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1.
H.H. Janzen K.A. Beauchemin Y. Bruinsma C.A. Campbell R.L. Desjardins B.H. Ellert E.G. Smith 《Nutrient Cycling in Agroecosystems》2003,67(1):85-102
Agroecosystems rely on inputs of nitrogen (N) to sustain productivity. But added N can leak into adjacent environments, affecting the health of other ecosystems and their inhabitants. Worries about global warming have cast further attention on the N cycle in farmlands because farms are a main source of N2O, and because carbon sequestration, proposed to help reduce CO2 loads, requires a build-up of N. Our objective was to estimate, as an illustrative example, the net N balance of Canadian agroecosystems in 1996 and then infer some hypotheses about the routes of N loss, their magnitude, and ways of reducing them. We defined agroecosystems as all agricultural lands in Canada including soil to 1 m depth and all biota, except humans. Only net flows of N across those boundaries were counted in our balance – all others represent internal cycling. Based on our estimates, about 2.35 Tg N entered Canadian agroecosystems from biological fixation, fertilizers, and atmospheric deposition (excluding re-deposited NH3). In the same year, about 1.03 Tg N were exported in crop products and 0.19 Tg were exported in animals and animal products. Consequently, N inputs exceed exports in products by about 1.13 Tg, a surplus that is either accumulating in agroecosystems or lost to the environment. Because potential soil organic matter gains can account for only a small part of the surplus N, most is probably lost to air or groundwater. Our finding, that N losses amount to almost half of N added, concurs with field experiments that show crop recovery of added N in a given year is often not more than 60%. Better management may reduce the fraction lost somewhat but, because N in ecosystems eventually cycles back to N2, substantive gains in efficiency may not come easily. As well as trying to reduce losses, research might also focus on steering losses directly to N2, away from more harmful intermediates. If some of the `missing N' can be assimilated into organic matter, agricultural soils in Canada may need little added N to achieve C sequestration targets. 相似文献
2.
Effect of crop residue management on nitrogen dynamics and balance in a lowland rice cropping system 总被引:5,自引:0,他引:5
Two field experiments were conducted in a rice–fallow–rice cropping sequence during consecutive dry and wet seasons of 1997 on a Fluvic Tropaquept to determine the fate and efficiency of broadcast urea in combination with three residue management practices (no residue, burned residue and untreated rice crop residue). Ammonia volatilization losses from urea (70 kg N ha–1) broadcast into floodwater shortly after transplanting for 11 d were 7, 12 and 8% of the applied N from no residue, burned residue and residue treated plots, respectively. During that time, the cumulative percent of N2 + N2O emission due to urea addition corresponded to 10, 4.3 and nil, respectively. The 15N balance study showed that at maturity of the dry season crop, fertilizer N recovery by the grain was low, only 9 to 11% of the N applied. Fifty to 53% of the applied 15N remained in the soil after rice harvest, mainly in the upper 0–5 cm layer. The unaccounted for 15N ranged from 27 to 33% of the applied N and was unaffected by residue treatments. Only 4 to 5% of the initial 15N-labeled urea applied to the dry season rice crop was taken up by the succeeding rice crop, to which no additional N fertilizer was applied. Grain yield and N uptake were significantly increased (P=0.05) by N application in the dry season, but not significantly affected by residue treatments in either season. 相似文献
3.
As a crucial factor of water eutrophication, nitrogen (N) discharge by agricultural non-point sources (NPS) has become a worldwide
concern, and so has its relationship to land use. This study was aimed at the quantitative relationships between N discharge
by stream and land use. It was conducted in the Meicun watershed of Xuancheng County, Anhui Province, in the subtropical low
hilly area of China. The study integrated dynamic monitoring of nutrient discharge by stream water and Geographic Information
Systems (GIS) analysis of land use of the watershed. Results showed that NO3−-N discharge ranged between 50 and 60% of the total nitrogen (T-N) and was 2.5–3.0 times as much as NH4+-N. There was a significant difference between forested and mixed sub-watersheds for NH4+-N, NO3−-N and T-N concentrations. Significant correlations existed between NH4+-N, NO3−-N and T-N concentrations in stream water and the area percentages of forest and paddy fields. The study found that the discharges
of NH4+-N, NO3−-N and T-N decreased exponentially with forest area increase, but a steep decline for NO3−-N and T-N seemed to occur when the forest percentage surpassed 70% and then there was almost no change. Similarly, the discharges
of NH4+-N, NO3−-N and T-N increased exponentially with the paddy fields increase, but a steep augmentation occurred for NO3−-N and T-N when the paddy percentage surpassed 20% and then it remained at a plateau. The study showed that in the subtropical
hilly region, paddy fields could increase N discharge due to farming management practices. Thus, primary measures to reduce
N in the receiving water body would include a change in farming management and building an ecological interception system
for paddy fields. 相似文献
4.
B. Ncube J. P. Dimes S. J. Twomlow W. Mupangwa K. E. Giller 《Nutrient Cycling in Agroecosystems》2007,77(1):53-67
Participatory on-farm trials were conducted for three seasons to assess the benefits of small rates of manure and nitrogen
fertilizer on maize grain yield in semi-arid Tsholotsho, Zimbabwe. Two farmer resource groups conducted trials based on available
amounts of manure, 3 t ha−1 (low resource group) and 6 t ha−1 (high resource group). Maize yields varied between 0.15 t ha−1 and 4.28 t ha−1 and both absolute yields and response to manure were strongly related to rainfall received across seasons (P < 0.001). The first two seasons were dry while the third season received above average rainfall. Maize yields within the
seasons were strongly related to N applied (R
2 = 0.77 in season 1, and R
2 = 0.88 and 0.83 in season 3) and other beneficial effects of manure, possibly availability of cations and P. In the 2001–2002
season (total rainfall 478 mm), application of 3 and 6 t ha−1 of manure in combination with N fertilizer increased grain yield by about 0.14 and 0.18 t ha−1, respectively. The trend was similar for the high resource group in 2002–2003 although the season was very dry (334 mm).
In 2003–2004, with good rainfall (672 mm), grain yields were high even for the control plots (average 1.2 and 2.7 t ha−1). Maize yields due to manure applications at 3 and 6 t ha−1 were 1.96 and 3.44 t ha−1, respectively. Application of 8.5 kg N ha−1 increased yields to 2.5 t ha−1 with 3 t ha−1 of manure, and to 4.28 t ha−1 with 6 t ha−1 of manure. In this area farmers do not traditionally use either manure or fertilizer on their crops, but they actively participated
in this research during three consecutive seasons and were positive about using the outcomes of the research in future. The
results showed that there is potential to improve livelihoods of smallholder farmers through the use of small rates of manure
and N under semi-arid conditions. 相似文献
5.
Nitrogen budget and surface water nitrogen load in Changshu: a case study in the Taihu Lake region of China 总被引:3,自引:0,他引:3
Chaopu Ti Yongqiu Xia Jianjun Pan Genmao Gu Xiaoyuan Yan 《Nutrient Cycling in Agroecosystems》2011,91(1):55-66
The Taihu Lake region in China is highly developed, but surface water pollution has become a serious environmental problem
in recent years, with nitrogen (N) a major pollutant. To assess the impact of human activity on N cycle and to evaluate the
sources of N in surface water in the region, a N-budget for Changshu, a representative county in the Taihu Lake region, was
established by using N-related human activities data from an intensive household survey conducted in 2007, measurement data
on N fluxes and literature data on other necessary parameters. Chemical fertilizer was the largest N input, accounting for
56.3%. Nearly 30% of the N ended up in surface water. Untreated human and animal waste was the major source of N transported
to water. The huge amount of N load to surface water would result in N concentrations of >6.0 mg N L−1 even after denitrification removal in wetlands. 相似文献
6.
There is an urgent need to establish sustainable nutrient cycling. Changes in amounts of N flow and separation of production and consumption sectors are becoming a serious environmental problem. In this study, the yearly N in- and outflow of a city in northern Japan from 1912 to 2002 was investigated based on the statistics and inventory data. Based on the characteristics of the N flow, the period was divided into manure-based period (MBP, 1912–1950), transition period from manure- to chemical fertilizer-based period (TP, 1950–1970), and chemical fertilizer-based period (CBP, 1970–2002). The highest amount of N inflow (up to 350 Mg N y–1) was observed at the end of the MBP, and the second peak (about 300 Mg N y–1) at the beginning of the CBP. The N application rate on farmland increased from 68 kg N ha–1 in 1912 to above 250 kg N ha–1 in the 1950s, then decreased to 168 kg N ha–1 in 2002. The farmland productivity increased from 30 kg N ha–1 at the end of the 1950s to 90 kg N ha–1 in 2002, due to improvement in crop varieties and management methods. In MBP surplus N in farmland and NH3 volatilization accounted for 90% of the N outflow from the city, then in CBP, disposal N and surplus N in farmland became the main N outflow. All these outputs are considered to increase the N concentration in rivers and/or underground water. In the case of surplus N in farmland, it exceeded the amount of optimum N management (<50 kg N ha–1; , Agricult. Ecosyst. Environ. 72: 35–52) during 1935–1970 and 1981–1997. In order to prevent degradation of the environment through artificially altered nutrient flow, we need to be aware of the environmental impact of the N flow and establish proper N management practices. 相似文献
7.
Influence of long-term changes in nitrogen flows on the environment: A case study of a city in Hokkaido, Japan 总被引:1,自引:0,他引:1
There is an urgent need to establish sustainable nutrient cycling. Changes in amounts of N flow and separation of production and consumption sectors are becoming a serious environmental problem. In this study, the yearly N in- and outflow of a city in northern Japan from 1912 to 2002 was investigated based on the statistics and inventory data. Based on the characteristics of the N flow, the period was divided into manure-based period (MBP, 1912–1950), transition period from manure- to chemical fertilizer-based period (TP, 1950–1970), and chemical fertilizer-based period (CBP, 1970–2002). The highest amount of N inflow (up to 350 Mg N y–1) was observed at the end of the MBP, and the second peak (about 300 Mg N y–1) at the beginning of the CBP. The N application rate on farmland increased from 68 kg N ha–1 in 1912 to above 250 kg N ha–1 in the 1950s, then decreased to 168 kg N ha–1 in 2002. The farmland productivity increased from 30 kg N ha–1 at the end of the 1950s to 90 kg N ha–1 in 2002, due to improvement in crop varieties and management methods. In MBP surplus N in farmland and NH3 volatilization accounted for 90% of the N outflow from the city, then in CBP, disposal N and surplus N in farmland became the main N outflow. All these outputs are considered to increase the N concentration in rivers and/or underground water. In the case of surplus N in farmland, it exceeded the amount of optimum N management (<50 kg N ha–1; , Agricult. Ecosyst. Environ. 72: 35–52) during 1935–1970 and 1981–1997. In order to prevent degradation of the environment through artificially altered nutrient flow, we need to be aware of the environmental impact of the N flow and establish proper N management practices. 相似文献
8.
The anodic dissolution of gold in alkaline thiosulfate solutions has been studied by using a rotating gold disc. Experimental results have shown that the gold dissolves at measurable rates in thiosulfate solutions at potentials above 0.2 V. It has been confirmed that dissolution occurs in parallel with oxidation of thiosulfate with a dissolution current efficiency that varies with time and with the experimental conditions and which is generally in the range of 0.3 to 0.6. Although oxygen could be used as an oxidant for gold in the thiosulfate system, the rate of the cathodic reduction of oxygen in the relevant potential region is too slow for practical purposes. It was found that in the potential region investigated, thiosulfate ions undergo oxidative decomposition leaving a sulfur-like film on the surface of gold, which inhibits the rate of dissolution of gold and results in a low anodic current efficiency for the dissolution of gold. The factors (temperature, pH, thiosulfate concentration and ammonia) have been found to have positive effects on the kinetics of gold dissolution. The rates of gold dissolution in oxygenated alkaline thiosulfate solutions have been estimated to be much lower than in the copper–ammonia–thiosulfate and cyanide systems. 相似文献
9.
煤燃烧是煤炭资源有效利用的重要方式之一。从原子分子水平上识别煤燃烧的机理,系统把握反应条件的影响,揭示含氮污染物迁移转化的路径对于煤炭高效清洁利用具有重要意义。以前期构建的宁东红石湾煤分子结构模型为基础,采用反应分子动力学模拟方法对煤大分子结构聚集态模型进行燃烧反应模拟,考察化学当量比和反应温度对燃烧过程中结构演变、燃烧反应物和产物的影响,探究有机氮转化路径。研究发现,随着反应不断进行,煤结构的断裂变化非常明显。不同化学当量比和不同温度下煤燃烧的结果表明,化学当量比越大、燃烧温度越高时,O2分子消耗速度越快,CO2的生成量也越多。对燃煤过程中含氮气体分子数量的分析表明,HCN是重要的含氮中间产物,NO、NO2是主要含氮气体产物。本研究还建立了煤燃烧过程中有机氮的转化路径,获得了HCN、NO和NO2的演变过程。 相似文献
10.
This paper reports Monte Carlo simulation results of a polymer melt of short, non-entangled chains which are embedded between two impenetrable walls. The melt is simulated by the bond-fluctuation lattice model under athermal conditions, i.e. only excluded volume interactions between the monomers and between the monomers and the walls are taken into account. In the simulations, the wall separation is varied from about one to about 15 times the bulk radius of gyration Rg. The confinement influences both static and dynamic properties of the films: Chains close to the walls preferentially orient parallel to it. This parallel orientation decays with increasing distances from the wall and vanishes for distances larger than about 2Rg. Strong confinement effects are therefore observed for film thicknesses D?4Rg. The preferential alignment of the chains with respect to the walls suppresses reorientations in perpendicular direction, whereas parallel reorientations take place in an environment of high monomer density. Therefore, they have a relaxation time larger than that of the bulk. On the other hand, the influence of confinement on the translation motion of the chains parallel to the walls is very weak. It almost coincides with the bulk behavior even if D≈1.5Rg. Despite these differences between translational and reorientational dynamics, their behavior can be well reproduced by a variant of Rouse theory which only assumes orthogonality of the Rouse modes and determines the necessary input from the simulation. 相似文献
11.
12.
Enhancement mechanism of mixing with baffle in agitated vessel using rotated two-bladed paddle impeller was investigated under a laminar condition. In mixing pattern, the toroidal isolated mixing region in the baffled vessel becomes distortive and much smaller than that of unbaffled vessel. From the visualization of streak cross-sections in the baffled vessel, interestingly, the renewed streak folds (streak lobes) are generated at the vicinity of baffles in both the vertical and horizontal cross-sections. These behaviors of streak are unlike the unbaffled case that the streak stretches straightforwardly. The streak lobe is known as the mixing template that its number and size are key factor for laminar mixing in agitated vessel. The results suggest that baffles can effectively transform the circumferential flow to vertical and/or radial flows. Consequently, in the baffled vessels, not only the vicinity of vessel wall but also the tip of baffles can become the origination of streak lobe formation, and folds of streak in the vertical and circumferential directions are further enhanced with baffles. 相似文献
13.
An analysis of farmers?? use of phosphorus fertiliser in industrial agriculture: a case study in the Bordeaux region (south-western France) 总被引:1,自引:1,他引:0
Thomas Nesme Samuel Brunault Alain Mollier Sylvain Pellerin 《Nutrient Cycling in Agroecosystems》2011,91(1):99-108
Global phosphorus (P) reserves are rapidly being depleted and agriculture is a major contributor to P consumption worldwide. Moreover, agricultural P use may lead to local excess of P in soils, resulting in water eutrophication. To remedy this situation, it is necessary to understand the key factors underlying P use by farmers within such a context. The objective of this article is to understand how farmers use P fertilisers under agricultural conditions. Particular attention was paid to the use of conventional decision-making tools such as soil analysis by farmers. Industrial agriculture, characterised by high levels of specialisation, input use and production yields, was considered as a case study. Forty farmers involved in industrial agriculture in south-western France were interviewed in 2008 about their P use. Soil analyses were collected when available. Input/output P balances were computed at the plot scale. Moreover, records of farmers?? fertilisation practices were analysed from a database developed by a technical advisory group. Results showed that 90% of farmers had a crop-specific and yearly P fertilisation strategy. This yielded very positive P balances for vegetable crops. Few farmers used recommendations based on soil analyses: in most cases, the amounts of P used by the farmers were lower than those recommended. When asked for their opinion about the role of P in agriculture, most farmers explained that P fertilisation helped boost the growth of spring crops in the event of adverse weather conditions. This revealed the emphasis placed on ??plant-based?? strategies for P management, as opposed to ??soil-based?? strategies, thus reinforcing annual fertilisation strategy. This study provides a foundation for understanding key factors underlying P consumption in agriculture. 相似文献
14.
F. Wendland H. Behrendt H. Gömann U. Hirt P. Kreins U. Kuhn R. Kunkel B. Tetzlaff 《Nutrient Cycling in Agroecosystems》2009,85(1):63-78
We used the interdisciplinary model network REGFLUD to predict the actual mean nitrate concentration in percolation water
at the scale of the Weser river basin (Germany) using an area—differentiated (100 m × 100 m) approach. REGFLUD combines the
agro-economic model RAUMIS for estimating nitrogen surpluses and the hydrological models GROWA/DENUZ for assessing the nitrate
leaching from the soil. The areas showing predicted nitrate concentrations in percolation water above the EU groundwater quality
standard of 50 mg NO3/l, have been identified as priority areas for implementing nitrogen reduction measures. For these “hot spot” areas a backward
modelling approach was used to quantify the maximal permissible nitrogen surplus levels in agriculture to guarantee a mean
long-term nitrate concentration in percolation water below 50 mg NO3/l. Research work will directly support the implementation of the EU-Water Framework Directive in the Weser basin, e.g. by
using the maximal permissible nitrogen surplus levels as a framework for the derivation of regionally adapted and hence effective
nitrogen reduction measures. 相似文献
15.
The Mode I fracture energy of a polyurethane adhesive with low Young’s modulus was investigated. Metal adherends in standardized double cantilever beam (DCB) tests are typically too stiff for soft adhesives, making it difficult to measure the fracture energy accurately. However, soft adhesives, such as a single-component polyurethane adhesive tested in this paper, are in high demand in the automobile industry. Thus, accurate measurement techniques must be established. Flexible substrates composed of spring steel were used for the DCB tests to accommodate the deformation of the adhesive layer. First, the applicability of the flexible substrates was discussed using specimens bonded with an epoxy adhesive. For soft adhesives, however, the deformation of the adhesive layer must be considered in the calculation methods of the fracture energy. Although the deformation effect on the DCB tests has been discussed with Winkler’s elastic foundation, the crack length must be measured along with the load and displacement. To overcome the difficulty of measuring the crack length, a calculation method based on Winkler’s elastic foundation was introduced applying the compliance-based beam method (CBBM). Finally, the fracture energy of the polyurethane adhesive was discussed by comparing the calculation methods with and without measuring the crack length. 相似文献
16.
This paper demonstrates the Retrofit Design Approach (RDA) and Response Surface Methodology (RSM) for the retrofit of industrial plants in which assessment of design options for improving existing processes in a site-wide and integrated manner is not straightforward, due to complex design interactions in the process. The design methodology applied in this study is based on the systematic use of a process simulator which is used to identify promising variables through sensitivity analysis. Hence, the most important factors are determined and a reduced model is constructed based on RSM. An optimization framework is then built using the reduced model based on key selected variables, which is optimized to find optimal conditions and performance of the process. This design methodology provides strategic guidelines for determining the most cost-effective design options. The retrofit of a hydrocarbon fractionation plant is presented as an industrial case study. This includes a large number of design options with different process configurations and operating conditions due to the interconnection of distillation columns in sequence and the integrated heat recovery within the plant. The case study results demonstrate the applicability of the proposed approach which is able to effectively deal with a large retrofit problems. This is possible with the aid of process simulation and RSM producing a reduced model which requires considerably less computational effort to solve. 相似文献
17.
To reduce metal ions in dilute solutions a new type of electrochemical reactor has been developed: the GBC-reactor. This reactor consists of a gas diffusion electrode coupled with a packed bed electrode. The working principle of the reactor is based upon two main reactions: the catalytic oxidation of hydrogen gas in the gas diffusion electrode and the simultaneous reduction of metal ions on the packed bed electrode. This process occurs spontaneously without the need for an external power supply when the Gibbs free energy of the total reaction is negative. To study the behaviour of the reactor the reduction of ferric ions was used as a model system. The experimental results, obtained from varying a number of key process parameters, could be described using a very simple macroscopic rate equation. It is concluded that the reduction of ferric ions in a GBC-reactor is controlled by both mass transfer and electrochemical kinetics. To carry out scale-up and optimization studies a reactor model incorporating the potential distribution in the packed bed electrode is, however, necessary. 相似文献
18.
Francesca D'Acunzo Donatella Capitani Giancarlo Masci Cristina Cherubini Ornella Ursini 《Polymer》2013
The search for innovative and well-defined organic–inorganic hybrid materials demands that the interplay between polymerization reactions, chemical reactivity, and physical adsorption be fully understood. We examine the case study of the thermal polymerization of styrene in the presence of silica, and the effect of pre-irradiation of silica with γ-rays. The effect of adsorption of styrene-derived radical species on the silica support and polymerization conditions on free and grafted (unextractable) polystyrenes and on polymerization mechanism is discussed on the basis of previous literature findings and new data. Evidence is provided of silica derivatization with various species besides polystyrene (13C CPMAS NMR), including products from a β-scission of the adsorbed radical intermediates ensuing oxygen donation from the silica network. The polymerization mechanism can be cationic as well as radical, depending on reaction conditions, and the cationic route prevails in the grafting of polystyrene from silica, resulting in a bimodal molecular weight distribution. 相似文献
19.
Serge Zhandarov 《Journal of Adhesion Science and Technology》2013,27(4):430-465
We compared two models of the pull-out specimen – the ‘equivalent cylinder’ and the platelet models in which the matrix droplet is represented as a set of thin parallel disks with the diameters varying along the embedded fiber to approximate the real droplet shape. Analytical expressions for the profiles of the fiber tensile stress and the interfacial shear stress have been derived for the matrix droplet in the shape of a spherical segment, including the effects of residual thermal stresses and interfacial friction. Using these expressions, we analyzed the process of crack initiation and propagation in the platelet model and investigated the effect of the specimen shape on the force–displacement curves. The interfacial stress near the loaded fiber end in the platelet model is higher than in the equivalent cylinder model, which gives rise to earlier crack initiation and smoother shape of the force–displacement curve. As a result, the calculated interfacial shear strength values may be underestimated by 10–20%, if the equivalent cylinder is used instead of the real droplet shape. A method of correction to the equivalent cylinder model in order to avoid this underestimation is proposed. 相似文献
20.
The flow in the wake and near-wake regions of individual Taylor bubbles rising through stagnant and co-current vertical columns of Newtonian liquids was studied, employing simultaneously particle image velocimetry (PIV) and pulsed shadowgraphy techniques (PST). Experiments were made with water and aqueous glycerol solutions covering a wide range of viscosities , in an acrylic column of 32 mm ID.Different wake structures (laminar, transitional and turbulent) are identified, in both stagnant and co-current flow conditions. In stagnant liquids, the wake flow pattern is only dependent on the dimensionless group Nf. The different types of wakes obtained are in accordance with the critical Nf numbers proposed in previous works. For co-current flow conditions, the flow patterns in the wake depend on the Reynolds number based on the relative (to the bubble) average velocity of the upward liquid flow, the laminar-transitional and transitional-turbulent limits being for the first time experimentally determined.The wake flow patterns are quantified by means of instantaneous and average flow fields. Values for the wake length and wake volume are also presented and compare well with correlations found in literature. Study of the flow in the near-wake zone enabled determination of the distance needed to recover the undisturbed liquid velocity profile.The detailed study of the flow in the wake and near-wake regions is an important contribution to better understanding the interaction and coalescence mechanisms between Taylor bubbles.The data reported are relevant to the validation of numerical simulation codes in the vertical slug flow regime. 相似文献