Modelling NH3 volatilisation within a urine patch using NZ-DNDC
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Urea concentrations in urine patches deposited during animal grazing can be over ten times higher than typical fertiliser application rates, potentially leading to large ammonia (NH3) losses. The process-based NZ-DNDC model was modified to better simulate soil pH changes and ammonia (NH3) emissions following urine application using data collected from a New Zealand field trial. After modification, simulated 30-day NH3 emissions decreased from 506 to 117 kg N ha−1 compared to measured emissions of 78 ± 3 kg N ha−1 (mean ± standard error) and the Nash–Sutcliffe Efficiency (NSE) for daily NH3 emissions increased from −7.11 to +0.97 for the parameterisation dataset. However, modified model correctly estimated the cumulative emissions for the first 7 days. Using the same parameterisation on an independent dataset from a nearby site gave cumulative 18-day NH3 emissions of 84 kg N ha−1 compared to the measured 48 ± 2 kg N ha−1 (mean ± standard error). However, the NSE for daily NH3 emissions was −0.71, indicating site specific parameterisation might be needed. The sensitivity of NH3 emissions to ±5 and ±10% errors in 4 model parameters was tested. The sensitivities ranged from −0.36 to +0.71. The highest sensitivity was to the rate of NH3 transfer from the soil solution to the atmosphere and the lowest sensitivity was to the rate of urea hydrolysis.
KeywordsUrine patch Ammonia (NH3) NZ-DNDC model
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