Nitrogen recycling across a spectrum of fertilization strategies: an assessment in olive groves

Abstract

Under the Zero pollution action, there is a strong emphasis on promoting management practices that enhance nitrogen (N) cycling to decrease reliance on inorganic N fertilizers and mitigate potential environmental impacts. Olive (Olea europaea L.) groves constitute the dominant landscape of many areas of Southern Mediterranean as Andalusia (Spain). Different farming models coexist in the olive sector, but they had not been studied in the context of N cycle. By integrating field and model data, we analyzed N flows, N balance, levels of N cycling closure, and N footprints in olive groves employing various cultivation models. Farms were categorized on: Org (organic external fertilization, high level of biomass recycling, temporary spontaneous cover crops), Tra (organic external fertilization, low recycling, bare soil), Int (inorganic external fertilization, low recycling, bare soil) and IPr (inorganic or organic external fertilization, low recycling, temporary spontaneous cover crops). Tree N demand, averaging 55 kg ha-1 y-1, correlated with fruit production and canopy area, making them suitable proxies for calculating N demand without exhaustive sampling. The N balance was positive for all farming models, with the Org model showing a more neutral balance (+7.4 kg N ha-1 y-1) and the IPr olive farms showing surpluses (+40.2 kg ha-1 y-1). In Org farms, annual N inputs were the lowest and rely on natural entries like N fixation, while Cycling Index was significantly higher than those of other groups (between 1.45 and 1.81 times). This was attributed to N recirculation mechanisms such as cover crops, shredding tree pruning, and returning harvested N through composted olive mill pomace. Soil erosion, often overlooked, emerged as a major N outflow, especially in Org farms with the highest soil N content, resulting in the highest N losses per liter of oil. Per liter of olive oil, Trad, Ipr, and Int farms use between 1.9 and 2.2 times more anthropogenic N inputs than Org farms. Overall, our results highlight significant potential to enhance internal N cycling, increase self-sufficiency, and extend the lifespan of N in olive farming by implementing scalable management practices that promote nature-based processes.