A win-win situation – Increasing protein production and reducing synthetic N fertilizer use by integrating soybean into irrigated Mediterranean cropping systems

Over the last decades, non-cereal crops have been displaced in European cropping systems leading to a significant dependency on imported soybean. Continuous maize cropping under Mediterranean irrigated conditions can lead to agronomic and environmental problems. The objective of this work was to assess diversified Mediterranean irrigated cropping systems to maximize protein production while reducing synthetic N fertilizer use. A field experiment was carried out from 2019 to 2021 in an irrigated area in NE Spain. Four cropping systems, (i) continuous maize (MM), (ii) soybean in a rotation one out of three years (MSrt), (iii) barley-maize double cropping system (BM), and (iv) barley-soybean double cropping system (BS) were assessed at the crop, pre-crop and cropping system level. Productivity in terms of grain, energy and protein yield was measured at the crop and calculated for the cropping system level. As well, synthetic N fertilizer use efficiency was calculated for each cropping system. At the pre-crop level, soybean introduction led to a 28% yield increase in the following cereal (maize or barley) mainly due to the residual N effect. At the cropping system level, soybean in rotation (MSrt) did not lead to a significant increase in total protein production compared to MM (from 895 to 947 kg ha−1 yr−1), but it mildly increased synthetic N fertilizer use efficiency. Protein production in the BS system (1778 kg protein ha−1 yr−1) was significantly higher than in all other cropping systems (990 kg protein ha−1 yr−1 on average). As well, BS was the cropping system with the highest synthetic N fertilizer use efficiency compared to the other cropping systems (251 and 88 kg grain kg synthetic N fertilizer−1). Our results demonstrate that introducing soybean as a double crop following barley is a successful strategy to reduce environmental impacts resulting from N fertilizer use and increase protein production, contributing to plant protein self-sufficiency and cropping systems diversification.