Towards efficient N cycling in intensive maize: role of cover crops and application methods of digestate liquid fraction

Digestate, a by-product of biogas production, is widely recognized as a promising renewable nitrogen (N) source with high potential to replace synthetic fertilizers. Yet, inefficient digestate use can lead to pollutant N losses as ammonia (NH3) volatilization, nitrous oxide (N2O) emissions and nitrate (NO - 3) leaching. Cover crops (CCs) may reduce some of these losses and recycle the N back into the soil after incorporation, but the effect on the N balance depends on the CC species. In a one--year field study, we tested two application methods (i.e., surface broadcasting, BDC; and shallow injection, INJ) of the liquid fraction of separated co-digested cattle slurry (digestate liquid fraction [DLF]), combined with different winter cover crop (CC) options (i.e., rye, white mustard or bare fallow), as starter fertilizer for maize. Later, side--dressing with urea was required to fulfil maize N--requirements. We tested treatment effects on yield, N--uptake, N--use efficiency parameters, and N--losses in the form of N2O emissions and NO - 3 leaching. CC development and biomass production were strongly affected by their contrasting frost tolerance, with spring--regrowth for rye, while mustard was winter killed. After the CCs, injection of DLF increased N2O emissions significantly compared with BDC (emission factor of 2.69% vs. 1.66%). Nitrous oxide emissions accounted for a small part (11%-13%) of the overall yield-scaled N losses (0.46-0.97 kg N Mg grain-1). The adoption of CCs reduced fall NO - 3 leaching, being 51% and 64% lower for mustard and rye than under bare soil. In addition, rye reduced NO - 3 leaching during spring and summer after termination by promoting N immobilization, thus leading to -57% lower annual leaching losses compared with mustard. DLF application method modified N-loss pathways, but not the cumulative yield-scaled N losses. Overall, these insights contribute to inform an evidence-based design of cropping systems in which nutrients are recycled more efficiently.

Automated summary

Digestate, a by-product of biogas production, has the potential to replace synthetic fertilizers as a renewable nitrogen source. However, the inefficient use of digestate can lead to nitrogen losses through ammonia volatilization, nitrous oxide emissions, and nitrate leaching. In this study, the effects of different application methods and winter cover crop options on maize production and nitrogen losses were tested.