Volatilization of ammonia from urine patches in a subtropical pasture

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.

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