Legacy effects of nitrogen pollution conversions modify community assembly and catastrophic shifts in riverine microorganisms

Nitrogen pollution frequency and severity are likely going to increase in many watersheds of the world. Recent evidence indicates that chronic pollution perturbation may produce legacy effects on sediment microbes. However, it is unclear whether the conversion of nitrogen pollution patterns lead to legacy effects on microbes, i.e., the capacity of past pollution patterns to influence current ecosystem response trajectories. Here, we focus on utilizing bioinformatics and hydrological modeling to parse specific nitrogen pollution patterns and distinguish certain conversions of pollution patterns between wet and dry seasons. We show that the varying degree of legacy effects induced by different types of pollution pattern conversions increases the discrepancy of microbial communities in the intra-, inter- diversity, and the microbial community assembly processes. Alternative stable states exist and legacy effects cause communities to be transient to alternative stable states on varying degrees under different types of pollution conversions. Our results indicate that legacy effects also change the microbial nitrogen-cycling functional traits, with consequences for river processes.