Variable root:shoot ratios and plant nitrogen concentrations discourage using just aboveground biomass to select legume service crops

Comparing different winter legume species by plant traits that determine their contributions to ecosystems as service crops. Since root biomass determines soil organic matter formation and biological nitrogen fixation (BNF) increases soil organic nitrogen stock, those plant traits are more suitable than shoot biomass to select service crops. We evaluated shoot and root biomass and BNF (using 15N natural abundance method) of 10 winter legume service crop species under field conditions, in two consecutive years. The legumes significantly varied in shoot (3.4 to 9.1 Mg ha−1 year−1) and root biomass (0.8 to 1.3 Mg ha−1 year−1) and in the proportion and the amount of biologically fixed nitrogen (36–61% and 43–121 kg of N ha−1, respectively). Whereas shoot biomass varied between years, root and BNF were rather constant determining a low correlation between the variables. Among the evaluated species, Trifolium pratense and T. alexandrinum were the top-ranked species for the evaluated conditions, showing the largest amount of root biomass (~ 1.3 Mg ha−1 year−1) and BNF (~ 112 kg of N ha−1 year−1). Selecting service crops species aimed to increase soil organic matter based only on shoot biomass may be insufficient. As a consequence, we highlight the importance of performing breeding programs aimed to increase, in addition to crop yield or shoot biomass production, other plant traits such as root biomass or biological nitrogen fixation that provide key regulating and supporting ecosystem services.