Nitric oxide and greenhouse gases emissions following the application of different cattle slurry particle size fractions to soil

The application to soil of different slurry particle size fractions may lead to variable gaseous soil emissions and associated differential environmental impacts. An incubation experiment was carried out during 70 d to assess the influence on nitric oxide (NO) and greenhouse gas (GHG; i.e. nitrous oxide, carbon dioxide and methane) emissions following incorporation of 4 particle size fractions, obtained through laboratorial separation from cattle slurry, to agricultural sandy loam soil (Dystric Cambisol). The response to these applied slurry fractions (>2000 μm, 2000–500 μm, 500–100 μm, <100 μm) was compared to other experimental treatments, including whole slurry (WS), ammonium sulphate (AS) and an unamended control (CON). The highest value of cumulated NO emissions (6.3 mg NO–N kg−1 dry soil) were observed from the AS treatment. The cumulated amount of NO emitted (∼1 mg NO–N kg−1 dry soil) was not significantly different between slurry fractions, thereby indicating that slurry particle size had no effect on NO emissions. The largest slurry fraction (>2000 μm) induced significantly higher N2O emissions (1.8 mg N2O–N kg−1 dry soil) compared to the other smaller sized fractions (1.0 mg N2O–N kg−1 dry soil). The >2000 μm, fraction, being more than 55% of the slurry by weight, was the major contributor to daily and cumulative N2O emissions. Hence, for N2O, the application of WS to agricultural soil is a better option that amendment with the >2000 μm, fraction. Low CH4 emissions (<200 μg CH4–C kg−1 dry soil d−1) were observed, but only in treatments amended with slurry or its fractions. The CH4 emissions were short-lived and rates returned to control levels within 3 d after the slurry application. Higher CO2 emissions were observed in soils amended with slurry fractions when compared to application with whole slurry. Clearly, slurry separation can increase soil CO2 emissions relative to whole slurry application.Overall, N2O contributed 10–30% to total GHG emissions, while that of methane was negligible. The present study suggested that mechanical separation of slurry into fractions and targeted application of the finest fractions to soil is a potential suitable management tool to reduce GHG emissions. However, the largest fractions have to be used for other purposes as anaerobic digestion rather than applied to soil.

► Slurry particle size has no effect on NO emissions following soil application. ► Slurry separation does not increase NO emissions after soil application. ► The grossest slurry fraction is the main contributor to N2O emissions when whole slurry is applied to soil. ► CO2 emissions should not be disregarded since it is still the major contributor to GHG emissions.