Ecologist U.S. Environmental Protection Agency Research Triangle Park, North Carolina, United States
Abstract: The views expressed are those of the authors and do not necessarily reflect the views or policies of the USEPA.
Atmospheric nitrogen (N) deposition from fossil fuel combustion, land-use change, and fertilizer application has been linked to significant changes in ecosystems. Anthropogenic N addition to terrestrial ecosystems has led to modifications in soil nutrient cycling processes and shifts in soil microbial community structure. Nitrogen deposition and climate change impact ecosystems simultaneously; therefore, understanding how climate mediates microbial community response to N deposition is necessary for informing critical load thresholds important for management and policy decisions. First, we synthesized recently published meta-analyses to determine whether microbial communities naturally occurring in more extreme climatic conditions (e.g., low precipitation, high temperature) were more sensitive to N deposition than communities under milder environmental conditions. We then searched the literature for studies which manipulated N deposition and at least one climate factor (e.g., increased temperature, reduced/supplemental precipitation, or elevated CO2) to determine whether experimental manipulations of climate provide consistent results with observational approaches. We used a multivariate, hierarchical Bayesian meta-analysis approach to examine whether microbial abundances and diversity responses to N deposition were modified under changes in temperature, precipitation, or elevated CO2.
We extracted ~3600 data points on microbial abundance and diversity indices from 65 studies published between 1990 and 2022. In a preliminary analysis, we found that across all studies, experimental N addition resulted in an increase in overall archaeal and fungal abundance, while bacterial abundance decreased under N addition. These shifts in abundance resulted in an increase in fungi to bacteria (F:B) ratio under N deposition. In contrast, previous meta-analyses have reported greater negative effects of N addition on fungal abundances relative to bacterial abundances, consequently leading to a negative effect on F:B ratio. Secondly, we found that on average, climate manipulations slightly influenced microbial community response to N deposition. For example, communities experiencing both an increase in temperature and N addition were less likely to exhibit significant changes in bacterial abundance, fungal abundance, and subsequently F:B ratio than communities receiving supplemental N alone. Interestingly, supplemental precipitation led to a slight increase in both fungal and bacterial abundances relative to controls, but when supplemental precipitation was combined with N deposition, the microbial community composition was similar to control communities. We will continue examining the relative effects of climate, length of experiment, and ecosystem characteristics (MAT, MAP) on microbial community response to N deposition.
