Overyielding is accounted for partly by plasticity and dissimilarity of crop root traits in maize/legume intercropping systems

Positive biodiversity-productivity relationships have been found in
biodiversity field experiments of grassland, forestry, and other natural
terrestrial ecosystems, where diversity effects were separated by
complementarity (CE) and selection effects (SE). However, we know little
about how CE and SE are related to root traits and root dissimilarity. A
four-year field experiment was carried out with a split-plot design, where
main plot was four nitrogen (N) applications (N0, N1, N2, N3) and five
cropping systems (maize (Zea mays L.)/soybean (Glycine max L. Merrill.),
maize/peanut (Arachis hypogaea L.) intercropped and the corresponding
monocultures) with three replicates. Roots were sampled in the N0 and N2
treatments in two years. Intercropping effects were analyzed based on
grain yield for four years and roots were sampled down to 60 cm depth, and
analyzed with morphological parameters at different crop growth stages in
two years. Intercropping significantly increased grain yield and
aboveground biomass in both intercropping systems under all N treatments.
The partitioning of the net intercropping effects showed that yield
advantage in intercropping was due to a positive CE under the N0
treatment, and to a positive SE with N application. Maize showed greater
root morphological plasticity than the legumes did, with greater changes
in root length density (RLD), root weight density (RWD) and total root
surface (TS) in intercropping than in monoculture. Intercropped maize
occupied a larger soil space, while lateral RLD distribution of legumes
was decreased by maize. The RLD, RWD, and TS of intercropped maize were
constant or increased in later growth stages. SE showed a significantly
positive relationship with root dissimilarity. Principal component
analysis showed mean root depth and specific root length of legumes drove
the positive CE in the absence of N fertilization. Root dissimilarity
determined by maize explained the selection effects in overyielding.
Complementarity effects under N0 were closely associated with specific
root traits such as mean root depth and specific root length. Linking
changes of root traits with intercropping effects aboveground helps
understand yield advantages in diverse agroecosystem. In general, a cereal
species with strong phenotypic plasticity intercropped with a legume
species with strong physiological plasticity can maximize the yield
advantage of intercropping.