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. Understanding future changes in the terrestrial carbon cycle is importantfor reliable projections of climate change and impacts on ecosystems. It iswell known that nitrogen (N) could limit plants' response to increasedatmospheric carbon dioxide and it is therefore important to include arepresentation of the N cycle in Earth system models. Here we present theimplementation of the terrestrial nitrogen cycle in the Joint UK LandEnvironment Simulator (JULES) – the land surface scheme of the UK EarthSystem Model (UKESM). Two configurations are discussed – the first one(JULES-CN) has a bulk soil biogeochemical model and the second one is adevelopment configuration that resolves the soil biogeochemistry withdepth (JULES-CNlayer). In JULES the nitrogen (N) cycle is basedon the existing carbon (C) cycle and represents all the key terrestrial Nprocesses in a parsimonious way. Biological N fixation is dependent on netprimary productivity, and N deposition is specified as an external input.Nitrogen leaves the vegetation and soil system via leaching and a bulk gasloss term. Nutrient limitation reduces carbon-use efficiency (CUE – ratioof net to gross primary productivity) and can slow soil decomposition. Weshow that ecosystem level N limitation of net primary productivity(quantified in the model by the ratio of the potential amount of C that canbe allocated to growth and spreading of the vegetation compared with theactual amount achieved in its natural state) falls at the lower end of theobservational estimates in forests (approximately 1.0 in the model comparedwith 1.01 to 1.38 in the observations). The model shows more N limitationin the tropical savanna and tundra biomes, consistent with the availableobservations. Simulated C and N pools and fluxes are comparable to thelimited available observations and model-derived estimates. Theintroduction of an N cycle improves the representation of interannualvariability of global net ecosystem exchange, which was more pronounced inthe C-cycle-only versions of JULES (JULES-C) than shown in estimates fromthe Global Carbon Project. It also reduces the present-day CUE from aglobal mean value of 0.45 for JULES-C to 0.41 for JULES-CN and 0.40 forJULES-CNlayer, all of which fall within the observational range.The N cycle also alters the response of the C fluxes over the 20thcentury and limits the CO2 fertilisation effect, such that thesimulated current-day land C sink is reduced by about 0.5 Pg C yr−1compared to the version with no N limitation. JULES-CNlayeradditionally improves the representation of soil biogeochemistry, includingturnover times in the northern high latitudes. The inclusion of aprognostic land N scheme marks a step forward in functionality and realismfor the JULES and UKESM models.

JULES-CN: a coupled terrestrial carbon–nitrogen scheme (JULES vn5.1)