Observed and modelled carbon fluxes at a grazed site with CO2 enrichment

New Zealand maintained a free-air CO₂ enrichment (FACE) facility from 1997 to 2020, with CO₂ concentration enriched by about 100 μmol mol^-1. It was unique by including regular sheep grazing. Here, we compared observed pasture productivity and soil organic carbon (SOC) with simulations by the CenW and APSIM models. On average, the pasture was observed to produce about 8 tDM ha^-1 yr^-1 grazed-biomass with variations from 4 to 12 tDM ha^-1 yr^-1 between years with different weather patterns. Seasonally, it varied between 10 kgDM ha^-1 d^-1 in winter and 45 kgDM ha^-1 d^-1 in spring. Summer productivity was sometimes even higher when sufficient rainfall was received. Simulations by both models conformed well to the observations, with average Nash-Sutcliffe model efficiencies for annual productivity of 0.53 for CenW and 0.48 for APSIM, and a model efficiency of 0.89 for seasonal productivity. Observed biomass productivity increased by about 10 % under CO₂ enrichment, with greater enhancement of 18 % simulated by the models, but the differences between observed and modelled enhancements were not statistically significant. There was large observed and modelled seasonal and interannual variability in the apparent enhancement without any consistent patterns. Both models showed slight increases in SOC under CO₂ enrichment by 1-3 tC ha^-1 over 23 years while our measurements to a depth of 25 cm showed no significant differences. We also conducted a scenario analysis of potential changes of growth and SOC under elevated CO₂ with adjusted grazing frequencies to match feed availability. It showed greater pasture productivity under increasing CO₂ that led to more frequent grazing and capture of a greater fraction of fixed carbon in grazing offtakes so that SOC remained nearly constant, or even decreased. These results, therefore, suggested that in grazed pasture systems, elevated CO₂ could enhance agricultural productivity with minimal SOC impacts.