Improved genotypes and fertilizers, not fallow duration, increase cassava yields without compromising arbuscular mycorrhizal fungus richness or diversity

Abstract

Arbuscular mycorrhizal fungi (AMF) are ubiquitous in agroecosystems, but their role in mediatingagricultural yield remains contested. Field experiments testing effects of realistic agronomic practices ofintensification on AM fungus composition and yields are scarce, especially in the low-input systems ofsub-Saharan Africa. A large, full-factorial field experiment was conducted in South-Kivu (DR Congo),testing effects of fallow duration (6 vs. 12 months), genotype (landrace vs. improved), and fertilizermanagement (control vs. five combinations omitting N, P, K, and/or secondary macro- andmicronutrients) on yields of cassava, an important staple crop strongly colonized by AMF. Furthermore,we used DNA-metabarcoding to evaluate effects of these agronomic practices on the AM fungalcommunities on the roots. The shorter fallow duration strongly increased diversity and richness of AMF,but this did not correspond with increased yields. Cassava yield was mainly determined by genotype,being largest for the improved genotype, which coincided with a significantly higher sum of AM fungalsequences. Effects of fertilizer or genotype on community composition were minor to absent. We foundno evidence that increased AMF richness and diversity enhanced cassava yields. In contrast, the use ofthe improved genotype and mineral fertilizers strongly benefitted yields, without compromising richnessor diversity of AMF. Cassava-AMF associations in this work appear robust to fertilizer amendments andmodern genotype improvement.