Faba bean introduction makes protein production less dependent on nitrogen fertilization in Mediterranean no-till systems

Context: Under Mediterranean rainfed areas, no-till cereal-based systems have been adopted to cope with water availability and increasing input costs. However, the increased risk of biotic stresses, high N-fertilizer dependence, and current EU policies warrant cropping systems re-design. Objective: Evaluate diversification and N fertilization as strategies to improve N use efficiency at the cropping system level and quantify its productivity. Methods: Four crop sequences combined with four levels of N fertilization were assessed in a three-year field experiment in semiarid rainfed north-eastern Spain. Crop sequences were continuous winter wheat (WCS) and three-year diversified rotations with pea (PCS), faba bean (FCS), or a multi-service cover crop (MSCS) and two years of cereals. Crop, pre-crop and cropping system levels were considered. Agronomic evaluation included crops above-ground biological N fixation (Ndfa), net N balance (Ndfa minus N removed by grain), soil N mineralisation productivity, energy to N tradeoff (ENT), and N use efficiency of protein (NUEp) production. Results: Pea yields ranged from 0 to 766 kg ha-1 and Ndfa from 24% to 54%. Faba bean yield ranged from 1378 to 4251 kg ha-1 and Ndfa from 32% to 72%. Net N balance was close to neutral for pea while in faba bean it ranged from 41 to -21 kg N ha-1. Alternative pre-crops led to greater soil N mineralisation (51 kg N ha-1, on average) and higher wheat yield (564 kg ha-1, on average) compared to wheat as the pre-crop. N fertilization increased protein yields, with FCS presenting the highest yields at all N fertilizer rates. This effect led to a stable NUEp (1.69 kg protein kg N supply-1), as the protein yield increased proportionally to N supply. Conclusions: Diversification improved the succeeding wheat performance and grain legumes N fixation exceeded grain N removal. Introducing legumes into cropping systems led to a decrease in energy productivity compared to the cereal-based system. However, protein production in the FCS was higher than in any other cropping system regardless of the N fertilizer rate. Implications or significance: Crop diversification adds challenges and risks in dry Mediterranean areas. However, the study shows that crop diversification with faba bean can decrease cropping system's N-fertilizer dependence and increase protein productivity, contributing to cropping systems' sustainability.