CLIMMANI/INTERFACE Joint Workshop: Nutrient Constraints on the Net Carbon Balance; Keflavik, Iceland, 15-17 June 2011 A challenge to improving predictions of the consequences of climate change is bringing empiricists and theoreticians together to work toward common goals. Critical to this challenge is ensuring that novel, process-based information is incorporated into the land surface portion of Earth system models and, at the same time, that field-based experiments are designed to explore model weaknesses. The Integrated Network for Terrestrial Ecosystem Research on Feedbacks to the Atmosphere and Climate (INTERFACE) and Climate Change: Manipulation Experiments in Terrestrial Ecosystems (ClimMani) joint meeting in Iceland fostered interdisciplinary interaction among land modelers, biogeochemists, and ecologists while focusing on nutrient constraints on the net carbon budget. Discussions at the meeting aimed to identify the status of current knowledge, major knowledge gaps, and the steps needed to move forward.