Soil microbial community structure and function in non-target and plant-influenced soils respond similarly to nitrogen enrichment

Plants rely on soil microbes, particularly those in their rhizosphere to access resources; however, these relationships are altered following disturbance, including nutrient enrichment. Plants also contribute to variation in resource availability by redirecting exudates as conditions change, but the ability to do this varies with species identity. In this study we compared the activity and composition of soil communities following nitrogen fertilization (10 g⁻¹ m⁻²) beneath plants in general (H1) and between Ratibida pinnata and Schizachyrium scoparium specifically (H2). We expected that the microbial structure and function would reflect the relatively C-rich environment of root rhizospheres under control conditions, but that N fertilization would homogenize microbial community composition and activity. Although several variables responded to either fertilization or plant input, we found few interactions between sample location and fertilization, which would indicate support for our hypotheses. For H1, which compared fertilization effects between bulk soil and plants generally, fertilization increased β-1,4-N acetylglucosaminidase activity in the plant-influenced soils, indicating that these rhizosphere microbes had reduced availability of labile carbon plant exudates compared with unfertilized plant rhizospheres. Furthermore, the higher ratio of Gram-positive to Gram-negative bacteria found in the unfertilized non-target condition suggests that the combination of low nitrogen and carbon resources of the bulk soil was uniquely stressful compared to other conditions. For H2, which compared the two plant species following fertilization, we found a reduced PLFA Metabolic Stress Index in the unfertilized rhizosphere of R. pinnata, which indicates a greater influx of labile carbon to these microbes. R. pinnata also maintained its relative cover with fertilization, indicating flexibility in reallocating resources, while relative cover of S. scoparium decreased. These plant-soil interactions occur within small volumes of soil yet scale to affect regional and global biogeochemical cycles and biodiversity. Although we found limited support for our hypotheses it is critical that we continue to study these processes to understand changes to our environment.