The roots of the rotation effect run deep

It is well-established that maize ( Zea mays L.) grown in extended rotations produces higher grain yields compared to maize grown in one- or two-phase rotations, even when nitrogen (N) is not limiting. Understanding the mechanisms driving this phenomenon, commonly referred to as ‘the rotation effect’, is important for designing cropping systems that use land and other resources efficiently. Differences in root systems can influence crop resource acquisition and therefore yield, but it is unknown if such differences play a role in the rotation effect. We hypothesized that maize grown in an extended rotation system exhibits a deeper root structure with less root mass compared to maize grown in a short rotation, and that these characteristics are correlated with differences in grain production. Using a long-term experiment established in 2001, we measured maize rooting depth across the growing season, root mass in 15 cm increments from 0 to 60 cm, and grain yields in the maize phase of two contrasting rotations: a 2-year rotation of maize/soybean ( Glycine max [L.] Merr) using inorganic sources of nitrogen (N) and maximum tillage depths of 15 cm (hereafter the ‘short rotation’), and a 4-year rotation of maize/soybean/oat ( Avena sativa L.)-alfalfa ( Medicago sativa L.)/alfalfa using a mix of organic and inorganic N sources and periodic inversion tillage to 25 cm (hereafter the ‘extended rotation’). Additionally, we measured soil penetration resistance and soil moisture, and performed a growth analysis on aboveground maize biomass. From 2013 to 2020, maize grain yields in the extended rotation were equal to or significantly higher than in the short rotation, averaging 8 % greater across eight years (11.0 and 10.2 dry Mg ha −1 , respectively). The timing (e.g., early season, late season) of the extended rotation’s maize growth advantage was not consistent across years, but in all three seasons of root measurements (2019–2021) the maximum rooting depth of maize in the extended rotation was significantly deeper than in the short rotation by an average of 11 % (82 versus 76 cm, respectively). At physiological maturity, the two systems had similar amounts of root mass from 0 to 60 cm soil depth, but maize grown in the extended rotation invested significantly less of that mass (30 % compared to 47 %) into the soil surface layer (0 to 15 cm). The soil penetration resistances of the two systems differed in a manner consistent with the differing tillage regimes of the two rotations, however the patterns did not align with root differences. We posit that the extended rotation’s ‘deeper and steeper’ maize root patterns did not guarantee higher maize yields, but rather bestowed the plant with more flexibility in resource acquisition which, in certain conditions, resulted in higher grain yields compared to maize grown in the short rotation. To our knowledge, this is the first report attempting to mechanistically link rooting patterns with plant growth in the context of the ‘rotation effect.’ This study enhances our understanding of how cropping system histories impact yields, and provides new data on yields and roots, both of which are highly relevant for sustainable intensification. While the present study focused on physical measurements, it suggests that more detailed exploration of how biological drivers impact root architecture is needed to gain a mechanistic understanding of the ‘rotation effect.’ • Maize grain yields in the four-year rotation were 8 % higher than in the two-year. • Maize roots extended 11 % deeper in the four-year compared with the two-year. • Maize had less shallow (0–15 cm) root biomass in the four-year compared with the two-year. • Four-year rotation maize roots were ‘deeper and steeper’ compared with the two-year. • Differences were not explained by physical soil measurements.