Irrigation but not fertilization increases yields and N2 fixation in organic faba bean in temperate climates

Understanding how to optimize water and nutrient management is crucial for improving crop productivity in organic farming systems. In this study, we examined the effects of irrigation and fertilization on yield and nitrogen fixation in organically managed faba bean crops in temperate climates in six field trials covering three sites and two years. Irrigated plots showed a 54 % yield increase and higher nitrogen fixation (up to 105 %), while fertilization with rock phosphate and potassium sulfate, with or without micronutrients, had no significant impact. Irrigation induced higher yields as well as a significant increase in pod number, grain count, and chlorophyll content of leaves, suggesting improved photosynthesis, flowering and pod filling. Despite low soil nutrient levels for P, Mo and B, mineral fertilization showed no effect on faba bean yields and nitrogen fixation, likely due to long-term organic fertilization with cattle manure. Our results also indicate that irrigation enhances farmgate nitrogen balance by increasing nitrogen fixation without depleting soil nitrogen reserves. These findings suggest that water, rather than nutrient supply, is essential for maintaining productivity and nitrogen fixation in organic faba bean cultivation also in temperate regions. Economically, however, irrigation proved cost-effective in only one of six trials, suggesting that irrigation rates need to be optimized.