Shifts in the structure and function of wheat root-associated bacterial communities in response to long-term nitrogen addition in an agricultural ecosystem

Abstract

Nitrogen (N) input in terrestrial ecosystems is increasing due to its deposition from anthropogenic activities, especially in agriculture, which can have pronounced effects on belowground microbial communities. However, our understanding of the long-term effects of N addition on microbial communities in the plant-soil system is still limited. Here, we used an Ion S5™ XL sequencing platform to explore the structural and functional characteristics of root-associated bacterial communities in winter wheat (Triticum aestivium) under long-term N addition. The N fertilization experiment involved five treatments (0, 90, 180, 270, and 360 kg N ha−1) in a farmland ecosystem over 14 years. Bacterial communities shifted significantly with increasing N level in three soil-root compartments (bulk soil, rhizosphere, and root interior) of wheat plants at two growth stages (jointing and filling). These shifts were associated with dynamic changes in soil properties, mainly pH, nitrate-N, and ammonium-N levels. There was a community shift between slow-growing oligotrophs and fast-grow copiotrophs with increasing N addition, indicating distinct survival strategies of bacteria across soil-root compartments and wheat growth stages. Potential biomarkers, including nitrifying, denitrifying, and N-fixing genera were identified to predict very high N concentration in each soil-root compartment. Significant variation was detected in the relative abundance of predicted functional genes related to N metabolism (denitrification, nitrification, and N fixation) across the three compartments, two growth stages, and five N levels. These results indicate that bacterial communities respond differentially to long-term high N input in the soil-root compartments of wheat plants during different growth stages.