Soil respiration and response of carbon source changes to vegetation restoration in the Loess Plateau, China.

Soil respiration is a large carbon flux from terrestrial ecosystems to the atmosphere, and small variations in soil respiration can prominently influence the global carbon (C) cycle. The vegetation changes could directly affect soil respiration. The large-scale "Grain for Green" project carried out on the Loess Plateau, China has importantly affected the contribution of soil respiration to atmospheric carbon dioxide (CO2). Therefore, it is important to study the effects of vegetation restoration on soil respiration. We selected four land-use types: crop, forest, shrub, and grassland in the Zhifanggou watershed to analyze variation in soil respiration during dry and rainy seasons. Furthermore, the source of CO2 emissions from soil respiration was identified using isotopes. The results showed that soil respiration in the rainy season was significantly higher than that in the dry season (P < .05). Soil respiration in the dry season was as follows: shrubland (1.04 μmol m-2 s-1) > cropland (0.72 μmol m-2 s-1) > forestland (0.44 μmol m-2 s-1) > grassland (0.33 μmol m-2 s-1). However, grass and forestland had significantly higher soil respiration than shrub and cropland in the rainy season (P < .05). Roots were the main source of soil respiration in cropland, which contributed >70% of CO2 emissions. Following revegetation, litter contributed more to soil respiration than roots or soil microorganisms at >68% of soil respiration. Our results provide a theoretical basis for assessing C balance in terrestrial ecosystems.