Evaluation of 11 terrestrial carbon-nitrogen cycle models against observations from two temperate Free-Air CO2 Enrichment studies - Zaehle,S; Medlyn,BE; De,Kauwe MG; Walker,AP; Dietze,MC; Hickler,T; Luo,Y; Wang,Y-P; El-Masri,B; Thornton,P; Jain,A; Wang,S; et al. (NEW PHYTOLOGIST, 2014)