COS 269-1 - Microbial community dynamics in the rhizosphere enhance nitrogen fixation and forage quality in novel perennial intercrops in Rwanda

Abstract: In low fertility tropical agroecosystems, intercropping with perennial legumes has the potential to maximize biological nitrogen fixation (BNF) and improve non-legume nitrogen (N) uptake. However, the microbial interactions that facilitate the beneficial effects of intercropping in perennial systems remain largely uncharacterized, particularly in the tropics. In this study, we explored the contribution of root-associated bacteria and fungi to N content and the nutritional characteristics of perennial grasses (Cenchrus purpureus and Brachiaria cv. Mulato II) and an annual cereal (Zea mays) intercropped with a perennial legume (Desmodium distortum or Desmodium intortum). We also evaluated how intercropping with these three non-legumes impacted BNF in Desmodium. We hypothesized that 1) intercropping would enhance BNF in Desmodium relative to single-cropped stands, 2) the presence of legume roots would alter bacterial and fungal rhizosphere community structure in the non-legume companion plant, and 3) altered community structure would explain N content and tissue quality in both legumes and non-legumes.



To assess intercropping-related changes to rhizosphere community structure and plant nutritional quality, we collected samples from replicated trials in three locations in Rwanda between March 2021 and December 2021. Total genomic DNA was extracted from rhizosphere soil and used to sequence bacterial (16S) and fungal (ITS2) amplicons for community analysis. Plant leaf tissue was collected and analyzed for N and ¹⁵N on an EA-IRMS. Finally, we quantified protein, lignin, ADF, and NDF in leaf tissue using near infrared spectroscopy. BNF was calculated using the ¹⁵N natural abundance method. Intercropped Desmodium exhibited higher rates of BNF (p< 0.01) than single stands regardless of location or timepoint. We also identified bacterial and fungal OTUs that were significantly enriched in the rhizospheres of intercropped plants compared to single cropped plants. These OTUs were correlated with leaf tissue N and forage quality metrics (p< 0.01). Individual plant species responded differently to the presence of a legume intercrop in terms of rhizosphere community structure. For instance, bacterial beta diversity differed the most between single cropped and intercropped rhizospheres of annual maize (p< 0.05) in two locations. Ongoing analyses will attempt to disentangle causal relationships between intercropping, rhizosphere community structure, and the nutritional characteristics of plant leaf tissue. This study highlights the complex linkages between plant species diversity, rhizosphere microbial dynamics, and tradeoffs for plant biomass characteristics. Our results, while not definitive, lend support for belowground microbial processes that facilitate plant N uptake and enhanced nutritional quality.