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.