土壤氧气可获得性对双季稻田温室气体排放通量的影响

为探讨土壤氧气可获得性(SOA)对双季稻田温室气体排放的影响,利用静态箱气相色谱法对多种管理措施影响下稻田温室气体排放通量和土壤氧化还原电位(Eh)、pH值及田间淹水深度(H)等3种SOA因子进行了观测。结果表明,甲烷(CH₄)排放最集中的Eh值、pH值和H范围分别为-100-0mV、5 < pH < 6和1-5cm,3个范围内分别观测到48.8%、61.1%和77.0%的CH₄排放,其中H对CH₄排放影响最明显,单独由其就可解释37.8%的CH₄排放通量(P < 0.0001)。对于氧化亚氮(N₂O),观测到较多的负通量,其纯排放最密集的3种SOA因子的范围分别是:0-100mV、5 < pH < 6和1-5cm,而200-300mV是其排放的临界Eh范围,高于此范围N₂O排放极少。厌氧的反硝化过程是双季稻田N₂O产生的主导过程。可为水稻田温室气体排放机理研究提供基础数据。     

Soil core method for direct simultaneous determination of N2 and N2O emissions from forest soils

In order to quantify N₂-emissions from a spruce and a beech site at the Höglwald Forest, a new measuring system was developed, that allowed simultaneous, direct determination of N₂- and N₂O-emission with high accuracy (detection limit approx. 10 g N m^–2 h^–1 for N₂ and <1 g for N₂O) using a gas-flow core method. This method requires exchange of the soil atmosphere with an artificial atmosphere, that differs only in that N₂ is substituted by He. The measuring system, the methodology of measurements and validation experiments are described in detail. Due to the huge heterogeneity of denitrification activity in different soil cores taken from our forest sites, no general trends of N₂ and N₂O production in relation to soil moisture and temperature could be demonstrated. Based on reasonable number of measurements, this work gives for the first time an estimate of the magnitude of N₂-losses from temperate forest soils. Both the magnitude of N₂-emissions (spruce: 7.2±0.7 kg N₂-N ha^–1 yr^–1; beech: 12.4±3.1 kg N₂-N ha^–1 yr^–1), as well as the N₂O–N₂ ratio (spruce: 0.136±0.04; beech: 0.52±0.19) were significantly higher for soils from the beech sites as compared to soils from the spruce site. The results suggests that N₂-emissions from N-saturated forest soils, still receiving high loads of atmospheric N-deposition, are approx. 30% of atmospheric N-input at the spruce site, and approx. 50% at the beech site. Our results demonstrate that losses of nitrogen in the form of N₂ cannot be neglected in the context of calculating N-balances for given forest sites.     

DOC and N<Subscript>2</Subscript>O dynamics in upland and peatland forest soils after clear-cutting and soil preparation

Forest clear-cutting followed by soil preparation means disturbance for soil microorganisms and disruption of N and C cycles. We measured fluxes of N₂O and dissolved organic carbon (DOC) in upland soil (podzol) and adjacent peat within a clear-cut forest catchment. Both soil types behaved in a similar way, showing net uptake of N₂O in the first year after the clear-cutting, and turning to net release in the second. The N₂O flux dynamics were similar to those of N content in logging residues, as reported from a nearby site. As organic matter is used in the food web of the decomposers, we attempted to explain the dynamics of N₂O uptake and release by measuring the concurrent dynamics of the low molecular weight (LMW) fraction and the aromaticity of DOC in a soil solution. The labile and most readily available LMW fractions of DOC were nearly absent in the year following the clear-cutting, but rose after two years. The more refractory high molecular weight (HMW) fraction of DOC decreased two years after the clear-cutting. The first year’s net uptake of N₂O could be accounted for by the growth of decomposer biomass in the logging residues and detritus from the degenerating ground vegetation, resulting in immobilization of nitrogen. Simultaneously, the labile, LMW fraction of DOC became almost completely exhausted. The low availability of the LMW fraction could retard the growth and cause the accumulated decomposer biomass to collapse. During the following winter and summer the fraction of LMW clearly increased, followed by increased N₂O emissions. The presence of LMW DOC fractions, not the concentration of DOC, seems to be an important controller for N₂O liberation after a major disturbance such as clear-cutting and site preparation. The complex connection between DOC characteristics, nitrification or denitrification merits further studies.     

Disentangling the long‐term effects of disturbance on soil biogeochemistry in a wet tropical forest ecosystem

Climate change is increasing the intensity of severe tropical storms and cyclones (also referred to as hurricanes or typhoons), with major implications for tropical forest structure and function. These changes in disturbance regime are likely to play an important role in regulating ecosystem carbon (C) and nutrient dynamics in tropical and subtropical forests. Canopy opening and debris deposition resulting from severe storms have complex and interacting effects on ecosystem biogeochemistry. Disentangling these complex effects will be critical to better understand the long‐term implications of climate change on ecosystem C and nutrient dynamics. In this study, we used a well‐replicated, long‐term (10 years) canopy and debris manipulation experiment in a wet tropical forest to determine the separate and combined effects of canopy opening and debris deposition on soil C and nutrients throughout the soil profile (1 m). Debris deposition alone resulted in higher soil C and N concentrations, both at the surface (0–10 cm) and at depth (50–80 cm). Concentrations of NaOH‐organic P also increased significantly in the debris deposition only treatment (20–90 cm depth), as did NaOH‐total P (20–50 cm depth). Canopy opening, both with and without debris deposition, significantly increased NaOH‐inorganic P concentrations from 70 to 90 cm depth. Soil iron concentrations were a strong predictor of both C and P patterns throughout the soil profile. Our results demonstrate that both surface‐ and subsoils have the potential to significantly increase C and nutrient storage a decade after the sudden deposition of disturbance‐related organic debris. Our results also show that these effects may be partially offset by rapid decomposition and decreases in litterfall associated with canopy opening. The significant effects of debris deposition on soil C and nutrient concentrations at depth (>50 cm), suggest that deep soils are more dynamic than previously believed, and can serve as sinks of C and nutrients derived from disturbance‐induced pulses of organic matter inputs.     

土壤磷含量与有效性对N2O排放量的影响元分析

土壤氧化亚氮(N₂O)主要产生于土壤微生物参与的氮循环过程,其排放量受磷含量及有效性影响。添加磷肥有助于缓解陆地生态系统磷限制,提升土壤有效磷含量,进一步影响土壤微生物对氮的利用,同时控制N₂O排放。然而不同独立实验中N₂O对外源磷添加的响应差异较大。研究从发表的中、英文文献中收集了54份关于施用磷肥与N₂O排放量的观测结果,采用元分析方法确定添加外源磷后N₂O排放量的响应差异及潜在影响因素。结果表明：(1)外源磷添加对土壤N₂O排放量影响不显著;但在磷肥施用量> 50 kg P/hm²的室外实验中土壤N₂O排放量显著降低了32.5%;施用NaH₂PO₄的室内实验中N₂O排放量显著降低了18.4%。(2)土壤N₂O对外源磷添加响应的高变异性是磷肥施用量和土壤含水量、土壤pH、土地利用类型、磷肥种类、纬度和实验时间多种影响...     

Allocation of carbon in a mature eucalypt forest and some effects of soil phosphorus availability

Pools and annual fluxes of carbon (C) were estimated for a mature Eucalyptus pauciflora (snowgum) forest with and without phosphorus (P) fertilizer addition to determine the effect of soil P availability on allocation of C in the stand. Aboveground biomass was estimated from allometric equations relating stem and branch diameters of individual trees to their biomass. Biomass production was calculated from annual increments in tree diameters and measurements of litterfall. Maintenance and construction respiration were calculated for each component using equations given by Ryan (1991a). Total belowground C flux was estimated from measurements of annual soil CO₂ efflux less the C content of annual litterfall (assuming forest floor and soil C were at approximate steady state for the year that soil CO₂ efflux was measured). The total C content of the standing biomass of the unfertilized stand was 138 t ha^-1, with approximately 80% aboveground and 20% belowground. Forest floor C was 8.5 t ha^-1. Soil C content (0–1 m) was 369 t ha^-1 representing 70% of the total C pool in the ecosystem. Total gross annual C flux aboveground (biomass increment plus litterfall plus respiration) was 11.9 t ha^-1 and gross flux belowground (coarse root increment plus fine root production plus root respiration) was 5.1 t ha^-1. Total annual soil efflux was 7.1 t ha^-1, of which 2.5 t ha^-1 (35%) was contributed by litter decomposition.The short-term effect of changing the availability of P compared with C on allocation to aboveground versus belowground processes was estimated by comparing fertilized and unfertilized stands during the year after treatment. In the P-fertilized stand annual wood biomass increment increased by 30%, there was no evidence of change in canopy biomass, and belowground C allocation decreased by 19% relative to the unfertilized stand. Total annual C flux was 16.97 and 16.75 t ha^-1 yr^-1 and the ratio of below- to aboveground C allocation was 0.43 and 0.35 in the unfertilized and P-fertilized stands, respectively. Therefore, the major response of the forest stand to increased soil P availability appeared to be a shift in C allocation; with little change in total productivity. These results emphasise that both growth rate and allocation need to be estimated to predict changes in fluxes and storage of C in forests that may occur in response to disturbance or climate change.     

Oxygen effects on methane production and oxidation in humid tropical forest soils

We investigated the effects of oxygen (O₂) concentration on methane (CH₄) production and oxidation in two humid tropical forests that differ in long‐term, time‐averaged soil O₂ concentrations. We identified sources and sinks of CH₄ through the analysis of soil gas concentrations, surface emissions, and carbon isotope measurements. Isotope mass balance models were used to calculate the fraction of CH₄ oxidized in situ. Complementary laboratory experiments were conducted to determine the effects of O₂ concentration on gross and net rates of methanogenesis. Field and laboratory experiments indicated that high levels of CH₄ production occurred in soils that contained between 9±1.1% and 19±0.2% O₂. For example, we observed CH₄ concentrations in excess of 3% in soils with 9±1.1% O₂. CH₄ emissions from the lower O₂ sites were high (22–101 nmol CH₄ m^?2 s^?1), and were equal in magnitude to CH₄ emissions from natural wetlands. During peak periods of CH₄ efflux, carbon dioxide (CO₂) emissions became enriched in ¹³C because of high methanogenic activity. Gross CH₄ production was probably greater than flux measurements indicated, as isotope mass balance calculations suggested that 48–78% of the CH₄ produced was oxidized prior to atmospheric egress. O₂ availability influenced CH₄ oxidation more strongly than methanogenesis. Gross CH₄ production was relatively insensitive to O₂ concentrations in laboratory experiments. In contrast, methanotrophic bacteria oxidized a greater fraction of total CH₄ production with increasing O₂ concentration, shifting the δ¹³C composition of CH₄ to values that were more positive. Isotopic measurements suggested that CO₂ was an important source of carbon for methanogenesis in humid forests. The δ¹³C value of methanogenesis was between ?84‰ and ?98‰, which is well within the range of CH₄ produced from CO₂ reduction, and considerably more depleted in ¹³C than CH₄ formed from acetate.     

The effect of different N substrates on biological N2O production from forest and agricultural light textured soils

N₂O emissions from two slightly alkaline sandy soils, from arable land and a woodland, were determined in a laboratory experiment in which the soils were incubated with different sources of nitrogen, with or without glucose, and with 0, 1 and 100 mL C₂H₂ L^-1. Large differences in the rate of N₂O production were observed between the two soils and between the different N treatments. The arable soil showed very low N₂O emissions derived from reduced forms of N as compared with the N₂O which was produced when the soil was provided with NO ₂ ^- or NO ₃ ^- and a C source, suggesting a very active denitrifier population. In contrast, the woodland soil showed a very low denitrification activity and a much higher N₂O production derived from the oxidation of NH ₄ ⁺ and reduction of NO ₂ ^- by some processes probably mediated by autotrophic or heterotrophic nitrifiers or dissimilatory NO ₂ ^- reducers. In both soils, the highest N₂O emissions were induced by NO ₂ ^- addition. Those emissions were demonstrated to have a biological origin, as no significant N₂O emissions were measured when the soil was autoclaved.     

Soil organic matter dynamics under decaying wood in a subtropical wet forest: effect of tree species and decay stage

Decaying wood is an important structural and functional component of forests: it contributes to generate habitat diversity, acts as either sink or source of nutrients, and plays a preponderant role in soil formation. Thus, decaying wood might likely have measurable effects on chemical properties of the underlying soil. We hypothesized that decaying wood would have a stronger effect on soil as decomposition advances and that such effect would vary according to wood quality. Twenty logs from two species with contrasting wood properties (Dacryodes excelsa Vahl. and Swietenia macrophylla King) and at two different decay stages (6 and 15 years after falling) were selected, and soil under and 50 cm away from decaying logs was sampled for soil organic matter (SOM) fractions [NaOH-extractable and water-extractable organic matter -(WEOM)] and properties (WEOM aromaticity). NaOH-extractable C and WEOM were higher in the soil influenced by 15-year-old logs, while the degree of aromaticity of WEOM was higher in the soil influenced by the 6-year-old logs. Decaying logs did influence properties of the underlying soil with differing effects according to the species since there was more NaOH-extractable C in the soil associated to D. excelsa logs and more WEOM in the soil associated to S. macrophylla older logs. It is proposed that such effects occurred through changes in the relative quantity and quality of different SOM fractions, as influenced by species and advancement in decomposition. Through its effect on SOM and nutrient dynamics, decaying wood can contribute to the spatial heterogeneity of soil properties, and can affect process of soil formation and nutrient cycling. Responsible Editor: Barbara Wick.     

Soil N and C trace gas fluxes and microbial soil N turnover in a sessile oak (Quercus petraea (Matt.) Liebl.) forest in Hungary

During two intensive field campaigns in summer and autumn 2004 nitrogen (N₂O, NO/NO₂) and carbon (CO₂, CH₄) trace gas exchange between soil and the atmosphere was measured in a sessile oak (Quercus petraea (Matt.) Liebl.) forest in Hungary. The climate can be described as continental temperate. Fluxes were measured with a fully automatic measuring system allowing for high temporal resolution. Mean N₂O emission rates were 1.5 μg N m⁻² h⁻¹ in summer and 3.4 μg N m⁻² h⁻¹ in autumn, respectively. Also mean NO emission rates were higher in autumn (8.4 μg N m⁻² h⁻¹) as compared to summer (6.0 μg N m⁻² h⁻¹). However, as NO₂ deposition rates continuously exceeded NO emission rates (−9.7 μg N m⁻² h⁻¹ in summer and −18.3 μg N m⁻² h⁻¹ in autumn), the forest soil always acted as a net NO _x sink. The mean value of CO₂ fluxes showed only little seasonal differences between summer (81.1 mg C m⁻² h⁻¹) and autumn (74.2 mg C m⁻² h⁻¹) measurements, likewise CH₄uptake (summer: −52.6 μg C m⁻² h⁻¹; autumn: −56.5 μg C m⁻² h⁻¹). In addition, the microbial soil processes net/gross N mineralization, net/gross nitrification and heterotrophic soil respiration as well as inorganic soil nitrogen concentrations and N₂O/CH₄ soil air concentrations in different soil depths were determined. The respiratory quotient (ΔCO_{2 resp} ΔO_{2 resp}⁻¹) for the uppermost mineral soil, which is needed for the calculation of gross nitrification via the Barometric Process Separation (BaPS) technique, was 0.8978 ± 0.008. The mean value of gross nitrification rates showed only little seasonal differences between summer (0.99 μg N kg⁻¹ SDW d⁻¹) and autumn measurements (0.89 μg N kg⁻¹ SDW d⁻¹). Gross rates of N mineralization were highest in the organic layer (20.1–137.9 μg N kg⁻¹ SDW d⁻¹) and significantly lower in the uppermost mineral layer (1.3–2.9 μg N kg⁻¹ SDW d⁻¹). Only for the organic layer seasonality in gross N mineralization rates could be demonstrated, with highest mean values in autumn, most likely caused by fresh litter decomposition. Gross mineralization rates of the organic layer were positively correlated with N₂O emissions and negatively correlated with CH₄ uptake, whereas soil CO₂ emissions were positively correlated with heterotrophic respiration in the uppermost mineral soil layer. The most important abiotic factor influencing C and N trace gas fluxes was soil moisture, while the influence of soil temperature on trace gas exchange rates was high only in autumn.     

Improving phosphorus availability in two upland soils of Vietnam using Tithonia diversifolia H.

Phosphorus was added to two acidic upland soils (a Cambisol and a Ferralsol) at two rates (9 mg P kg⁻¹ and 145 mg P kg⁻¹) either in an inorganic P form (KH₂PO₄) or as a green manure (Tithonia diversifolia H. at 2.5 g kg⁻¹ and 40 g kg⁻¹). The effect of P source on the chemical availability of P was assessed in an incubation experiment by measuring resin extractable P, soluble molybdate reactive (DMR-P) and unreactive P (DMU-P). Soil pH and extractable Al were monitored during the incubation period of 49 days. Green manure addition caused an immediate and sustained increase in soil pH and an immediate and sustained decrease in extractable Al. Labile P (resin P + DMR-P + DMU-P) was increased more by P added as a green manure than when added in inorganic form in one soil (Ferralsol), while it decreased or did not differ in the other one (Cambisol). In both soils, the concentrations of soluble DMU-P were frequently higher where Tithonia had been added. The effects of green manure amendment on physical factors governing the phosphorus supply through diffusive transport were also investigated. Aggregate size distribution was substantially changed by green manure amendment due to a shift in the percentage of microaggregates (<250 μm in diameter) to larger sizes. Changes in soil aggregation as a consequence of green manure amendment led to a reduction in specific surface area (SSA) of the whole soil. Coupled with the large increase in effective cation exchange capacity caused by green manure amendment in both soils, and the decrease in SSA, there was an increase in the net negative surface charge density in both soils. In summary, at a large addition rate – and in addition to the well-known effect derived from the extra supply in P, green manure amendment may improve the chemical availability and diffusive supply of P through the following mechanisms: (i) an increase in soil pH increasing the solubility of phosphate sources; (ii) a decrease in extractable Al reducing the fixation of added P; (iii) increased macro-aggregation and reduced specific surface area and porosity leading to fewer sorption sites for P and hence enhanced diffusion rates; and (iv) increased negative charges and reduced positive charges at the soil surface resulting in a net increase in repulsive force for P. The induced changes in most measured soil properties were smaller in the Ferralsol than in the Cambisol. This revised version was published online in June 2005 with a corrected article title.     

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。     

干湿交替频率对不同土壤CO2和N2O释放的影响

干旱、半干旱和地中海气候区,乃至一些湿润地区,由干湿交替引起的土壤碳、氮的短暂脉冲式释放很大程度上决定着长时间尺度温室气体释放的总量,是土壤碳、氮温室气体释放的关键过程.选择我国降雨梯度下的森林、农田、草地和荒漠生态系统,采集土样进行实验室统一控制条件下的多重干湿交替循环,对比探讨不同生态系统土壤干湿交替频率对CO2和N2O释放的影响模式.结果表明:(1)干湿交替能够显著的激发土壤中CO2和N2O的释放,森林、农田、草地和荒漠土壤CO2和N2O释放速率对干湿交替的响应模式基本一致,其响应强度与土壤本底中碳和氮的含量有关;(2)在一定培养时间内,随着干湿交替频率的增加,土壤再湿润阶段CO2释放速率降低,但是,气体释放的总量较之于恒湿对照组有所增加.(3)不同土壤N2O的释放总量对于湿交替频率的响应模式表现出很大的差异,其中农田和荒漠土壤响应模式类似.     

Carbon biogeochemistry and climate change

The rapid increase of atmospheric CO₂ resulting from anthropogenic activites has stimulated a great deal of interest in the carbon cycle. Important decisions need to be made about future tolerable levels of atmospheric CO₂ content, as well as the land and fossil fuel use strategies that will permit us to achieve these goals. The vast amount of new data on atmospheric CO₂ content and ancillary properties that has become available during the last decade, and the development of models to interpret these data, have led to significant advances in our capacity to deal with such issues. However, a major continuing source of uncertainty is the role of photosynthesis in providing a sink for anthropogenic emissions. It is thus appropriate that a new evaluation of the status of our understanding of this issue should be made at this time.The aim of this paper is to provide a setting for the papers that follow by giving an overview of the role of carbon dioxide in climate, the biogeochemical processes that control its distribution, and the evolution of carbon dioxide through time from the origin of the earth to the present. We begin with a discussion of relevant processes. We then proceed to a more detailed discussion of the time periods that are best documented: the late Pleistocene (during which time large continental ice sheets waxed and waned) and the modern era of anthropogenic impact on the carbon cycle.     

Nitrogen fixation by non-legumes in tropical agriculture with special reference to wetland rice

Summary Of the 143 million hectares of cultivated rice land in the world, 75% are planted to wetland rice. Wet or flooded conditions favour biological nitrogen fixation by providing (1) photic-oxic floodwater and surface soil for phototrophic, free-living or symbiotic blue-green algae (BGA), and (2) aphotic-anoxic soil for anaerobic or microaerobic, heterotrophic bacteria. TheAzolla-Anabaena symbiosis can accumulate as much as 200 kg N ha^–1 in biomass. In tropical flooded fields, biomass production from a singleAzolla crop is about 15 t fresh weight ha^–1 or 35 kg N ha^–1. Low tolerance for high temperature, insect damage, phosphorus requirement, and maintenance of inoculum, limit application in the tropics. Basic work on taxonomy, sporulation, and breeding ofAzolla is needed. Although there are many reports of the positive effect of BGA inoculation on rice yield, the mechanisms of yield increase are not known. Efficient ways to increase N₂-fixation by field-grown BGA are not well exploited. Studies on the ecology of floodwater communities are needed to understand the principles of manipulating BGA. Bacteria associated with rice roots and the basal portion of the shoot also fix nitrogen. The system is known as a rhizocoenosis. N₂-fixation in rhizocoenosis in wetland rice is lower than that ofAzolla or BGA. Ways of manipulating this process are not known. Screening rice varieties that greatly stimulate N₂-fixation may be the most efficient way of manipulating the rhizocoenosis. Stimulation of N₂-fixation by bacterial inoculation needs to be quantified.     

Cropping intensity and rainfall effects on upland rice yields in northern Laos

Two fractions of earthworm humic substances, differing in nominal molecular weight, containing a very low amount of free IAA, and exhibiting auxin-like properties, were prepared and characterised by infrared and ¹H-NMR spectroscopy. In this study we investigate their effects on stomatal opening, as influenced by phospholipase A₂, in leaf of the Argenteum mutant of pea (Pisum sativum L.) Both of the humic fractions caused stomatal opening in the epidermal peels. The response showed a broad biphasic dose dependence and the effective concentrations were similar for the two fractions. The maximal stomatal apertures in response to both humic substances were similar to that caused by IAA and somewhat less than the response to white light or fusicoccin. Two inhibitors of phospholipase A₂ selectively blocked the response of stomata to both IAA and humic substances, without affecting the response to light or fusicoccin. We conclude that stomatal opening in response to auxin and humic substances involves activation of a phospholipase A₂ that is not involved in signalling the response to light or fusicoccin.     

东北温带次生林和落叶松人工林土壤CH4吸收和N2O排放通量

2007年6月-2008年6月,在帽儿山用静态箱/气相色谱法测定了相邻次生林和落叶松人工林土壤CH4和N2O通量,结果表明:次生林转变为落叶松人工林后土壤年CH4吸收和年N2O排放通量均显著增加,分别为次生林的1.2倍和3.6倍.两林分CH4和N2O通量表现相似的季节动态,生长季土壤CH4吸收通量和N2O排放通量均高于非生长季.次生林和落叶松人工林土壤CH4吸收通量与土壤温度均呈正相关关系,而与土壤含水量呈负相关关系.土壤N2O排放通量与土壤温度和土壤铵态氮含量均呈正相关关系,而与土壤含水量没有明显相关性.次生林转变为落叶松人工林后,落叶松林地较厚的凋落物层改变了林地土壤水分的格局,影响了土壤的CH4和N2O通量.     

Short term effect of ploughing a permanent pasture on N2O production from nitrification and denitrification

Soils are an important source of N₂O, which can be produced both in the nitrification and the denitrification processes. Grassland soils in particular have a high potential for mineralization and subsequent nitrification and denitrification. When ploughing long term grassland soils, the resulting high supply of mineral N may provide a high potential for N₂O losses. In this work, the short-term effect of ploughing a permanent grassland soil on gaseous N production was studied at different soil depths. Fertiliser and irrigation were applied in order to observe the effect of ploughing under a range of conditions. The relative proportions of N₂O produced from nitrification and denitrification and the proportion of N₂ gas produced from denitrification were determined using the methyl fluoride and acetylene specific inhibitors. Irrespectively to ploughing, fertiliser application increased the rates of N₂O production, N₂O production from nitrification, N₂O production from denitrification and total denitrification (N₂O + N₂). Application of fertiliser also increased the denitrification N₂O/N₂ ratio both in the denitrification potential and in the gaseous N productions by denitrification. Ploughing promoted soil organic N mineralization which led to an increase in the rates of N₂O production, N₂O production from nitrification, N₂O production from denitrification and total denitrification (N₂O + N₂). In both the ploughed and unploughed treatments the 0–10 cm soil layer was the major contributing layer to gaseous N production by all the above processes. However, the contribution of this layer decreased by ploughing, gaseous N productions from the 10 to 30 cm layer being significantly increased with respect to the unploughed treatment. Ploughing promoted both nitrification and denitrification derived N₂O production, although a higher proportion of N₂O lost by denitrification was observed as WFPS increased. Recently ploughed plots showed lower denitrification derived N₂O percentages than those ploughed before as a result of the lower soil water content in the former plots. Similarly, a lower mean nitrification derived N₂O percentage was found in the 10–30 cm layer compared with the 0–10 cm.     

Isotopic signatures of N2O produced by ammonia-oxidizing archaea from soils

N₂O gas is involved in global warming and ozone depletion. The major sources of N₂O are soil microbial processes. Anthropogenic inputs into the nitrogen cycle have exacerbated these microbial processes, including nitrification. Ammonia-oxidizing archaea (AOA) are major members of the pool of soil ammonia-oxidizing microorganisms. This study investigated the isotopic signatures of N₂O produced by soil AOA and associated N₂O production processes. All five AOA strains (I.1a, I.1a-associated and I.1b clades of Thaumarchaeota) from soil produced N₂O and their yields were comparable to those of ammonia-oxidizing bacteria (AOB). The levels of site preference (SP), δ¹⁵N^bulk and δ¹⁸O -N₂O of soil AOA strains were 13–30%, −13 to −35% and 22–36%, respectively, and strains MY1–3 and other soil AOA strains had distinct isotopic signatures. A ¹⁵N-NH₄⁺-labeling experiment indicated that N₂O originated from two different production pathways (that is, ammonia oxidation and nitrifier denitrification), which suggests that the isotopic signatures of N₂O from AOA may be attributable to the relative contributions of these two processes. The highest N₂O production yield and lowest site preference of acidophilic strain CS may be related to enhanced nitrifier denitrification for detoxifying nitrite. Previously, it was not possible to detect N₂O from soil AOA because of similarities between its isotopic signatures and those from AOB. Given the predominance of AOA over AOB in most soils, a significant proportion of the total N₂O emissions from soil nitrification may be attributable to AOA.     

Mechanisms of carbon and nutrient release and retention in beech forest gaps

Fluxes of CO₂ and N₂O were measured along a microclimatic gradient stretching from the centre of a gap into a mature beech stand using an automated chamber method. Simultaneously the regulating factors like soil water tensions, soil temperatures, nitrate concentrations were measured along the gradient. The daily mean values of the fluxes of CO₂ and N₂O were divided into classes of temperature and furthermore subdivided into classes of soil water tension to assess the significance of each regulating factor.Soil respiration at the centre of the gap was 40% lower compared to the rooted mature stand. The difference was explained by root respiration. At both sites soil respiration was primarily controlled by the soil temperature with an average Q₁₀ value of 2.3 over the different classes of temperature and soil water tension. Soil water tension reduced the soil respiration by up to 20% only by soil water tension above 400–600 hPa at the mature stand. The formation of N₂O was reduced when the soil temperature was below 10°C or the soil water tension exceeded 200 hPa. Therefore the N₂O emission was 6 times higher at the unrooted centre of the gap due to the high moisture content in the growing season. Higher nitrate concentration doubled the N₂O emission at the unrooted edge of the canopy and resulted in losses of 6.4 kg N ha^-1 within six months. Above 10°C and below 200 hPa the N₂O emission depended strongly upon the temperature with varying Q₁₀ values over the different classes of temperature and soil water tension. High Q₁₀ values up to 14.4 have been calculated below 14°C and were explained by several processes with synergetic effects.