Fig. 2: Overall effect, variable importance, and the effect of two most important factors determining the legume pre-crop effect on main crop yield.

a Histogram of natural log response ratios (lnRRs). b Importance of factors in determining the yield benefit of the legume-based cropping system. c The yield effect of legume pre-crop decreased significantly with increasing N fertilizer rate. d The yield effect of legume pre-crop decreased significantly with increasing crop diversity. Crop diversity is defined as the number of crop species × number of crop functional groups × number of crop species per year. The dashed line, solid red line, and the shaded area in a represent lnRR = 0, the mean, and the standard errors of the mean on a natural log scale. The numbers denote the total observations with the number of studies in parentheses. The relative importance in b is quantified based on a meta-forest model. MAT, MAP, SOC, and STN stands for mean annual temperature, mean annual precipitation, initial soil organic carbon and total nitrogen content in topsoil, respectively. Linear mixed-effects model fit tests used Satterthwaite approximations for denominator degrees of freedom. The effects are quantified as the percentage changes in legume- versus non-legume-based cropping systems. Colored lines in c, d represent the average N fertilizer rate- and diversity-specific responses, respectively, with their bootstrapped 95% confidence intervals indicated by shading.