Digestate as a Driver of Nitrogen Action: Nutrient Economy of Winter Oilseed Rape in the Seed-Filling Period

Digestate, a by-product of methane fermentation in agricultural biogas plants, is a valuable source of nitrogen (D-N). The mode of action of this specific (mineral/organic) N carrier has not yet been elucidated. Strategies for D-N yield-forming action in winter oilseed rape (WOSR) were determined based on a three-year series of field experiments conducted between 2016 and 2018. The field experiment was arranged as a two-factorial design. The first factor was the fertilization system (FS), based on two N carriers: (i) ammonium nitrate, 34-0-0 (acronym AN-FS, and (ii) digestate (D-FS). The N doses were: 0 (absolute N control), 60, 120, 180, and 240 kg ha-1. Nutrient: (i) mass, (ii) balance, and (iii) remobilization quantity were determined at two stages defining the seed-filling period (S-FP): the onset of flowering and physiological maturity of the rapeseed. These traits and developed indices were used to predict seed yield (SEY), the number of seeds in the pods of lateral branches (SeSB) per m², and N mass at the onset of flowering (NAF) and at physiological maturity (NAH). The type of N source, averaged over N doses and years did not differentiate yield (≈2.75 t ha-1). The main reason was the lack of differences in the primary component of the yield structure, SeSB per m² (R² = 0.81. The reason for the lower yield efficiency of the ammonium nitrate (AN-FS) compared to the D-FS was the higher unit productivity of the N accumulated by the crop at the onset of flowering (NAF). This difference amounted to approximately 20% in favor of the digestate system (15.8 vs. 18.9 kg of seeds per kg of N). The WOSR production strategy based on the use of AN, which involved saturating the rapeseed canopy with N during the onset of flowering, proved ineffective for yield formation. The higher yield-forming efficiency of D-FS resulted from the higher efficiency of the canopy N resulted from both the interaction between N and K (remobilization) and net soil uptake of phosphorus (P), as well as micronutrients, including zinc (Zn) and copper (Cu) during S-FP. It is important to highlight the ameliorative effect of digestate regarding drought. Indeed, at the onset of flowering, increasing N rates in the digestate stabilized the Ca mass in the rapeseed canopy, whereas AN caused a progressive increase. Studies have shown that digestate can be considered a significant element of sustainable winter rapeseed production.