O. T. Denmead

A sampler for measuring atmospheric ammonia flux

Abstract This paper describes the design, construction and testing of a simple sampling device for determining ammonia fluxes in the atmosphere. Sampler performance was predicted theoretically and tested in a wind tunnel, the laboratory and in the field. The results showed that air flowed through the sampler at a rate linearly proportional to the external wind speed and that the ammonia it contained was absorbed quantitatively by the sampler. The mass of ammonia, M , collected by the instrument during a sampling period, t , is thus proportional to the mean convective flux density of ammonia, ( uϱ N ) since M = ( uϱ N )t , where u is the wind velocity, ϱ N the ammonia density, A is the effective cross-sectional area of the sampler, and the overbar represents a time-mean.

Carbon dioxide and methane fluxes from an intermittently flooded paddy field

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 , <50% of the mean for the flooded days. These differences in the CO2 budget were mainly due to increased CO2 emissions from the soil surface under drained conditions resulting from the removal of the diffusion barrier caused by the floodwater. Small changes in canopy photosynthesis observed between flooded and drained paddies had little influence on the CO 2 budget and could be explained by sensitivity of stomata to humidity saturation deficit. The CH 4 flux for the drained paddy showed distinct diurnal variation with a maximum of 1.3m gC H 4 m 2 s 1 in the afternoon, but after reflooding the peak flux decreased to <0.9m gC H 4 m 2 s 1 . Mean daily CH4 emissions were 28% larger for the drained paddy than when it was flooded. As with the CO 2 flux, the larger CH 4 flux on the drained days can be attributed to reduced resistance of CH4 transfer from the soil to air by removal of the floodwater.

Plant physiological methods for studying evapotranspiration: problems of telling the forest from the trees

Abstract This review is concerned with methods for quantifying physical and physiological processes within plant communities which influence evapotranspiration. Micrometeorological approaches have not been very fruitful because of their reliance on classical flux-gradient relationships, but new perceptions and eddy correlation techniques promise real progress in the near future. Alternative plant physiological approaches including chamber systems, tracer techniques and so-called combination methods, are considered. Perceived problems with chamber systems are the difficulties of mimicking the natural environment inside the chamber and the effects of chamber air pressure on vapour transfer from the soil. Tracer techniques (using pulses of heat or radioactive isotopes) avoid these problems but require exact knowledge of the water pathway in the stem. Combination methods also do not disturb the natural environment, but many difficult measurements are required and the sampling problems are large. They can be applied with least equivocation to layers of leaves. Application to whole plant communities is circumspect because of changing microclimates within the community, the large vertical spread of sources and sinks, and the fact that much of the evaporated water may not pass through plants at all. Many physiological approaches measure only transpiration. Some attention is given to the neglected component, soil evaporation. An analysis of evaporation from the floor of a pine forest suggests that the litter limits the vapour loss to perhaps half what it might be if the soil surface was exposed.

Rainfall interception and evaporation from soil below a wheat canopy

Abstract 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.

Factors controlling ammonia loss from trash covered sugarcane fields fertilized with urea

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.In the dry climatic zone, where no rain or dew fell, water addition to the trash by condensation of evaporated soil moisture was not sufficient to dissolve much urea so very little ammonia was lost. In the cool and warm moist zones, small additions of water to the trash from dew, light rain and condensation maintained a slow but steady pattern of ammonia loss over a period of six weeks and resulted in losses of 32% and 39% of the applied nitrogen. At the site in the wet zone, heavy rainfall apparently washed the urea from the trash layer into the soil and limited ammonia loss to 17% of the applied nitrogen.Substitution of ammonium sulfate for urea reduced ammonia loss to less than 1.8% of the applied nitrogen.

Effects of heat and water vapor transport on eddy covariance measurement of CO2 fluxes

Flux densities of carbon dioxide were measured over an arid, vegetation-free surface by eddy covariance techniques and by a heat budget-profile method, in which CO2 concentration gradients were specified in terms of mixing ratios. This method showed negligible fluxes of CO2, consistent with the bareness of the experimental site, whereas the eddy covariance measurements indicated large downward fluxes of CO2. These apparently conflicting observations are in quantitative agreement with the results of a recent theory which predicts that whenever there are vertical fluxes of sensible or latent heat, a mean vertical velocity is developed. This velocity causes a mean vertical convective mass flux (= ρcw for CO2, in standard notation). The eddy covariance technique neglects this mean convective flux and measures only the turbulent flux ρ′c w′. Thus, when the net flux of CO2 is zero, the eddy covariance method indicates an apparent flux which is equal and opposite to the mean convective flux, i.e., ρ′c w′ = −ρc w. Corrections for the mean convective flux are particularly significant for CO2 because ρcw and ρ′c w′ are often of similar magnitude. The correct measurement of the net CO2 flux by eddy covariance techniques requires that the fluxes of sensible and latent heat be measured as well.

Effect of fertilizer placement on nitrogen loss from sugarcane in tropical Queensland

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.

Identifying sources and sinks of scalars in a corn canopy with inverse Lagrangian dispersion analysis. I. Heat.

Abstract 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.

Fate of urea nitrogen applied to a banana crop in the wet tropics of Queensland

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.

Nitrous oxide emissions from grazed pastures: measurements at different scales

Abstract 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.