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

<p class="MsoNormal"><span>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 (NO₃^-) leaching. Cover crops may reduce some of these N losses and recycle the N back into the soil after incorporation, but the net effect on the N balance depends on the cover crop species. In a one-year field study, we tested the effects of two application methods (i.e., surface broadcasting, BDC; and shallow injection, INJ) of the liquid fraction of separated co-digested cattle slurry (DLF), combined with different winter cover crop options (CCs, i.e., rye, white mustard or bare fallow), as starter fertilizer for maize. Later in the season, side-dressing with urea was required to fulfill 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 NO₃^- leaching. Cover crop development and biomass production were strongly affected by their contrasting frost tolerance, with spring-regrowth for rye, while mustard was winter killed. After the cover crops, injection of DLF increased N₂O emissions significantly compared with BDC (emission factor of 2.69 <em>vs.</em> 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 NO₃^- leaching, <span>being 51% and 64% lower for mustard and rye than under bare soil. In addition, rye reduced </span>NO₃^- leaching during spring and summer after termination by promoting N immobilization, thus leading to -57% lower annual leaching losses compared to mustard.<span> Our study confirms the potential of CCs to reduce </span>NO₃^-<span> leaching, but it also highlights that their residue can increase N losses once terminated and that they may cause yield reductions. 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.</span></span></p>