Carbon (C) and nitrogen (N) are important elements of forest ecosystems, and quantifying C and N storage and distribution is a critical step for better understanding of C and N cycles in forest ecosystems. C and N pools in forests vary greatly, depending on the forest type, forest growth stage, and site quality. The mechanisms controlling the spatial and temporal variation of C and N accumulation in forest ecosystems, especially in the northwestern China, have been sporadically investigated in the past. In this paper, the content, storage and distribution of C and N were investigated in four http: / / www.ecologica.cn typical forests at the Liupan Mountains in Ningxia, northwestern China. The four forest types included Larix principis鄄 rupprechtii plantation, Pinus armandii secondary forest, Betula platyphylla secondary forest, and Prunus salicina shrubs. The concentrations of C and N were measured in different organs of the trees, and the contents of C and N in the vegetation layers of trees, shrubs, grasses, and layers of humus, and soil (0-100 cm depth) were estimated. Storage, composition, and distribution of C and N in these ecosystems were examined. The results showed that the concentrations of C did not significantly differ among the different organs and tree species, but the concentration of N varied. The highest N concentration was found in the leaves and the lowest in the trunk. The contents of C and N were higher in the above鄄ground biomass than in the below鄄ground biomass for both shrubs and grasses. The C contents decreased in the order of tree layer > shrub layer > grass layer, but N contents increased in the order of tree layer < shrub layer < grass layer in the studied forests. The C content was lower in the humus layer than in each of the vegetation layers, but the situation was opposite for the N content. The contents of both C and N decreased with increasing soil depth. The total C storage in the four forest ecosystems was in the following order: B. platyphylla secondary forest (640. 02 t / hm 2) > P. armandii secondary forest (450.98 t / hm 2) > L. principis鄄rupprechtii plantation (364.56 t / hm 2) > P. salicina shrub (196.55 t / hm 2). The total N storage decreased from 47.02 t / hm 2 in B. platyphylla secondary forest to 36.19 t / hm 2 in P. armandii secondary forest, 27.86 t / hm 2 in L. principis鄄rupprechtii plantation and 15.99 t / hm 2 in P. salicina shrub ecosystems in the study sites. The amount of C and N in the soil (0-100 cm depth) accounted for the major proportion of the total C and N storage in the studied forest ecosystems, with the range of 84.7%-93.9% for C and 98.1%-98.6% for N for the four forest types. On average, the C / N ratio in the soil and in the entire ecosystem was 12.48 and 13.52 in B. platyphylla secondary forest, 11.84 and 13.12 in L. principis鄄rupprechtii plantation, 11.70 and 12.29 in P. salicina shrub, and 10.76 and 12.56 in P. armandii secondary forest, respectively. In addition, the C / N ratio decreased gradually from the tree layer to shrub layer, grass la...
