Influence of soil management and previous crop nitrogen fertilization on alfalfa productivity in Mediterranean agroecosystems

Intensive maize ( Zea mays L.) cultivation in Mediterranean irrigated systems has led to soil degradation and excessive nitrogen (N) accumulation, increasing the risk of environmental contamination. Alfalfa ( Medicago sativa L.), a perennial legume with low input requirements, offers a sustainable alternative that can improve soil structure and reduce residual N. However, limited research has assessed its establishment under different tillage systems and its capacity for biological N fixation in Mediterranean conditions. This study aimed to evaluate the impact of transitioning from intensive maize cultivation to alfalfa under different tillage systems. Specifically, we hypothesized that (i) No-tillage enhances alfalfa establishment and yield, (ii) biological N fixation is higher under no-tillage, and (iii) lower residual N levels from maize would favors biological N fixation and alfalfa productivity. A three-year field experiment (2019–2021) was conducted in northeastern Spain using a randomized block design with two factors: tillage system (conventional tillage, CT, vs. no-tillage, NT) and the residual soil nitrogen levels resulting from fertilization of the preceding maize crop (0, 200, and 400 kg N ha⁻¹). Alfalfa biomass yield was measured at each harvest. Soil mineral N and moisture were monitored, and biological N fixation was quantified using the natural ¹ ⁵N abundance method. Soil structure was assessed through macroaggregate stability analysis. NT significantly improved initial alfalfa establishment, resulting in a 32 % higher biomass yield in the first year (3190 vs. 2415 kg ha⁻¹ under NT and CT, respectively). However, by the second year, yield differences between tillage systems disappeared. Biological N fixation was higher in NT during the first year (111 vs. 77 kg N ha⁻¹ in CT) but equalized across treatments over time. Residual N fertilization delayed nodulation and biological N fixation, leading to lower yields in fertilized plots by the end of the first year. The duration of the experiment was sufficient to improve soil macroaggregate stability under CT, reaching values similar to those observed in NT by the end of the study. NT accelerated alfalfa establishment and enhanced biological N fixation in early growth stages, while alfalfa itself restored soil properties over time, reducing differences between tillage systems. Excessive residual N from maize hindered biological N fixation and negatively impacted alfalfa yields. This study underscores the benefits of integrating no-tillage alfalfa into Mediterranean irrigated systems as a sustainable strategy to improve soil health, optimize N use, and enhance long-term productivity in intensive crop rotations. • Alfalfa enhances sustainability in intensive crop rotations by optimizing soil properties. • No-tillage improved initial alfalfa establishment and first-year biomass yield. • Alfalfa accelerated soil structure recovery, reducing differences between tillage systems. • Residual N from the preceding crop delayed biological N fixation and reduced yields.