Urea fertilization and grass species alter microbial nitrogen cycling capacity and activity in a C₄ native grassland

Soil microbial transformation of nitrogen (N) in nutrient-limited native C₄ grasslands can be affected by N fertilization rate and C₄ grass species. Here, we report in situ dynamics of the population size (gene copy abundances) and activity (transcript copy abundances) of five functional genes involved in soil N cycling (nifH, bacterial amoA, nirK, nirS, and nosZ) in a field experiment with two C₄ grass species (switchgrass (Panicum virgatum) and big bluestem (Andropogon gerardii)) under three N fertilization rates (0, 67, and 202 kg N ha^-1). Diazotroph (nifH) abundance and activity were not affected by N fertilization rate nor grass species. However, moderate and high N fertilization promoted population size and activity of ammonia oxidizing bacteria (AOB, quantified via amoA genes and transcripts) and nitrification potential. Moderate N fertilization increased abundances of nitrite-reducing bacterial genes (nirK and nirS) under switchgrass but decreased these genes under big bluestem. The activity of nitrous oxide reducing bacteria (nosZ transcripts) was also promoted by moderate N fertilization. In general, high N fertilization had a negative effect on N-cycling populations compared to moderate N addition. Compared to big bluestem, the soils planted with switchgrass had a greater population size of AOB and nitrite reducers. The significant interaction effects of sampling season, grass species, and N fertilization rate on N-cycling microbial community at genetic-level rather than transcriptional-level suggested the activity of N-cycling microbial communities may be driven by more complex environmental factors in native C₄ grass systems, such as climatic and edaphic factors.