title: Getting to the roots: Mechanisms of plant nitrogen and phosphorus acquisition in intercropping
Creators: name: Schwerdtner, Ulrike
Publisher: name: University of Bayreuth
Year: 2022
Subjects: subject: intercropping
subject: phosphorus
subject: nitrogen
subject: plant nutrient acquisition
subject: nutrient mobilization
subject: rhizosphere
subject: 550

description: The major challenge of agriculture is to increase food production while simultaneously reducing environmental impacts and resource use. Intercropping, i.e., the simultaneous cultivation of at least two plant species in close proximity, is expected to be a promising approach as it potentially produces higher yields (referred to as overyielding) on less land and with fewer resource inputs (e.g., fertilizers) compared to monocropping. Overyielding in intercropping is supposed to result from above- and belowground interspecific plant interactions comprising the “4C” of competition, compensation, complementarity, and cooperation (facilitation). Intercropping has also been shown to increase plant nutrient contents, although the underlying mechanisms of plant nutrient acquisition are still not fully understood. The present thesis investigated, therefore, how belowground mechanisms of plant nitrogen (N) and particularly phosphorus (P) acquisition contribute to maize overyielding in intercropping. To investigate the effects of intercropping on plant nutrition and productivity, four case studies were conducted combining different experimental setups and several species combinations with likely contrasting nutrient acquisition mechanisms. A two-year field experiment (Study I, with further explorations in Study II) was accompanied by three greenhouse experiments, of which one was conducted with soil from the field experiment (i.e., with various N and P sources; Study II) and two with mineral substrate (i.e., with defined N and P sources; Studies III and IV). In all experiments, maize (Zea mays L.) was cultivated as the main crop, while faba bean (Vicia faba L.), soy (Glycine max (L.) Merr.), blue lupin (Lupinus angustifolius L.), and white mustard (Sinapis alba L.) were cultivated as companion crops. The thesis showed that intercropping resulted in maize overyielding and enhanced maize N and P contents in the field, especially in soy/maize and lupin/maize intercropping, as compared to maize monocropping. Smaller but still positive intercropping effects on maize productivity were also found in faba bean/maize (when simultaneously sown in 2019) and mustard/maize intercropping. Maize overyielding was mainly caused by belowground interspecific interactions in legume/maize intercropping and by aboveground interspecific interactions in mustard/maize intercropping. Legumes enhanced maize N acquisition in intercropping due to their ability to symbiotically fix atmospheric N2 which was in part transferred to the maize plants, suggesting both N complementarity and N facilitation. Up to 20% of maize aboveground biomass N content was thus derived from legumes in the field. In addition, mustard slightly enhanced maize N acquisition in intercropping compared to monocropping, which was likely associated with compensation and/or complementarity. Further, the thesis showed that all companion species had generally higher P contents (per plant) and/or higher P concentrations (per gram biomass) than maize, indicating that they mobilized P from sparingly soluble sources more effectively than maize. The three legumes had high phosphomonoesterase activities in the rhizosphere and exuded high amounts of dissolved organic carbon (DOC). The legumes also exuded high amounts of low molecular weight organic acid anions (LMWOA) into the rhizosphere. Faba bean additionally decreased while mustard increased its rhizosphere pH. These changes in the companions’ rhizosphere likely mobilized P from organic (via high phosphomonoesterase activities and perhaps stimulation of microorganisms through DOC) and inorganic P sources (via rhizosphere pH changes and high LMWOA exudation). The large root lengths of faba bean, soy, and mustard probably promoted plant P uptake, at least once P was mobilized. Overall, the companion species used species-specific mechanisms of P mobilization, which were likely associated with P mining (exudation of P-mobilizing compounds), root foraging, and stimulation of beneficial microorganisms. In intercropping, these mechanisms were likely also beneficial for maize P acquisition due to P complementarity and P facilitation among the intercropped plant species. Moreover, the thesis showed for the first time that a high LMWOA concentration in the rhizosphere in intercropping is not only caused by high LMWOA release of the companion species but also by an increased LMWOA exudation of maize, at least when grown together with lupin. With this, the thesis challenges the common view that legume/cereal intercropping is advantageous over monocropping due to the high P mobilization capacity of legumes from which the cereals simply benefit. Hence, the finding that the presence of lupin affected the exudation of maize provides new insights into the mechanisms underlying P acquisition in intercropping. Taken together, the enhanced maize productivity in intercropping was likely the result of reduced competition for N and P due to the combined effects of compensatory, complementary, and facilitative plant interactions. Hence, intercropping with its positive effects on plant productivity and plant N and P acquisition is promising in achieving food sovereignty and reducing the reliance on industrial fertilizers like those derived from finite phosphate rock. Therefore, intercropping should be considered an integral part of an overall agricultural transformation to meet future needs while staying within humanities’ safe (and just) operating space.