Diversifying maize rotation with industrial crops can improve maize yield, soil nutrient stocks, and nitrate leaching losses depending on time since adoption and crop species

Maize (Zea mays L.) is a major crop worldwide, commonly used as monoculture and with high nitrate leaching losses. Diversified maize rotations can improve the environmental problems of maize monoculture without reducing yields and soil nutrients. Previous research focused on maize-legume or maize-cereal rotations, with limited exploration of rotations with industrial crops for biorefining. Furthermore, long-term field trials are rare, hindering our understanding of the crop performance and nutrient dynamics over time. In a four-year rotation system of maize, hemp (Cannabis sativa L.), beet (Beta vulgaris L.), and triticale (Triticosecale) established in 2012 on a sandy soil in Denmark, quadruplicated for maize to appear each year, we examined dry matter yield, yield stability, biomass nitrogen (N) and biomass N stability of maize in rotation compared to monoculture across two rotation cycles. We also quantified nitrate leaching, soil carbon (C) and N stocks in the root zone of 0-100 cm. Moreover, the period between the main crops was covered with “secondary crops”- winter rye (Secale cereale L.), winter rape (Brassica napus L.), grass/clover (Festuca rubra L. – Trifolium repens L.). The results showed that in the initial four years, the aboveground biomass yield of maize in rotation (15.5 Mg ha^-1) was significantly lower (by 7%) than that in monoculture (16.6 Mg ha^-1), but this difference disappeared in the following four years (17 and 16.5 Mg ha^-1). The maize biomass N yield in rotation (194.5 kg ha^-1) was similar to that in monoculture (196.6 kg ha^-1) in the first cycle and was significantly higher (by 8%) in the second cycle (195.5 and 165.7 kg ha^-1). Nitrate leaching showed interannual variability affected by double-cropping, being almost halved by the diverse rotation compared to the monoculture at the start of the rotation, but increasing at the onset of the second cycle when the preceding winter rape did not survive in the winter. Also, winter rye following maize reduced nitrate leaching, except when the preceding secondary crop was grass-clover or poorly thriving winter rape. During the whole period, the rotation system can increase both soil C and N stocks. This study shows that several of the benefits of diverse crop rotations in comparison to monoculture require several years to take place, and that the management of the secondary crops is particularly vital for reducing nitrateleaching.