Nitrogen balance studies in pastures have shown unexplained losses of applied N with much of the loss assumed to be denitrification and NH3 volatilization. This project studied soil, plant, and microclimate effects on diurnal and seasonal fluctuations of gaseous NH3 flux in a subtropical pasture fertilized with urea. Soil and microclimate measurements were taken concurrently with NH3 flux density determinations to relate these factors to magnitude and direction of NH3 transport. Average daily NH3 volatilization varied with seasonal soil and microclimate conditions and time after area application. A short period of large NH3, efflux was observed after urea application during warmer seasons whereas smaller efflux for longer duration was observed during cooler seasons. Soil water content and rainfall after urea application modified seasonal NH3 losses. Diurnal NH3 cycling was apparent, with large efflux occurring during daytime and small efflux or influx at night. Prior to each urea application, average diurnal NH3 transport was into the soil‐plant system possibly due to low soil mineral and plant N. Brief periods of NH, influx were common throughout all seasons, particularly around sunset and sunrise. Soil surface temperature was the most highly correlated factor influencing NH3 flux density during the summer season. During the remainder of the year evapotranspiration had the highest correlation, although the increased midday fluxes were probably due to paralleled increase in soil temperature and windspeed. All of the major influencing factors are. interrelated through their dependence on solar radiation. Rainfall distribution and amount after urea application appeared to control the total NH3 losses from applied urea. The rainfall influence was probably the result of rainfall dispersing urea which prevented high concentrations of NH3 and NH4+ from building up around urea‐prills.
Wilson, J.D., Catchpole, V.R., Denmead, O.T. and Thurtell, G.W., 1983. Verification of a simple micrometeorological method for estimating the rate of gaseous mass transfer from the ground to the atmosphere. Agric. Meteorol., 29: 183--189.Wilson et al. proposed a simple micrometeorological method of estimating the rate of gaseous mass transfer to the atmosphere from a small circular plot which required measurements of time-average species-concentration and horizontal windspeed at a single height. This method has been applied here to estimate the rate of volatilization of ammonia from a 25 m radius plot treated with urea fertilizer. The emission rates thus obtained agreed satisfactorily with estimates based on a mass balance which employed concentration and windspeed measurements at five levels.
Ammonia volatilization from artificially applied cattle urine was measured in a fertilized (374 kg nitrogen ha-1 year-1) and grazed (4 beef steers ha-1) pasture of Nandi setaria (Setaria sphacelata var. sericea) in south-eastern Queensland. Microplots (0.5 by 0.5 m) of pasture were treated with 1.4 1. of urine (= 37-48 g nitrogen m-2) and evolved ammonia measured with a flow-through chamber system that maintained near ambient temperatures and air flows. Calculated volatilization over a 14-day period was 18.8 % of the applied urine nitrogen in June, 14.4 % in November-December and 28.4 % in February-March. More than half of the volatilization occurred within 48 h of applying the urine. Measurements of soil mineral nitrogen in February-March showed that more than 80% of the urinary urea was hydrolysed within 2 h and only traces remained unhydrolysed after 24 h. Soil ammonium declined from 700 to 1000 �g nitrogen cm-3 in the surface 2.5 cm on the first day to near-background levels on the 14th day. Accumulation of nitrate was rapid after the 2nd day, and accounted for one-third of the applied nitrogen by the 14th day. Nitrite reached a maximum of 2.3 % of the applied nitrogen on the 7th day and had virtually disappeared by the 14th day. Only traces of nitrite were detected below 2.5 cm depth.
The fate of urea-N during the 14 d after four seasonal applications to a Setaria sphacelata cv. Nandi pasture in south-eastern Queensland was traced. The aim was to explain the variations in the responses by grass, which have been observed after applications of urea. The total losses of urea-15N from the soil-plant system and the fluxes of NH, into the atmosphere were respectively 29 and 12% of the urea-N applied in summer, 45 and 42% in autumn, 23 and 13% in winter, and 20 and 9% in spring. Losses of nitrogen by leaching were probably negligible as mineral-N derived from the fertilizer remained mostly in the 0-2 cm layer of soil. Hydrolysis of urea to NH4+-N was complete after 1 d in summer, 3 d in spring, 6 d in autumn and 7 d in winter. Losses of urea-N were influenced by the water content of the surface soil (0-0.5 cm) at the time of the application and by the subsequent pattern of rainfall. The loss was large when urea was applied to wet soil and rainfall during the next 7 d did not exceed 1 mm, as in autumn. Losses were much smaller when rain (5 mm or more) fell soon after the applications, as in spring and summer. In winter urea was applied to dry soil and fluxes of NH, continued at a moderate rate until rain fell on the sixth day. Losses of 15N were generally about double the losses of NH,. Work on this discrepancy is needed to fully understand the fate of urea-N broadcast onto pastures. Losses of NH, from urea broadcast onto pastures could be minimized in practice by making applications to dry soil just before rain is expected or before irrigation.
Nitrogen (N) fertilizer is being lost from sugarcane soils following application to the crop. This study was conducted to estimate the quantity of N being lost from the soil through biological denitrification and to determine the proportion of gaseous N being emitted either as N2O or as N2. Field studies were conducted on four different soils (humic gley, alluvial massive earth, red earth and gleyed podzolic), and on different crop management systems, by installing plastic (PVC) cylinders (23.5 cm diam., 25 cm long) in the soil to a depth of 20 cm beside the plant row in a ratoon sugarcane crop. 15N-labelled KNO3 was applied as a band across each cylinder to a depth of 2.5 cm at a rate of 160 kg N/ha. After rainfall or irrigation, the cylinders were capped for 3 h intervals and gas in the headspace sampled in the morning and afternoon, for up to 4 days. Denitrification losses from the humic gley ranged from 247 g N/ha.day for cultivated plots to 1673 g N/ha.day for no-till plots. Over the sampling period, this was equivalent to 3.2% and 19.7% of the N applied, respectively. Nitrous oxide accounted for 46% to 78% of the total N lost. For the alluvial, massive earth and the red earth and gleyed podzolic, losses over the sampling period ranged from 25 to 117 g N/h.day and represented <1% of the N applied. Recovery of 15N in the soil ranged from 67% at the first sampling on the red earth soil to 4.9% at the third sampling on the alluvial, massive earth soil. In a glasshouse study, intact soil cores (23.5 cm diam., 20 cm long), taken from the humic gley and the alluvial, massive earth, were waterlogged after band application of 15N-labelled KNO3 at a rate of 160 kg N/ha. Gas samples from the headspace were taken after 3 h, and then morning and afternoon for the next 14 days. Denitrification losses ranged from 13.2 to 38.6% of N applied with the majority of gaseous N loss occurring as N2. Total recoveries after 14 days, including the evolved gases, ranged from 68.7 to 88.2%. We conclude that denitrification is a major cause of fertilizer N loss from fine-textured soils, with nitrous oxide the major gaseous N product when soil nitrate concentrations are high.
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