Soil Organic Carbon Density And Its Influencing Factors In The Riparian Woodland Of The Lijiang River, Guangxi Province

Riparian woodlands have recently been recognized as important carbon storage regions with considerable potential to sequester carbon by the global warming. Understanding soil organic carbon (SOC) content in the riparian woodland and differences of SOC content between riparian woodlands and adjacent land use types has an important significance in effectively assessing the SOC storage of riparian woodlands and influences of riparian land-use change on SOC pool.In this study, the investigation focused on the riparian woodland of the Lijiang River, which is located in a typical karst geomorphology region. The goals of this study were to quantify the soil organic carbon density (SOCD) (0-20 cm) in riparian woodlands (N=54, average 1131.75±143.44 m2/samplings), and to reveal its spatial distribution characteristics (N=4, 3250.00±2217.36 m2/samplings), and to assess differences of SOCD between riparian woodlands and other adjacent land use types and influences of different factors (soil texture, soil litter, root biomass, human activities, et al.) on SOCD of the woodland.Based on soil surveys and laboratory analyses, the results showed that:(1) The dominated trees of the upper layer of the canopy in riparian woodlands of the Lijiang River (DBH>5 cm) were Pterocarya stenoptera, bamboo, camphor tree and Chinese tallow tree. The basal area of trees and shurbs in the riparian woodland was 25.88± 10.63 m2/ha (DBH±2.5 cm). The density of all trees and shurbs was 4662.36±3036.54 individuals/ha. The richness, Shannon diversity and evenness indexes of all trees and shurbs in the riparian woodland were 3.34±1.24,2.23±0.47 and 0.75±0.09, respectively. For difference types of riparian woodlands, the density and basal area of trees and shurbs in the middle layer of the canopy of bamboo woodlands was significantly lower than that in non-bamboo woodlands, by 87.61% and 86.42%, respectively (p<0.01). In the upper and middle layer of the canopy, the richness and Shannon diversity of all trees and shurbs in the bamboo woodland also were significantly lower than that in the non-bamboo woodland (p<0.01). Therefore, the diversity of trees and shurbs in the middle layer of the canopy of the bamboo woodland was significantly lower than that in the non-bamboo woodland.(2) The topsoil SOCD of riparian woodlands (0-20 cm) was 35.79±9.51 t/ha, and was significantly higher than that in adjacent farmlands (N=34) and grasslands (N=13) by 42.45% and 28.88%, respectively (p<0.01). While the SOCD in bamboo woodlands was significant lower than that in non-bamboo woodlands by 19.10%(p<0.05), and was only significantly higher than that in the orchard, by 40.45%(p=0.012). Therefore, the riparian woodland in the Lijiang River has more SOCD than that in adjacent farmlands and grasslands. The conversion of riparian woodlands nito farmlands and grassland can cause the loss of SOC, especially for the conversion of non-bamboo woodlands. And also, the planting of bamboo in the riparian could not promote the SOC sequestration.(3) The distribution of the SOCD in riparian woodlands (N=4) dispersed with an increase in the distance from the main river channels, and existed strong distribution pattern of spatial heterogeneity. For the different land use types, the high value of SOCD was concentrated in the riparian woodland. Therefore, compared to adjacent farmlands and grasslands, the riparian woodland has a higher SOCD.(4) The SOCD in riparian woodlands showed a significantly positive correlation with the silt (r= 0.55) and clay content (r= 0.57), but a significantly negative correlation with the sand content (r=-0.64) (p<0.01). In non-bamboo woodlands, significantly positive correlations between the SOCD and the soil litter (r=0.44) and soil root biomass (r=0.38) (p<0.05) were found. The SOCD in riparian digging sand lands was 3.49 t/ha (0-40 cm), and was lower than that in adjacent woodlands by 96.08%. Artificial digging of riparian sand can cause serious losses of SOC.In summary, the future woodland management of the Lijiang River riparian should pay more attention to protect and restoration riparian woodlands, and decrease the loss of SOC by deforestation for agricultural, economic and energy purposes. Meanwhile it should take some pertinence measures to promote the SOC sequestration in bamboo woodlands to construct a bamboo ecosystem with harmony in landscape, economic and ecology.