Sugarcane Straw Removal and Irrigation System Shape Arbuscular Mycorrhizal Fungi and Rhizosphere Microorganisms in the Cerrado

Abstract

Removing sugarcane straw for second-generation bioenergy may compromise soil biological quality, but the effect is rarely evaluated together with the irrigation system that farms already operate. We assessed a 3 × 3 factorial combining straw removal time, that is no removal, one year and two consecutive years, with reel, centre pivot and rainfed systems in commercial fields of the Brazilian Cerrado. Arbuscular mycorrhizal fungi were characterized morphologically, and six beneficial taxa and three soil-borne pathogens were quantified by real-time PCR. Mycorrhizal responses depended on straw removal alone, with no effect of irrigation and no interaction. Spore density and family richness fell to a minimum after one year and recovered by the second year to values indistinguishable from undisturbed soil, whereas root colonization did not change. Evenness declined only after two removals, as Acaulosporaceae rose disproportionately and came to dominate the community. Molecular targets behaved differently: most were governed by interactions between straw management and irrigation, so that a single removal effect cannot be stated for them. Pseudomonas fluorescens was the exception, disappearing wherever straw was removed regardless of system. The ratio between beneficial organisms and pathogens declined after two removals across all systems. Sporulation, colonization and spore-derived richness therefore diverge, and none of them alone reports soil biological quality reliably.