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More than 56000 check dams have been built in the Loess Plateau, which capture around 0.95 Pg of organic carbon and act as an important carbon sink. However, the decomposition mechanism of organic carbon in the sediment in these dams is still poorly understood, and thus it is difficult to quantify their role in terrestrial carbon cycling. In this study, the mineralization culture was used as a simulated environment for the natural sediment environment. With the observations in the simulated environment, the decomposition rates of sediment organic carbon (SOC) were compared under different conditions to investigate the factors influencing the decomposition rate of SOC. The results showed that the average SOC decomposition rate of sediment under anoxic and aerobic conditions was (6.47±4.06) and (56.66±17.78) mg C·kg -1 ·d -1 , respectively. The decomposition rate of SOC in dam sedi-ment under burial conditions was only 11.4% of that under the assumed aerobic condition, indicating that burial condition significantly reduced SOC decomposition. Under anoxic conditions, chemical compositions in the sediment had a greater effect on the decomposition rate of SOC than the microorga-nisms. In contrast, the effect of microorganisms on the decomposition rate of SOC was more significant under aerobic conditions. The physical properties of sediment had little effect on the decomposition rate of SOC under both anoxic and aerobic conditions. Under natural conditions, the siltation dam acted as a carbon sink. When the dam breaks, SOC stored in the sedimentary anoxic condition would be quickly exposed to the air, followed by a significant increase in the decomposition rate, and thus acting as a carbon source.

Understanding the relationship between soil erosion and the changes in landscape patterns is important for guiding the management and development of watersheds. The Nangou watershed in Ansai County, Shaanxi on the Loess Plateau, is an area with the implement of "Grain for Green", ecological agriculture, ecological tourism and the demonstration of science and technology for landscape management. We quantified the spatial and temporal variations of landscape pattern and soil erosion from 1981 to 2018 using the GIS and the universal soil loss equation (USLE). The relationship between the soil erosion modulus and nine landscape pattern indices in three categories was analyzed using the principal component analysis at both plot and landscape levels. The results showed that, among the five landscape types, the spatial and temporal changes of cultivated land and woodland dominated the evolution of landscape patterns, which affected the concentration and distribution degree of the whole watershed. Soil erosion in the Nangou watershed decreased annually, with erosion area, erosion modulus and soil erosion intensity decreasing by 29.7%, 61.2%, and 73.4% from 1981 to 2018, respectively. The variation of cultivated land and forest land areas determined the changes of soil erosion modulus of the whole watershed. The change trend of landscape pattern index was consistent with that of soil erosion. "Grain for Green" Project was the major dri-ving force for the changes in the landscape pattern and for the reduction of soil erosion. The characteristic development and management could weaken soil erosion intensity in parts of the study area. The rational configuration of landscape types could effectively control soil erosion in a watershed. The combination of rational configuration and characteristic management could help achieve the goals for sustainable and high quality development of the watershed.

In order to understand the spatial variation of heavy metals in Shaanxi section of Weihe River, the contents and sources of Cd, Co, Cu, Pb, Sb, As, Ni, Cr, Zn, Mn in sediments at 17 sampling sites in Shaanxi section of Weihe River and its tributaries were analyzed. The results showed that the average contents of Cd, Co, Cu, Pb, Sb, As, Ni, Cr, Zn, Mn were 0.10, 1.24, 11.73, 11.95, 24.90, 24.91, 29.31, 54.18, 72.74, 626.85 mg·kg -1 , respectively. Except for that of Cd being greater than 1, the coefficient of variation for each of other heavy metals was less than 0.5. The peak contents of Cd, Pb and Cr were found at the sampling site of Bahe River, of Co and Mn at Heihe River, of Cu and Zn at Qingjiang River, and of Sb, As and Ni at Shawang Ferry, Xianyang Railway Bridge and Linjia Village, respectively. According to the results of correlation analysis, principal component analysis and cluster analysis, the elements of Cd, Co, Cu, Pb, Ni, Cr, Zn and Mn mainly originated from industrial and domestic sources, but Sb and As mainly originated from agricultural and geochemical sources.

Soil erosion in the ecologically fragile zone is severely affected by the intensified coal mining activities in the Yushenfu Mining Region of the Loess Plateau. Soil erosion controlling projects such as the "Grain for Green" program are continuously implemented by the government, which has effectively mitigated or even controlled soil erosion. Coal mining results in new environmental problems, which has changed the erosion characteristics and sediment transport in watersheds. In this study, we identified the sediment source trapped behind the check dam using a composite fingerprinting approach and clarified the impact of "Grain for Green" program and coal mining on variation of the amount and sources of sediment during two decades, in a representative check-dam which controlled the watershed of Laoyeman. The two decades were divided into two stages, i.e. an earlier decade (1990-1999) and a later decade (2000-2010) with respect to the implement of the "Grain for Green" program in the flood couplets sequence established during the running of the dam. The annual average thickness of flood couplets during 2000-2010 decreased from (42.7±29.9) cm to (16.6±13.7) cm during 1990-1999, and the annual sediment deposition per unit area (square meter) at the sampling site reduced from 579.9 kg·m -2 to 245.8 kg·m -2 . The main source of sediment was gully wall (averaged 79.2%±10.1%) during the study period. However, compared with sediment sources during 1990-1999, the contribution of sediment from slope during 2000-2010, which was greatly affected by the "Grain for Green" program, showed an increasing trend (average annual contribution increased by 7.4%). According to the coaling mining and the variation of rainfall erosivity, our results indicated that under the combined effects of the "Grain for Green" program and decreases of rainfall erosivity, the intensity of sediment production in the watershed was substantially decreased. Coal mining had important impacts on sediment contribution from the slope, and obviously affected soil erosion controlling effects of the "Grain for Green" program. It is obvious that coal mining has increased the risk of erosion and sediment production on the slope, and that soil erosion control on the slope should focus on areas affected by coal mining.

To effectively utilize the vegetation on grain-for-green slopes in the wind-water erosion crisscross region, it is necessary to determine the reasonable vegetation utilization intensity. We set up runoff plots on slopes which were not cultivated and were closed for many years in the Liudaogou catchment, a typical catchment in wind-water erosion crisscross region of the Loess Plateau. With artificial simulated rainfall experiments, the characteristics of runoff and sediment yield on slopes (10°, 20° and 30°) under different utilization intensity of vegetation were studied to select the reasonable utilization intensity. The results showed that the runoff rate in the process of rainfall simulation could be divided into two periods: rapidly increasing in the initial period and slow increasing or quasi-steady state in the middle and late periods. The variation of erosion rate during the rainfall was dependent on the slope. The utilization intensity had a significant effect on the runoff yield, which increased with the increasing utilization intensities. The slope gradient had a significant effect on the sediment yield, with the variation of sediment yield with slope gradient being: 20°>30°>10°. Compared with the unused (natural) plots, the relative runoff and sediment increased with increasing utilization intensities. Predicted based on the rainfall data, annual soil erosion amount on the slope would be basically lower than the tolerance level of soil loss when the vegetation cover on the slope surface reached 25% in 15 years after abandoning reclamation. More attention should be paid to the restoration and management of vegetation on the slope of 20° in this area.