Optimising irrigation scheduling for winter wheat-soybean relay intercropping in temperate Europe

Crop diversification can buffer climate extremes and support biodiversity and has been reported to meet both production and sustainability needs. In Germany, winter wheat is the primary crop, often grown in relatively simple rotations with other cereals, rapeseed, and maize. Intercropping practices such as winter wheat-soybean relay intercropping (RC) with high spatial-temporal differentiation have the potential to enhance sustainability. However, the establishment of soybeans into a winter cereal remains a challenge due to high competition for water indicating the need for supplementary irrigation for successful RC in dry regions. This study presents an irrigation scheduling framework for winter wheat-soybean RC in temperate regions. We calibrated and validated the process-based agroecosystem model MONICA using three-year field data from 2020 to 2023. The model accurately reproduced aboveground biomass (R2 = 0.88, n = 66) and grain yield for both crops in RC (R2 = 0.70, n = 16), with root mean square errors of 2091 kg ha−1 (biomass) and 372 kg ha−1 (grain yield). The validated model was used to test the effect of irrigation timing, dosage, and strategy on RC yield performance across two soil types (594 simulations in total). A sensitivity test of 20 mm irrigation increments across phenological stages showed that soybean grain yield was most responsive to irrigation at the first pod stage, resulting in intercropped soybean yield increases up to 47 % (+249 kg ha−1) compared to rain-fed. Yield gains from irrigation increment of 20 mm peaked at moderate application rates of 80 mm in sandy and 120 mm in loamy soils, then declined. Contrasting soil texture simulations indicated that loamy soils required higher irrigation volumes but achieved greater land-use efficiency. Land equivalent ratio (LER) reached 1.21 with cumulative seasonal total irrigation of 140 mm, while sandy soils benefited from low-volume, high-frequency irrigation (LER = 1.11 with 140 mm), eliminating 64 % of drought-induced yield loss versus 48 % in loamy soils. These results highlight the importance of tailored irrigation strategies based on soil texture and crop phenology, providing a practical foundation for supplementary irrigation in cereal-legume RC systems and supporting crop diversification in temperate climates.