Changes in microbial communities in soil treated with organic or conventional N sources

Despite the extensive use of N fertilizers in agricultural soils, we are yet to fully understand their impact on soil microbial communities that mediate important soil processes. A 3-yr field study was undertaken in Georgia, where sweet corn (Zea mays L.) was grown under conventional or organic systems. Nitrogen (N) was supplied with ammonium sulfate at 112 kg N ha^-1 (AS100) or 224 kg N ha^-1 (AS200) or a combination of poultry litter, cover crop, and blood meal at 112 kg N ha^-1 (PL100) or no N (control). Soil samples were collected from field plots to assess the impact of treatments on bacteria, fungi, and ammonia oxidizers using molecular methods that targeted 16S RNA, ITS2, and amoA genes, respectively. Treatment had significant impact on bacterial but not fungal composition. The AS200 significantly changed the relative abundances of Verrucomicrobia and Acidobacteria and decreased bacterial alpha diversity as compared with control. Beta-diversity analysis showed clear separation of microbial communities in AS200 vs. control and PL100. The abundance of ammonia-oxidizing bacteria (AOB) was more responsive to N input than ammonia oxidizing-archaea. It was also significantly and positively correlated with nitrification potential and soil nitrate with increasing N rates, indicating AOB's dominance in driving nitrification under high N input. Overall, the results indicated that AS200 changed bacterial composition and diversity, suggesting corresponding changes in soil processes related to N mineralization and nitrification. Understanding such changes in microbial communities can help us predict changes in soil processes to adopt sustainable management systems.