The impacts of manure application methods on emissions of ammonia, nitrous oxide and on crop response—A review

Based on simple averages of the reported abatement efficiencies of NH3 emissions, abatement is greater from the use of trailing shoe (TS) (65%) and open-slot injection (OSI) (70–80%) machines than from the trailing hose (TH) (35%). There is considerable variation in the efficiencies reported, especially for trailing hose (0–75%) but also open-slot injection (23–99%). Variation in emissions following the use of the trailing shoe appeared to be somewhat less (38–74%), although this may be due to there being fewer studies reported of the trailing shoe. When slurries or solid manures are applied to arable land immediate incorporation by plough is the most effective abatement technique reducing emissions by at least 90%. Even short (4–6 h) delays in incorporating manures after application will greatly reduce the efficacy of rapid incorporation as a means of NH3 abatement. These reduced-NH3 emission application techniques will also increase crop uptake of manure-N, increasing the value of manures and reducing the net cost of reduced-NH3 application techniques. While there are circumstances under which reduced-NH3 application techniques may increase emissions of N2O, such increases are not inevitable and concern over such emission trade offs should not be allowed to compromise advice on reducing emissions of NH3. The rapid incorporation of solid manures may reduce emissions of NH3 while not increasing, or even reducing, those of N2O. Slurry needs to be injected to depths which increase the diffusion path to the soil surface sufficiently to lead to the majority of denitrified N being emitted as di-nitrogen in order to avoid increasing emissions of N2O.