Explore the vast range of titles available.

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.

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.

Received for publication June 26, 2009. Open cattle feedlots are a source of air pollutants that include particular matter (PM). Over 24 h, exposure to ambient concentrations of 50 µg m–3 of the coarse-sized fraction PM (aerodynamic diameter <10 µm [PM10]) is recognized as a health concern for humans. The objective of our study was to document PM10 concentration and emissions at two cattle feedlots in Australia over several days in summer. Two automated samplers were used to monitor the background and in-feedlot PM10 concentrations. At the in-feedlot location, the PM10 emission was calculated using a dispersion model. Our measurements revealed that the 24-h PM10 concentrations on some of the days approached or exceeded the health criteria threshold of 50 µg m–3 used in Australia. A key factor responsible for the generation of PM10 was the increased activity of cattle in the evening that coincided with peak concentrations of PM10 (maximum, 792 µg m–3) between 1930 and 2000 h. Rain coincided with a severe decline in PM10 concentration and emission. A dispersion model used in our study estimated the emission of PM10 between 31 and 60 g animal–1 d–1. These data contribute to needed information on PM10 associated with livestock to develop results-based environmental policy.

Ammonia volatilization to the atmosphere is a complex process affected by a combination of biological, chemical and physical factors. Primarily, the rate of volatilization must be affected by the amount and form of the source, whether it be native soil organic matter, plant residues, animal excretions, added organic materials or fertilizers. Ammonia concentration in biological systems is controlled by such diverse factors as immobilization-mineralization reactions, urease activity, nitrification and leaching. The balance between ammonium and ammonia also determines the rate and extent of ammonia loss; this equilibrium is affected by pH, buffering capacity of soil or water, presence of calcium carbonate, formation of insoluble precipitates, cation exchange capacity, soil texture, fixation on clay minerals or organic matter, temperature and water loss. Other factors include the movement of ammonia in soil water or soil air by diffusion or convection, the ammonia concentration in the adjacent atmosphere, wind speed, and the presence or absence of plants. Many of these processes are affected by the proximity of the source to the soil or water surface. While most of the processes affecting ammonia volatilization have been well documented from laboratory observations, there is a great need for field data on the actual rates and amounts of volatilization from plant communities and cultivated crops in their natural environments.

The paper is concerned mainly with nitrous oxide, methane and carbon dioxide, which account for more than 70% of predicted greenhouse warming. All three have significant sources in the soil-plant environment and principal sinks in the atmosphere or the oceans. The emphasis is on methodological problems associated with measuring source and sink strengths, but the biogeochemistry of individual gases and problems of scaling to longer times and larger areas are addressed also. Nitrous oxide accounts for some 6% of predicted greenhouse warming. Its atmospheric concentration is 315 ppbv and is increasing at 0.25% per year. The principal sink appears to be destruction through photochemical processes in the stratosphere. The main causes of the N₂O increase are thought to be biomass burning, fossil fuel combustion processes, and what now seem to be substantial emissions from soils associated with increased nitrogen inputs, irrigation and tropical land clearing. Uncertainty about the strengths of the soil sources is due largely to our reliance on enclosure techniques for flux measurement, and the lack of appropriate scaling procedures. Methane now accounts for 18% of anticipated greenhouse warming. Its atmospheric concentration is 1.7 ppmv and is increasing at 1% per year. Its greenhouse effect seems likely to increase over the next 50 years. The biggest sink appears to be oxidation in the atmosphere, but some oxidation occurs in soils as well. The main sources are rice fields, wetlands, biomass burning, ruminants, land fills, natural gas production, and coal mining. As for N₂O, there is much uncertainty about individual source strengths and there are urgent needs for better measurement and scaling techniques. Increased CO₂ concentrations account for 49% of the greenhouse effect. The present atmospheric CO₂ concentration is 350 ppmv, increasing at 0.4% per year. Over 80% of the increase is due to fossil fuels, and the rest to deforestation and biomass burning. Atmospheric fluxes of CO₂ can be measured much more precisely than those of N₂O and CH₄, by micrometeorological techniques, but the scaling problem still remains. The largest known sink for CO₂ is the oceans, but recent calculations point to a large 'missing' sink for CO₂, which may be as yet unidentified sequestering processes in terrestrial ecosystems.

A micrometeorological technique has been used to measure the flux of ammonia and related gaseous nitrogen compounds into the atmosphere from a pasture grazed by sheep. During 3 weeks in late summer, the average daily flux density of nitrogen in these forms was 0.26 kilogram per hectare. This is a substantial part of the nitrogen turnover in grazed pastures.