title: Warming and drought reduce temperature sensitivity of nitrogen transformations.
abstract: Shifts in nitrogen (N) mineralization and nitrification rates due to global changes can influence nutrient availability, which can affect terrestrial productivity and climate change feedbacks. While many single-factor studies have examined the effects of environmental changes on N mineralization and nitrification, few have examined these effects in a multifactor context or recorded how these effects vary seasonally. In an old-field ecosystem in Massachusetts, USA, we investigated the combined effects of four levels of warming (up to 4 °C) and three levels of precipitation (drought, ambient, and wet) on net N mineralization, net nitrification, and potential nitrification. We also examined the treatment effects on the temperature sensitivity of net N mineralization and net nitrification and on the ratio of C mineralization to net N mineralization. During winter, freeze-thaw events, snow depth, and soil freezing depth explained little of the variation in net nitrification and N mineralization rates among treatments. During two years of treatments, warming and altered precipitation rarely influenced the rates of N cycling, and there was no evidence of a seasonal pattern in the responses. In contrast, warming and drought dramatically decreased the apparent Q10 of net N mineralization and net nitrification, and the warming-induced decrease in apparent Q10 was more pronounced in ambient and wet treatments than the drought treatment. The ratio of C mineralization to net N mineralization varied over time and was sensitive to the interactive effects of warming and altered precipitation. Although many studies have found that warming tends to accelerate N cycling, our results suggest that warming can have little to no effect on N cycling in some ecosystems. Thus, ecosystem models that assume that warming will consistently increase N mineralization rates and inputs of plant-available N may overestimate the increase in terrestrial productivity and the magnitude of an important negative feedback to climate change.
authors.affiliation: Department of Forestry and Natural Resources, Purdue University, West Lafayette, IN 47907, USA. novema@gmail.com
authors.forename: Dolaporn S
authors.initials: DS
authors.lastname: Novem  Auyeung
authors.forename: Vidya
authors.initials: V
authors.lastname: Suseela
authors.forename: Jeffrey S
authors.initials: JS
authors.lastname: Dukes
mesh_terms: D055864:Droughts; D057232:Global Warming; D009584:Nitrogen / Q000737:chemistry*; D013696:Temperature
chemical_list: D009584:Nitrogen