Water Chemistry and Microbial Nitrogen Cycling in Wild Rice Wetlands

Abstract

Wild rice (Zizania palustris), a native emergent aquatic plant, has various ecological functions and high cultural and economic values across eastern and north-central North America. Due to environmental contaminants and disruptions of habitat, the abundance and distribution of wild rice has been declining over decades. Possible impediments such as water level fluctuations, changing geochemistry, and herbivory could be reasons for declining wild rice stands. Recent collaborative restoration efforts have resulted in improved wildlife habitat and increased opportunities for wild rice harvest but have shown varied success in the reestablishment of self-sustaining beds. Soil-plant-microbe interactions may be important determinants in wild rice allocations but remain underexplored in wild rice wetlands. Particularly, prokaryotic communities involved in processing key limiting nutrients, such as nitrogen, presumably influence the re-establishment and perpetuation of wild rice populations in estuaries and other wetlands. This study aims to examine the different forms of nitrogen assimilated and cycled by wild rice. Water, sediment, and plant samples were collected throughout the wild rice life stages from self-sustaining and restored wild rice wetlands and lakes in northern Minnesota. Biophysical parameters including surface/porewater chemistry and nutrients were analyzed and prokaryotic communities were assessed with high-throughput DNA sequencing analysis. Additionally, the prokaryotic communities were further evaluated using Functional Annotation of Prokaryotic Taxa, a nitrogen function gene library, and quantitative Polymerase Chain Reaction. Water chemistry characteristics, particularly total nitrogen, total phosphorus, and conductivity were strongly correlated with wild rice density, indicating certain water chemistry is conducive for wild rice populations. Self-sustaining sites had higher concentrations of total nitrogen, ammonia, and nitrate when compared to restored sites, but all sites fell within the range of oligotrophic lakes and rivers/streams in Minnesota. Wild rice-associated bacterial and archaeal communities were distinct from those found in the water compared to those in the roots and sediment. Nitrogen fixation and nitrate respiration had the highest total reads of operational taxonomic units for all the nitrogen metabolic functions. The N library indicated there are more nitrogen fixers in the sediment and roots than in the water samples. Most of the genera influencing the high abundance of nitrogen fixation are archaeal methanogens. Wild rice roots at self-sustaining sites had higher abundances of nitrogen cycling functional genes. These findings have given us more insight into how carbon and nitrogen play a crucial role in the fitness and population of wild rice and provides us with better restoration strategies for the future.