Browsing by Author "Zhao, Wenwu"
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Item Distribution of Shrubland and Grassland Soil Erodibility on the Loess Plateau(MDPI, 2018-06) Zhang, Xiao; Zhao, Wenwu; Wang, Lixin; Liu, Yuanxin; Feng, Qiang; Fang, Xuening; Liu, Yue; Earth Sciences, School of ScienceSoil erosion is one of the most severe problems facing environments and has increased throughout the 20th century. Soil erodibility (K-factor) is one of the important indicators of land degradation, and many models have been used to estimate K values. Although soil erodibility has been estimated, the comparison of different models and their usage at a regional scale and, in particular, for different land use types, need more research. Four of the most widely distributed land use types were selected to analyze, including introduced and natural grassland, as well as introduced and natural shrubland. Soil particle size, soil organic matter and other relevant soil properties were measured to estimate soil erodibility in the Loess Plateau. The results show that: (1) the erosion productivity impact calculator (EPIC) model and SHIRAZI model are both suitable for the Loess Plateau, while the SHIRAZI model has the advantage of fewer parameters; (2) introduced grassland has better ability to protect both the 0–5 cm soils and 5–20 cm soils, while the differences between introduced and natural shrubland are not obvious at a catchment scale; (3) the K values of introduced grassland, natural grassland, introduced shrubland and natural shrubland in the 0–5 cm layer vary from 0.008 to 0.037, 0.031 to 0.046, 0.012 to 0.041 and 0.008 to 0.045 (t·hm2·h/(MJ·mm·hm2)), while the values vary from 0.009 to 0.039, 0.032 to 0.046, 0.012 to 0.042 and 0.008 to 0.048 (t·hm2·h/(MJ·mm·hm2)) in the 5–20 cm layer. The areas with a mean multiyear precipitation of 370–440 mm are the most important places for vegetation restoration construction management at a regional scale. A comprehensive balance between water conservation and soil conservation is needed and important when selecting the species used to vegetation restoration. This study provides suggestions for ecological restoration and provides a case study for the estimate of soil erodibility in arid and semiarid areas.Item Drylands contribute disproportionately to observed global productivity increases(Elsevier, 2023-01-30) Wang, Shuai; Fu, Bojie; Wei, Fangli; Piao, Shilong; Maestre, Fernando T.; Wang, Lixin; Jiao, Wenzhe; Liu, Yanxu; Li, Yan; Li, Changjia; Zhao, Wenwu; Earth and Environmental Sciences, School of ScienceDrylands cover about 40% of the terrestrial surface and are sensitive to climate change, but their relative contributions to global vegetation greening and productivity increase in recent decades are still poorly known. Here, by integrating satellite data and biosphere modeling, we showed that drylands contributed more to global gross primary productivity (GPP) increase (65% ± 16%) than to Earth greening (33% ± 15%) observed during 1982–2015. The enhanced productivity per unit leaf area, i.e., light-use efficiency (LUE), was the mechanism behind this pattern. We also found that LUE was more sensitive to soil moisture than to atmospheric vapor pressure deficit (VPD) in drylands, while the opposite was observed (i.e., LUE was more sensitive to VPD) in humid areas. Our findings suggest the importance of using different moisture stress metrics in projecting the vegetation productivity changes of dry versus humid regions and highlight the prominent role of drylands as key controllers of the global carbon cycle.Item Quantitative synthesis on the ecosystem services of cover crops(Elsevier, 2018-10) Daryanto, Stefani; Fu, Bojie; Wang, Lixin; Jacinthe, Pierre-André; Zhao, Wenwu; Earth Sciences, School of ScienceThe maintenance of soil health in agro-ecosystems is essential for sustaining agricultural productivity. Through its positive impacts on various soil physical and biological processes, cover cropping can be an important component of sustainable agricultural production systems. However, the practice of cover cropping can be complex, and possible trade-offs between the benefits and side effects of cover crops have not been examined. To evaluate these benefits and potential trade-offs, we quantitatively synthesized different ecosystem services provided by cover crops (e.g., erosion control, water quality regulation, soil moisture retention, accumulation of soil organic matter and microbial biomass, greenhouse gas (GHG) emission, weed and pest control, as well as yield of the subsequent cash crop) using data from previous publications. We used a simple indicator (δ), defined as the ratio of an observed variable (i.e., ecosystem service) under cover crop and under fallow condition, to evaluate the impacts of cover crops on a given ecosystem service. Our results showed that cover crops provided beneficial ecosystem services in most cases, except for an increase in GHG emission (δCO2 = 1.46 ± 0.47 and δN2 O = 1.49 ± 1.22; ± SD) and in pest (nematode) incidence (δnematode abundance = 1.29 ± 1.61). It is also important to highlight that, in some cases, tillage could offset the extent of ecosystem service benefits provided by cover crops. Based on this synthesis, we argue that cover crops should be incorporated into modern agricultural practices because of the many environmental benefits they offer, particularly the maintenance of soil and ecosystem health. More importantly, there was generally an increase in cash crop yield with cover cropping (δyield = 1.15 ± 0.75), likely due to improvement in various soil processes. Despite its benefits, the complexity of cover crop management should not be overlooked, and site-specific factors such as climate, soil type, cover crop species and tillage practices must be considered in order to optimize the benefits of cover cropping. In addition to crop yield, detailed economic analyses are needed to calculate the direct (e.g., reduction in the amount of chemical fertilizer) and indirect monetary benefits (e.g., the improvement of soil quality) of cover crops. Such a comprehensive analysis could serve as incentive for producers to integrate cover crops into their management practices.Item Relationship between soil water content and soil particle size on typical slopes of the Loess Plateau during a drought year(Elsevier, 2019-01) Zhang, Xiao; Zhao, Wenwu; Wang, Lixin; Liu, Yuanxin; Liu, Yue; Feng, Qiang; Earth Sciences, School of ScienceIn the context of global climate change as well as local climate warming and drying on the Loess Plateau of China, understanding the relationship between soil particle size and soil water distribution during years of atypical precipitation is important. In this study, fractal geometry theory is used to describe the mechanical composition and texture of soils to improve our understanding of hydropedology and ecohydrology in the critical zone on the Loess Plateau. One grassland slope and two shrubland slopes were selected in the hilly and gully region of the Loess Plateau, and soils were sampled along hillslope transects at depths of 0–500 cm. Fractal theory and redundancy analysis (RDA) were used to identify relationships between the fractal dimension of soil particle-size distributions and the corresponding van Genuchten parameters for the soil-water-characteristic curves. The oven-drying method was used to measure soil water content, and the high-speed centrifugation method was used to generate soil-water-characteristic curves. The results show that (1) the soil water that can be used by Caragana korshinskii during a drought year is distributed below 2 m from the surface, whereas the soil water that can be used by grass is below 1.2 m; (2) Caragana korshinskii promotes the conservation of fine soil particles more than does natural restored grass, and the soil particle-size distribution fractal dimension changes with depth and position; and (3) soil hydraulic properties correlate strongly with soil pedological properties such as bulk density and the soil particle-size distribution fractal dimension. These results provide a case study of the relationships among soil distributions, hydrologic and geomorphic processes for vegetation restoration in drylands with a thick vadose zone. More studies on soil property changes are needed to provide case studies and empirical support for ecological restoration in the Loess Plateau of China.Item The spatial distribution and temporal variation of desert riparian forests and their influencing factors in the downstream Heihe River basin, China(2017) Ding, Jingyi; Zhao, Wenwu; Daryanto, Stefani; Wang, Lixin; Fan, Hao; Feng, Qiang; Wang, Yaping; Earth Science, School of ScienceDesert riparian forests are the main restored vegetation community in Heihe River basin. They provide critical habitats and a variety of ecosystem services in this arid environment. Since desert riparian forests are also sensitive to disturbance, examining the spatial distribution and temporal variation of these forests and their influencing factors is important to determine the limiting factors of vegetation recovery after long-term restoration. In this study, field experiment and remote sensing data were used to determine the spatial distribution and temporal variation of desert riparian forests and their relationship with the environmental factors. We classified five types of vegetation communities at different distances from the river channel. Community coverage and diversity formed a bimodal pattern, peaking at the distances of 1000 and 3000 m from the river channel. In general, the temporal normalized difference vegetation index (NDVI) trend from 2000 to 2014 was positive at different distances from the river channel, except for the region closest to the river bank (i.e. within 500 m from the river channel), which had been undergoing degradation since 2011. The spatial distribution of desert riparian forests was mainly influenced by the spatial heterogeneity of soil properties (e.g. soil moisture, bulk density and soil particle composition). Meanwhile, while the temporal variation of vegetation was affected by both the spatial heterogeneity of soil properties (e.g. soil moisture and soil particle composition) and to a lesser extent, the temporal variation of water availability (e.g. annual average and variability of groundwater, soil moisture and runoff). Since surface (0–30 cm) and deep (100–200 cm) soil moisture, bulk density and the annual average of soil moisture at 100 cm obtained from the remote sensing data were regarded as major determining factors of community distribution and temporal variation, conservation measures that protect the soil structure and prevent soil moisture depletion (e.g. artificial soil cover and water conveyance channels) were suggested to better protect desert riparian forests under climate change and intensive human disturbance.Item Spatial Variations of Soil Moisture under Caragana korshinskii Kom. from Different Precipitation Zones: Field Based Analysis in the Loess Plateau, China(MDPI, 2016-02) Liu, Yuanxin; Zhao, Wenwu; Wang, Lixin; Zhang, Xiao; Daryanto, Stefani; Fang, Xuening; Department of Earth Sciences, School of ScienceSoil moisture scarcity has become the major limiting factor of vegetation restoration in the Loess Plateau of China. The aim of this study is: (i) to compare the spatial distribution of deep (up to 5 m) soil moisture content (SMC) beneath the introduced shrub Caragana korshinskii Kom. under different precipitation zones in the Loess Plateau and (ii) to investigate the impacts of environmental factors on soil moisture variability. Soil samples were taken under C. korshinskii from three precipitation zones (Semiarid-350, Semiarid-410, Semiarid-470). We found that the highest soil moisture value was in the 0–0.1 m layer with a large coefficient of variation. The soil water storage under different precipitation zones increased following the increase of precipitation (i.e., Semiarid-350 < Semiarid-410 < Semiarid-470), although the degree of SMC variation was different for different precipitation zones. The SMC in the Semiarid-350 zone initially increased with soil depth, and then decreased until it reached the depth of 2.8-m. The SMC in the Semiarid-410 zone showed a decreasing trend from the top soil to 4.2-m depth. The SMC in the Semiarid-470 zone firstly decreased with soil depth, increased, and then decreased until it reached 4.6-m depth. All SMC values then became relatively constant after reaching the 2.8-m, 4.2-m, and 4.6-m depths for Semiarid-350, Semiarid-410, and Semiarid-470, respectively. The low but similar SMC values at the stable layers across the precipitation gradient indicate widespread soil desiccation in this region. Our results suggested that water deficit occurred in all of the three precipitation zones with precipitation, latitude, field capacity, and bulk density as the main environmental variables affecting soil moisture. Considering the correlations between precipitation, SMC and vegetation, appropriate planting density and species selection should be taken into account for introduced vegetation management.Item Variations of deep soil moisture under different vegetation types and influencing factors in a watershed of the Loess Plateau, China(2016-08) Fang, Xuening; Zhao, Wenwu; Wang, Lixin; Feng, Qiang; Ding, Jingyi; Liu, Yuanxin; Zhang, Xiao; Department of Earth Sciences, School of ScienceSoil moisture in deep soil layers is a relatively stable water resource for vegetation growth in the semi-arid Loess Plateau of China. Characterizing the variations in deep soil moisture and its influencing factors at a moderate watershed scale is important to ensure the sustainability of vegetation restoration efforts. In this study, we focus on analyzing the variations and factors that influence the deep soil moisture (DSM) in 80–500 cm soil layers based on a soil moisture survey of the Ansai watershed in Yan'an in Shanxi Province. Our results can be divided into four main findings. (1) At the watershed scale, higher variations in the DSM occurred at 120–140 and 480–500 cm in the vertical direction. At the comparable depths, the variation in the DSM under native vegetation was much lower than that in human-managed vegetation and introduced vegetation. (2) The DSM in native vegetation and human-managed vegetation was significantly higher than that in introduced vegetation, and different degrees of soil desiccation occurred under all the introduced vegetation types. Caragana korshinskii and black locust caused the most serious desiccation. (3) Taking the DSM conditions of native vegetation as a reference, the DSM in this watershed could be divided into three layers: (i) a rainfall transpiration layer (80–220 cm); (ii) a transition layer (220–400 cm); and (iii) a stable layer (400–500 cm). (4) The factors influencing DSM at the watershed scale varied with vegetation types. The main local controls of the DSM variations were the soil particle composition and mean annual rainfall; human agricultural management measures can alter the soil bulk density, which contributes to higher DSM in farmland and apple orchards. The plant growth conditions, planting density, and litter water holding capacity of introduced vegetation showed significant relationships with the DSM. The results of this study are of practical significance for vegetation restoration strategies, especially for the choice of vegetation types, planting zones, and proper human management measures.Item Vegetation responses and trade‐offs with soil‐related ecosystem services after shrub removal: A meta‐analysis(Wiley, 2019) Daryanto, Stefani; Wang, Lixin; Fu, Bojie; Zhao, Wenwu; Wang, Shuai; Earth Sciences, School of ScienceAim To assess the sustainability of different shrub control practices (fire, mechanical, and chemical), based on their efficacy to control shrubs and their effects on multiple ecosystem service provisions, including possible trade‐off and/or synergy. Methods Using a meta‐analysis approach, this study synthesized results from global shrub removal experiments. Log response ratio (lnR) between the outcome of shrub removal and that of the untreated control was used to estimate proportional changes in soil and vegetation properties resulting from each shrub control practice. Results When forage provisioning is the only service considered, shrub removal could achieve this desirable outcome as indicated by increasing herbaceous biomass. However, observable decreases in litter, biological crust cover, and soil nutrients, as well as increases in bare soil indicated long‐term potential trade‐offs with other ecosystem services (e.g., erosion control service, nutrient cycling); the degree may be influenced by different shrub control methods. Synergistic properties were probably limited to a short‐term boost of herb productivity resulting from short‐term increase in herb biomass and diversity as well as nutrient availability. Conclusion Human‐induced drivers manifested in shrub control practices may change vegetation response. However, management also changed non‐targeted processes, generating potential reduction in several regulating ecosystem services. Continuous monitoring to assess landscape conditions should therefore become the key for adaptive management. Sustainable forage production should focus on strategies to maintain multiple ecosystem services because consideration of those services can lead to long‐term protection of the landscape and provide a broader range of environmental benefits.