Balancing cover crop benefits and economic realities in Mediterranean rice farming

Abstract

Cover crops (CC) have the potential to reduce the dependency of rice (Oryza sativa L.) production on chemical N fertilizers and the associated environmental and economic risks. While extensively studied in tropical and subtropical systems, their potential in Mediterranean lowlands remains underexplored. This study evaluated whether CC could reduce chemical N fertilization in a Mediterranean rice system (Ebro Delta, Spain). Rice was grown during three seasons (2020–2023) preceded by a bare fallow, hairy vetch (Vicia villosa Roth), or ryegrass (Lolium multiflorum Lam.) during the winter period, and with 4 different fertilizer N rates (0, 120, 180, and 240 kg N ha⁻¹) under a split-plot design. Besides testing rice productivity, we developed a simple but effective indicator to assess the economic impact of cover crops by adapting the marginal net return of grain production to include cover cropping costs. Differences in biomass accumulation between the CC species were variable across years, with ryegrass being more dependent on precipitation, but also negatively affected by the N fertilization for the rice from the previous season. Due to its sole reliance on soil N uptake, ryegrass never produced more biomass N than hairy vetch. Rice yields were 13% lower and N use efficiency 16% lower after ryegrass than after bare fallow across fertilization levels. Hairy vetch tended to improve rice development (i.e., NDVI and panicle density) but did not translate into higher yields or N use efficiency compared to bare fallowing, potentially due to inhibited N mineralization under anaerobic conditions in flooded environments. Consequently, CC implementation significantly reduced profitability, showing how economic incentives are needed to encourage adoption. Implementing hairy vetch as CC increased the soil total N by 10%, potentially benefiting long-term rice production. These findings offer practical insights for agronomists and policymakers focused on enhancing the sustainability of Mediterranean rice systems.