The Effect of N Deposition on Nitrous Oxide and Nitric Oxide Emissions from Temperate Forest Soils

Long-term and short-term N deposition effects on N2O and NO emissions from forest soils were compared. Long-term NH3 deposition (> 20 years) from a poultry farm to a downwind woodland (decreasing from 73 to 18 kg N ha-1 y-1, 30 to 110 m downwind of the farm) resulted in the re-emission of 6% and 14% of NH3-N deposited as N2O-N and NO-N, respectively. However, when in short-term (2-3 years) field experiments the atmospheric N deposition to mature conifer plantations was raised by fumigation with NH3 to 15 kg N ha-1 y-1 or by acid mist to 48 and 96 kg N ha-1 y-1 the N deposited was immobilised. In the acid mist experiment more than 2 years of acid mist (48 and 96 kg N ha-1 y-1) were required to significantly increase N2O emissions from -0.3 μg N2O-N m-2 h-1 (control) to 0.5 and 5.7 μg N2O-N m-2 h-1, respectively. This suggests, that N deposition simulation studies in soil ecosystems, which have previously not been exposed to high rates of N (by deposition or fertilisation), need to be long-term. Also, measurements of N2O and/or NO may be a non-destructive, quick indicator of the N status of the soil.