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Climate models and satellite remote sensing have been used to determine the effects of afforestation on soil temperature at regional and global scales. However, the coarse spatial resolution of both methods have made them insensitive to local topography, which is a controlling factor of the surface energy budget, especially in areas with complex topography. While typically applied only locally at representative sites, long-term field measurements can shed light on the role of topography on soil temperature after afforestation and provide direct evidence to verify and calibrate the results from modeling and satellite observation. In this study, a pair of neighboring catchments (one afforested and the other with natural regrowth of grasses) on the Chinese Loess Plateau (CLP) was selected to assess the effects of afforestation on soil and ambient air temperature at different slope positions with an entire year of continuous measurements collected every 10 min. The results showed that the uphill slope in both catchments experienced higher soil and air temperatures than the downhill gully, where less solar radiation was received at the ground surface due to canopy interception and topographic shading. For example, the annual average soil (10-100 cm depth) and air temperature in the uphill slope in the forestland catchment was 1.09 and 1.22 degrees C higher than in the downhill gully, respectively. The effects of topography on soil and air temperature varied at different times of the year, and these effects interacted with the growth status of vegetation. In winter and spring, topography significantly affected soil and air temperature due to varied solar radiation received at different slope positions. By contrast, in summer, the effect of vegetation increased along with higher evapotranspiration and more solar radiation interception by the canopy, especially in the forestland catchment, which showed cooler soils than the other catchment during both the days and nights at all investigated slope positions. However, compared with the grassland catchment, the uphill slope of the forestland catchment had cooler air temperatures during the day, but warmer temperatures at night. We have concluded that both topography and afforestation influence the spatial variation and temporal dynamics of soil and air temperature, and they thereby the surface energy balance of the CLP. More local studies are warranted in order to continue to calibrate regional models and remote sensing data.

A megaproject, “Gully Land Consolidation” (GLC), was launched on the Chinese Loess Plateau in 2011 to combat land degradation and create new farmlands. However, the newly-claimed farmlands have suffered soil salinization, and the mechanism of salinization has been inadequately addressed. In this study, a creek valley that completed the GLC project in 2014 was selected and a total of 17 new farmlands along the reshaped creek valley were investigated in 2016. Moreover, the hillslope farmlands without conducting the GLC project, the loess parent material that used to fill the stream channel, and the water samples from reservoirs, irrigation wells and spring outlets in the reshaped valley were sampled and chemically analyzed. We found that the new farmlands experienced a significant rise in the water table that played a key role on soil salinization. After GLC, >80% of the sampling sites (14 out of 17) showed a shallow depth (≤3 m) from the soil surface down to the groundwater influence zone. Moreover, the closer the downstream farmlands to the newly-built dams, the shallower the water table, indicating considerable impacts of constructing dams on the water table depth in the near-dam farmlands. At the study site, more than half of the shallow soil samples (20 cm depth) underwent light soil salinization, and the most significant soil salinity occurred in the upper stream farmlands. The interplay between a near-surface water table, a high evaporation rate, landform features, and the alkaline properties of the loessial soil has led to the significant soil salinization in the upper stream of the valley. To mitigate soil salinization and ensure the sustainability of the newly-created farmlands in GLC, drainage trenches in the downstream near-dam farmlands and the drainage canals on both sides of the reshaped valley are suggested, which improve the drainage efficiency and control the water table depth.

Storm runoff is important for maintaining surface water resources, while this function is significantly affected by land use and land cover changes, e.g., afforestation and reforestation. The Chinese Loess Plateau (CLP) has undergone large-scale vegetation rehabilitation, especially afforestation over the past 20 years. We hypothesize that afforestation has profoundly changed the amounts and mechanisms of storm runoff generation in headwater catchments on the CLP. To test this hypothesis, rainfall, soil moisture, and streamflow were monitored in a grass catchment and an adjacent forest catchment for two consecutive years. The objective of this study was to elucidate the varied mechanisms of storm runoff generation in these two contrasting revegetated catchments. Results showed that (1) average runoff coefficient in the grassland catchment (0.042) was approximately ten times higher than that in the forestland catchment (0.004), confirming the impact of catchment afforestation on the suppression of storm runoff generation. (2) Peak rainfall intensity was the first-order control of the runoff coefficient in the grassland catchment, but not in the forestland catchment. (3) Threshold values for antecedent soil moisture (~18%) and the sum of antecedent soil moisture index and event precipitation (~210 mm) were identified in the grassland catchment, above which storm runoff significantly increased. (4) Two extraordinary high runoff coefficient events were observed in the grassland catchment, one due to high peak rainfall intensity and strong surface runoff and the other due to high rainfall amount and high antecedent soil moisture. We conclude that long-term afforestation has changed the mechanisms and patterns of storm runoff generation, and different conditions of rainfall intensity, rainfall amount and antecedent soil moisture determined the hydrological connectivity between the upper hillslope and downhill gully in the catchment. This study deepens our understanding of the mechanisms and thresholds of storm runoff generation in headwater catchments on the CLP.

Remote rural areas, which are generally the source regions for drinking water in small towns in China, experience nitrate pollution due to intensive agricultural activities. However, little is known about the nitrate sources of these waterbodies and how human activities affect the water quality in these rural areas. Here, a pair of rural watersheds on the Chinese Loess Plateau (CLP) were selected for study. One of the watersheds, which provides drinking water for local villages and a nearby small town, has experienced valley reshaping, damming and agricultural land consolidation, whereas the other one was not affected by human activities. Water chemistry monitoring and environmental isotope analysis were combined to identify the nitrate sources of the surface and groundwater in the dammed and natural watersheds and to assess the effects of landform transformation and agricultural and living activities on water quality. The results demonstrated that nitrate concentrations in both watersheds were lower than 10 mg L−1. However, the seasonal nitrate contents in the surface (0.42–0.52 mg L−1) and groundwater (1.86–3.65 mg L−1) of the dammed watershed were significantly lower than those of the natural watershed (surface water: 0.92–2.07 mg L−1; groundwater: 2.73–8.21 mg L−1). Isotope analysis demonstrated that nitrate fertilizer, soil ammonium, and manure and septic waste were the main sources of nitrate in the surface water of the two watersheds, while the nitrate in the groundwater mainly originated from the nitrification of soil ammonium and manure and septic waste. Compared to the natural watershed, the dammed watershed had increased inputs of agricultural fertilizers in the surface water. However, there was no significant difference in the groundwater nitrate sources between the two watersheds. The results also demonstrated that denitrification and dissimilatory nitrate reduction to ammonium (DNRA) probably occurred in the groundwater during the wet season, while nitrification likely occurred during the dry and snowmelt seasons. Hierarchical cluster analysis (HCA) showed that there were strong interactions between the surface runoff and the reservoir water in the dammed watershed, while in the natural watershed, streamflow had a close relationship with the groundwater. We concluded that valley reshaping, damming and agricultural land consolidation increased the nitrate input from mineral fertilizers in the watershed, but decreased the nitrate concentrations in the surface water through changing the transformation and transportation processes of nitrate. However, these activities had little influence on the groundwater, while the time lag between land consolidation and the groundwater quality response should be considered in the future. This study aids in developing strategies for the comprehensive management of watershed and drinking water sources in rural regions.

In order to study the relationship between trace element concentrations and coal-forming environment in the No. 6 Coal, Haerwusu Mine, Inner Mongolia, China, 39 bench samples were taken from the opencast mine, following Chinese Standard Method GB/T 482–2008. The samples were analyzed by an optical microscope, a scanning electron microscope in conjunction with an energy-dispersive X-ray spectrometer (SEM-EDX) analysis, inductively coupled plasma mass spectrometric (ICP-MS). The results indicate that the average contents of vitrinite, inertinite, liptinite are 54.59%, 34.84% and 5.8%, respectively. The concentrations of Li and Ga in the No. 6 coal are higher than those of in world coals, and most of trace elements in the No. 6 coal are correlated with minerals, which are mainly clay minerals, carbonate minerals and sulfide minerals. Two samples were identified as dry forest swamp facies (FtM I), 19 samples as shallow water forest swamp facies (FtM II), and 13 samples as reed swamp facies (RM). Compared with the average contents of world coals, trace elements in No. 6 coal of Haerwusu mine are rich in Li and V, while contents of the other elements are close or lower than the average contents of world coals. However, the variations of CC (Concentration Coefficients) among each swamp type are small and irregular, which may indicate that the concentration of the selected trace elements in the coal is mainly dominated by regional coal-forming environment, not coal-forming microenvironment.