Nitrogen limitation and acidification: Reconciling soil organic matter responses to fertilization through microbial physiology, soil mineralogy, and soil acidity

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Savannah R. Adkins, Department of Biology, Utah State University, LOGAN, UT, Bonnie G. Waring, Imperial College London, London, UT, United Kingdom, John M. Stark, Department of Biology, Utah State University, Logan, UT and Samantha Weintraub, INSTAAR and Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, CO

Background/Question/Methods
Soils are the largest terrestrial carbon (C) stock and therefore provide one of the most promising opportunities for enhanced C sequestration. Yet, global change factors, such as nitrogen (N) deposition, have uncertain implications for the terrestrial C sink. N fertilization experiments across independent sites indicate conflicting soil organic matter responses to N addition, ranging from soil C gains to soil C losses. Nitrogen added in excess is a probable cause of this variability, as it can lead to soil acidification. Therefore, there is a threshold at which N addition transitions from increasing soil C to causing soil C losses. The magnitude and direction at which N addition effects soil C stocks is dependent on interactions between microbial physiology, soil mineralogy, and soil acidity, which varies across ecosystems and is further complicated by opposing responses of separate soil C pools (particulate and mineral-protected organic matter). To better understand the mechanisms driving soil C pool responses to N deposition, we recorded soil C and N cycle changes in long term fertilization sites throughout North America and employed a lab incubation experiment to decouple the effects of N addition from acidification.
Results/Conclusions
Soils were collected from tundra, desert, shrub steppe, temperate forest and tropical forest ecosystems with mean annual temperature and precipitation ranging from -3 to 38 oC and 305 to 1500 mm, respectively. Preliminary results demonstrate that C mineralization was highest in control soils, lowest under N treatments, and intermediate under acid addition, although the magnitude at which nitrogen addition and acidification effect soil C is site specific. Because the magnitude of microbial responses to N fertilization was site dependent, other factors, such as soil mineralogy or microbial community composition, may influence soil C pool responses to N addition. Reconciling the conflicts between site-to-site soil C responses is vital to mitigating global change. This work will influence biogeochemical and terrestrial ecosystem models by increasing our ability to project C sequestration potentials, and through helping shape on the ground climate change mitigation strategies.