Interactions between soil properties, fungal communities, the soybean cyst nematode, and crop yield under continuous corn and soybean monoculture

Corn (Zea mays) and soybean (Glycine max) production forms an integral part of economies worldwide, but yields are limited by biotic and abiotic factors associated with short rotations and long-term monocultures. The objectives of this study were (i) to investigate the role of corn-soybean crop rotations and continuous monocultures in shaping bulk soil fungal communities, ii) to identify fungal taxa or functional guilds correlated with SCN density, and (iii) to characterize relationships between biotic and abiotic factors and their effects on corn and soybean yields. The study utilized a long-term rotation site with corn and soybean planted in annual rotation, five-year rotation, and long-term monoculture. High throughput sequencing of the ITS1 region of fungal rDNA revealed that soil fungal community structure varied significantly by crop sequence, with fungal communities under five consecutive years of monoculture becoming progressively similar to corresponding communities in long-term monoculture plots. Total fungal alpha diversity was greater under corn, but patterns of diversity and relative abundance of specific fungal functional guilds differed by crop, with more nematophagous fungi proliferating under soybean and more arbuscular mycorrhizal fungi (AMF) proliferating under corn. The relative abundance of nematode-trapping fungi and several putative nematode egg parasites was positively correlated with SCN density at several time points, suggesting that these fungi may proliferate as a result of the availability of the SCN as a nutrition source. Soil properties also varied by crop sequence, with higher pH and P under continuous soybean and higher Fe, Mn, and Cu under continuous corn. Lower levels of P corresponded with the relative abundance of several orders of fungi with roles in P uptake and transfer to plants (Glomerales, Paraglomerales, and Sebacinales), while higher P levels corresponded with the relative abundance of Mortierellales, a fungal order containing phosphate-solubilizing fungi. Structural equation modeling identified the SCN and soil nitrogen as the most important variables explaining soybean yield and fungal pathogens of corn and soil nitrogen as the most important variables explaining corn yield.