Variable response of inorganic carbon and consistent increase of organic carbon as a consequence of afforestation in areas with semiarid soils

Abstract Soil inorganic carbon (SIC) is the most common form of carbon in arid and semiarid regions, where afforestation is used to combat soil degradation. However, response of SIC to afforestation has been studied rarely, which hinders any prediction about soil carbon dynamics. To determine the response of SIC and soil organic carbon (SOC) to afforestation in semiarid soils and whether such responses vary with soil aggregates, soil texture, and climatic conditions, we measured SIC and SOC in bulk soils and aggregate fractions under paired afforested lands and farmlands at five sites across the Loess Plateau. The relationship of these responses to soils and climatic variables was analyzed. Afforestation increased SOC at all sites, with accumulation rates of 0.21, 0.23, 0.34, and 0.09 g kg −1 yr −1 in bulk soils, macroaggregates (>0.25 mm), microaggregates (0.25–0.053 mm), and silt + clay fraction, respectively, when averaged across sites and soil depths. However, the response of SIC to afforestation varied with site, with increased SIC in drier sites and decreased SIC in wetter sites. Change in SIC was negatively correlated with change in SOC either across or within bulk soils and aggregate fractions. The SIC decreased and SOC increased with increasing soil clay content, precipitation, temperature, and site aridity index. These results indicated consistent increase in SOC but variable response of SIC to afforestation among sites in the Loess Plateau. Such variable response of SIC should be incorporated into carbon cycle models to reduce uncertainties when predicting sequestration dynamics of soil carbon.