Environmental drivers of soil carbon and nitrogen accumulation in global drylands

We are far from understanding the spatial patterns of dryland soil carbon and nitrogen stocks and how they vary among different land cover types. We used data from 12,000 sites from 129 countries in global drylands to estimate soil organic carbon (SOC) and total nitrogen (STN) stocks in different land cover types, explore the factors driving their spatial distribution, and predict the trends under different climate scenarios in global drylands. SOC and STN stocks in the upper 100 cm reached 419.5 and 38.2 Pg, respectively, with the upper 0-30 cm accounting for half of them. The largest SOC stocks were found in forests, shrublands and grasslands, while STN stocks peaked in forests, bare areas and croplands. The factors driving the spatial patterns of SOC and STN varied among soil depths, with mean annual temperature, pH and aridity being the main factors driving the spatial patterns in SOC and STN density for 0-30 cm, and soil texture the strongest factor for 60-100 cm. Under the Representative Concentration Pathways (RCP) 4.5 scenario, SOC and STN stocks were predicted to decrease by 3.6 % and 4.0 %, respectively, from 2020 to 2100, whereas under the RCP 8.5 scenario, the projected decreases were 5.9 % and 6.4 % respectively. Our results indicate that if we want to accurately predict C and N accumulation, and design effective mitigation measures in terrestrial ecosystems under future climatic scenarios, we need to better explore the drivers that operate at the deeper soil depths, which also accumulate a significant amount of SOC and STN.