Nitrogen Inhibits Microbial Respiration in Long-Term Nitrogen and Phosphorus Addition Experiments across North-American Grasslands.

Abstract:

Grasslands contain a globally significant amount of carbon (C) stored as soil organic matter (SOM). Therefore, understanding how anthropogenic nitrogen (N) and phosphorus (P) deposition impact SOM decomposition in grasslands is critical to modeling the global C cycle. Previous work indicates that N addition may reduce microbial respiration from grassland soils by reducing microbial biomass; however, whether this effect generalizes across diverse grasslands and over decadal timescales remains unclear. We hypothesized that 1) N addition would decrease microbial respiration, 2) P addition would increase microbial respiration, and 3) N and P effects at different sites would depend on soil texture, plant biomass, and climate. We collected soils from nine grasslands in North America that received factorial N and P additions for ten years as a part of the Nutrient Network. We analyzed the soils for microbial respiration, biomass, and potential enzyme activity in the laboratory. After ten years of nutrient addition, N reduced microbial respiration in all but one site. This reduction in overall respiration was associated with an inhibition of late-stage SOM decomposition, and a stimulation of early-stage decomposition. Suppression of late-stage decomposition was associated with a reduction in microbial biomass under N addition, possibly due to N-derived soil acidification. N effects on respiration were not associated with changes in microbial enzyme activity. Meanwhile, P addition also reduced microbial biomass, but not respiration. Also contrary to expectations, the strength of the N effect on respiration across sites was unrelated to soil texture, plant community measures, or climate variables, but was more pronounced at sites with higher soil C. Overall, this study demonstrates that long-term anthropogenic N deposition, through effects on microbial biomass, has the capacity to reduce CO₂ flux to the atmosphere from diverse grasslands, depending on the existing soil C content.