In Northwestern China, the carbon fixing capacity of black locust (Robinia pseudoacacia) has been questioned because of its slow growth following the return of unproductive farmland to forest. To explore the effects of stand age on the carbon sequestration potential of R. pseudoacacia in a semi-arid, ecologically fragile area, parameters related to carbon fixation were investigated in plots of three stand ages (5, 10, and 25 years). Each plot was divided into four subsystems: R. pseudoacacia, understory vegetation, litter, and soil, and the carbon stored capacity of each subsystem was estimated. The organic carbon density of R. pseudoacacia, understory vegetation, and litter ranged from 3.4-16.8% and increased gradually with increasing stand age. Soil organic carbon increased with increasing stand age and accounted for 83.2-96.6% of the total carbon stored. Soil CaCO 3 content also increased with increasing soil depth and stand age. Because total plant and soil carbon storage increased with increasing age of R. pseudoacacia stands, the 25-year-old R. pseudoacacia community had the highest carbon fixation capacity, which was substantial even in this arid region. 116Vol. 61, 2015, No. 3: 116-121 Plant Soil Environ.doi: 10.17221/931/2014-PSE Premature senescence in R. pseudoacacia resulting from severe soil moisture deficit is of particular concern, as this species is the most widely used for afforestation. In addition, there are few data on the relationship between carbon fixation and stand age or on the carbon fixation capacity of man-made forests in the Loess Plateau. In this study, R. pseudoacacia plots in stands of three ages (5, 10, and 25 years) were selected in the Zhifanggou valley and each plot was divided into four subsystems: R. pseudoacacia, understory vegetation, litter, and soil. Our aim was to compare carbon storage in the plots and to estimate the carbon fixation capacity of R. pseudoacacia forests that were established on retired farmland. We also examined how carbon sequestration in R. pseudoacacia forests on arid retired farmland varies with stand age. MATERIAL AND METHODSStudy site. The study was conducted in the Zhifanggou valley (36°43'34''N, 109°15'28''E) in the Ansai county, Shaanxi province, in the Loess Plateau hinterland, China. Mean annual temperature and precipitation are 9.4°C and 465.2 mm, respectively. R. pseudoacacia was planted as the major forest type on gently sloped retired farmland in 1984, 1999, and 2004.Biomass measurement. Plots with the same aspect and slope containing 5, 10, and 25-yearold R. pseudoacacia trees were selected for biomass investigation. Each plot contained a grid of R. pseudoacacia with 21 columns and 24 rows. The columns and rows were spaced 3 m apart; tree density was 0.145 individuals/m 2 . In each plot, three 10 m × 10 m quadrats were established, and the height and diameter at breast height (DBH) of each R. pseudoacacia tree was recorded and used to calculate average DBH and tree height. These averages were used to calculate R. pseudoacacia ...