The interaction between types of cover crop residue and digestate application methods affects ammonia volatilization during maize cropping season

Organic and mineral fertilizers are important sources of ammonia (NH₃ ) emissions from agricultural fields. The objectives of this study were (a) to evaluate how different cover crop (CC) residues (i.e., rye [Secale cereale L.], white mustard [Sinapis alba L.], and bare soil as control) in combination with different application methods of digestate (surface broadcast vs. shallow injection) affect NH₃ volatilization before planting maize (Zea mays L.) and (b) to assess the residual effect of previous CCs on NH₃ volatilization after urea top-dress application at the V5-V6 phonological stage of maize. Ammonia volatilization was measured using semi-static chambers for 14 d (335 h) after planting and for 6 d (150 h) at the V5-V6 stage. Overall, NH₃ emissions decreased by 67-77% with digestate injection compared with surface broadcasting. However, the reduction in NH₃ volatilization using the injection method was significantly lower with mustard residue (6.72 kg NH₃ -N ha^-1 ) than with rye residue (14.15 kg NH₃ -N ha^-1 ), which allowed for more volatilization by increasing the exposure of digestate to the air. Broadcast digestate method did not affect the cumulative NH₃ -N losses obtained with different CC types. After urea top-dressing at the V5-V6 stage of maize, the cumulative losses of NH₃ (during 150 h) were 2.99 kg NH₃ -N ha^-1 with rye as previous CC and 2.49 kg NH₃ -N ha^-1 with mustard. Our study shows that digestate injection before maize planting and urea top-dressing application followed immediately by irrigation (15 mm) could be considered as useful strategies to mitigate NH₃ volatilization and increase N use efficiency in maize.