Fig. 1. Effect of topsoil depth on grain yields of maize and soybean in 2016, 2017 and 2018. The main effects of topsoil depth, crop species and cropping year, and their interactions are all highly significant (P < 0.001). Values with means ± standard error (n=3). Within a growing season of each panel, values with a same letter are not significantly different (P > 0.05).