Denitrification in a semi-arid grazing ecosystem

The effect of large herbivores on gaseous N loss from grasslands, particularly via denitrification, is poorly understood. In this study, we examined the influence of native migratory ungulates on denitrification in grasslands of Yellowstone National Park in two ways, by (1) examining the effect of artificial urine application on denitrification, and (2) comparing rates inside and outside long-term exclosures at topographically diverse locations. Artificial urine did not influence denitrification 3 and 12 days after application at hilltop, mid-slope, and slope-bottom sites. Likewise, grazers had no effect on community-level denitrification at dry exclosure sites, where rates were low. At mesic sites, however, ungulates enhanced denitrification by as much as 4 kg N ha⁻¹ year⁻¹, which was double atmospheric N inputs to this ecosystem. Denitrification enzyme activity (DEA, a measure of denitrification potential) was positively associated with soil moisture at exclosure sites, and herbivores stimulated DEA when accounting for the soil moisture effect. Glucose additons to soils increased denitrification and nitrate additions had no influence, suggesting that denitrification was limited by the amount of labile soil carbon, which previously has been shown to be enhanced by ungulates in Yellowstone. These results indicate that denitrification can be an ecologically important flux in portions of semi-arid landscapes, and that there is a previously unsuspected regulation of this process by herbivores. Received: 6 March 1998 / Accepted: 28 August 1998     

开展高寒退化生态系统恢复与重建技术研究,助力西藏生态安全屏障保护与建设

西藏是青藏高原的核心,是我国重要的生态安全屏障。由于高、寒、旱的特点,西藏高寒生态系统极为脆弱,在自然和人为因素影响下极易发生退化,治理难度大。草地退化和土地沙化治理一直是西藏生态安全屏障保护与建设的重中之重。为此,国家在\"典型脆弱生态修复与保护研究\"重点专项里启动了\"西藏退化高寒生态系统恢复与重建技术及示范(2016YFC0502000)\"项目,旨在研究生态系统演变规律和影响机理的基础上,针对西藏高原不同的退化区域,重点研发高寒退化草地恢复、沙化土地治理、生态产业及生态畜牧业发展等技术与模式,开展县域水平的集成示范,实现高寒生态系统功能的提升与适应性优化管理的目标,为西藏生态安全屏障保护与建设提供技术支撑。     

退化草地恢复过程中土壤氮素状况以及与植被地上绿色生物量形成关系的研究

 研究了在不同放牧率下形成的不同退化阶段的草地各形态氮素（全氮、硝态氮、铵态氮、无机氮和微生物氮）的变化情况,同时也研究了植被地上绿色生物量与各形态氮素季节变化的同步性关系。土壤全氮含量相对稳定,随草地植被状况和植物生长时期变化不大,说明土壤总氮库有相当的弹性。土壤硝态氮（NO－3-N）、铵态氮（NH+4-N）、无机氮（IN）和微生物氮（Micro-N）季节变化明显。土壤Micro-N和NO-3-N含量随植物生长逐渐降低,到植物枯黄期含量又回复到较高的水平;土壤NH+4-N含量随植物生长有逐渐升高的趋势;IN则随着植物的生长出现低-高-低-高的特点,且与植被地上绿色生物量呈显著负相关（R=-0.247, p<0.01）。在放牧条件下草原植物优先利用NO－3-N,NO－3-N与植被地上绿色生物量有显著的负相关性,是形成草原植被地上绿色生物量的有效性氮素。Micro-N能解释土壤IN 22.3%的变异（R2=0.223, p<0.01）,Micro-N是土壤无机氮的重要来源。土壤NH+4-N与Micro-N呈显著负相关（R=-0.222, p<0.01）,说明土壤微生物对土壤NH+4-N有偏好吸收。总体上,不同形态的氮素在各土壤层次间差异显著,随土壤层次的加深含量逐步降低。连续放牧11年恢复两年后,各氮素组分对放牧压力消除的响应并不一致。土壤全氮含量与停止放牧前相比变化差异不显著;而Micro-N对放牧压力消失的响应在不同处理下整个生长季的结果比较一致,即以前过度和中度放牧处理的Micro-N含量较高,无牧和轻牧含量较低;IN、NH+4-N和NO－3-N变化比较复杂,在不同放牧恢复处理上结果并不一致。总的来看,以前中度和过度放牧的IN、NH+4-N和NO－3-N含量较高,存在潜在损失的可能。经过两年的恢复,植被地上绿色生物量（8月）过牧处理与无牧处理差异不显著。     

Stable carbon and nitrogen isotope signatures of decomposing tropical macrophytes

The Pantanal of Mato Grosso, Brazil, is a large, seasonal wetland, which exhibits high macrophyte productivity at the beginning of the rainy season, when the floodplain becomes flooded. During inundation, from December through May, there is rapid turnover of decomposing macrophyte litter, which is subsequently colonized and consumed by various organisms. In this paper, the variation in the carbon and nitrogen isotope signatures of decomposing macrophytes and detritus was determined to provide an isotopic baseline for the elucidation of higher trophic levels. Seven abundant macrophyte species, Cyperaceae sp., Pontederia lanceolata, Cabomba furcata, Salvinia auriculata, Eichhornia crassipes, Nymphaea amazonum and Paspalum repens, were exposed in mesocosm decomposition experiments lasting 21 or 100 days. Stable isotope ratios of carbon and nitrogen and the atomic C/N ratios were determined for decomposing plant material, particulate organic matter (POM), the microbial film, and aquatic invertebrate larvae. The ¹³C values for the macrophytes did not change during decomposition. However, the variability of ¹⁵N was high (range of ± 6 ) due to microbial activity. There was no consistent difference in the isotopic signatures of macrophytes and POM. C/N ratios decreased from 17 to 50 in macrophytes, to 7 to 12 in POM. The isotopic signatures and C/N ratios of the microbial film were the same as those of POM. We concluded that heterotrophic processes did not fractionate stable carbon isotopes but caused an increase in the variability of stable nitrogen ratios and a change in the C/N ratios in our experimental system. Therefore, it was not possible to distinguish fresh and senescent material or even POM when used as a food source. The ¹³C values of the aquatic larvae were closely coupled to those of the carbon source provided.     

The sensitivity of a species-rich flood-meadow plant community to fertilizer nitrogen: the Lužnice river floodplain,Czech Republic

Concerns over the relative stability or sensitivity of biologicallydiverse ecosystems in relation to environmental change include the effects ofland-use intensification on diverse plant communities. This paper examines thesensitivity of a floristically diverse flood-meadow under hay-cutting managementto nitrogen enrichment, this being a key component of intensified agriculturalmanagement. A gradient of fertilizer nitrogen treatments was applied to a sitein the Czech Republic in two successive seasons and plant community response wasmonitored using measures of species diversity, cover and above-ground primaryproduction. Results show that diversity was supported by annual hay-cuttingmanagement and that the community was highly sensitive to nitrogen enrichment.Fertilization at rates consistent with intensive agricultural practice reducedspecies richness significantly within eight weeks, with forbs and mossparticularly susceptible. The cover and biomass of some grasses were stimulatedby fertilization until constrained by litter accumulation. Over two seasons,fertilization significantly reduced species diversity and simplified communitystructure as inter-specific competitive relations shifted. Biologically diverseflood-meadows therefore seem to be vulnerable to agricultural intensificationand other human activities that promote enhanced nitrogen levels.     

Nitrogen cycling in grasslands at Kanpur,India

Summary The present paper deals with the distribution of nitrogen in the different plant compartments and in the top 30 cm soil among the protected, semi-protected and open-grazed grasslands at Kanpur (26° 26 N latitude and 80° 22 E longitude).The protected site indicated greater nitrogen content (g N m^–2) in the aboveground and belowground plant parts as compared to those of semi-protected and open-grazed sites. Nitrogen content in the combined live and dead herbage varied from 2.6 to 53.5 g m^–2 in protected community, 1.6 to 27.6 g m^–2 in semi-protected and 0.9 to 17.4 g m^–2 in open-grazed community. The content ranged between 1.0 to 17.7, 0.5 to 9.7 and 0.4 to 6.6 g m^–2 for belowground and from 0.1 to 1.1, 0.1 to 0.5 and 0.1 to 0.3 g m^–2 for litter compartments in protected, semi-protected and open-grazed community respectively.A significant positive relationship was found with the nitrogen per gram dry weight in combined live and dead herbage of the study sites and the average monthly relative humidity.The distribution pattern of nitrogen in plant/soil system indicated that the major portion of nitrogen (91 per cent in the protected, 95 per cent in the semi-protected and 96 per cent in the opengrazed stands) in the system was retained in the soil while a small fraction of it (9 per cent, 5 per cent and 4 per cent on protected, semi-protected and open-grazed area respectively) resided in plant compartments. Partitioning, uptake, transfer and release of nitrogen have been discussed in detail for all three sites.     

Preliminary observations on nitrogen transport during summer in a small spring-fed Ontario stream

Nitrogen transport in a 2 km-long, spring-fed stream was studied during the summer months by analyzing weekly water samples from four stations. The water at the spring had a consistently high level of nitrate-N ranging from about 7 mg/l in late spring to about 3 mg/l in early fall. However, over the length of the stream, 60% (about 97 kg) of the incoming nitrate-N is lost from the water during the summer period. The loss, which does not appear to be attributable to the uptake by aquatic macrophytes or to immobilization, is thought to result from denitrification.     

Denitrifying population dynamic and evolution of nitrogen gas during a composting process

Abstract Little information exists about nitrogen losses through microbial activity during treatment of solid urban waste (SUW) by processes such as composting. In the present study, in addition to evaluating the pattern of nitrogen losses by denitrification at different stages of the process, a comparison between the method of Pochon and Tardieux, and an improved gas chromatographic method for estimating denitrifying populations was undertaken, Though the MPN (Most Probable Number) enumerations were higher using the colorimetric method than the gas chromatographic one, the patterns of the two graphs showing numbers of denitrifiers during composing were the same. The highest numbers were revealed immediately after loading the reactor (10⁷–10⁸/g d.w.), lower numbers of denitrifiers were found in the second sampling corresponding to the thermophilic phase (10³–10⁴/g d.w.). These numbers increased gradually as the waste material stabilized (10th to 123rd day of composting) to again reach values of 10⁷–10⁸/g d.w.     

Plant and soil nitrogen dynamics in mediterranean grasslands: a comparison of annual and perennial grasses

Summary The predominance of annual species in the rangelands of southwestern Spain is not due only to climatic factors but is also strongly influenced by grazing management. Manipulating the grazing system in an experimental plot gave a vegetation structure with patches of annual grasses (mainly Vulpia ssp. and Bromus hordeaceus) and patches of perennial grasses (mainly Phalaris aquatica). This vegetation change allowed us to test the hypothesis that life-cycle differences between annual and perennial grasses affect soil nitrogen availability and plant uptake. Nitrogen availability, measured by in situ incubation, and nitrogen uptake were measured through the growing period (October to June). Amounts of in situ mineralized nitrogen over the whole growth phase were more important for soils supporting perennials (37 ppm) than for soils supporting annuals (27 ppm). The difference between the mineral nitrogen produced in situ and the mineral nitrogen accumulated during the same time in the soil allowed an estimation of the maximum mineral nitrogen quantity which can be taken up by the vegetation during each incubation period. The quantities accumulated over the year were 47 and 38 ppm (or 103 and 83 kg/ha) for soils supporting perennials and annuals respectively. For the same period, amounts of nitrogen immobilized in biomass production were 90 and 70 kg/ha for perennials and annuals respectively. During the autumn, a large proportion of mineral nitrogen was leached from soils supporting annual plants which had only just commenced germination. By contrast, the ability to use mineral nitrogen as soon as autumn rains occurred gave a competitive advantage to the perennial species, but only if they were protected from grazing during this period. The higher mineralization and use of this nitrogen reserve by perennials indicates that they made more efficient use of nitrogen resources than annuals, and validate the initial hypothesis.     

Nitrogen transformation processes in relation to improved cultural practices for lowland rice

Summary Inappropriate method and timing of N fertilizer application was found to result in 50–60% N losses. Recent nitrogen transformation studies indicate that NH₃ volatilization in lowland rice soils is an important loss mechanism, causing a 5–47% loss of applied fertilizer under field conditions. Estimated denitrification losses were between 28 and 33%. Ammonia volatilization losses from lowland rice can be controlled by i) placement of fertilizer in the reduced layer and proper timing of application, ii) using phenylphosphorodiamidate (PPD) to delay urease activity in flooded soils, and iii) using algicides to help stabilize changes in floodwater pH. Appropriate fertilizer placement and timing is probably the most effective technique in controlling denitrification at the farm level. The effectivity of nitrification inhibitors as another method is still being evaluated. With 60–80% of N absorbed by the crop derived from the native N pool, substantial yield gains in lowland rice are highly possible with resources already in the land. Extensive studies on soil N and its management, and an understanding of soil N dynamics will greatly facilitate the decrease in immobilization and ammonium fixation in the soil and the increase in N availability to the rice crop. Critical research needs include greater emphasis on N transformation processes in rainfed lowland rice which is grown under more harsh and variable environmental regimes than irrigated lowland rice.     

Controls and models for estimating direct nitrous oxide emissions from temperate and sub-boreal agricultural mineral soils in Europe

Based on a review of N₂O field studies in Europe, major soil, climate and management controls of N₂O release from agricultural mineral soils in the European Union have been identified. Data for these N₂O emission drivers can easily be gathered from statistical services. Using stepwise multivariate linear regression analysis, empirical first order models of N₂O emissions have been established which allow – in contrast to existing large-scale approaches – a regionally disaggregated estimation of N₂O emissions at sub-national, national and continental level in the temperate and boreal climate regions of Europe. Arable soils showed lower mean and maximum emissions in oceanic temperate climate (Temperate West) than in pre-alpine temperate and sub-boreal climate (Sub-boreal Europe). Therefore, two separate regression models were developed. Nitrous oxide emissions from arable soils the Temperate West amount to an average flux rate below 2 kg N₂O-N ha^–1 yr^–1 and rarely exceed 5 kg N₂O-N ha^–1 yr^–1. They are modelled by the parameters fertiliser, topsoil organic carbon and sand content. In Sub-boreal European arable soils, N₂O emissions vary in a much wider range between 0 and 27 kg N₂O-N ha^–1 yr^–1 in dependence of available nitrogen, represented in the model by fertiliser and topsoil nitrogen content. Compared to existing methods for large scale inventories, the regression models allow a better regional fit to measured values since they integrate additional driving forces for N₂O emissions. For grasslands, a fertiliser-based model was established which yields higher emission estimates than existing ones. Due to an extreme variability, no climate, soil nor management parameters could be included in the empirical grasslands model.     

A 3-year exposure to CO2 and O3 induced minor changes in soil N cycling in a meadow ecosystem

We investigated the individual and interactive effects of moderately elevated CO₂ (ambient air + 100 ppm) and/or O₃ (40–50 ppb) on soil N cycling and microbial biomass N in a 3-year open-top chamber experiment conducted in meadow mesocosms. The results show that elevated O₃ decreased the concentrations of mineral N and NH₄⁺-N in the mesocosm soil in the last growing season (2004). Total N, NO₃⁻-N, microbial biomass N, decomposition rate, potential nitrification and denitrification were not affected by elevated O₃ and/or CO₂. It is thus concluded that the proposed future ambient O₃ and CO₂ levels, such as used in this experiment, may not induce major changes in the below-ground N processes in N-poor northern European hay meadow ecosystems.     

Evaluation of pilot-scale modified A2O processes for the removal of nitrogen compounds from sewage

In this study, pilot-scale experiments for the removal of nitrogen from sewage obtained from a county Y sewer system were performed using modified A₂O processes. Using this approach, the total amount of nitrogen discharged during denitrification of the influent was average 38.6 mg/L and a level of average 10.8 mg/L was maintained throughout the denitrification process, which resulted in an average removal efficiency that was greater than 72%. The nitrogen components in the effluent water consisted of 22% ammonia nitrogen, 6% nitrite nitrogen and 72% nitrate nitrogen, reaching a nitrification efficiency of 94%. In conclusion, since these advanced treatment methods, which involve modified A₂O processes, were successfully employed to remove nitrogen from sewage discharge, they hold promise for wide spread use by treatment plants.     

Plant community responses to resource availability and heterogeneity during restoration

Availability and heterogeneity of resources have a strong influence on plant community structure in undisturbed systems, as well as those recovering from disturbance. Less is known about the role of resource availability and heterogeneity in restored communities, although restoration provides a valuable opportunity to test our understanding of factors that influence plant community assembly. We altered soil nitrogen (N) availability and soil depth during a prairie restoration to determine if the availability and/or heterogeneity of soil resources influenced plant community composition in restored grassland communities. Plant community responses to three levels of N availability (ambient, enriched by fertilization, and reduced by carbon amendment) and two levels of soil depth (deep and shallow) were evaluated. In addition, we evaluated plant community responses to four whole plot heterogeneity treatments created from the six possible combinations of soil N availability and soil depth. The soil depth treatment had little influence on community structure during the first 3 years of restoration. Total diversity and richness declined over time under annual N enrichment, whereas diversity was maintained and richness increased over time in soil with reduced N availability. Non-native species establishment was lowest in reduced-N soil in the initial year, but their presence was negligible in all of the soil N treatments by the second year of restoration. Panicum virgatum, a native perennial C₄ grass, was the dominant species in all soil N treatments by year three, but the magnitude of its dominance was lowest in the reduced-N soil and highest in enriched-N soil. Consequently, the relative cover of P. virgatum was strongly correlated with community dominance and inversely related to diversity. The differential growth response of P. virgatum to soil N availability led to a higher degree of community similarity to native prairie in the reduced-N treatment than in the enriched-N treatment. There were no differences in plant community structure among the four whole plot-level heterogeneity treatments, which all exhibited the same degree of similarity to native prairie. Diversity and community heterogeneity in the whole-plot treatments appeared to be regulated by the dominant species effect on light availability, rather than soil N heterogeneity per se. Our results indicate that a strong differential response of a dominant species to resource availability in a restored community can regulate community structure, diversity, and similarity to the native (or target) community, but the importance of resource heterogeneity in restoring diversity may be dampened in systems where a dominant species can successfully establish across a range of resource availability.     

Nitrogen attenuation in the Connecticut River,northeastern USA; a comparison of mass balance and N<Subscript>2</Subscript> production modeling approaches

Two methods were used to measure in-stream nitrogen loss in the Connecticut River during studies conducted in April and August 2005. A mass balance on nitrogen inputs and output for two study reaches (55 and 66 km), at spring high flow and at summer low flow, was computed on the basis of total nitrogen concentrations and measured river discharges in the Connecticut River and its tributaries. In a 10.3 km subreach of the northern 66 km reach, concentrations of dissolved N₂ were also measured during summer low flow and compared to modeled N₂ concentrations (based on temperature and atmospheric gas exchange rates) to determine the measured “excess” N₂ that indicates denitrification. Mass balance results showed no in-stream nitrogen loss in either reach during April 2005, and no nitrogen loss in the southern 55 km study reach during August 2005. In the northern 66 km reach during August 2005, however, nitrogen output was 18% less than the total nitrogen inputs to the reach. N₂ sampling results gave an estimated rate of N₂ production that would remove 3.3% of the nitrogen load in the river over the 10.3 km northern sub-reach. The nitrogen losses measured in the northern reach in August 2005 may represent an approximate upper limit for nitrogen attenuation in the Connecticut River because denitrification processes are most active during warm summer temperatures and because the study was performed during the annual low-flow period when total nitrogen loads are small.     

基于文献计量分析的草原生态补偿研究进展

草原生态补偿是指在合法利用草原资源过程中,草原使用者或受益者对草原资源的所有权者或为草原生态环境保护付出代价者支付相应费用的行为。如何实现补偿机制的合理化及其长期可持续发展是我国草原生态补偿一直以来的研究热点和重点。基于文献计量方法,系统归纳总结了草原生态补偿领域的研究现状和发展模式,探讨了草原生态补偿的特点及其与森林、河流、农田生态补偿之间的差异和联系。研究结果表明早期草原生态补偿研究主要聚焦于草地质量提升、草原物种丰富度提升、草原景观维护等方面;随着草原生态建设的发展和深入,草原生态补偿逐渐成为经济学、社会学等多学科理论和方法交叉的研究领域。目前,草原管理政策与牧民满意度的耦合关系、草原生态补偿政策效益评估和草原地区生态补偿标准测定是草原生态补偿研究的热点和难点。各领域生态补偿的差异主要体现在机制与效益评价两方面。草原生态补偿机制的建立受限于经济价值认知不足等问题,难以像森林补偿那样形成相对完善的市场机制,也无法适用流域生态补偿中常见的筹资方式。此外,生态系统的独特性也对草原生态补偿效益评价过程中需要进行针对性的指标选择和权重分配做出了要求。在探讨草原生态补偿研究态势及热点前沿的基础上,研究结合我国国情分析了草原生态补偿定量研究中各方法的适用性及优缺点,以期为草原生态补偿后续研究及相关政策的制定实施提供思路和参考。