Drought turns a Central European Norway spruce forest soil from an N2O source to a transient N2O sink

Based on current climate scenarios, a higher frequency of summer drought periods followed by heavy rainfall events is predicted for Central Europe. It is expected that drying/rewetting events induce an increased matter cycling in soils and may contribute considerably to increased emissions of the greenhouse gas N₂O on annual scales. To investigate the influence of drying/rewetting events on N₂O emissions in a mature Norway spruce forest in the Fichtelgebirge area (NE Bavaria, Germany), a summer drought period of 46 days was induced by roof installations on triplicate plots, followed by a rewetting event of 66 mm experimental rainfall in 2 days. Three nonmanipulated plots served as controls. The experimentally induced soil drought was accompanied by a natural drought. During the drought period, the soil of both the throughfall exclusion and control plots served as an N₂O sink. This was accompanied by subambient N₂O concentrations in upper soil horizons. The sink strength of the throughfall exclusion plots was doubled compared with the control plots. We conclude that the soil water status together with the soil nitrate availability was an important driving factor for the N₂O sink strength. Rewetting quickly turned the soil into a source for atmospheric N₂O again, but it took almost 4 months to turn the cumulative soil N₂O fluxes from negative (sink) to positive (source) values. N₂O concentration and isotope analyses along soil profiles revealed that N₂O produced in the subsoil was subsequently consumed during upward diffusion along the soil profile throughout the entire experiment. Our results show that long drought periods can lead to drastic decreases of N₂O fluxes from soils to the atmosphere or may even turn forest soils temporarily to N₂O sinks. Accumulation of more field‐scale data on soil N₂O uptake as well as a better understanding of underlying mechanisms would essentially advance our knowledge of the global N₂O budget.     

Predicting in situ soil N2O emission using NOE algorithm and soil database

This paper presents a new algorithm, Nitrous Oxide Emission (NOE) for simulating the emission of the greenhouse gas N₂O from agricultural soils. N₂O fluxes are calculated as the result of production through denitrification and nitrification and reduction through the last step of denitrification. Actual denitrification and nitrification rates are calculated from biological parameters and soil water‐filled pore space, temperature and mineral nitrogen contents. New suggestions in NOE consisted in introducing (1) biological site‐specific parameters of soil N₂O reduction and (2) reduction of the N₂O produced through nitrification to N₂ through denitrification. This paper includes a database of 64 N₂O fluxes measured on the field scale with corresponding environmental parameters collected from five agricultural situations in France. This database was used to test the validity of this algorithm. Site per site comparison of simulated N₂O fluxes against observed data leads to mixed results. For 80% of the tested points, measured and simulated fluxes are in accordance whereas the others resulted in an important discrepancy. The origin of this discrepancy is discussed. On the other hand, mean annual fluxes measured on each site were strongly correlated to mean simulated annual fluxes. The biological site‐specific parameter of soil N₂O reduction introduced into NOE appeared particularly useful to discriminate the general level of N₂O emissions from site to site. Furthermore, the relevance of NOE was confirmed by comparing measured and simulated N₂O fluxes using some data from the US TRAGNET database. We suggest the use of NOE on a regional scale in order to predict mean annual N₂O emissions.     

Hippuric acid and benzoic acid inhibition of urine derived N2O emissions from soil

Atmospheric concentrations of the greenhouse gas nitrous oxide (N₂O) have continued to rise since the advent of the industrial era, largely because of the increase in agricultural land use. The urine deposited by grazing ruminant animals is a major global source of agricultural N₂O. With the first commitment period for reducing greenhouse gas emissions under the Kyoto Protocol now underway, mitigation options for ruminant urine N₂O emissions are urgently needed. Recent studies showed that increasing the urinary concentration of the minor urine constituent hippuric acid resulted in reduced emissions of N₂O from a sandy soil treated with synthetic bovine urine, due to a reduction in denitrification. A similar effect was seen when benzoic acid, a product of hippuric acid hydrolysis, was used. This current laboratory experiment aimed to investigate these effects using real cow urine for the first time. Increased concentrations of hippuric acid or benzoic acid in the urine led to reduction of N₂O emissions by 65% (from 17% to <6% N applied), with no difference between the two acid treatments. Ammonia volatilization did not increase significantly with increased hippuric acid or benzoic acid concentrations in the urine applied. Therefore, there was a net reduction in gaseous N loss from the soil with higher urinary concentrations of both hippuric acid and benzoic acid. The results show that elevating hippuric acid in the urine had a marked negative effect on both nitrification and denitrification rates and on subsequent N₂O fluxes. This study indicates the potential for developing a novel mitigation strategy based on manipulation of urine composition through ruminant diet.     

Nonlinear response of N2O flux to incremental fertilizer addition in a continuous maize (Zea mays L.) cropping system

The relationship between nitrous oxide (N₂O) flux and N availability in agricultural ecosystems is usually assumed to be linear, with the same proportion of nitrogen lost as N₂O regardless of input level. We conducted a 3‐year, high‐resolution N fertilizer response study in southwest Michigan USA to test the hypothesis that N₂O fluxes increase mainly in response to N additions that exceed crop N needs. We added urea ammonium nitrate or granular urea at nine levels (0–292 kg N ha^?1) to four replicate plots of continuous maize. We measured N₂O fluxes and available soil N biweekly following fertilization and grain yields at the end of the growing season. From 2001 to 2003 N₂O fluxes were moderately low (ca. 20 g N₂O‐N ha^?1 day^?1) at levels of N addition to 101 kg N ha^?1, where grain yields were maximized, after which fluxes more than doubled (to >50 g N₂O‐N ha^?1 day^?1). This threshold N₂O response to N fertilization suggests that agricultural N₂O fluxes could be reduced with no or little yield penalty by reducing N fertilizer inputs to levels that just satisfy crop needs.     

Soil 13C–15N dynamics in an N2-fixing clover system under long-term exposure to elevated atmospheric CO2

Reduced soil N availability under elevated CO₂ may limit the plant's capacity to increase photosynthesis and thus the potential for increased soil C input. Plant productivity and soil C input should be less constrained by available soil N in an N₂‐fixing system. We studied the effects of Trifolium repens (an N₂‐fixing legume) and Lolium perenne on soil N and C sequestration in response to 9 years of elevated CO₂ under FACE conditions. ¹⁵N‐labeled fertilizer was applied at a rate of 140 and 560 kg N ha^?1 yr^?1 and the CO₂ concentration was increased to 60 Pa pCO₂ using ¹³C‐depleted CO₂. The total soil C content was unaffected by elevated CO₂, species and rate of ¹⁵N fertilization. However, under elevated CO₂, the total amount of newly sequestered soil C was significantly higher under T. repens than under L. perenne. The fraction of fertilizer‐N (f_N) of the total soil N pool was significantly lower under T. repens than under L. perenne. The rate of N fertilization, but not elevated CO₂, had a significant effect on f_N values of the total soil N pool. The fractions of newly sequestered C (f_C) differed strongly among intra‐aggregate soil organic matter fractions, but were unaffected by plant species and the rate of N fertilization. Under elevated CO₂, the ratio of fertilizer‐N per unit of new C decreased under T. repens compared with L. perenne. The L. perenne system sequestered more ¹⁵N fertilizer than T. repens: 179 vs. 101 kg N ha^?1 for the low rate of N fertilization and 393 vs. 319 kg N ha^?1 for the high N‐fertilization rate. As the loss of fertilizer‐¹⁵N contributed to the ¹⁵N‐isotope dilution under T. repens, the input of fixed N into the soil could not be estimated. Although N₂ fixation was an important source of N in the T. repens system, there was no significant increase in total soil C compared with a non‐N₂‐fixing L. perenne system. This suggests that N₂ fixation and the availability of N are not the main factors controlling soil C sequestration in a T. repens system.     

Initial cultivation of a temperate-region soil immediately accelerates aggregate turnover and CO2 and N2O fluxes

The immediate effects of tillage on protected soil C and N pools and on trace gas emissions from soils at precultivation levels of native C remain largely unknown. We measured the response to cultivation of CO₂ and N₂O emissions and associated environmental factors in a previously uncultivated U.S. Midwest Alfisol with C concentrations that were indistinguishable from those in adjacent late successional forests on the same soil type (3.2%). Within 2 days of initial cultivation in 2002, tillage significantly (P=0.001, n=4) increased CO₂ fluxes from 91 to 196 mg CO₂‐C m^?2 h^?1 and within the first 30 days higher fluxes because of cultivation were responsible for losses of 85 g CO₂‐C m^?2. Additional daily C losses were sustained during a second and third year of cultivation of the same plots at rates of 1.9 and 1.0 g C m^?2 day^?1, respectively. Associated with the CO₂ responses were increased soil temperature, substantially reduced soil aggregate size (mean weight diameter decreased 35% within 60 days), and a reduction in the proportion of intraaggregate, physically protected light fraction organic matter. Nitrous oxide fluxes in cultivated plots increased 7.7‐fold in 2002, 3.1‐fold in 2003, and 6.7‐fold in 2004 and were associated with increased soil NO₃^? concentrations, which approached 15 μg N g^?1. Decreased plant N uptake immediately after tillage, plus increased mineralization rates and fivefold greater nitrifier enzyme activity, likely contributed to increased NO₃^? concentrations. Our results demonstrate that initial cultivation of a soil at precultivation levels of native soil C immediately destabilizes physical and microbial processes related to C and N retention in soils and accelerates trace gas fluxes. Policies designed to promote long‐term C sequestration may thus need to protect soils from even occasional cultivation in order to preserve sequestered C.     

中国农田土壤N2O排放通量分布格局研究

中国作为世界上一个重要农业大国,对全球大气中N2O浓度的影响正在引起人们的普遍关注。该研究采用针对农业土壤痕量气体排放估算建立的,基于N2O的产生、传输和消耗机理的反硝化分解（DNDC）模型,在建立了有关中国气候、农业土壤和农业生产的分县数据库基础上,估计了我国各县农业土壤N2O的排放通量,发现我国农田土壤N2O排放通量有较明显的地区差异,西北地区较低,东南地区较高。还发现无论温度升高,还是施肥量变化,对我国农田土壤N2O排放通量的影响,都存在区域差异,表现为东南地区的变化幅度较西北地区大,这可能与我国气候的干湿变化有较密切的关系。     

稻麦轮作生态系统中土壤湿度对N2O产生与排放的影响

通过对太湖地区稻麦轮作生态系统的N₂O排放及土壤湿度进行系统观测和开展一系列模拟实验,研究了降雨和土壤湿度对N₂O排放和产生过程的影响.结果表明,春季和秋季麦田N₂O排放与降雨量呈明显正相关,但水稻田和冬季麦田的N₂O排放不受降雨影响.稻麦轮作周期内的N₂O排放较强烈地受土壤湿度制约,土壤湿度为田间持水量的97～100%或84～86%WFPS(土壤体积含水量与总孔隙度的百分比)时,N₂O排放最强,低于此湿度范围时,N₂O排放通量与土壤湿度呈正相关,反之,则呈负相关.田间N₂O排放随土壤湿度的变化形式与模拟条件下培养土壤样品的N₂O产生率变化非常相似,但前者的最佳湿度范围比后者窄,而且偏小.     

Impact of altering the water table height of an acidic fen on N2O and NO fluxes and soil concentrations

The impact of experimentally intensified summer drought and precipitation on N₂O and NO turnover and fluxes was investigated in a minerotrophic fen over a 2‐year period. On three treatment plots, drought was induced for 6 and 10 weeks by means of roofs and drainage and decreased water table levels by 0.1–0.3 m compared with three nonmanipulated control plots. When averaged over the three treatment plots, both N₂O and NO emission showed only little response to the drought. On the single plot scale, however, a clear impact of the treatment on N₂O and NO fluxes could be identified. On the plot with the weakest water table reduction hardly any response could be observed, while on the plot with the greatest drainage effect, N₂O and NO fluxes increased by 530% and 270%, respectively. Rewetting reduced NO emissions to background levels (0.05–0.15 μmol m^?2 h^?1), but heavily enhanced N₂O emission (18–36 μmol m^?2 h^?1) for several days in the plots with largest water table reduction. These peaks contributed up to 40% to the cumulative N₂O fluxes and were caused by rapid N₂O production according to isotope abundance data. According to N₂O concentrations and isotope abundance analysis N₂O was mostly produced at depths between 0.3 and 0.5 m. During water table reduction net N₂O production in 0.1 m depth steadily increased in the most effectively dried plot from 2 up to 44 pmol cm^?3 day^?1. Rewetting immediately increased net N₂O production in the topsoil of the drought plots, showing rates of 18–174 pmol cm^?3 day^?1. This study demonstrates that drought and rewetting can temporarily increase N₂O emission to levels that have to date only been reported from nutrient rich and degraded fens that have been drained for agricultural purposes.     

间作对旱地CO2和N2O排放影响的研究进展

农田温室气体排放是近年来科学界的研究热点,采用合适的种植模式是减少农田温室气体排放的有效途径之一.本文综述了作物间作对旱地土壤CO₂和N₂O排放的影响及机理.合理间作能够提高土壤有机碳(SOC)含量、促进不同作物秸秆向SOC转化、降低SOC矿化速率,从而减少CO₂排放.禾本科与豆科作物间作能够在维持作物产量的情况下,减少化学氮肥投入、土壤有效氮残留及还田秸秆产生的无机氮,降低N₂O排放.间作作物的互作、田间小气候环境的改善也是影响土壤温室气体排放的重要因素.今后,要增加土壤温室气体监测时长并对影响因子进行综合、全面的分析,尤其是从分子水平探究间作模式下土壤微生物对温室气体产生过程的作用机理,为构建环境友好型农业模式提供科学依据.     

有机肥和无机肥对菜地土壤N2O排放及其来源的影响

以北京地区白菜地为研究对象进行肥料试验,结合乙炔抑制试验设置有机肥乙炔组、有机肥无乙炔组、无机肥(尿素)乙炔组、无机肥无乙炔组4个处理,利用N₂O稳定同位素自然丰度技术,研究菜地土壤N₂O排放通量和同位素特征值的变化规律,旨在明确在不同肥料作用下N₂O产生和消耗的微生物途径,为减少菜地N₂O排放和科学施肥提供理论依据.结果表明: 在白菜生长期内有机肥乙炔组、有机肥无乙炔组、无机肥乙炔组、无机肥无乙炔组的N₂O总排放量分别为(374±37)、(283±34)、(458±36)、(355±41) g·m^-2,有机肥处理的N₂O通量显著低于无机肥处理,乙炔组的N₂O通量显著高于无乙炔组;两种肥料处理的N₂O还原程度基本相同,而无机肥处理的硝化作用更高;乙炔仅抑制部分硝化作用,在N₂O排放高峰期也仅抑制部分N₂O还原,当排放降低时才能完全抑制N₂O还原.可见,无机肥尿素能够促进硝化作用进而增加N₂O排放,高浓度的N₂O对乙炔抑制N₂O还原起拮抗作用.因此,在北京地区菜地使用有机肥替代部分无机肥可以有效减少硝化作用触发的土壤N₂O排放,同时在使用乙炔抑制方法研究N₂O来源时应考虑合理的N₂O浓度阈值.     

紫色土菜地生态系统土壤N2O排放及其主要影响因素

应用静态箱/气相色谱法对种菜历史超过20a的紫色土菜地进行了一年N2O排放的定位观测, 分析了菜地N2O排放特征及施氮、土壤温度、土壤湿度和蔬菜参与对N2O排放的影响. 结果表明, 紫色土菜地生态系统在不施氮和施氮（N150kg?hm-2）情况下N2O平均排放通量为50.713.3和168.437.3g?m-2?h-1, N2O排放系数为1.86%. 菜地生态系统N2O排放强度高于当地粮食作物农田,其主要原因在于菜地较高的养分水平和频繁的施肥、浇水等田间管理措施. 从菜地N2O排放总量的季节分配来看, 有64%的N2O排放量来自于土壤水热条件较好的夏秋季蔬菜生长期, 冬春季蔬菜生长期N2O排放量较少, 仅占34%. 因此, 土壤水热条件不同是造成菜地N2O排放量季节分配差异的重要原因. 氮肥对增加N2O排放的效应因蔬菜生育期内单位时间施肥强度不同而异, 蔬菜生育期越短, 施氮对增加N2O排放的效应越明显.不施氮和常规施氮菜地N2O排放通量与地下5cm处土壤温度呈显著的正相关, 但不种蔬菜的空地两者之间的关系不显著, 并且常规施氮菜地土壤温度（T）对N2O排放通量（F）的影响可用指数方程F＝11.465e0.032T（R＝0.26, p<0.01）表示. 土壤湿度对菜地N2O排放的影响存在阈值效应, 当土壤含水空隙率（WFPS）介于60%-75％时更易引发N2O高排放. 因此, 依据蔬菜生育期特点, 结合土壤水分状况调节施肥量与施肥时间可能会减少菜地N2O排放.     

不同灌溉量对华北平原菜地N2O排放及其来源的影响

以华北平原菜地为研究对象,通过控制灌溉量设置不同的土壤水分对照,利用稳定同位素¹⁵N自然丰度法,结合传统的乙炔抑制试验,对不同土壤水分条件下的N₂O排放、N₂O同位素特征值以及同位素异位体位嗜值(SP值)变化规律进行分析,以阐明不同水分条件下N₂O的排放规律及其来源.结果表明:水分条件显著影响N₂O排放,相比于50%土壤孔隙含水率(WFPS),70%WFPS的水分条件下N₂O的排放较高.N₂O的排放集中在施肥前期,在施肥中后期迅速减弱.50%WFPS条件下,N₂O的排放最初以硝化作用为主,占比约为90%,随后硝化作用迅速下降,反硝化变成主导作用,施肥7 d后即达到80%以上;而70%WFPS条件下初期则以反硝化为主,占比约为70%,随后下降至40%左右,施肥10 d后逐渐升高至80%.整体上,N₂O的排放主要以反硝化作用为主,不同水分处理对土壤硝化、反硝化作用的影响主要体现在施肥前期,后期均以反硝化为主.综上,建议华北地区的菜地生产应适当降低灌溉量,以减少N₂O排放.     

Regional application of PnET-N-DNDC for estimating the N2O source strength of tropical rainforests in the Wet Tropics of Australia

In contrast to the significant importance of tropical rainforest ecosystems as one of the major sources within the global atmospheric N₂O budget (2.2–3.7 Tg N yr^?1), regional estimates of their N₂O source strength are still limited and highly uncertain. To contribute toward more reliable estimates of the N₂O source strength of tropical rainforest ecosystems on a regional scale, we modified a process‐oriented biogeochemical model, PnET‐N‐DNDC, and parameterized it to simulate C and N turnover and associated N₂O emissions in and from tropical rainforest ecosystems. Model modifications included: (1) new parameterizations associated with plant physiology and soil hydrology and the addition of algorithms relating daily leaf litterfall to water stress as well as to daily rainfall to account for the effects of heavy rainfall damage; (2) the development of a denitrifier activity index that depends on soil moisture conditions and influences N turnover by denitrification; and (3) the addition of a biological N fixation algorithm. Daily simulated N₂O emissions based on site data were in good agreement (model efficiencies up to 0.83) with field observations in the Wet Tropics of Australia and Costa Rica. The model was even able to reproduce the highly dynamic pattern of N₂O emissions with short‐term increases during the wet season. Sensitivity analyses demonstrated that the PnET‐N‐DNDC model was sensitive to changes in soil properties such as pH, clay content, soil organic carbon and climatic factors such as rainfall and temperature. By linking the PnET‐N‐DNDC model to a geographic information systems database, tropical rainforests in a 9000 km² area of the Wet Tropics of Australia are estimated to emit 962 t N₂O‐N yr^?1 (2.4 kg N₂O‐N ha^?1 yr^?1) between July 1997 and June 1998.     

Water availability controls microbial temperature responses in frozen soil CO2 production

Soil processes in high-latitude regions during winter are important contributors to global carbon circulation, but our understanding of the mechanisms controlling these processes is poor and observed temperature response coefficients of CO₂ production in frozen soils deviate markedly from thermodynamically predicted responses (sometimes by several orders of magnitude). We investigated the temperature response of CO₂ production in 23 unfrozen and frozen surface soil samples from various types of boreal forests and peatland ecosystems and also measured changes in water content in them after freezing. We demonstrate that deviations in temperature responses at subzero temperatures primarily emanates from water deficiency caused by freezing of the soil water, and that the amount of unfrozen water is mainly determined by the quality of the soil organic matter, which is linked to the vegetation cover. Factoring out the contribution of water limitation to the CO₂ temperature responses yields response coefficients that agree well with expectations based on thermodynamic theory concerning biochemical temperature responses. This partitioning between a pure temperature response and the effect of water availability on the response of soil CO₂ production at low temperatures is crucial for a thorough understanding of low-temperature soil processes and for accurate predictions of C-balances in northern terrestrial ecosystems.     

施氮对水稻土N2O释放及反硝化功能基因(narG/nosZ)丰度的影响

以紫潮泥和红黄泥两种不同质地的水稻土壤作为研究对象,通过室内培养试验,分析施用硝态氮肥对N2O释放和反硝化基因(narG/nosZ)丰度的影响,并探讨反硝化基因丰度与N2O释放之间的关系。结果表明,施用硝态氮显著增加两种水稻土的N2O释放量。在72h培养过程中,施氮改变了紫潮泥反硝化基因(narG/nosZ)的丰度,但并未明显影响红黄泥反硝化基因(narG/nosZ)丰度。通过双变量相关分析发现,除了紫潮泥narG基因外,其它的反硝化基因丰度和N2O释放之间并没有显著相关性。     

短期落干对水稻土反硝化微生物丰度和N2O释放的影响

为了探明水稻土落干过程对温室气体排放和反硝化微生物的影响,通过模拟水稻土淹水落干过程,系统监测了落干开始后24 h内N2O的释放和氧化还原电位(Eh)的变化,并利用实时PCR(qPCR)方法测定了反硝化功能基因narG和nosZ的丰度.结果表明：落干开始后4 h N₂O释放量就明显增加,在24 h时N₂O的释放量比淹水对照增加了5倍多;narG和nosZ基因丰度也随着落干过程的推移而快速增加;而且N₂O排放通量与narG基因呈极显著相关(P<0.01).表明水稻土短期淹水落干过程中,含narG基因反硝化微生物是驱动N₂O释放的主要功能微生物.     

稳定性氮肥配合秸秆还田对水稻产量及N2O和CH4排放的影响

以中国科学院辽宁沈阳农田生态系统国家野外科学观测研究站连续两年的试验平台为依托,以潮棕壤为供试土壤,开展了稳定性氮肥配合秸秆还田对水稻产量及N₂O和CH₄排放的影响研究,设置对照(CK)、尿素(U)、尿素+脲酶抑制剂+硝化抑制剂(U+I)、秸秆还田(S)、秸秆还田+尿素(S+U)、秸秆还田+尿素+脲酶抑制剂+硝化抑制剂(S+U+I)6个处理.结果表明: 与CK相比,尿素显著提高了水稻产量、N₂O和CH₄累积排放及全球增温潜势.硝化抑制剂和脲酶抑制剂与尿素配施可显著减缓N₂O的累积排放.秸秆还田显著增加了N₂O和CH₄累积排放、全球增温潜势和温室气体排放强度.S+U+I处理水稻产量最高,但温室气体排放强度也显著高于其他处理;U+I处理产量略低于S+U+I,但温室气体排放强度最小.秸秆单独还田处理作物产量与对照相比无显著差异.在东北潮棕壤发育的水田中,S+U+I和U+I是相对较优的施肥模式.     

UV-B辐射增强对小麦秸秆化学成分及其施用后土壤N2O排放的影响

在室外试验的基础上,研究了地表UV-B辐射增强条件下小麦秸秆成分的变化,及在室内不同培养条件下施用UV-B辐射处理后小麦秸秆对土壤N₂O排放的影响.室外试验结果表明：地表UV-B辐射增强减少了小麦地上部分生物量,显著增加了小麦秸秆木质素和全氮含量,增幅分别达94.2%和12.3%,降低了其C/N.室内培养试验结果表明：与常规小麦秸秆相比,UV-B辐射处理后的小麦秸秆显著提高了旱地和淹水条件下N₂O的排放量;施用秸秆同时伴施硝态氮条件下,经过UV-B辐射处理的小麦秸秆显著促进了旱地条件下N₂O的排放,其排放量为常规秸秆处理的3.2倍,但在淹水条件下对N₂O排放无显著影响;无论何种培养条件下,经过UV-B辐射处理的小麦秸秆对土壤呼吸都未产生显著影响.