Evidence for large carbon sink and long residence time in semi-arid forests based on 15-year flux and inventory records

The rate of change in atmospheric CO2 is significantly affected by the terrestrial carbon sink, but the size and spatial distribution of this sink, and the extent to which it can be enhanced to mitigate climate change are highly uncertain. We combined carbon stock (CS) and eddy covariance (EC) flux measurements that were collected over a period of 15 years (2001-2016) in a 55-year-old 30 km2 pine forest growing at the semi-arid timberline (with no irrigating or fertilization). The objective was to constrain estimates of the carbon (C) storage potential in forest plantations in such semi-arid lands, which cover ~ 18% of the global land area. The forest accumulated 145-160 g C m-2 y-1 over the study period based on the EC and CS approaches, with a mean value of 152.5 ± 30.1 g C m-2 y-1 indicating 20% uncertainty in carbon uptake estimates. Current total stocks are estimated at 7,943 ± 323 g carbon m-2 and 372 g nitrogen m-2 . Carbon accumulated mostly in the soil (~71% and 29% for soil and standing biomass carbon, respectively) with long soil carbon turnover time (59 y). Regardless of un-expected disturbances beyond those already observed at the study site, the results support a considerable carbon sink potential in semi-arid soils and forest plantations, and imply that afforestation of even 10% of semi-arid land area under conditions similar to that of the study site, could sequester ~ 0.4 Pg C y-1 over several decades.