Empirical studies show that nitrogen (N) addition often reduces microbial decomposition of soil organic matter (SOM) and carbon dioxide (CO2) production via microbial respiration. Although predictions from theoretical models support these findings, the mechanisms that drive this response remain unclear. To address this uncertainty, we sampled soils of three grassland sites in the U.S. Central Great Plains that each have received seven years of continuous experimental nutrient addition in the field. Nitrogen addition significantly decreased the decomposition rate of slowly cycling SOM and the cumulative carbon (C) respired per mass soil C. We evaluated whether this effect of N addition on microbial respiration resulted from: 1) increased microbial carbon use efficiency (CUE), 2) decreased microbial oxidative enzyme activity, or 3) decreased microbial biomass due to plant and/or soil mediated responses to N enrichment. In contrast to our hypotheses - as well as results from N addition studies in forest ecosystems and theoretical predictions - N did not increase microbial CUE or decrease microbial oxidative enzyme activity. Instead, reduced microbial biomass likely caused the decreased respiration in response to N enrichment. Identifying what factors drive this decreased microbial biomass response to N should be a priority for further inquiry.
Bibliographical noteFunding Information:
This work would not have been possible without the generous support of the Nutrient Network . Numerous individuals contributed to establishing and maintaining the NutNet experimental sites sampled in this study, including: Jean Knops at Cedar Point (Nebraska); Kim LaPierre at Konza Prairie (Kansas); and Dana Blumenthal, Cynthia Brown, and Julia Klein at Shortgrass Steppe (Colorado). Chris Buyarski, Lynn Hu, Joey Krenz, and Jacob Olbrich assisted in the lab. This work was supported by a National Science Foundation Graduate Research Fellowship (Grant No. 00039202 ) and a National Science Foundation Doctoral Dissertation Improvement Grant (Grant No. 1401082 ) to CER. The Nutrient Network has been supported by funding to Elizabeth Borer and Eric Seabloom from the National Science Foundation Research Coordination Network (NSF-DEB-1042132) and the Long Term Ecological Research programs (NSF-DEB-1234162 to Cedar Creek Long Term Ecological Research Program), as well as the University of Minnesota’s Institute on the Environment (DG-0001-13).
© 2016 Elsevier Ltd.
- Microbial biomass
- Microbial respiration
- Nutrient Network