Search

Changes in soil moisture influence the water availability to crop plants and soil ecological processes like carbon and nutrient cycling, impacting on crop productivity and environmental performance (greenhouse gas emissions, leaching) of agricultural systems. While traditional soil moisture measurements are done using point-based methods, the recent development of the cosmic-ray soil moisture neutron sensor (CRNS) offers the opportunity to measure soil water at the field scale. However, due to its shallow (

This paper reports ammonia (NH3) emissions from irrigated, fertilized pastures grazed intensively by dairy cattle in Victoria, Australia. Irrigation bays (65 × 365 m) were grazed in rotation by 315 dairy cattle and daily emissions were measured in summer and autumn. After grazing for a few days, the bays were fertilized with urea at 50 kg N ha−1 and irrigated with 50 mm of water. Passive NH3 samplers were used to measure horizontal fluxes of NH3 from different sectors of the bays directly and the corresponding surface emissions were calculated from the sampler data using a backward Lagrangian stochastic (bLs) model of atmospheric dispersion. Ammonia emissions from the whole herd during the grazing phases were 14.8 kg NH3−N in summer, but only 2.0 kg NH3−N in autumn, corresponding to emission factors of 12% and 1% for an assumed annual input of nitrogen (N) in excreta of 150 kg N head−1 yr−1. Only 1% of the urea-N was lost through NH3 volatilization when irrigation followed fertilization immediately, but 6% was lost when irrigation was delayed for 3 days. The bLs model appears to be a useful tool for assessing gas emissions from dairy farm operations.

Nitrogen (N) deficiency is responsible for a large proportion of the exploitable yield gap that exists in Australian wheat production. In order to maximise N use efficiency (NUE), growers attempt to match N fertiliser to seasonal water limited yield potential, which in the southern grain growing region is largely determined by in-crop rainfall. Australia’s variable rainfall makes water limited yield potential very difficult to predict, and growers tend to under fertilise, fearing economic and environmental losses of N if crops are over-fertilised. However, environmental losses of excess N are low in the semi-arid, winter dominant rainfall environments with high water holding capacity soils that comprise much of the wheat producing regions of south eastern Australia. This is particularly the case when high carbon (C) to N ratio crop residues are retained which facilitate rapid immobilisation of any mineral N surplus to crop requirements. We therefore propose that the exploitable yield gap in wheat could be closed if a longer-term strategy of N fertiliser application was adopted. In this strategy, growers would simply use N fertiliser to maintain an environmentally appropriate base level of inorganic N (N bank) rather than attempting to match fertiliser inputs to seasonal conditions. We used field studies with 15N labelled fertiliser and cropping systems simulations to demonstrate that fertiliser N applied at rates calculated to maintain fixed thresholds of inorganic N (N bank criteria) and top-dressed immediately prior to the period of rapid crop uptake can substantially increase yields and profit in comparison to current practice, even when applications exceed crop demand in some years. A further benefit of this strategy is an increase in soil organic matter. Both field and simulation studies indicate that the environmental cost of such a strategy would be minimal, with only a small increase in leaching and denitrification despite higher overall rates of N application. This strategy needs to be compared with current practices in longer term field experiments to confirm production increases and lack of environmental impacts.

Beef cattle feedlots are a major source of ammonia (NH3) emissions from livestock industries. We investigated the effects of lignite surface applications on NH3 and nitrous oxide (N2O) emissions from beef cattle feedlot pens. Two rates of lignite, 3 and 6kgm(-2), were tested in the treatment pen. No lignite was applied in the control pen. Twenty-four Black Angus steers were fed identical commercial rations in each pen. We measured NH3 and N2O concentrations continuously from 4th Sep to 13th Nov 2014 using Quantum Cascade Laser (QCL) NH3 analysers and a closed-path Fourier Transform Infrared Spectroscopy analyser (CP-FTIR) in conjunction with the integrated horizontal flux method to calculate NH3 and N2O fluxes. During the feeding period, 16 and 26% of the excreted nitrogen (N) (240gNhead(-1)day(-1)) was lost via NH3 volatilization from the control pen, while lignite application decreased NH3 volatilization to 12 and 18% of the excreted N, for Phase 1 and Phase 2, respectively. Compared to the control pen, lignite application decreased NH3 emissions by approximately 30%. Nitrous oxide emissions from the cattle pens were small, 0.10 and 0.14gN2O-Nhead(-1)day(-1) (

As farmers in southern Australia typically apply nitrogen (N) to cereal crops by top-dressing with ammonia (NH3) based fertilizer in late winter or early spring there is the potential for large losses of NH3. This paper describes the results of micrometeorological measurements to determine NH3 loss and emission factors following applications of urea, urea ammonium nitrate (UAN), and ammonium sulfate (AS) at different rates to cereal crops at two locations in southern Australia. The amounts of NH3 lost are required for farm economics and management, whilst emission factors are needed for inventory purposes. Ammonia loss varied with fertilizer type (urea > UAN > AS) and location, and ranged from 1.8 to 23 % of N applied. This compares with the emission factor of 10 % of applied N advocated by IPCC ( 2007). The variation with location seemed to be due to a combination of factors including soil texture, soil moisture content when fertilizer was applied and rainfall after fertilizer application. Two experiments at one location, 1 week apart, demonstrated how small, temporal differences in weather conditions and initial soil water content affected the magnitude of NH3 loss. The results of these experiments underline the difficulties farmers face in timing fertilization as the potential for loss, depending on rainfall, can be large.

The Great Barrier Reef World Heritage Area contains highly sensitive ecosystems that are threatened by the effects of anthropogenic activity including eutrophication. The nearby sugarcane plantations of tropical north Queensland are fertilised annually and there has been ongoing concern about the magnitude of the loss of applied nitrogen to the environment. Previous studies have considered the potential of rainwater run-off to deposit reactive nitrogen species into rivers and ultimately into the Great Barrier Reef Lagoon, but have neglected the possibility of transport via the atmosphere. This paper reports the results of a modelling study commissioned by Australia’s National Heritage Trust aimed at assessing whether or not atmospheric deposition of reactive nitrogen from Queensland’s sugarcane plantations posed a potential threat to the Great Barrier Reef Lagoon. Atmospheric dispersion modelling was undertaken using The Air Pollution Model, developed by Australia’s Commonwealth Scientific and Industrial Research Organisation. Despite the predominance of onshore southeasterly winds, the dispersion model results indicate that 9% of the time during the sugarcane fertilization season (in the modeled years 2001–2006) the meteorological conditions resulted in emissions from the coastal regions of north Queensland being transported out over the ocean around the Great Barrier Reef. The results suggest that there may be a greater efficiency for transport out over the reef during October than for November and December. For the 2 months that exhibited the greatest potential for transport of coastal pollution to the Great Barrier Reef, the modeled deposition of nitrogen oxides (NOX) into the Great Barrier Reef lagoon was less than 1% of the total emissions from the sugarcane plantations, but was not zero. Our model has a simple chemical scheme that does not cover the full chemistry of all reactive nitrogen compounds and so the results are only indicative of the potential levels of deposition. Nevertheless, our study shows that small amounts of NOX that originate from sugarcane fertilization may be transported and dry deposited into the Great Barrier Reef lagoon. Other pathways not included in the modeling scheme may provide a more efficient transport mechanism. Whilst modern practices for the application of fertilizer to sugarcane plantations have drastically reduced emissions, the potential efficiency of transport of pollutants via the atmosphere may be of concern for other more highly polluting agricultural industries.

NO x and N 2 O emissions from coastal acid sulfate soils (CASS) cultivated for sugarcane production were investigated on the coastal lowlands of northern New South Wales, Australia. Two series of short-term measurements were made using chambers and micrometeorological techniques. Series 1 occurred during the wet season, the water-filled pore space (WFPS) was between 60%-80% and the site flooded during the measurements. Measurements were made directly after the harvest of soybean crop, which fixed an estimated 100 kg N ha −1 , and the emission amounted to 3.2 kg NO x -N ha −1 (12 d) and 1.8 kg N 2 O-N ha −1 (5d). Series 2 was made towards the end of the dry season when the WFPS was less than 60%. In Series 2, after an application of 50 kg N ha −1 , emissions were markedly less, amounting to 0.9 kg N ha −1 over 10 d. During both series when the soil was moist, emissions of NO x were larger than those of N 2 O. The emission of NO x appeared to be haphazard, with little time dependence, but there was a clear diurnal cycle for N 2 O, emphasising the need for continuous measurement procedures for both gases. These results suggest that agricultural production on CASS could be important sources of greenhouse gases and nitrogen practices will need to be optimised to reduce the offsite effects of atmospheric warming, acidification or nitrification. Many questions still remain unanswered such as the emissions during the soybean bean filling stage and crop residue decomposition, the longer-term losses following the fertiliser application and emissions from CASS under different land uses.

Climatic conditions and cultural practices in the sub-tropical and tropical high-rainfall regions in which sugarcane is grown in Australia are conducive to rapid carbon and nitrogen cycling. Previous research has identified substantial exchanges of methane (CH 4 ) and nitrous oxide (N 2 O) between sugarcane soils and the atmosphere. However, that research has been mostly short-term. This paper describes recent work aimed at quantifying exchanges of CH 4 and N 2 O from fertilised sugarcane soils over whole growing seasons. Micrometeorological and chamber techniques provided continuous measurements of gas emissions in whole-of-season studies in a burnt-cane crop on an acid sulfate soil (ASS) that was fertilised with 160 kg nitrogen (N) ha −1 as urea in the south of the sugarcane belt (Site 1), and in a crop on a more representative trash-blanketed soil fertilised with 150 kg urea-N ha −1 in the north (Site 2). Site 1 was a strong source of CH 4 with a seasonal emission (over 342 days) of 19.9 kg CH 4 ha −1 . That rate corresponds to 0.5–5% of those expected from rice and wetlands. The many drains in the region appear to be the main source. The net annual emission of CH 4 at Site 2 over 292 days was essentially zero, which contradicts predictions that trash-blankets on the soil are net CH 4 sinks. Emissions of N 2 O from the ASS at Site 1 were extraordinarily large and prolonged, totalling 72.1 kg N 2 O ha −1 (45.9 kg N ha −1 ) and persisting at substantial rates for 5 months. The high porosity and frequent wetting with consequent high water filled pore space and the high carbon content of the soil appear to be important drivers of N 2 O production. At Site 2, emissions were much smaller, totalling 7.4 kg N 2 O ha −1 (4.7 kg N ha −1 ), most of which was emitted in less than 3 months. The emission factors for N 2 O (the proportion of fertiliser nitrogen emitted as N 2 O–N) were 21% at Site 1 and 2.8% at Site 2. Both factors exceed the default national inventory value of 1.25%. Calculations suggest that annual N 2 O production from Australian sugarcane soils is around 3.8 kt N 2 O, which is about one-half a previous estimate based on short-term measurements, and although ASS constitute only about 4% of Australia's sugarcane soils, they could contribute about 25% of soil emissions of N 2 O from sugarcane. The uptake of 50–94 t CO 2 ha −1 from the atmosphere by the crops at both sites was offset by emissions of CH 4 and N 2 O to the atmosphere amounting to 22 t CO 2 -e ha −1 at Site 1 and 2 t CO 2 -e ha −1 at Site 2.

This study conducted in the Wimmera, a major cropping area in western Victoria Australia, evaluated a backward Lagrangian stochastic (bLs) dispersion model for measuring ammonia (NH 3 ) loss and compared NH 3 losses from a wheat crop after top-dressing with urea or “Green Urea”. Green Urea contained 45.8% nitrogen (N) as urea and “Agrotain” (N-(n-butyl) thiophosphorictriamide) @ 5.0 L/t. The two products (80 kg N ha −1 ) were applied to circular plots of 25 m radius and losses were determined for a period of 23 days using mass balance micrometeorological methods. When the NH 3 concentration in the air at the stability independent height, 0.8 m above the crop, was used there was a strong relationship between the vertical flux density of NH 3 as determined by the full profile method and that determined by the bLs method ( r = 0.86). Rates of ammonia loss from the urea treatment ranged from 0.2 to 2.1 μg N m −2 s −1 , while those from the Green Urea treatment never exceeded 0.35 μg N m −2 s −1 . Cumulative NH 3 losses for the urea and Green Urea treatments were 7.6 kg N ha −1 (9.5% of applied N) and 0.8 kg N ha −1 (1.0% of applied N), respectively. The results indicate that use of Green Urea instead of regular urea in Victorian wheat growing could substantially reduce NH 3 emission.

We first present the elements of an inverse Lagrangian model of gas transport in plant canopies. The model allows the inference of sites of gas exchange in the canopy and their source and sink strengths from measured profiles of mean gas concentration and statistics of the canopy turbulence. The practical application of the model is demonstrated through a case study of the fate of ammonia volatilized from fertilizer applied to the floor of a sugarcane crop. Some of the lost ammonia was absorbed by the foliage of the crop; the rest was lost to the atmosphere above. While there was excellent agreement between model predictions of the net flux from the canopy and independent micrometeorological measurements of ammonia flux in the air-layer above it, verification of flux predictions within the canopy was much more difficult. Appeal was made to a process-based model of canopy gas exchange that describes gas transport to and from foliage surfaces in terms of diffusion across aerodynamic, boundary-layer and stomatal resistances in response to a difference in ammonia concentration between the air and leaf sub-stomatal cavities. There was acceptable agreement between the two models in their predictions of foliage ammonia uptake. Next, we apply the process model to a study of the recapture of volatilized ammonia by sugarcane crops with different leaf area indices (LAI). The study indicated recoveries increasing almost linearly with LAI and suggested probable recoveries in excess of 20% for canopies with LAIs of 2 or more. These and other published studies of ammonia exchange between canopy and atmosphere that employed both the inverse Lagrangian and process models suggest that their coupling provides a powerful tool for studying canopy gas exchange.

The theory, applications, strengths and weaknesses of approaches commonly used for measuring trace gas fluxes are reviewed. Chambers, representing the smallest scale (∼1 m2), are the most common tools. Their operating principle is simple, they can be highly sensitive, the cost can be low and field requirements small. Problems include leaks, stickiness of some gases, inhibition of fluxes through concentration build-up, pressure effects and spatial and temporal variability in gas fluxes. Mass balance techniques are suitable for small, defined source areas, typically tens to thousands of square metres in extent. Emissions are calculated from the difference in the rates at which the gas is carried into a control volume above the source area by the wind and carried out. The required primary data are profiles of gas concentration on the downwind boundaries as well as the wind speed profile, the wind direction and the upwind background gas concentration. They have been used to measure gas emissions from landfills, treated fields and small animal herds. Circular test areas make the method independent of wind direction. A newly developed technique based on a backward Lagrangian stochastic dispersion model is also applicable to small, well-defined source areas of any shape. The surface flux is calculated form measurements of atmospheric turbulence and stability and the gas concentration at any height downwind. Implementation of the method is aided greatly by a software package WindTrax. The combination provides a powerful new tool for measuring gas emissions from treated areas and intensive animal production systems. Finally, techniques suitable for measuring gas emissions on large landscape scales (ha) are discussed. Eddy covariance is the micrometeorologist’s preferred technique for this scale. The method uses fast response anemometers and gas sensors to make direct measurements of the vertical gas flux at a point, several times a second. However, it is not feasible for many trace gases for a variety of reasons. These are discussed. Relaxed eddy accumulation is an alternative technique that retains the attraction of eddy covariance by providing a direct point measurement. It removes the need for a fast response gas sensor by substituting for it a fast solenoid valve sampling system. Flux–gradient methods are in more common use. Fluxes are calculated as the product of an eddy diffusivity and the vertical concentration gradient of the gas or the product of a transfer coefficient and the difference in gas concentration between two heights. Assumptions of the method and precautions in its application are discussed.

Nitrous oxide emissions from flood-irrigated pastures grazed for dairy production were measured over 2 years using micrometeorological techniques. Turbulent atmospheric transfer coefficients derived from fluxes of sensible heat and momentum were used to estimate the N 2 O fluxes from concentrations measured at two heights. Careful data selection excluded data for periods of very low wind-speed and strong stability when transfer coefficients were unreliable. The measurements demonstrated the dynamic nature of N 2 O emissions following grazing, fertiliser application and irrigation. Daily-averaged emissions ranged from 2 O-N ha −1 day −1 . High emissions due to denitrification occurred when soil temperatures were >15 °C, nitrogen was available following grazing or fertiliser application and the water filled pore space of the soil was in the range 65–95%. N 2 O emission factors of 0.23 ± 0.06% for grazing were similar to those for fertiliser application plus grazing (0.32 ± 0.12%).

The adage: “There is no such thing as a free lunch”, is relevant to land-use hydrology in Australia. Changes in land use to achieve greater productivity of food and fibre may have an adverse effect on the water balance and hence on the natural resource capital of a catchment. An altered regime of catchment outflow accompanies those land-use changes which, together with land degradation, impairs available water resources in quantity and quality and threatens enterprise sustainability, notwithstanding the initial improvement in productivity. Central to any hydrological change is an altered pattern of seasonal and annual water use by vegetation that has become modified in function with an amended transpiration fraction of daily evapotranspiration. In Australia, since measurement of evapotranspiration became feasible, the hydrological consequences of changes in land use have been determined, allowing the benefits in terms of plant productivity achieved through enhanced water use efficiency to be weighed against changed catchment outflows, diminished in either quantity or quality. Four case studies are presented as examples of ecological and hydrological changes: two deal with the upland forest environment and two with arable lowlands. In an upland eucalypt forest, following wildfire with subsequent regeneration from natural seedling establishment, substantial reduction in water yield occurred throughout a 50-year period of succession in the even-aged stand. In comparison, the effect of converting eucalypt forest to pine plantations was less detrimental to the yield of water from the catchments, with substantial growth increases over 30 years. In the lowlands, agricultural productivity, both as annual pasture and as crop, far exceeds that of natural perennial grassland and woodland. This increase in productivity comes not so much from any change to the yield of total water outflow but at the expense of water quality, compromised with increased material transport in suspension and solution resulting from accelerated erosion in association with outbreaks of soil salinity and acidity. The present study is aimed at optimising management to give plant production outcomes that ensure environmental protection through resource conservation. In the uplands, harvesting of water is the dominant consideration so that conservative management with limited plant productivity is sought. In the lowlands, the objective is to devise novel ecosystems with profitable plant production that exercises due control on outflow in maintaining the chemical and physical integrity of the edaphic environment.

In open beef feedlot systems, more than 50% of dietary nitrogen (N) is lost as ammonia (NH3). Here we report an effective and economically-viable method to mitigate NH3 emissions by the application of lignite. We constructed two cattle pens (20 × 20 m) to determine the effectiveness of lignite in reducing NH3 emissions. Twenty-four steers were fed identical commercial rations in each pen. The treatment pen surface was dressed with 4.5 kg m−2 lignite dry mass while no lignite was applied in the control pen. We measured volatilised NH3 concentrations using Ecotech EC9842 NH3 analysers in conjunction with a mass balance method to calculate NH3 fluxes. Application of lignite decreased NH3 loss from the pen by approximately 66%. The cumulative NH3 losses were 6.26 and 2.13 kg N head−1 in the control and lignite treatment, respectively. In addition to the environmental benefits of reduced NH3 losses, the value of retained N nutrient in the lignite treated manure is more than $37 AUD head−1 yr−1, based on the current fertiliser cost and estimated cost of lignite application. We show that lignite application is a cost-effective method to reduce NH3 loss from cattle feedlots.

In trials of a mass balance method for measuring methane (CH 4 ) emissions, sonic anemometers and an open-path laser were used to measure the transport of CH 4 released from a ground-level source across a downwind face 50 m long and 6 m high. Release rates matched emissions expected from dairy herds of 2 to 40 cows. The long laser path permitted inferences from measurements in only two planes, one upwind and one downwind, while the fast-response instruments allowed calculation of instantaneous horizontal fluxes rather than fluxes calculated from mean wind speeds and mean concentrations. The detection limit of the lasers was 0.02 ppmv, with the separation between the transmitters and reflectors being about 50 m. The main conclusions from the 23 trials were: (1) Emissions calculated from mean wind speeds and concentrations overestimated the true emissions calculated from instantaneous measurements by 5%. (2) Because of small changes in methane concentration, the minimum sample size in animal trials would be 10 dairy cows, producing about 40 mg CH 4 s −1 . (3) For release rates greater than 40 mg CH 4 s −1 and with sufficient replication, the technique could detect a change in production rate of 9% ( P = 0.05 ). (4) Attention to perceived weaknesses in the present technique should help towards detecting changes of 5%.

This paper introduces the micrometeorological field campaigns known asOASIS (Observations At Several Interacting Scales) and then summarizesseveral companion studies that have used the OASIS dataset. Instrumentedtowers, aircraft and atmospheric sondes were used for measurements overthree paired sites (crops and pastures), approximately equi-spaced along an88-km transect in south-eastern New South Wales, Australia, during the australsprings of 1994 and 1995. Measurements included standard meteorologicaldata and the fluxes of solar and net radiation, sensible heat, water vapour andthe greenhouse gases CO2, N2O, CH4. Descriptions of the site, andthe spatial and temporal variations of climate fields and fluxes, are presented.There were strong contrasts in fluxes and surface conductances, evaporationratios and water use efficiencies between the 1994 drought year and the normalrainfall year of 1995. Despite greater incoming solar radiation in 1994 associatedwith less cloud cover, net radiation was lower than in 1995 because of greateroutgoing thermal radiation caused by higher surface temperatures. In 1994 dailysensible heat fluxes were about 50% higher and evaporation rates about half thosefor 1995. Rainfall in the three-month growing season prior to the field campaignswas the key determinant of leaf area index, surface conductances and the fluxes ofsensible and latent heat and CO2. Antecedent rainfall distribution also controlled variation in fluxes and surface properties along the transect within each year. There was a net loss of CO2 to the atmosphere at the drier central sites in 1994, and a net uptake at the wetter north-eastern sites. Both sites recorded uptake of CO2 in 1995, but values were lower at the central site than at the north–east site due to the strong rainfall gradient along the transect in the three months prior to each fieldcampaign. Differences in fluxes between crops and pastures at each site were smallerthan between sites.

Two methods are examined for combining measurements from instrumented aircraftand towers to estimate regional scale evapotranspiration. Aircraft data provided spatially averaged values of properties of the surface, the evaporative fraction and maximum stomatal conductance. These quantities are less sensitive to meteorological conditions than the turbulent fluxes of heat and water vapour themselves. The methods allowed aircraft data collected over several days to be averaged and thus to reduce the random error associated with the temporal under-sampling inherent in aircraft measurements. Evaporative fraction is estimated directly from the aircraft data, while maximum stomatal conductance is estimated by coupling the Penman–Monteith equation to a simple model relating surface conductance to the incoming shortwave radiation and specific humidity saturation deficit. The spatial averages of evaporative fraction and maximum stomatal conductance can then be used with routine tower data to estimate the regional scale evapotranspiration. Data from aircraft flights and six ground based sites during the OASIS field campaign in south–east New South Wales in 1995 have been used to check the methods. Both the evaporative fraction and the maximum stomatal conductance derived from the aircraft data give information on the spatial variability of the surface energy budget at scales from 10 to 100 km. Daily averaged latent heat fluxes estimated using these methods for the OASIS study region agree with the available observations in quasi-stationary conditions or in weakly non-stationary conditions when the data from several aircraft flights are averaged to reduce the impact of short term imbalances in the surface energy budget.

Animal production systems are major agricultural sources of the greenhouse gases, methane and nitrous oxide, of pollutants such as ammonia and hydrogen sulfide, and of odours. Verifying inventory estimates of their production, and monitoring, regulating and reducing their emission all require the ability to make precise, non-obtrusive measurements of their fluxes, and in the case of odours, an ability to predict concentrations close to the source. However, these requirements present special problems. The source areas are often small or irregularly shaped, the sources may be distributed heterogeneously, and emissions may be transient. Odours present an additional problem in that it is difficult to quantify their intensity since they are usually due to a mixture of more than 100 volatile organic compounds. The use of tracer gases to follow odour dispersion is discussed, but quantifying odour intensity is a pressing problem. The paper considers measurement techniques appropriate for sources of different shapes and sizes. Most attention is given to small and irregularly shaped sources for which conventional, large-area micrometeorological approaches are inapplicable: line-sources, strips, small plots, and waste storage areas such as ponds and manure piles. Mass balance techniques appear to be particularly useful for these purposes. Various applications of the methodology are considered in some detail. The advent of open-path gas lasers that can measure concentrations rapidly and over distances of hundreds of meters is seen as a significant development. Finally, the paper considers the use of a Lagrangian dispersion model that allows emissions from sources of any geometry to be linked to concentrations at any location downwind. In forward mode, it can predict downwind concentrations for given source scenarios, while in backward mode, it can infer fluxes from measurements of concentration and wind speed at just one height downwind. It is a very attractive tool for use in many dispersion problems relevant to the theme of the paper.

Micrometeorological flux-gradient and nocturnal boundary layer methods were combined with Fourier transform infrared (FTIR) spectroscopy for high-precision trace gas analysis to measure fluxes of the trace gases CO2, CH4 and N2O between agricultural fields and the atmosphere. The FTIR measurements were fully automated and routinely obtained a precision of 0.1–0.2% for several weeks during a measurement campaign in October 1995. In flux-gradient measurements, vertical profiles of the trace gases were measured every 30 min from the ground to 22 m. When combined with independent micrometeorological measurements of water vapour fluxes, trace gas fluxes from the underlying surface could be determined. In the nocturnal boundary layer method the rate of change in mass storage in the 0–22 m layer was combined with fluxes measured at 22 m to estimate surface fluxes. Daytime fluxes for CO2 were −0.78±0.40 (1σ) mg CO2 m−2 s−1. Daytime fluxes of N2O and CH4 were very small and difficult to measure reliably using the flux-gradient technique, despite the high precision of the concentration measurements. Mean daytime flux for N2O was 17±48 ng N m−2 s−1, while the corresponding flux for CH4 was 47±410 ng CH4 m−2 s−1. The mean nighttime flux of CO2 estimated using the nocturnal boundary layer method was +0.15±0.05 mg CO2 m−2 s−1, in good agreement with chamber measurements of respiration rates. Nighttime fluxes of CH4 and N2O from the nocturnal boundary layer method were 109±69 ng CH4 m−2 s−1 and 2±3.2 ng N m−2 s−1, respectively, in good agreement with chamber measurements and inventory estimates based on the sheep and cattle stocking rates in the region. The suitability of FTIR-based methods for long term monitoring of spatially and temporally averaged flux measurements is discussed.

This paper reports on the fate of nitrogen (N) in a first ratoon sugarcane (Saccharum officinarum L.) crop in the wet tropics of Queensland when urea was either surface applied or drilled into the soil 3–4 days after harvesting the plant cane. Ammonia volatilization was measured with a micrometeorological method, and fertilizer N recovery in plants and soil, to a depth of 140 cm, was determined by mass balance in macroplots with 15N labelled urea 166 and 334 days after fertilizer application. The bulk of the fertilizer and soil N uptake by the sugarcane occurred between fertilizing and the first sampling on day 166. Nitrogen use efficiency measured as the recovery of labelled N in the plant was very low. At the time of the final sampling (day 334), the efficiencies for the surface and subsurface treatments were 18.9% and 28.8%, respectively. The tops, leaves, stalks and roots in the subsurface treatment contained significantly more fertilizer N than the corresponding parts in the surface treatment. The total recoveries of fertilizer N for the plant-trash-soil system on day 334 indicate significant losses of N in both treatments (59.1% and 45.6% of the applied N in the surface and subsurface treatments, respectively). Drilling the urea into the soil instead of applying it to the trash surface reduced ammonia loss from 37.3% to 5.5% of the applied N. Subtracting the data for ammonia loss from total loss suggests that losses by leaching and denitrification combined increased from 21.8% and 40.1% of the applied N as a result of the change in method of application. While the treatment resulted in increased denitrification and/or leaching loss, total N loss was reduced from 59.1% to 45.6%, (a saving of 13.5% of the applied N), which resulted in an extra 9.9%of the applied N being assimilated by the crop.

Source/sink strengths and vertical flux distributions of carbon dioxide within and above a rice canopy were modelled using measured mean concentration profiles collected during an interna- tional rice experiment in Okayama, Japan (IREX96). The model utilizes an Eulerian higher-order closure approach that permits coupling of scalar and momentum transport within vegetation to infer sources and sinks from mean scalar concentration profiles; the so-called 'inverse problem'. To compute the required velocity statistics, a Eulerian second-order closure model was considered. The model well reproduced measured first and second moment velocity statistics inside the canopy. Using these modelled velocity statistics, scalar fluxes within and above the canopy were computed and compared with CO2 eddy-correlation measurements above the canopy. Good agreement was obtained between model calculations of fluxes at the top of the canopy and measurements. Close to the ground, the model predicted higher respiratory fluxes when the paddy was drained compared to when it was flooded. This is consistent with the floodwater providing a barrier to diffusion of CO2 from the soil to the atmosphere. The Eulerian sources and flux calculations were also compared to source and flux distributions estimated independently using a Lagrangian Localized Near Field theory, the first study to make such a comparison. Some differences in source distributions were predicted by these analyses. Despite this, the calculated fluxes by the two approaches compared well provided a closure constant, accounting for the influence of 'near-field' sources in the Eulerian flux transport term, was given a value of 1.5 instead of the value of 8 found in laboratory studies.

This paper reports a study in the wet tropics of Queensland on the fate of urea applied to a dry or wet soil surface under banana plants. The transformations of urea were followed in cylindrical microplots (10.3 cm diameter × 23 cm long), a nitrogen (N) balance was conducted in macroplots (3.85 m × 2.0 m) with 15N labelled urea, and ammonia volatilization was determined with a mass balance micrometeorological method. Most of the urea was hydrolysed within 4 days irrespective of whether the urea was applied onto dry or wet soil. The nitrification rate was slow at the beginning when the soil was dry, but increased greatly after small amounts of rain; in the 9 days after rain 20% of the N applied was converted to nitrate. In the 40 days between urea application and harvesting, the macroplots the banana plants absorbed only 15% of the applied N; at harvest the largest amounts were found in the leaves (3.4%), pseudostem (3.3%) and fruit (2.8%). Only 1% of the applied N was present in the roots. Sixty percent of the applied N was recovered in the soil and 25% was lost from the plant-soil system by either ammonia volatilization, leaching or denitrification. Direct measurements of ammonia volatilization showed that when urea was applied to dry soil, and only small amounts of rain were received, little ammonia was lost (3.2% of applied N). In contrast, when urea was applied onto wet soil, urea hydrolysis occurred immediately, ammonia was volatilized on day zero, and 17.2% of the applied N was lost by the ninth day after that application. In the latter study, although rain fell every day, the extensive canopy of banana plants reduced the rainfall reaching the fertilized area under the bananas to less than half. Thus even though 90 mm of rain fell during the volatilization study, the fertilized area did not receive sufficient water to wash the urea into the soil and prevent ammonia loss. Losses by leaching and denitrification combined amounted to 5% of the applied N.

Source distributions for heat, water vapour, CO 2 and CH 4 within a rice canopy were derived using measured concentration profiles, a prescribed turbulence field and an inverse Lagrangian analysis of turbulent dispersion of scalars in plant canopies. Measurements were made during IREX96, an international rice experiment in Okayama, Japan. Results for the cumulative fluxes of heat, water vapour and CH 4 at the canopy top were satisfactory once their respective concentration profiles were smoothed using simple analytic functions. According to the inverse analysis, water vapour was emitted relatively uniformly by each of five equi-spaced layers within the canopy, whereas sensible heat fluxes were small ( −2 ) and of either sign. Methane fluxes were predicted to be emitted most strongly in the lower 50% of the canopy, as expected from the distribution of micropores along leaves and leaf sheaths, the major pathway for CH 4 loss from the soil–crop system. No smoothing was required for CO 2 concentration profiles and the inverse analysis provided close correspondence between the turning point in the concentration profile is the changeover from respiration by the soil/paddy water and lower canopy to net photosynthesis by the upper canopy. These results could only be obtained by including both the near- and far-field contributions of sources to the total concentration profile. Neglect of the near-field contribution in the inverse analysis led to spurious source distributions. Excellent agreement was obtained between cumulative fluxes of heat, water vapour, CO 2 and CH 4 at the top of the canopy from the inverse analysis and direct eddy covariance measurements when the friction velocity u * >0.1 m s −1 , and atmospheric stability was approximately neutral. Nocturnal fluxes of CO 2 and CH 4 from the inverse method exceeded micrometeorological measurements above the canopy by a factor of 2–3 when u * −1 and stable atmospheric conditions prevailed within and above the canopy. Neglect of these stability effects will lead to an underestimate of the dispersion coefficients (dimension of resistances) in the transport model and hence an overestimate of the fluxes. Further work is required to establish the correct procedure for incorporating stability effects into the inverse analysis.

The paper examines the strengths and weaknesses of a rangeof meteorological flux measurement techniques that mightbe used to verify predictions of greenhouse gas inventories.Recent research into emissions of methane (CH4)produced by enteric fermentation in grazing cattle and sheepis used to illustrate various methodologies. Quantifying thisimportant source presents special difficulties because the animalsconstitute moving, heterogeneously distributed, intermittent, pointsources. There are two general approaches: one, from the bottom up,involves direct measurements of emissions from a known number ofanimals, and the other, from the top down, infers areal emissions ofCH4 from its atmospheric signature. A mass-balance methodproved successful for bottom-up verification. It permits undisturbedgrazing, has a simple theoretical basis and is appropriate for fluxmeasurements on small plots and where there are scattered pointsources. The top-down methodologies include conventional flux-gradientapproaches and convective and nocturnal boundary-layer (CBL and NBL)budgeting schemes. Particular attention is given to CBL budget methods inboth differential and integral form. All top-down methodologies require ideal weather conditions for their application, and they suffer from the scattered nature of the source, varying wind directions and low instrument resolution. As for mass-balance, flux-gradient micrometeorological measurements were in good agreement with inventory predictions of CH4 production by livestock, but the standard errors associated with both methods were too large to permit detection of changes of a few per cent in emission rate, which might be important for inventory, regulatory or research purposes. Fluxes calculated by CBL and NBL methods were of the same order of magnitude as inventory predictions, but more improvement is needed before their use can be endorsed. Opportunities for improving the precision of both bottom-up and top-down methodologies are discussed.

An inverse Lagrangian dispersion analysis was used to infer sources and sinks of ammonia (NH 3 ) in the canopy of a dense corn crop (LAI∼5) subject to frequent sprinkler irrigation with dairy effluent. Source strengths were calculated for four canopy layers using statistics of the canopy turbulence and measurements of atmospheric NH 3 concentrations at six heights within the canopy and two above it. The analysis was performed for 4 days of measurement, two immediately following and two several days after effluent application. The analysis provided estimates of the net loss of NH 3 from the canopy, which agreed well (within 20%) with conventional aerodynamic estimates of the flux of NH 3 in the crop boundary layer. In addition, it permitted an examination of processes of loss within the canopy. Small, but not insignificant NH 3 losses were inferred for the soil. These ranged from 4% of the total loss several days after effluent application to 30% on the day of application. Unexpectedly large losses occurred from the foliage in the top half of the canopy. Calculations of apparent ammonia compensation points for the corn leaves indicated that even days after the surface water had evaporated, compensation points were too high for the loss to be explained by diffusion through stomata. It was surmised that the loss was from residues remaining on foliage surfaces. Volatilization losses from the developed crop were estimated to be about 30% of the N applied, 18% coming from soil and foliage and 12% from spray losses during effluent application.

Estimates made by the Intergovernmental Panel on Climate Change (IPCC) and the Australian National Greenhouse Gas Inventory Committee (NGGIC) suggest that grazed pastures are substantial anthropogenic sources of nitrous oxide (N2O), contributing 28% of all anthropogenic N2O emissions globally and >43% for Australia. These estimates are based almost wholly on extrapolations of enclosure experiments to the field scale and uncertainty levels are high. Verification with direct field measurements is needed. This paper reports micrometeorological studies of N2O emissions from Australian grazed pastures made at the same location on a variety of space scales. They included a mass-balance study employing a small test plot approximately 0.05 ha in area in which 14 sheep were grazed, tower-based flux measurements representing areas between 25 ha and 5 km2 and convective boundary-layer budgets representing regions of order 100 km2. The mass-balance study, which was considered to be the most reliable micrometeorological approach, gave an average emission over 8 days of 1.87 g N2O–N head−1 d−1 corresponding to 11.5% of the nitrogen (N) voided by the animals in urine and dung. However, the data set included two days after rain on which emissions were an order of magnitude larger than on the other days in the study. For the latter, the emission of N2O accounted for 3.9% of the N excreted. Although uncertainty levels remain high due to large temporal and spatial variability, the micrometeorological measurements suggested that N2O emissions might be considerably larger than those predicted by NGGIC algorithms which use emission factors of 0.4% for urine and 1.25% for dung, but appear to be predicted more closely by IPCC algorithms which use 2% for both. The study has indicated ways to improve the precision of relevant micrometeorological approaches.

To assess the role of floodwater in controlling the exchanges of CO 2 and CH4 from soil, floodwater and the canopy in intermittently flooded rice paddies, an intensive field campaign (IREX96) was conducted in Japan during August 1996. Eddy covariance was employed to measure fluxes of heat, water vapor and CO 2. The flux-gradient method was used to determine CH4 fluxes from measured profiles of CH 4 concentrations, with the required eddy diffusivity estimated using a modified aerodynamic approach or CO 2 as a reference scalar. When the paddy was drained, net CO2 uptake from the atmosphere during daytime was 23% less, and nighttime CO2 emissions were almost twice as great, than when the paddy was flooded. The mean daily CO 2 uptake on the drained days was 14.5 g m 2

Methane emissions from a flock of 14, 1-year old sheep grazing on a grass and legume pasture were measured using a micrometeorological mass-balance method and a sulphur hexaflouride (SF 6 ) tracer technique. The former measured the mean emission, over 45 min intervals, from all the sheep within a fenced 24 m×24 m enclosure, from the enrichment of methane (CH 4 ) in air as it passed over the sheep. The tracer technique measured emissions from a subset of 7 individual animals over 24 h periods from measurements of CH 4 and SF 6 concentrations in air exhaled by the sheep, and from the known rate of release of SF 6 from small permeation tubes placed in the animals’ rumens. Both methods gave highly similar results for 4 out of 5 days. When the species composition of dietary intake was steady during the last two days of measurement, the mean emission rate from the mass-balance method was 11.9±1.5 (SEM) g CH 4 sheep -1 d -1 , while the rate from the tracer technique was 11.7±0.4 (SEM) g CH 4 sheep -1 d -1 . These rates are for sheep with mean live mass of 27 kg, with a measured dry matter intake of 508 g sheep -1 d -1 and pasture dry matter digestibility of 69.5%. There was close agreement between these measurements and estimates from algorithms used to predict methane emissions from sheep for the Australian National Greenhouse Gas Inventory.

A mass balance method is described for calculating gas production from a surface or volume source in a small test plot from measurements of differences in the horizontal fluxes of the gas across upwind and downwind boundaries. It employs a square plot, 24 m×24 m, with measurements of gas concentration at four heights (up to 3.5 m) along each of the four boundaries. Gas concentrations are multiplied by the appropriate vector winds to yield the horizontal fluxes at each height on each boundary. The difference between these fluxes integrated over downwind and upwind boundaries represents production. Illustrations of the method, which involve exchanges of methane and carbon dioxide, are drawn from experiments with landfills, pastures and grazing animals. Tests included calculation of recovery rates from known gas releases and comparisons with a conventional micrometeorological approach and a backward dispersion model. The method performed satisfactorily in all cases. Its sensitivity for measuring exchanges of CO2, CH4 and N2O in various scenarios was examined. As employed by us, the mass balance method can suffer from errors arising from the large number of gas analyses required for a flux determination, and becomes unreliable when there are light winds and variable wind directions. On the other hand, it is non-disturbing, has a simple theoretical basis, is independent of atmospheric stability or the shape of the wind profile, and is appropriate for flux measurement in situations where conventional micrometeorological methods can not be used, e.g. for small plots, elevated point sources, and heterogeneous surface sources.

Lagrangian analysis of transport within the canopy space leads to a physical description by which the mean concentration profiles can be predicted for any given source distribution through knowledge of the turbulence statistics. The concentration profiles are related to the source-sink distributions through a matrix of linear equations, a ‘dispersion matrix’, in which the coefficients are functions of σw the standard deviation of the vertical wind velocity, and TL, the Lagrangian time-scale. The Inverse Lagrangian analysis inverts the dispersion matrix to calculate the source/sink distributions from the mean concentration profiles and profiles of σw and TL. Thus the technique allows separate identification of, say, the respiration of CO2 at the soil (or paddy water) surface and the uptake of atmospheric CO2 by plants in the canopy. The observations and the computational scheme required for such applications are relatively simple. The paper describes the use of the technique to infer sources of heat, water vapour and CO2 in a wheat crop.

The paper considers the theory and application of budget techniques for regional scalar flux estimation using the daytime convective boundary layer (CBL) and the nocturnal boundary layer (NBL). CBL techniques treat the well mixed layer of air between heights of, say, 100 m and 1000 m as an integrator of surface fluxes along the path of a column of air moving over the landscape. They calculate the average surface flux from the scalar concentration in and above the mixed layer, and the CBL height. The flux estimates are averaged over regions of 10–104 km2 extending 10 to 100 km upwind. An integral form of the CBL budget is used to estimate daily regional rates of CO2 uptake and evaporation from three data sets. There was plausible agreement between the estimates and locally measured fluxes. CBL budgets have great potential for estimating regional scalar fluxes, but there is an urgent need for validation through direct measurements of fluxes and budget parameters. NBL budgets are useful when low-level, radiative inversions inhibit vertical mixing. Surface scalar fluxes can then be estimated from the rate of concentration change below the inversion. An example application for estimating the nocturnal CO2 flux is given. While simple in concept, NBL budgets are more difficult to apply in practice because of the unpredictability of the depth of the layer and sometimes, its absence altogether. On the other hand, the depth of the atmospheric mixing chamber is better defined, few assumptions are required and the concentration changes usually will be larger and hence more easily detectable than in CBL budgetting.

Soil evaporation is often an important component of the total water loss from plant canopies, but is poorly estimated in many SVAT models. This paper reports the use of micrometeorological and lysimetric methods to measure soil evaporation (Es) in the canopy of a wheat crop over a period of 7 weeks as the crop matured and green leaf area index (L) declined from 3.8 to 0.2. Simultaneous measurements of total canopy evaporation (E) were made by eddy correlation. The within-canopy micrometeorological methods employed a variance ratio approach and an inverse Lagrangian analysis. The pooled micrometeorological and lysimetric data conformed very well to the relationship E s E = exp(−0.61L) with an r2 value of 0.95. The relationship should be a useful empirical means for estimating daily or weekly Es in SVAT studies.

The paper deals with flux measurements in two contexts: small plots and plant canopies. Mass balance methods have been developed for small experimental plots with lateral dimensions of tens of metres rather than the 1 m typical of chambers or the hundreds of metres required for conventional micrometeorological estimates. The general method relies on the conservation of mass to equate the differences in horizontal fluxes across upwind and downwind boundaries of a test plot with the surface flux within the plot along the line of the wind. Applications to soil and animal experiments are discussed. Lagrangian descriptions of transport now supplant older, but inappropriate gradient-diffusion theory for inferring fluxes and source-sink distributions of scalars in plant canopies. An inverse Lagrangian theory due to M. R. Raupach provides a relatively simple observational and computational scheme for making such inferences from measurements of mean concentration profiles and canopy turbulence. The scheme and a range of applications are described.

Components of the seasonal pattern of water use by a wheat crop at Wagga Wagga, N.S.W. are presented. Total evapotranspiration was estimated using a combination of Time Domain Reflectometry and Neutron Moisture Meter measurements. Miniature lysimeters were used to measure evaporation from soil below the canopy. Evaporation of intercepted rain was calculated using an aerodynamic technique in conjunction with a canopy storage coefficient. Detailed measurements of soil evaporation and rainfall interception commenced 80 days after sowing (DAS), when L ∼ 1, until 165 DAS. Interception losses accounted for 33% of rain during this period (114 mm), while soil evaporation was 48% of rainfall. Combined interception and soil evaporation losses for the whole growing season accounted for 49% of total evapotranspiration. Scope exists to reduce soil evaporation by managing crops for early canopy closure, but this will mean greater rainfall interception losses and reduced replenishment of soil water.

The paper reviews methods for measuring the exchanges of gases between soil-plant systems and the atmosphere, with emphasis on the greenhouse gases CO2 CH4 and N2O The methods vary in scale from chambers covering

Open cattle feedlots are a source of air pollutants that include particular matter (PM). Over 24 h, exposure to ambient concentrations of 50 microg m(-3) of the coarse-sized fraction PM (aerodynamic diameter

Ammonia losses following surface applications of urea to trash covered sugar cane fields were investigated in four climatic zones of tropical Queensland. Volatilization of ammonia and evaporation of water were determined by micrometeorological techniques. The results showed that the pattern, rate and extent of ammonia loss were controlled by the availability of water in the trash and its evaporation. Water added by dewfall, rainfall or condensation of evaporated soil moisture dissolved some of the urea and allowed it to be hydrolyzed to ammonia by the urease enzyme in the sugarcane residues; when the water evaporated, ammonia was lost to the atmosphere.

A technique combining open-path Fourier transform infrared spectroscopy with an inverse-dispersion model was used to quantify methane (CH4), nitrous oxide (N2O) and ammonia (NH3) emissions from an isolated cattle pen in south-eastern Australia. Twenty-eight Angus steers (1-year old, initial average liveweight 404 kg) were fed a 60% grain diet and kept in a pen (20 × 20 m) for 41 days. Gas concentrations were measured downwind of the pen using an open-path Fourier transform infrared spectroscopy with a path length of 100 m, having a detection sensitivity of 2, 0.3 and 0.4 ppb for CH4, N2O and NH3, respectively. Daily emission rates were 232, 14 and 192 g/cattle.day for CH4, N2O and NH3, respectively. The measured CH4 emissions were in agreement with predictions based on Australian National Inventory recommendations, however, measured N2O and NH3 emissions were much higher than the predicted values. Extrapolation of our measurements would mean that CH4 and N2O emissions from beef feedlot cattle contribute 3.1% and 5.9% of the Australian agricultural CH4 and N2O emissions, respectively.

A thorough understanding of the physical and chemical processes involved in NH 3 emission from inorganic N fertilizers and fertilized crops is required if reliable and operational NH 3 emission factors and decision support systems for inorganic fertilizers are to be developed, taking into account the actual soil properties, climatic conditions and management factors. For this reason, the present review focuses on processes involved in NH 3 volatilization from inorganic nitrogen fertilizers and the exchange of ammonia between crop foliage and the atmosphere. The proportion of nitrogen lost from N fertilizers due to NH 3 volatilization may range from ≈0 to >50%, depending on fertilizer type, environmental conditions (temperature, wind speed, rain), and soil properties (calcium content, cation exchange capacity, acidity). The risk for high NH 3 losses may be reduced by proper management strategies including, e.g., incorporation of the fertilizer into the soil, use of acidic fertilizers on calcareous soils, use of fertilizers with a high content of carbonate-precipitating cations, split applications to rice paddies or application to the soil surface beneath the crop canopy. The latter takes advantage of the relatively low wind speed within well-developed canopies, reducing the rate of vertical NH 3 transport and increasing foliar NH 3 absorption. Conversely, NH 3 is emitted from the leaves when the internal NH 3 concentration is higher than that in the ambient atmosphere as may often be the case, particularly during periods with rapid N absorption by the roots or during senescence induced N-remobilization from leaves. Between 1 and 4% of shoot N may be lost in this way.

Methane (CH4) emissions from animals represent a significant contribution to anthropogenically produced radiatively active trace gases. Global and national CH4 budgets currently use predictive models based on emission data from laboratory experiments to estimate the magnitude of the animal source. This paper presents a method for measuring CH4 from animals under undisturbed field conditions and examines the performance of common models used to simulate field conditions. A micrometeorological mass difference technique was developed to measure CH4 production by cattle in pasture and feedlot conditions. Measurements were made continuously under field conditions, semiautomatically for several days, and the technique was virtually nonintrusive. The method permits a relatively large number of cattle to be sampled. Limitations include light winds (less than approximately 2 m/s), rapid wind direction changes, and high-precision CH4 gas concentration measurement. Methane production showed a marked periodicity, with greater emissions during periods of rumination as opposed to grazing. When the cattle were grazed on pasture, they produced .23 kg CH4 x animal(-1) x d(-1), which corresponded to the conversion of 7.7 to 8.4% of gross energy into CH4. When the same cattle were fed a highly digestible, high-grain diet, they produced .07 kg CH4 x animal(-1) x d(-1), corresponding to a conversion of only 1.9 to 2.2% of the feed energy to CH4. These measurements clearly document higher CH4 production (about four times) for cattle receiving low-quality, high-fiber diets than for cattle fed high-grain diets. The mass difference method provides a useful tool for "undisturbed" measurements on the influence of feedstuffs and nutritional management practices on CH4 production from animals and for developing improved management practice for enhanced environmental quality.

Summary Meteorological techniques for measuring trace gas fluxes on three important scales not well catered for by conventional methods have been discussed: an inverse Lagrangian dispersion method appropriate for the canopy scale, mass balance methods for small and heterogeneous ecosystems and boundary layer budgeting schemes for the regional scale. The inverse Lagrangian analysis offers a relatively simple measurement scheme for inferring fluxes of trace gases and their source-sink distributions within plant canopies. Inputs are the profiles of mean gas concentration and turbulence within and above the canopy. The analysis provides a bridge between chamber and cuvette measurements on soil and foliage elements and flux measurements on a field scale. Mass balance methods are appropriate for flux measurements in small ecosystems, tens of meters in lateral extent. Fluxes from areas of known geometry are calculated from the rate at which the wind transports gas across the upwind and downwind boundaries of the designated area. The method can fill the gap between chambers of, say, 1 m 2 in area and conventional micrometeorological methods representing, say, 10 4 m 2 . It can suffer from errors arising from the large number of gas analyses required for a flux determination and may become unreliable when there are light winds and variable wind directions. On the other hand, it is non-disturbing, has a simple theoretical basis, smooths over surface heterogeneity and is independent of atmospheric stability or the shape of the wind profile. Convective and nocturnal boundary layer (CBL and NBL) budgeting techniques are discussed in the context of a recent experiment to estimate regional fluxes of carbon dioxide, methane and nitrous oxide in a rural area of southeast Australia. CBL techniques estimate the average surface flux over regions of order 100 km 2 through the buildup or drawdown of gas concentration in the atmospheric mixed layer and its depth. An integral form of CBL budgeting was used to estimate daily fluxes. Input data were gas concentrations at 22 m and CBL heights obtained with radiosondes. The atmospheric gas concentrations above the CBL were assumed to be the current clean-air baseline values. It was concluded that even with this simplified obsevation scheme, CBL budgeting can be a very useful survey tool and in regions that are homogeneous in the large, can provide better than order of magnitude estimates of trace gas fluxes. NBL budgeting techniques follow the change of gas storage in the surface layer at night when low-level radiative inversions inhibit vertical mixing. The footprint is difficult to estimate, but is of order 1 to 5 km. On one occasion during the experiment, balloon measurements were made up to a height of 100 m, but routinely, tower-based measurements were made to 22 m. It was concluded that for gases whose emissions do not exhibit marked diurnal cycles, NBL budgets may be simpler alternatives than either CBL budgets or conventional micrometeorological measurements made by day. When diurnal variation is large, both day and night measurements are needed to define the 24-hour flux.

Publisher Summary This chapter provides a report on how the fluxes of trace gases can be validated among different scales. A variety of techniques determine the trace gas fluxes, including chamber, micrometeorological, and airborne methods. Micrometeorological measurements of oceanic fluxes pose additional problems. For many trace gases, the fluxes between the ocean and the atmosphere are small compared to fluxes between terrestrial ecosystems and the atmosphere. This causes technical problems related to the detection limits of instruments. Two distinct methods exist to derive fluxes using atmospheric models—that is, forward simulations and inverse methods. In forward atmospheric modeling, emissions are used to drive the atmospheric model to predict concentrations and fluxes. Inverse methods use observations of atmospheric concentrations to derive flux estimates. Both methods can be applied at all scale levels. In many cases, fluxes can be determined with different techniques, used simultaneously. Inverse methods use observations of atmospheric concentrations to derive flux estimates.

We have used FTIR spectroscopy for high precision trace gas analysis combined with micrometeorological flux-gradient and chamber methods to measure the fluxes of trace gases CO2, CH4, N2O and CO between agricultural soils and the atmosphere. In flux gradient measurements, vertical profiles of the trace gases were measured every 30 minutes from the ground to 22 m. When combined with independent micrometeorological measurements of heat, water vapor and radiative fluxes, trace gas fluxes from the underlying surface could be determined. In chamber measurements, a closed chamber was placed over the soil surface and the air in the chamber monitored continuously by FTIR spectroscopy. Fluxes were calculated from the time rate of change of concentrations in the chamber after closure. The FTIR measurements were fully automated and ran reliably for several weeks, routinely obtaining precision of 0.1–0.5%.

Much of the fertiliser nitrogen (N) used in agriculture is lost to the atmosphere as nitric oxide and nitrogen dioxide (collectively referred to as NOx), ammonia (NH3), and nitrous oxide (N2O). The lost N is not only an economic problem for the farmer; it also contaminates the environment and affects human health. Because the values obtained for NOx and NH3 loss to the atmosphere from agriculture in Monsoon Asia have been questioned, we quantitatively determined, using new techniques, the emission of these gases from a maize crop fertilised with urea in northern China. The fertiliser was deep-placed by traditional farmers’ practice and emissions of NOx and NH3were determined with a chemiluminescence analyser and a backward Lagrangian stochastic dispersion technique. The emission measurements indicate that 1.2% of the applied N was lost as NOx. This loss is far greater than measured or derived by other researchers, and we suggest that this is because our measurements were made continuously rather than as spot measurements with static chambers. The results for NH3 show that, although the fertiliser was placed below the soil surface, a small amount (7% of the applied N) was still lost to the atmosphere. Soil analyses indicate that the rate of nitrification in this soil was low, and the maximum nitrate (NO3–) concentration found in the soil (31.4 µg N/g) was only 3.9% of the fertiliser N added. Thus, there is little potential for NO3– to be leached down the profile. A study using soil cores and acetylene inhibition to measure denitrifying activity suggested that the rate of denitrification in this soil was also very low. The results suggest that in this soil with slow nitrification and denitrification rates and little potential for leaching, deep placement of the urea to limit NH3 volatilisation is an effective method for increasing fertiliser use efficiency.

A mechanistic simulation model was used to examine the expected responses of wheat to a doubling in atmospheric CO2 concentration in combination with a 2°C rise in temperature. The model produced satisfactory simulations of total biomass, leaf area and canopy transpiration under present climatic conditions. However, soil evaporation was underestimated by up to 35% by the simulations. Model predictions suggested that increased CO2 levels should lead to increased biomass and potential yields of wheat up to about 550ppm, after which photosynthesis under full sunlight increases only gradually with CO2. Higher temperatures are expected to decrease biomass production by shortening the growing season and by increasing plant respiration. A doubling of CO2 concentration and a temperature increase of 2°C is expected to cause only marginal changes in biomass production. Transpiration use efficiency may be enhanced by -20% with a doubling of CO2 above current levels.