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Application of the micrometeorological mass balance method to the determination of ammonia loss from a grazed sward
Author(s) -
Ryden John C.,
McNeill James E.
Publication year - 1984
Publication title -
journal of the science of food and agriculture
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.782
H-Index - 142
eISSN - 1097-0010
pISSN - 0022-5142
DOI - 10.1002/jsfa.2740351206
Subject(s) - fetch , wind speed , flux (metallurgy) , atmospheric sciences , wind direction , environmental science , tracer , ammonia , chemistry , hydrology (agriculture) , meteorology , physics , geology , oceanography , geotechnical engineering , organic chemistry , nuclear physics
The micrometeorological mass balance method has been assessed with a view to its use in the determination of ammonia (NH 3 ) loss from grazed swards. The method involves the measurement of wind speed and the concentration of NH 3 in air at different heights above the sward at its windward boundary and a position leeward of the grazed area. The flux of NH 3 is calculated from these measurements and a continuous record of wind direction. Quantitative recovery of NH 3 from air flows up to 10 litre min −1 was achieved using a simple trap containing dilute orthophosphoric acid and a gas dispersion tube. Wind speed measured at a height of 0.25 m at six different positions above a previously grazed sward varied by <2%. The NH 3 concentration at the same positions was affected by the distance between the windward edge of the sward and the sampling position (i.e. the fetch) and by heterogeneity in the distribution of sources of NH 3 (urine‐ or dung‐affected areas). However, the normalised horizontal flux through 0.25 m (wind speedXNH 3 concentration/fetch) varied by less than ±8.1% probably due to mixing through fluctuations in wind direction (approximately ±30° on the mean) as air passed over the sources of NH 3 during each measurement period. Plots of wind speed or NH 3 concentration versus logarithm of height indicated that each approximated a linear relationship. This facilitated the calculation of the NH 3 flux per unit land area and reduced the total error to about 10%. The total loss of NH 3 derived by summing losses during individual sampling intervals of 2 to 18 h within a 24 h period was essentially the same as that derived by averaging wind speeds and NH 3 concentrations measured continuously over the same 24 h period. The flux of NH 3 from a ryegrass sward grazed by yearling steers ranged from 0.01 to 0.14 kg N ha −1 h −1 during a 2 day grazing period within a 28 day rotation and during the 5 days following removal of animals. A pronounced diurnal variation was observed in the flux of NH 3 , the maximum occurring between 13.00 and 20.00 hours on each day. Rainfall and low rates of evapotranspiration reduced the flux to <0.02 kg N h −1 h −1 . The total loss of NH 3 during 28 days was 20.7 kg N ha −1 .