Current year N fertilization rate, more than prior N history, dictates biomass and diversity of arbuscular mycorrhizal fungi in soil at maize reproduction

This study used a 10-year field study in Ontario, Canada, that investigated the impact of N fertilization rate on soil AMF biomass and community structure in a rainfed, tilled, maize monoculture system during R2/R3 stage of reproductive growth. Treatments included six historical or continuous (CON) N fertilization rates and 3 rates that were shocked up (SKU) to 188 kg N ha-1 or shocked down (SKD) to 0 kg N ha-1 every five years. As previously found, AMF biomass in soil was inversely related to current year N rate regardless of whether that rate was historical or shocked. Amplicon sequencing of the 18S rRNA gene (V9 region) identified 16 unique amplicon sequence variants (ASVs) belonging to six Glomeromycota genera in descending abundance: Septoglomus, Claroideoglomus, Rhizophagus, Paraglomus, Ambispora, and Glomus. Septoglomus, Rhizophagus and Paraglomus were more abundant under low CON (<57 kg N ha-1) while Claroideoglomus and Ambispora were more abundant under high CON (>115 kg N ha-1). Although the relative abundance of dominant AMF genera showed no consistent trend with CON fertilization rate distinct shifts within genera of specific ASV’s were noted among low CON, high CON and with N fertilization disturbance (i.e. shock). While AMF beta diversity responded to historical N rate (CON 0_0 N vs other N rates; P = 0.002), when N disturbance was included in the analysis, only current year N fertilization rate (P = 0.0003) was significant, with no interaction with historical rate (P = 0.0683). Thus, we conclude that current year N fertilization rate, regardless of whether historical or disturbed, was the main determinant of AMF biomass and community diversity. Our findings also demonstrate that under maize monoculture, zero to low N fertilization regimes, regardless of N history, supports higher AMF biomass in soil and a slightly more diverse and stable AMF community. However, this may come at the cost of maize productivity. Reducing N inputs for economic and environmental benefits and increasing cropping system complexity through rotations and/or cover crops may better balance the interplay between crop productivity, AMF’s contribution to plant nutrient acquisition and soil C sequestration.