Abstract
Nitrogen (N) enrichment is an element of global change that could influence the growth and abundance of many organisms. In this meta-analysis, I synthesized responses of microbial biomass to N additions in 82 published field studies. I hypothesized that the biomass of fungi, bacteria or the microbial community as a whole would be altered under N additions. I also predicted that changes in biomass would parallel changes in soil CO2 emissions. Microbial biomass declined 15% on average under N fertilization, but fungi and bacteria were not significantly altered in studies that examined each group separately. Moreover, declines in abundance of microbes and fungi were more evident in studies of longer durations and with higher total amounts of N added. In addition, responses of microbial biomass to N fertilization were significantly correlated with responses of soil CO2 emissions. There were no significant effects of biomes, fertilizer types, ambient N deposition rates or methods of measuring biomass. Altogether, these results suggest that N enrichment could reduce microbial biomass in many ecosystems, with corresponding declines in soil CO2 emissions.
Highlights
Microbes control atmospheric concentrations of a number of important greenhouse gases (Conrad 1996)
Studies that measured microbial biomass via chloroform fumigation were associated with a significant decline of 18%
Other studies have reported declines in the abundance of actinomycetes (Siguenza et al 2006), ectomycorrhizal fungi (Lilleskov et al 2001; Nilsson et al 2007) and certain gram-negative bacteria (Nilsson et al 2007) along increasing gradients of ambient N deposition. These findings indicate that microbial biomass can be affected by the current levels of anthropogenic N deposition when analyses are constrained to a smaller geographic scale
Summary
Microbes control atmospheric concentrations of a number of important greenhouse gases (Conrad 1996). I used the ÔNitrogen deposition onto the United States and Western EuropeÕ dataset (Holland et al 2005) to estimate background rates of wet N deposition for applicable studies This dataset includes 0.5 · 0.5 degree resolution maps of wet NH4+ and NO3) deposition, as interpolated from a network of in situ measurements collected in 1978–1994. I was able to include North American or Western European studies for which latitude and longitude, or sufficient information to determine latitude and longitude, were reported For each of these sites, I summed the wet deposition rates for NH4+ and NO3) that were reported for the corresponding latitude ⁄ longitude grid cell (Appendix S1).
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