Seasonal variation in pathways of CH4 production and in CH4 oxidation in rice fields determined by stable carbon isotopes and specific inhibitors

Flooded rice fields, which are an important source of the atmospheric methane, have become a model system for the study of interactions between various microbial processes. We used a combination of stable carbon isotope measurements and application of specific inhibitors in order to investigate the importance of various methanogenic pathways and of CH₄ oxidation for controlling CH₄ emission. The fraction of CH₄ produced from acetate and H₂/CO₂ was calculated from the isotopic signatures of acetate, carbon dioxide (CO₂) and methane (CH₄) measured in porewater, gas bubbles, in the aerenchyma of the plants and/or in incubation experiments. The calculated ratio between both pathways reflected well the ratio determined by application of methyl fluoride (CH₃F) as specific inhibitor of acetate‐dependent methanogenesis. Only at the end of the season, the theoretical ratio of acetate: H₂ = 2 : 1 was reached, whereas at the beginning H₂/CO₂‐dependent methanogenesis dominated. The isotope discrimination was different between rooted surface soil and unrooted deep soil. Root‐associated CH₄ production was mainly driven by H₂/CO₂. Porewater CH₄ was found to be a poor proxy for produced CH₄. The fraction of CH₄ oxidised was calculated from the isotopic signature of CH₄ produced in vitro compared to CH₄ emitted in situ, corrected for the fractionation during the passage from the aerenchyma to the atmosphere. Isotope mass balances and in situ inhibition experiments with difluoromethane (CH₂F₂) as specific inhibitor of methanotrophic bacteria agreed that CH₄ oxidation was quantitatively important at the beginning of the season, but decreased later. The seasonal pattern was consistent with the change of potential CH₄ oxidation rates measured in vitro. At the end of the season, isotope techniques detected an increase of oxidation activity that was too small to be measured with the flux‐based inhibitor technique. If porewater CH₄ was used as a proxy of produced CH₄, neither magnitude nor seasonal pattern of in situ CH₄ oxidation could be reproduced. An oxidation signal was also found in the isotopic signature of CH₄ from gas bubbles that were released by natural ebullition. In contrast, bubbles stirred up from the bulk soil had preserved the isotopic signature of the originally produced CH₄.     

Effects of raised CO2 on potential CH4 production and oxidation in, and CH4 emission from, a boreal mire

1 In a glasshouse experiment we studied the effect of raised CO₂ concentration (720 p.p.m.) on CH₄ emission at natural boreal peat temperatures using intact cores of boreal peat with living vascular plants and Sphagnum mosses. After the end of the growing season half of the cores were kept unnaturally warm (17–20 °C). The potential for CH₄ production and oxidation was measured at the end of the emission experiment.
2 The vascular cores ('Sedge') consisted of a moss layer with sedges, and the moss cores (' Sphagnum ') of Sphagnum mosses (some sedge seedlings were removed by cutting). Methane efflux was 6–12 times higher from the Sedge cores than from the Sphagnum cores. The release of CH ₄ from Sedge cores increased with increasing temperature of the peat and decreased with decreasing temperature. Methane efflux from Sphagnum cores was quite stable independent of the peat temperatures.
3 In both Sedge and Sphagnum samples, CO₂ treatment doubled the potential CH₄ production but had no effect on the potential CH₄ oxidation. A raised concentration of CO₂ increased CH₄ efflux weakly and only at the highest peat temperatures (17–20 °C).
4 The results suggest that in cool regions, such as boreal wetlands, temperature would restrict decomposition of the extra substrates probably derived from enhanced primary production of mire vegetation under raised CO₂ concentrations, and would thus retard any consequent increase in CH₄ emission.     

Methane emissions from rice microcosms: The balance of production, accumulation and oxidation

In rice microcosms (Oryza sativa, var. Roma, type japonica),CH₄ emission, CH₄ production, CH₄oxidation and CH₄ accumulation were measured over an entirevegetation period. Diffusive CH₄ emission was measured inclosed chambers, CH₄ production was measured in soil samples,CH₄ oxidation was determined from the difference between oxicand anoxic emissions, and CH₄ accumulation was measured byanalysis of porewater and gas bubbles. The sum of diffusiveCH₄ emission, CH₄ oxidation, andCH₄ accumulation was only 60% of the cumulativeCH₄ production. The two values diverged during the first 50days (vegetative phase) and then again during the last 50 days (latereproductive phase and senescence) of the 150 day vegetation period. Duringthe period of day 50–100 (early reproductive phase/flowering), theprocesses were balanced. Most likely, gas bubbles and diffusion limitationare responsible for the divergence in the early and late phases. The effectof rice on CH₄ production rates and CH₄concentrations was studied by measuring these processes also in unplantedmicrocosms. Presence of rice plants lowered the CH₄concentrations, but had no net effect on the CH₄ productionrates.     

Nitrogen-regulated effects of free-air CO2 enrichment on methane emissions from paddy rice fields

Using the free‐air CO₂ enrichment (FACE) techniques, we carried out a 3‐year mono‐factorial experiment in temperate paddy rice fields of Japan (1998–2000) and a 3‐year multifactorial experiment in subtropical paddy rice fields in the Yangtze River delta in China (2001–2003), to investigate the methane (CH₄) emissions in response to an elevated atmospheric CO₂ concentration (200±40 mmol mol^?1 higher than that in the ambient atmosphere). No significant effect of the elevated CO₂ upon seasonal accumulative CH₄ emissions was observed in the first rice season, but significant stimulatory effects (CH₄ increase ranging from 38% to 188%, with a mean of 88%) were observed in the second and third rice seasons in the fields with or without organic matter addition. The stimulatory effects of the elevated CO₂ upon seasonal accumulative CH₄ emissions were negatively correlated with the addition rates of decomposable organic carbon (P<0.05), but positively with the rates of nitrogen fertilizers applied in either the current rice season (P<0.05) or the whole year (P<0.01). Six mechanisms were proposed to explain collectively the observations. Soil nitrogen availability was identified as an important regulator. The effect of soil nitrogen availability on the observed relation between elevated CO₂ and CH₄ emission can be explained by (a) modifying the C/N ratio of the plant residues formed in the previous growing season(s); (b) changing the inhibitory effect of high C/N ratio on plant residue decomposition in the current growing season; and (c) altering the stimulatory effects of CO₂ enrichment upon plant growth, as well as nitrogen uptake in the current growing season. This study implies that the concurrent enrichment of reactive nitrogen in the global ecosystems may accelerate the increase of atmospheric methane by initiating a stimulatory effect of the ongoing dramatic atmospheric CO₂ enrichment upon methane emissions from nitrogen‐poor paddy rice ecosystems and further amplifying the existing stimulatory effect in nitrogen‐rich paddy rice ecosystems.     

Effects of free-air CO2 enrichment (FACE) on CH4 emission from a rice paddy field

Methane (CH₄) is a particularly potent greenhouse gas with a radiative forcing 23 times that of CO₂ on a per mass basis. Flooded rice paddies are a major source of CH₄ emissions to the Earth's atmosphere. A free‐air CO₂ enrichment (FACE) experiment was conducted to evaluate changes in crop productivity and the crop ecosystem under enriched CO₂ conditions during three rice growth seasons from 1998 to 2000 in a rice paddy at Shizukuishi, Iwate, Japan. To understand the influence of elevated atmospheric CO₂ concentrations on CH₄ emission, we measured methane flux from FACE rice fields and rice fields with ambient levels of CO₂ during the 1999 and 2000 growing seasons. Methane production and oxidation potentials of soil samples collected when the rice was at the tillering and flowering stages in 2000 were measured in the laboratory by the anaerobic incubation and alternative propylene substrates methods, respectively. The average tiller number and root dry biomass were clearly larger in the plots with elevated CO₂ during all rice growth stages. No difference in methane oxidation potential between FACE and ambient treatments was found, but the methane production potential of soils during the flowering stage was significantly greater under FACE than under ambient conditions. When free‐air CO₂ was enriched to 550 ppmv, the CH₄ emissions from the rice paddy field increased significantly, by 38% in 1999 and 51% in 2000. The increased CH₄ emissions were attributed to accelerated CH₄ production potential as a result of more root exudates and root autolysis products and to increased plant‐mediated CH₄ emissions because of the larger rice tiller numbers under FACE conditions.     

A moderate increase in the annual CH4 efflux by raised CO2 or NH4NO3 supply in a boreal oligotrophic mire

Natural wetlands release about 20% of global emissions of CH₄, an effective greenhouse gas contributing to the total radiative forcing. Thus, changes in the carbon cycle in wetlands could have significant impacts on climate. The effect of raised supply of CO₂ or NH₄NO₃ on the annual CH₄ efflux from the lawn of a boreal oligotrophic mire was investigated over two years. Ten study plots were enclosed with mini‐FACE rings, five vented with CO₂‐enriched air and the other five with ambient air. In addition, five plots were sprayed with NH₄NO₃ so that the cumulative addition of N was 3 g m^?2 y^?1; and five plots were controls. The CO₂ enrichment (target concentration 560 ppm_v) increased CH₄ efflux about 30–40%, but half of this increase seemed to be caused by the air‐blowing system. The increasing atmospheric concentration of CO₂ would promote CH₄ release in boreal mires, but the increase in CH₄ efflux would be clearly smaller than that reported in studies made in temperate or subtropical temperature conditions. Addition of N enhanced the annual release of CH₄ only slightly. At least over the short‐term, the increase in N deposition would have little effect on CH₄ effluxes. The increase in CH₄ release would probably increase radiative forcing and thus accelerate climate change. However, CH₄ effluxes are only a small part in the whole matter balance in mires and thus further studies are needed to define the net effects of raised supply of CO₂ or N for carbon accumulation, trace gas fluxes and radiative forcing.     

黑土稻田CH4与N2O排放及减排措施研究

通过对黑土稻田CH₄和N₂O排放的观测,发现水稻生长季CH₄和N₂O排放量低于全国其它地区稻田.CH₄和N₂O排放之间存在互为消长关系(r=-0.513,P<0.05).但在同样施肥水平条件下,间歇灌溉与长期淹灌相比,CH₄排放明显减少而N₂O略有增加,其相对综合温室效应被大大减少且水稻产量未受影响.为此,间歇灌溉可作为减少稻田温室气体排放的水分管理措施.另外,通过对CH₄和N₂O排放的相关微生物过程探讨,揭示产甲烷菌数与CH₄排放间呈显著性正相关(R²=0.82,P<0.05),硝化菌数和反硝化菌数与N₂O排放有重要关系.     

Relationship between C2H2 reduction, H2 evolution and 15N2 fixation in root nodules of pea (Pisum sativum)

The quantitative relationship between C₂H₂ reduction, H₂ evolution and ¹⁵N₂ fixation was investigated in excised root nodules from pea plants ( Pisum sativum L. cv. Bodil) grown under controlled conditions. The C₂H₂/N₂ conversion factor varied from 3.31 to 5.12 between the 32nd and the 67th day after planting. After correction for H₂ evolution in air, the factor (C₂H₂-H₂)/N₂ decreased to values near the theoretical value 3, or in one case to a value significantly ( P < 0.05) below 3. The proportion of the total electron flow through nitrogenase, which is not wasted in H₂ production but used for N₂ reduction, is often stated as the relative efficiency (1-H₂/C₂H₂). This factor varied significantly ( P < 0.05) during the growth period. The actual allocation of electrons to H₂ and N₂, expressed as the H₂/N₂ ratio, was independent of plant age, however. This discrepancy and the observation that the (C₂H₂-H₂)/N₂ conversion factor tended to be lower than 3, suggests that the C₂H₂reduction assay underestimates the total electron flow through nitrogenase.     

稻田CH4和N2O的排放及养萍和施肥的影响

用箱法对我国东北稻田CH₄和N₂O排放进行观测研究表明,东北稻田的CH₄排放通量比南方稻田小,平均日排放通量和生长季节排放总量分别为0.07和7.4g·m^-2.稻田淹水期几乎没有N₂O的净排放,但在非淹水期内却有大量N₂O排放(平均通量59μgN₂O·m^-2·h^-1).稻田养萍和施肥明显促进CH₄和N₂O排放。稻田CH₄和N₂O排放之间存在消长关系。制定稻田温室气体减排技术措施时应充分注意这一关系。     

A review of nitrogen enrichment effects on three biogenic GHGs: the CO2 sink may be largely offset by stimulated N2O and CH4 emission

Anthropogenic nitrogen (N) enrichment of ecosystems, mainly from fuel combustion and fertilizer application, alters biogeochemical cycling of ecosystems in a way that leads to altered flux of biogenic greenhouse gases (GHGs). Our meta-analysis of 313 observations across 109 studies evaluated the effect of N addition on the flux of three major GHGs: CO₂, CH₄ and N₂O. The objective was to quantitatively synthesize data from agricultural and non-agricultural terrestrial ecosystems across the globe and examine whether factors, such as ecosystem type, N addition level and chemical form of N addition influence the direction and magnitude of GHG fluxes. Results indicate that N addition increased ecosystem carbon content of forests by 6%, marginally increased soil organic carbon of agricultural systems by 2%, but had no significant effect on net ecosystem CO₂ exchange for non-forest natural ecosystems. Across all ecosystems, N addition increased CH₄ emission by 97%, reduced CH₄ uptake by 38% and increased N₂O emission by 216%. The net effect of N on the global GHG budget is calculated and this topic is reviewed. Most often N addition is considered to increase forest C sequestration without consideration of N stimulation of GHG production in other ecosystems. However, our study indicated that although N addition increased the global terrestrial C sink, the CO₂ reduction could be largely offset (53–76%) by N stimulation of global CH₄ and N₂O emission from multiple ecosystems.     

Effects of elevated atmospheric carbon dioxide on amino acid and NH4+-N cycling in a temperate pine ecosystem

Rising atmospheric carbon dioxide (CO₂) is expected to increase forest productivity, resulting in greater carbon (C) storage in forest ecosystems. Because elevated atmospheric CO₂ does not increase nitrogen (N) use efficiency in many forest tree species, additional N inputs will be required to sustain increased net primary productivity (NPP) under elevated atmospheric CO₂. We investigated the importance of free amino acids (AAs) as a source for forest N uptake at the Duke Forest Free Air CO₂ Enrichment (FACE) site, comparing its importance with that of better‐studied inorganic N sources. Potential proteolytic enzyme activity was monitored seasonally, and individual AA concentrations were measured in organic horizon extracts. Potential free AA production in soils ranged from 190 to 690 nmol N g⁻¹ h⁻¹ and was greater than potential rates of soil NH₄⁺ production. Because of this high potential rate of organic N production, we determined (1) whether intact AA uptake occurs by Pinus taeda L., the dominant tree species at the FACE site, (2) if the rate of cycling of AAs is comparable with that of ammonium (NH₄⁺), and (3) if atmospheric CO₂ concentration alters the aforementioned N cycling processes. A field experiment using universally labeled ammonium (¹⁵NH₄⁺) and alanine (¹³C₃H₇¹⁵NO₂) demonstrated that ¹⁵N is more readily taken up by plants and heterotrophic microorganisms as NH₄⁺. Pine roots and microbes take up on average 2.4 and two times as much NH₄^{+ 15}N compared with alanine ¹⁵N 1 week after tracer application. N cycling through soil pools was similar for alanine and NH₄⁺, with the greatest ¹⁵N tracer recovery in soil organic matter, followed by microbial biomass, dissolved organic N, extractable NH₄⁺, and fine roots. Stoichiometric analyses of ¹³C and ¹⁵N uptake demonstrated that both plants and soil microorganisms take up alanine directly, with a ¹³C : ¹⁵N ratio of 3.3 : 1 in fine roots and 1.5 : 1 in microbial biomass. Our results suggest that intact AA (alanine) uptake contributes substantially to plant N uptake in loblolly pine forests. However, we found no evidence supporting increased recovery of free AAs in fine roots under elevated CO₂, suggesting plants will need to acquire additional N via other mechanisms, such as increased root exploration or increased N use efficiency.     

Links between methanotroph community composition and CH4 oxidation in a pine forest soil

The main gap in our knowledge about what determines the rate of CH₄ oxidation in forest soils is the biology of the microorganisms involved, the identity of which remains unclear. In this study, we used stable-isotope probing (SIP) following ¹³CH₄ incorporation into phospholipid fatty acids (PLFAs) and DNA/RNA, and sequencing of methane mono-oxygenase ( pmoA ) genes, to identify the influence of variation in community composition on CH₄ oxidation rates. The rates of ¹³C incorporation into PLFAs differed between horizons, with low ¹³C incorporation in the organic soil and relatively high ¹³C incorporation into the two mineral horizons. The microbial community composition of the methanotrophs incorporating the ¹³C label also differed between horizons, and statistical analyses suggested that the methanotroph community composition was a major cause of variation in CH₄ oxidation rates. Both PLFA and pmoA -based data indicated that CH₄ oxidizers in this soil belong to the uncultivated 'upland soil cluster α'. CH₄ oxidation potential exhibited the opposite pattern to ¹³C incorporation, suggesting that CH₄ oxidation potential assays may correlate poorly with in situ oxidation rates. The DNA/RNA-SIP assay was not successful, most likely due to insufficient ¹³C-incorporation into DNA/RNA. The limitations of the technique are briefly discussed.     

植物排放N2O和CH4的研究

N₂O和CH₄是2种重要的温室气体, 但其排放源尚未得到充分鉴别。1990年和2006年先后报道植物能排放N₂O和CH₄, 并日益受到广泛的关注。然而, 迄今为止对植物排放这2种气体的研究均是分开单独进行的。该文以8种陆生草本植物为研究对象, 首次同步考察了新鲜离体植物地上部排放N₂O和CH₄的通量。研究结果表明: 8种植物均能排放这2种气体。其中, 黑麦草(Lolium perenne)、抱茎苦荬菜(Ixeridium sonchifolium)和菠菜(Spinacia oleracea)的CH₄通量较高, 分别为165.38、 52.28和21.64 ngCH₄·g^–1dw·h^–1; 抱茎苦荬菜、蒙古蒿(Artemisia mongolica)、大豆(Glycine max)和菠菜的N₂O通量较高, 分别为7.19、6.92、5.44和4.05 ngN₂O·g^–1dw·h^–1。研究结果不仅为植物本身既能排放N₂O又能排放CH₄在植物中可能具有普遍性提供了进一步的实验依据, 而且为深入研究其机理找到了几种适宜的植物种(如抱茎苦荬菜、菠菜)。     

The effect of termite biomass and anthropogenic disturbance on the CH4 budgets of tropical forests in Cameroon and Borneo

The exchange of CH₄ between tropical forests and the atmosphere was determined by simultaneously measuring the net CH₄ flux at the soil surface and assessing the flux contribution from soil-feeding termite biomass, both within the soil profile and in mounds. In Cameroon the flux of CH₄ ranged from a net emission of 40.7 ng m^–2 s^–1 to a net CH₄ oxidation of –53.0 ng m^–2 s^–1. Soil-inhabiting termite biomass was significantly correlated with CH₄ flux. Termite mounds emitted up to 2000 ng s^–1 mound^–1. Termite-derived CH₄ emission reduced the soil sink strength by up to 28%. Disturbance also had a strong effect on the soil sink strength, with the average rate of CH₄ oxidation, at – 17.5 ng m^–2 s^–1, being significantly smaller (≈ 36%) at the secondary forest site than the –27.2 ng m^–2 s^–1, observed at the primary forest site. CH₄ budgets calculated for each site indicated that both forests were net sinks for CH₄ at – 6.1 kg ha^–1 y^–1 in the near-primary forest and – 3.1 kg ha^–1 y^–1 in the secondary forest. In Borneo, three forest sites representing a disturbance gradient were examined. CH₄ oxidation rates ranged from 0 to – 32.1 ng m^–2s^–1 and a significant correlation between the net flux and termite biomass was observed only in an undisturbed primary forest, although the biomass was insufficient to cause net emission of CH₄. Rates of CH₄ oxidation were not significantly different across the disturbance gradient but were, however, larger in the primary forest (averaging – 15.4 ng m^–2 s^–1) than in an old-growth secondary forest (–13.9 ng m^–2s^–1) and a young secondary re-growth (– 10.8 ng m^–2s^–1). CH₄ flux from termite mounds ranged from net oxidation in an abandoned mound to a maximum emission of 468 ng s^–1 mound^–1. CH₄ budgets calculated for each site indicated that CH₄ flux from termite mounds had an insignificant effect on the budget of CH₄ at the regional scale at all three forest sites. Annual oxidation rates were – 4.8, – 4.2 and – 3.4 kg ha^–1 y^–1 in the primary, secondary and young secondary forests, respectively.     

加气灌溉对不同施氮水平的设施甜瓜土壤CO2和N2O排放的影响

为探讨不同加气灌溉施氮模式下设施甜瓜土壤CO₂和N₂O排放的动态变化规律及其与土壤温度、湿度的关系,本研究采用密闭静态箱-气相色谱法对加气灌溉不同施氮水平下土壤CO₂和N₂O排放进行监测,并分析了加气灌溉对不同施氮量下土壤CO₂和N₂O排放的影响.试验采用加气灌溉(AI)和不加气灌溉(CK)两种灌溉方式,施氮量设不施氮(N₁)、传统施氮量的2/3(150 kg·hm^-2,N₂)和传统施氮量(225 kg·hm^-2,N₃)3个施氮水平.结果表明:加气灌溉土壤CO₂和N₂O排放量高于不加气灌溉处理,但是差异不显著;相同灌溉模式下,CO₂和N₂O排放量随施氮量的增加而显著增加,施氮量是土壤CO₂和N₂O排放的主要影响因素.加气灌溉条件下,不同施氮处理N₂O排放通量与土壤温度和湿度呈显著正相关,CO₂排放通量与土壤温度呈显著正相关.加气减氮处理在氮肥减少1/3的情况下,甜瓜产量提高了6.9%,温室气体排放引起的增温潜势值从9544.82 kg·hm^-2下降到9340.72 kg·hm^-2.综上,通过加气灌溉减少氮肥施用量来抑制农业生产系统中温室气体排放是可行的.     

Landscape controls of CH4 fluxes in a catchment of the forest tundra ecotone in northern Siberia

Terrestrial ecosystems in northern high latitudes exchange large amounts of methane (CH₄) with the atmosphere. Climate warming could have a great impact on CH₄ exchange, in particular in regions where degradation of permafrost is induced. In order to improve the understanding of the present and future methane dynamics in permafrost regions, we studied CH₄ fluxes of typical landscape structures in a small catchment in the forest tundra ecotone in northern Siberia. Gas fluxes were measured using a closed‐chamber technique from August to November 2003 and from August 2006 to July 2007 on tree‐covered mineral soils with and without permafrost, on a frozen bog plateau, and on a thermokarst pond. For areal integration of the CH₄ fluxes, we combined field observations and classification of functional landscape structures based on a high‐resolution Quickbird satellite image. All mineral soils were net sinks of atmospheric CH₄. The magnitude of annual CH₄ uptake was higher for soils without permafrost (1.19 kg CH₄ ha⁻¹ yr⁻¹) than for soils with permafrost (0.37 kg CH₄ ha⁻¹ yr⁻¹). In well‐drained soils, significant CH₄ uptake occurred even after the onset of ground frost. Bog plateaux, which stored large amounts of frozen organic carbon, were also a net sink of atmospheric CH₄ (0.38 kg CH₄ ha⁻¹ yr⁻¹). Thermokarst ponds, which developed from permafrost collapse in bog plateaux, were hot spots of CH₄ emission (approximately 200 kg CH₄ ha⁻¹ yr⁻¹). Despite the low area coverage of thermokarst ponds (only 2.1% of the total catchment area), emissions from these sites resulted in a mean catchment CH₄ emission of 3.8 kg CH₄ ha⁻¹ yr⁻¹. Export of dissolved CH₄ with stream water was insignificant. The results suggest that mineral soils and bog plateaux in this region will respond differently to increasing temperatures and associated permafrost degradation. Net uptake of atmospheric CH₄ in mineral soils is expected to gradually increase with increasing active layer depth and soil drainage. Changes in bog plateaux will probably be much more rapid and drastic. Permafrost collapse in frozen bog plateaux would result in high CH₄ emissions that act as positive feedback to climate warming.     

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CH4和N2O排放关系及其微生物学机理和一些影响因子

用静态箱法原位观测和分析了我国北方稻田3～12月CH₄和N₂O的排放及其关系,并研究了这一关系发生的微生物学机理.同时,监测了土壤湿度、pH、水分及Eh的变化.结果表明,稻田CH₄和N₂O排放之间存在着互为消长的关系(R²=0.0494).土壤湿度、pH及Eh变化范围分别在0～24℃、6.87～7.02和415～300mv之间,水分从非淹水期的38～72%FC至5～10cm浅水淹灌.土壤Eh对CH₄和N₂O的释放起重要的调控作用.在整个观测期内,与CH₄和N₂O释放密切相关的6种菌群(发酵细菌、产氢产乙酸细菌、产甲烷细菌、甲烷氧化菌、硝化细菌、反硝化细菌)各有其数量消长及酶活性变化规律,稻田CH₄和N₂O排放之间互为消长的关系受这些相关微生物数量及酶活性变化的共同调控.     

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.