased by about 15% (from 31 to 36 TgCH₄ year⁻¹) when the top 20% models were used, although we consider this result moderately uncertain given the unevenly distributed global observations. Our analyses demonstrate that model performance ranking is subject to benchmark selection due to large inter-site variability, highlighting the importance of expanding coverage of benchmark sites to diverse environmental conditions. We encourage future development of wetland CH₄ models to move beyond static benchmarking and focus on evaluating site-specific and ecosystem-specific variabilities inferred from observations.     

Elevated CO2 and nutrient addition after soil N cycling and N trace gas fluxes with early season wet-up in a California annual grassland

We examined the effects of growth carbon dioxide (CO₂)concentration and soil nutrient availability on nitrogen (N)transformations and N trace gas fluxes in California grasslandmicrocosms during early-season wet-up, a time when rates of Ntransformation and N trace gas flux are high. After plant senescenceand summer drought, we simulated the first fall rains and examined Ncycling. Growth at elevated CO₂ increased root productionand root carbon:nitrogen ratio. Under nutrient enrichment, elevatedCO₂ increased microbial N immobilization during wet-up,leading to a 43% reduction in gross nitrification anda 55% reduction in NO emission from soil. ElevatedCO₂ increased microbial N immobilization at ambientnutrients, but did not alter nitrification or NO emission. ElevatedCO₂ did not alter soil emission of N₂O ateither nutrient level. Addition of NPK fertilizer (1:1:1) stimulatedN mineralization and nitrification, leading to increased N₂Oand NO emission from soil. The results of our study support a mechanisticmodel in which elevated CO₂ alters soil N cycling and NOemission: increased root production and increased C:N ratio in elevatedCO₂ stimulate N immobilization, thereby decreasingnitrification and associated NO emission when nutrients are abundant.This model is consistent with our basic understanding of how C availabilityinfluences soil N cycling and thus may apply to many terrestrial ecosystems.     

Carbon emission along a eutrophication gradient in temperate riverine wetlands: effect of primary productivity and plant community composition

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Nitrous oxide emissions from surface flow and subsurface flow constructed wetland microcosms: Effect of feeding strategies

The effects of continuous and intermittent feeding strategies on nitrogen removal and N₂O emission from surface flow and subsurface flow constructed wetlands were evaluated in this study. Microcosm wetlands planted with Phragmites australis were constructed and operated with different feeding strategies for the 4-month experiment. Results showed the intermittent feeding strategy could enhance the removal of ammonium effectively in the subsurface flow constructed wetlands, although it had no significant effect for the surface flow wetlands. And the intermittent feeding mode could promote the emission of N₂O. The amount of N₂O-N emission from the subsurface flow constructed wetlands with intermittent feeding mode was about 5 times higher than that with continuous feeding strategy and the emission rate ranged from 0.09 ± 0.03 to 7.33 ± 1.49 mg/m²/h. Compared with the surface flow constructed wetlands, the N₂O emission in the subsurface flow constructed wetlands was affected significantly by the intermittent feeding mode.