稻田甲烷排放模型研究——模型及其修正

在过去十多年内 ,关于稻田甲烷排放的模拟已经进行了不少有益的探索并且开发出了数个有关的模型。模型的成功研制是准确定量估计不同区域范围内稻田甲烷排放的前提。以往大部分模型由于模拟精度不高 ,或者是其要求太多的输入参数 ,因而限制了它在大尺度范围内的广泛应用。在一个比较成熟的模型基础上 ,进行了必要的修正与扩充。增加了稻田甲烷通过气泡方式排放的模拟模块 ,并修正了原模型中关于土壤氧化还原电位变化的模拟 ,使之能适应于多种稻田水管理方式。新修正的模型 (CH4 MOD)不仅保留了原模型输入参数较少和易于获得的优点 ,而且能适应多种水稻耕作方式 ,这为进一步利用模型技术准确估计大尺度区域稻田甲烷排放提供了一种新的科学方法     

东北黑土地区稻田甲烷排放时空演变及排放潜力分析

稻田甲烷排放是农业源甲烷排放的主要来源。东北黑土地区是我国最大的粮食生产基地,农业温室气体减排是实现黑土地永续利用的关键议题之一。运用稻田甲烷排放模型(CH4MOD)核算并分析了2009-2018年东北黑土地区稻田甲烷排放的时空演变特征,结合GOSAT卫星遥感数据探究了水稻生产与区域甲烷排放的时空动态联系,进一步量化了稻田甲烷对区域甲烷排放的贡献程度及不同情景下的排放潜力。结果表明,受水稻生产面积扩张和排放强度提高的影响,东北黑土地区稻田甲烷排放总量从2009年的39.05万t增加到2018年的79.53万t。东北黑土地区区域甲烷排放在季节变化和栅格单元上表现出与稻田甲烷排放较为一致的时空动态,大规模的稻田耕作可能会增加水稻生产与区域甲烷排放直接相关的可能性。随着水稻持续扩种稳产,2018年东北黑土地区水稻生产贡献了区域甲烷排放总量的15.04%,其中黑龙江省的贡献率高达31.06%。在基准发展情景下,预计2035年东北黑土地区稻田CH₄排放量较2018年增加19.5%;在粮食供给保障情景下,维持当前稻田耕作面积,水稻生产集约化程度提高,预计其稻田CH₄排放量较2018年减少0.88%;在此基础上,采取促进秸秆还田、增施有机肥、实施节水间歇灌溉等稻田管理措施将使稻田CH₄排放量增加17.8%-63.6%。以满足膳食需求和供给保障为导向,优化水稻种植结构、控制稻田耕作面积,推动技术进步、品种改良以提升单产水平,采取化肥和有机肥搭配施用、节水间歇灌溉等途径能够缓解稻田甲烷排放。研究综合运用自上而下的遥感数据和自下而上的模型运算,刻画了水稻生产与区域甲烷排放的时空联系,进一步评估了稻田甲烷的排放潜力及减排措施的减排效果,为促进东北黑土地区农业甲烷减排和生产布局优化提供了理论依据和决策参考。     

基于模型和GIS技术的中国稻田甲烷排放估计

将一个比较成熟的稻田甲烷排放模型CH4MOD和GIS空间化数据库结合,模拟估计了中国大陆2000年水稻生长季稻田甲烷的排放。模型的空间输入参数包括:逐日气温、耕层土壤砂粒含量、外源有机质施用量、稻田水分管理模式、水稻移栽期与收获期、水稻种植面积与单产,空间分辨率为10km×10km。模拟结果表明:2000年稻田甲烷排放量为6.02Tg,其中:早稻生长季排放1.63Tg、晚稻1.46Tg、单季稻2.93Tg。提高区域稻田甲烷排放估计精度的进一步目标应放在减小输入参数误差和提高空间数据精度上,在现有数据库基础和模型———GIS技术下探讨我国稻田甲烷排放估计的不确定性范围是必要的。     

华南丘陵区冬闲稻田二氧化碳、甲烷和氧化亚氮的排放特征

采用静态箱 气相色谱法对收获后冬闲稻田CO₂、CH₄和N₂O排放进行了田间原位测定,探讨了越冬稻田3种温室气体的排放规律.结果表明,残茬稻田和裸田的CO₂的排放峰值分别出现在18:00和16:00左右.日间CH₄排放为净值,夜间表现为弱吸收.残茬稻田和裸田N₂O夜间排放分别为日间平均的1.79和1.58倍.残茬稻田的昼夜CO₂平均排放通量显著高于裸田(P＜0.05).在测定期间,残茬稻田CO₂排放随温度升高而增高.相关分析表明,CO₂排放与土温、地表温度和气温均呈显著相关,表明温度是影响收获后稻田CO₂排放的主要因素.在11月10日至翌年1月18日测定期间,残茬稻田的CO₂和CH₄平均排放通量分别为(180.69±21.21) mg·m^-2·h^-1和(-0.04±0.01) mg·m^-2·h^-1,CO₂排放通量较裸田高13.06％,CH₄吸收增高50%.残茬稻田的N₂O排放通量为(21.26±19.31) μg·m^-2·h^-1,较裸田低60.75％.由此说明华南丘陵区冬闲稻田是大气CO₂和N₂O的源,CH₄的汇.     

太阳辐射对稻田甲烷排放的影响

太阳辐射减弱是气候变化的主要特征之一,而太阳辐射减弱对稻田甲烷(CH4)排放的影响尚不明确,且缺少高光谱遥感用于估算稻田CH4排放的研究.通过田间模拟试验,研究了不同遮阴强度对稻田CH4排放和水稻冠层光谱特征的影响,并基于冠层高光谱数据估算了CH4排放通量.采用单因子试验设计,遮阴强度设3个水平,即对照(不遮阴,CK)、轻度遮阴(S1,单层遮阴,遮阴率为60%)和重度遮阴(S2,双层遮阴,遮阴率为84%).结果表明:与对照相比,遮阴明显降低了稻田CH4排放,但重度遮阴下CH4排放高于轻度遮阴;近红外波段水稻冠层反射率表现为CK>S2>S1;水稻冠层光谱反射率(699~1349 nm)与CH4排放通量呈极显著正相关,最高相关系数达0.64,6种植被指数与CH4排放通量也呈极显著相关,其中比值植被指数(RVI)与CH4排放通量的相关系数最大,达0.84;建立了以RVI、归一化植被指数(NDVI)和507 nm原始反射率(ρ507)为参数估算CH4排放通量的逐步回归模型,决定系数R2分别为0.86和0.85,利用该模型可为开展区域稻田温室气体排放的遥感监测提供试验依据.     

华南丘陵地区农林复合生态系统晚稻田甲烷和氧化亚氮排放

采用静态箱-气相色谱法对晚稻田甲烷(CH4)和氧化亚氮(N2O)排放进行田间原位测定。结果表明,有植株参与的稻田CH4排放通量季节变化与地下5cm温度呈显著正相关关系。稻田CH4和N2O季节平均排放通量在有植株参与时分别为1.16±0.38mgm-2h-1和42.33±20.00μgm-2h-1,而无植株参与的分别为0.15±0.11mgm-2h-1和51.69±15.87μgm-2h-1。水稻种植对CH4的排放影响较大,对N2O的排放影响较小,有植株参与的稻田CH4平均排放量显著高于无植株参与的稻田,N2O的平均排放量无显著差异。     

微生物生物量C、土壤呼吸的季节性变化与黑土稻田甲烷排放

对黑土水稻田一个生长季中土壤微生物生物量C、土壤呼吸和甲烷排放通量进行了监测。结果表明,在水稻生长初期,长效尿素能显著抑制微生物生物量C和土壤呼吸(P<0．05),间歇灌溉措施对二者几乎没有影响,间歇灌溉能减少稻田甲烷的排放,平均排放量比对照减少了32．5％,长效尿素的施用稻田使甲烷的排放略有增加,施用长效尿素的处理,微生物生物量C与甲烷排放量之间呈显著正相关关系。     

猪粪与沼气渣对双季稻田甲烷排放的影响

随着环境温度的升高,稻田甲烷排放通量增加。早稻期间甲烷排放通量随着水稻生育期的增加而逐步加快,而晚稻甲烷排放主要集中在水稻生长的前中期,而且排放量很高。一天中甲烷排放具有很强的周期性,在6：00～8：00时,甲烷排放通量进入谷底,14：00时甲烷排放通量达到峰值。稻田甲烷排放通量与土壤5cm处的温度及土壤水溶解甲烷含量具有较高的相关性。猪粪和沼气渣的施用分别提高稻田甲烷排放量22．14％和4．40％。在早稻期间,施用猪粪和沼气渣分别提高土壤水溶解甲烷含量40．3％和11．9％,而晚稻期间仅分别提高23．9％和5．04％。     

铁炉渣施加对稻田甲烷产生、氧化与排放的影响

为了阐明铁炉渣施加对稻田甲烷产生、氧化与排放的影响,采用静态箱-气相色谱法对对照(CK)、2 Mg/hm2(FeⅠ)、4Mg/hm2(FeⅡ)和8 Mg/hm2(FeⅢ)铁炉渣施加后稻田甲烷产生、氧化与排放进行了测定与分析.研究结果表明:观测期内,CK、Fe Ⅰ、FeⅡ和FeⅢ样地甲烷产生量分别为0.06-8.87、0.12-8.28、0.15-7.84、0.17-7.82 mg·m-2·h-1,平均产生量分别为4.68、3.92、3.14、2.76 mg·m-2·h-1;甲烷氧化量分别是0.02-1.27、0.09-0.95、0.09-1.54、0.09-2.79 mg·m-2·h-1,平均氧化量为0.46、0.47、0.59、0.55 mg·m-2·h-1;甲烷排放分别是0.04-7.99、0.03-7.33、0.06-6.30、0.08-5.12 mg· m-2·h-1,平均值分别为3.11、2.29、1.76、1.59 mg·m-2·h-1.铁炉渣的施加降低了甲烷产生量和排放通量,提高了甲烷氧化量.     

开放式空气CO2增高对稻田CH4和N2O排放的影响

在FACE(free aircarbondioxideenrichment)平台上 ,采用静态暗箱 气相色谱法观测研究了大气CO2 浓度增加对稻田CH4和N2 O排放的影响 .结果表明 ,在 15 0和 2 5 0kgN·hm-2 两种氮肥水平下大气CO2 浓度增加 2 0 0 μmol·mol-1均明显促进水稻生长 ,水稻生物量积累 .大气CO2 浓度增加对 15 0和 2 5 0kgN·hm-2 两种氮肥水平下稻田CH4排放均无显著影响 ,并简要分析了与现有文献报道结果不一致的原因 .大气CO2 浓度增加也未导致 15 0和 2 5 0kgN·hm-2 两种氮肥水平下稻田N2 O排放的明显变化 ,与大多数研究结果一致 .     

稻田甲烷排放模型研究——模型灵敏度分析

模型方法对区域稻田甲烷排放估计的不确定性主要源于模型参数在区域范围内的误差,这种误差导致的估计不确定性由模型灵敏度决定.采用一种动力学分析与统计分析相结合的方法对稻田甲烷模型CH4MOD进行了参数灵敏度分析,结果表明,稻田水管理方式的灵敏度最高,灵敏度指数为O.64,其次为稻田土壤的砂粒含量参数,灵敏度指数0.50,灵敏度最低的参数是水稻移栽期地上生物量.以模型灵敏度指数为基础,建立了模型估计值不确定性与模型参数区域化误差间的数量关系,利用这一量化关系得出我国2000年稻田甲烷排放的不确定性范围为3.09～10.61Tg.此外,模型灵敏度参数的大小也反映了模型要素对稻田甲烷排放影响的大小,因而分析的结果对于采取合理措施减少稻田甲烷排放具有指导意义.     

Model estimates of methane emission from irrigated rice cultivation of China

A model developed by the authors ( Huang et al. 1998 ) was further validated against field measurements from various regions of the world and calibrated to estimate methane emission from irrigated rice cultivation of China. On the basis of available information on rice cultivated area, growth duration, grain yield, soil texture and temperature, methane emission from Chinese rice paddies was estimated for 28 rice cultivated provinces in mainland. The calculated daily methane emission rates, on a provincial scale, ranged from 0.15 to 0.86 g m^–2 with an average of 0.32 g m^–2. Five of the top six locations with higher daily methane emissions are located at a latitude between 28° and 31° N. A total amount of 9.66 Tg (1 Tg = 10¹² g) CH₄ per year, ranging from 7.19 to 13.62, was estimated to be released from Chinese rice paddy soils. Of the total, 45% is emitted from the single-rice growing season, and 19% and 36% are from the early-rice and the late-rice growing seasons, respectively. Approximately 70% of the total is emitted in the region located at latitude between 25° and 32° N. The emissions from rice fields in Sichuan and Hunan Province were calculated to be 2.85 Tg y^–1, accounting for ≈ 30% of the total. Comparisons of the estimated and the observed emission rates show that the estimates were, in general, close to the measurements at most locations.     

Impact of gas transport through rice cultivars on methane emission from rice paddy fields

Two Italian rice (Oryza sativa var. japonica) cultivars, Lido and Roma, were tested in the field for methane production, oxidation and emission. In two consecutive years, fields planted with the rice cultivar Lido showed methane emissions 24–31% lower than fields planted with the cultivar Roma. This difference was observed irrespective of fertilizer treatment. In contrast to methane emissions, differences in methane production or oxidation were not observed between fields planted with the two cultivars. Plant-mediated transport of methane from the sediment to the atmosphere was the dominating pathway of methane emission. During the entire vegetation period, the contribution of this pathway to total methane emission amounted to c. 90%, whereas the contribution of gas bubble release and of diffusion through the water column to total methane emission was of minor significance. Results obtained from transport studies of tracer gas through the aerenchyma system of rice plants demonstrated that the root–shoot transition zone is the main site of resistance to plant-mediated gas exchange between the soil and the atmosphere. The cultivar Lido, showing relatively low methane emissions in the field, had a significantly lower gas transport capacity through the aerenchyma system than the cultivar Roma. Thus, the observed differences in methane emissions in the field between the cultivars Lido and Roma can be explained by different gas transport capacities. Apparently, these differences in gas transport capacities are a consequence of differences in morphology of the aerenchyma systems, especially in the root–shoot transition zone. It is, therefore, concluded that identification and use of high-yielding rice cultivars which have a low gas transport capacity represent an economically feasible, environmentally sound and promising approach to mitigating methane emissons from rice paddy fields.     

A semi-empirical model of methane emission from flooded rice paddy soils

Reliable regional or global estimates of methane emissions from flooded rice paddy soils depend on an examination of methodologies by which the current high variability in the estimates might be reduced. One potential way to do this is the development of predictive models. With an understanding of the processes of methane production, oxidation and emission, a semi-empirical model, focused on the contributions of rice plants to the processes and also the influence of environmental factors, was developed to predict methane emission from flooded rice fields. A simplified version of the model was also derived to predict methane emission in a more practical manner. In this study, it was hypothesized that methanogenic substrates are primarily derived from rice plants and added organic matter. Rates of methane production in flooded rice soils are determined by the availability of methanogenic substrates and the influence of environmental factors. Rice growth and development control the fraction of methane emitted. The amount of methane transported from the soil to the atmosphere is determined by the rates of production and the emitted fraction. Model validation against observations from single rice growing seasons in Texas, USA demonstrated that the seasonal variation of methane emission is regulated by rice growth and development. A further validation of the model against measurements from irrigated rice paddy soils in various regions of the world, including Italy, China, Indonesia, Philippines and the United States, suggests that methane emission can be predicted from rice net productivity, cultivar character, soil texture and temperature, and organic matter amendments.     

赤红壤早稻田甲烷排放通量及其影响因素

用封闭箱法对广东省赤红壤早稻田CH4排放通量进行了观测。结果表明,CH4排放有明显的季节变化规律,3个排放高峰分别出现在水稻分蘖末期、孕穗抽穗期和乳熟期,平均通量为5.7mg.m-2.h-1。在测定期内,CH4排放与5和10cm土壤温度呈显著正相关,与土壤Eh呈显著负相关,与土壤pH值、水层深浅相关不明显。     

Estimates of methane emissions from Chinese rice paddies by linking a model to GIS database

Methane is one of the principal greenhouse gases. Irrigated rice paddies are recognized as contributing to atmospheric methane concentration. Methane emissions from rice paddies are among the most uncertain estimates in rice-growing countries. Efforts have been made over the last decade to estimate CH₄ emissions from Chinese rice paddies via the model method. However, these estimates are very vague due to different models and upscaling methods. A reduction in these uncertainties may be achieved by coupling field-scale models with regional databases. The objective of this article is to develop a methodology of coupling a CH₄ emission model with regional databases by which CH₄ emissions from Chinese rice paddies can then be estimated. CH4MOD, a model for simulating CH₄ emissions from rice paddies with minimal input by using commonly available parameters, is of great potential in terms of upscaling as it has provided a realistic estimate of the observed results from various soils, climates and agricultural practices. By linking spatial databases to CH4MOD, CH₄ emissions from Chinese rice paddies in the 2000 rice-growing season were simulated on a day-by-day basis. The spatial databases were created by GIS with a spatial resolution of 10km10km, including soil sand percentage, amounts of crop straw and roots from the previous season and farm manure, the water management pattern, dates of rice transplanting and harvesting, acreage of rice planted, rice grain yield and daily air temperature. ARCGIS software was used to meet all GIS needs, including data access, projection definition, overlaying of different vector layers, creation of grids (a raster format of ARCGIS software) by converting vector data, and the data conversion between grids and ASCII formats. Methane emissions from rice paddies in mainland China in the 2000 rice-growing season were estimated to be 6.02 Tg (1 Tg = 10⁹ kg). Of the total, approximately 49% (2.93Tg) is emitted during the single rice-growing season, and 27% (1.63Tg) and 24% (1.46Tg) are from the early and late rice-growing seasons respectively. It was concluded that regional CH4 emissions from rice paddies could be estimated by coupling CH4MOD with regional databases with a high spatial resolution. A further effort should be made to improve the quality of the spatial databases, especially in terms of the amount of added organic matter and the water regime. It is also necessary to evaluate the uncertainties of the present estimates in order to improve the overall accuracy.     

Options for mitigating methane emission from a permanently flooded rice field

Permanently flooded rice fields, widely distributed in south and south‐west China, emit more CH₄ than those drained in the winter crop season. For understanding CH₄ emissions from permanently flooded rice fields and developing mitigation options, CH₄ emission was measured year‐round for 6 years from 1995 to 2000, in a permanently flooded rice field in Chongqing, China, where two cultivations with four treatments were prepared as follows: plain‐cultivation, summer rice crop and winter fallow with floodwater layer annually (convention, Ch‐FF), and winter upland crop under drained conditions (Ch‐Wheat); ridge‐cultivation without tillage, summer rice and winter fallow with floodwater layer annually (Ch‐FFR), and winter upland crop under drained conditions (Ch‐RW), respectively. On a 6‐year average, compared to the treatments with floodwater in the winter crop season, the CH₄ flux during rice‐growing period from the treatments draining floodwater and planting winter crop was reduced by 42% in plain‐cultivation and by 13% in ridge‐cultivation (P < 0.05), respectively. The reduction of annual CH₄ emission reached 68 and 48%, respectively. Compared to plain‐cultivation (Ch‐FF), ridge‐cultivation (Ch‐FFR) reduced annual CH₄ emission by 33%, and which was mainly occurred in the winter crop season. These results indicate that draining floodwater layer for winter upland crop growth was not only able to prevent CH₄ emission from permanently flooded paddy soils directly in the winter crop season, but also to reduce CH₄ emission substantially during the following rice‐growing period. As an alternative to the completely drainage of floodwater layer in the winter crop season, ridge‐cultivation could also significantly mitigate CH₄ emissions from permanently flooded rice fields.     

Estimates of methane emissions from Chinese rice paddies by linking a model to GIS database

Methane is one of the principal greenhouse gases. Irrigated rice paddies are recognized as contributing to atmospheric methane concentration. Methane emissions from rice paddies are among the most uncertain estimates in rice-growing countries. Efforts have been made over the last decade to estimate CH₄ emissions from Chinese rice paddies via the model method. However, these estimates are very vague due to different models and upscaling methods. A reduction in these uncertainties may be achieved by coupling field-scale models with regional databases. The objective of this article is to develop a methodology of coupling a CH₄ emission model with regional databases by which CH₄ emissions from Chinese rice paddies can then be estimated. CH4MOD, a model for simulating CH₄ emissions from rice paddies with minimal input by using commonly available parameters, is of great potential in terms of upscaling as it has provided a realistic estimate of the observed results from various soils, climates and agricultural practices. By linking spatial databases to CH4MOD, CH₄ emissions from Chinese rice paddies in the 2000 rice-growing season were simulated on a day-by-day basis. The spatial databases were created by GIS with a spatial resolution of 10km×10km, including soil sand percentage, amounts of crop straw and roots from the previous season and farm manure, the water management pattern, dates of rice transplanting and harvesting, acreage of rice planted, rice grain yield and daily air temperature. ARCGIS software was used to meet all GIS needs, including data access, projection definition, overlaying of different vector layers, creation of grids (a raster format of ARCGIS software) by converting vector data, and the data conversion between grids and ASCII formats. Methane emissions from rice paddies in mainland China in the 2000 rice-growing season were estimated to be 6.02 Tg (1 Tg = 10⁹ kg). Of the total, approximately 49% (2.93Tg) is emitted during the single rice-growing season, and 27% (1.63Tg) and 24% (1.46Tg) are from the early and late rice-growing seasons respectively. It was concluded that regional CH4 emissions from rice paddies could be estimated by coupling CH4MOD with regional databases with a high spatial resolution. A further effort should be made to improve the quality of the spatial databases, especially in terms of the amount of added organic matter and the water regime. It is also necessary to evaluate the uncertainties of the present estimates in order to improve the overall accuracy.     

水稻植株对稻田甲烷排放的影响

稻田CH4排放是稻田土壤中CH4产生、氧化和传输不同过程的净效应,水稻植株强烈影响稻田CH4的产生、氧化和传输过程,是导致稻田CH4排放季节性变化规律的一个重要因素,本文综述了水稻植株对稻田CH4排放过程的不同影响,水稻植株根系分泌物和脱落物作为产甲烷前体促进稻田土壤中CH4的产生,在水稻生长后期,植株根系分泌物和脱落物被认为是稻田土壤甲烷产生的主要基质,是导致这一时期稻田CH4高排放通量的主要原因;水稻植株根系泌氧在根际环境形成一个微氧区域氧化稻田甲烷,整个水稻生长季稻田土壤中产生的CH4大约36％～90％在植株根际环境中被氧化;约80％甚至更多的稻田CH4通过水稻植株的通气组织进入大气圈,植株对稻田CH4的传输具有十分重要的意义。