Slowing of nitrogen cycling and increasing nitrogen use efficiency following afforestation of semi-arid shrubland

Evidence for large carbon sink and long residence time in semiarid 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 semiarid timberline (with no irrigating or fertilization). The objective was to constrain estimates of the carbon (C) storage potential in forest plantations in such semiarid lands, which cover ~18% of the global land area. The forest accumulated 145-160 g C m^-2  year^-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  year^-1 indicating 20% uncertainty in carbon uptake estimates. Current total stocks are estimated at 7,943 ± 323 g C/m² and 372 g N/m² . Carbon accumulated mostly in the soil (~71% and 29% for soil and standing biomass carbon, respectively) with long soil carbon turnover time (59 years). Regardless of unexpected disturbances beyond those already observed at the study site, the results support a considerable carbon sink potential in semiarid soils and forest plantations, and imply that afforestation of even 10% of semiarid land area under conditions similar to that of the study site, could sequester ~0.4 Pg C/year over several decades.