Kaibo Wang

Long-term natural succession improves nitrogen storage capacity of soil on the Loess Plateau, China

Land-use change resulting from natural succession enhances the nitrogen (N) accumulation capacity of terrestrial ecosystems. Toexplore thosefactors thatfosterchangesinsoil Nstorage under evolvingconditionsofvegetation succession, a study on N storage at differing stages along a 150-year chronosequence was conducted in the Ziwuling Forest RegioninthecentralpartoftheLoessPlateau,China.A principal findingwas therapid increaseinNstorageinthe0-60cm soil layer, which achieves a stable value after the shrub community stage (~50-60 years), leading to the overall long-term (~150 years) accumulation of soil stored N in the post-abandonment secondary forest. Soil N accumulated mainly in the pioneer stage and showed a significant increase before the shrub community stage (P 20cm). Inthetopsoil (0-20cm), Nstorage valuesshowed a markedlypositive correlation with soil organic carbon(SOC), totalsoil Nand fineroots.Inthedeepersoil layers(20-40and40-60cm)therewas a correlation only with TN. Soil bulk density, soil water content and soil pH were not the determining factors behind N storage values in the topsoil (0-20cm), although they did show negative, positive and negative correlations, respectively. In addition, they showed no consistent correlations in the lower soil layer (<20cm). The results suggest that changes to N storage values were the result of the accumulation of SOC, total N and primary productivity during the process of forest succession, and this capacity is positively related to post-abandonment forest succession on the Loess Plateau, China.

How many check dams do we need to build on the Loess Plateau

F more than 400 years, check dams have been constructed on the Loess Plateau of China. Over the past several hundred years, people have increasingly realized the advantages of check dams for capturing sediments, improving gully slope stabilities, and increasing croplands. In Environmental Science and Technology, Wang et al. summarized the advantages of check dams for environmental services and food security. The report demonstrated that about 110,000 check dams have been built on the Loess Plateau over the past 50 years and approximately 21 billion m of sediments have been captured by these dams. Moreover, the filled check dams can be reclaimed as croplands, and by 2002, approximately 320,000 hectares of dam croplands had been created. The significant role of check dams in soil conservation and cropland expansion inspires the passions of policy makers. As early as 2003, the Ministry of Water Resources of P.R. China (CMWR) initiated a program for check dams in the Loess Plateau, and 163,000 check dams are planned and an investment of 83.06 billion RMB of funding is required for the period 2003−2020. Policy makers consider that the Loess Plateau has the capacity to allow the construction of as many as 334,000 check dams and will therefore require an even greater amount of investment. The Loess Plateau is currently undergoing a great leap forward in check dam construction. The CMWR has set a target of 47,000 check dams, built at an investment of more than 20 billion RMB, from 2010 to 2015. Increasing numbers of environmental scientists are concerned about the negative effects of such large-scale engineering on the balance of the water cycle and sediment load in the Yellow River. The CMWR projects that, by 2020, check dam construction will lead to a 4.3−5.5 billion m decrease in water yield to the Yellow River. However, the amount of this decrease is highly uncertain. The actual amount may exceed the projected amount due to trends in climate warming and extensive human activities. The sediment load and streamflow in the Yellow River have dramatically decreased in recent years. Huang et al. reported that the sediment load delivered from the Yellow River to the sea decreased sharply to 0.15 billion tons per year between 2000 and 2005, representing only 14% of the widely cited estimate of 1.08 billion tons per year. The data released by the Yellow River Sediment Bulletin show that the sediment load gauged by the Huayuankou hydrologic station decreased substantially to 0.107 billion tons per year between 2000 and 2010. This value represents only 10% of the sediment load of 1.054 billion tons per year occurring between 1950 and 2000. Moreover, the annual streamflow in the Yellow River averaged 40.05 billion m between 1950 and 2000, whereas it decreased to 22.65 billion m during the past decade. Many scientists conclude that human intervention is the primary factor that caused the decrease in the sediment load and streamflow in the Yellow River. However, we still have not determined a suitable sediment load and streamflow that would keep the Yellow River healthy, and therefore determine the appropriate number of check dams that should be built on the Loess Plateau. The Loess Plateau covers an area of 648,700 km, including 200,000 km of highplain plateau, 140,000 km of hilly plateau, 107,200 km of rocky mountains, 63,600 km of Fen River− Wei River fault depression valley, 79,2000 km of deserts, and 58,700 km of Hetao alluvial plains (Figure 1). The different geographical regions play different roles in soil and water conservation and ecosystem services. The hilly plateau regions suffer the most severe soil erosion in the Loess Plateau and should therefore be considered critical areas for check dam engineering. The rocky mountain regions are appropriate for planting and should therefore be considered critical areas for afforestation and water conservation. We suggest that the planning of check dam engineering should comply with the landforms and geographical function zones. Currently, many significant problems occur in the planning and engineering of check dams. First, there is a lack of critical discussion on the appropriate density and distribution of check dams. The contiguous area of Shanxi-Shaanxi-Inner Mongolia is the central erosion area of the hilly plateau and is therefore the most critical target area for check dam construction. However, a large number of check dams are still planned for construction in the highplain plateau and rocky mountains. Second, effective soil and water conservation measures and climate change significantly decreased soil erosion in the Loess Plateau and

Comparing watershed black locust afforestation and natural revegetation impacts on soil nitrogen on the Loess Plateau of China

This study examined a pair of neighbouring small watersheds with contrasting vegetations: artificial forestland and natural grassland. Since 1954, afforestation which mainly planted with black locust has been conducted in one of these watersheds and natural revegetation in the other. The differences in soil total N, nitrate, ammonium, foliar litterfall δ15N and dual stable isotopes of δ15N and δ18O in soil nitrate were investigated in the two ecosystems. Results showed that there was no significant difference in soil total N storage between the two ecosystems, but the black locust forestland presented higher soil nitrate than the grassland. Moreover, the foliar litterfall N content and δ15N of the forestland were significant higher than the grassland. These results indicate that 60 years of watershed black locust afforestation have increased soil N availability. The higher nitrate in the forestland was attributed to the biological N fixation of black locust and difference in ecosystem hydrology. The dual stable isotopes of δ15N and δ18O revealed that the two ecosystems had different sources of soil nitrate. The soil nitrate in the forestland was likely derived from soil N nitrification, while the soil nitrate in the grassland was probably derived from the legacy of NO3− fertiliser.

Dynamics of ecosystem carbon stocks during vegetation restoration on the Loess Plateau of China

In the last few decades, the Loess Plateau had experienced an extensive vegetation restoration to reduce soil erosion and to improve the degraded ecosystems. However, the dynamics of ecosystem carbon stocks with vegetation restoration in this region are poorly understood. This study examined the changes of carbon stocks in mineral soil (0–100 cm), plant biomass and the ecosystem (plant and soil) following vegetation restoration with different models and ages. Our results indicated that cultivated land returned to native vegetation (natural restoration) or artificial forest increased ecosystem carbon sequestration. Tree plantation sequestered more carbon than natural vegetation succession over decades scale due to the rapid increase in biomass carbon pool. Restoration ages had different effects on the dynamics of biomass and soil carbon stocks. Biomass carbon stocks increased with vegetation restoration age, while the dynamics of soil carbon stocks were affected by sampling depth. Ecosystem carbon stocks consistently increased after tree plantation regardless of the soil depth; but an initial decrease and then increase trend was observed in natural restoration chronosequences with the soil sampling depth of 0–100 cm. Moreover, there was a time lag of about 15–30 years between biomass production and soil carbon sequestration in 0–100 cm, which indicated a long-term effect of vegetation restoration on deeper soil carbon sequestration.

Biomass Components and Environmental Controls in Ningxia Grasslands

Grassland plays an important role in the global carbon cycle and climate regulation. However, there are still large uncertainties in grassland carbon pool and also its role in global carbon cycle due to the lack of measured grassland biomass at regional scale or global scale with a unified survey method, particular for below-ground biomass. The present study, based on a total of 44 grassland sampling plots with 220 quadrats across Ningxia, investigated the characteristics of above-ground biomass (AGB), below-ground biomass (BGB), litter biomass (LB), total biomass (TB) and root:shoot ratios (R:S) for six predominantly grassland types, and their relationships with climatic factors. AGB, BGB, LB and TB varied markedly across different grassland types, the median value ranging from 28.2-692.6 g m-2 for AGB, 130.4-2 036.6 g m-2 for BGB, 9.2-82.3 g m-2 for LB, and 168.0-2 681.3 g m-2 for TB. R:S showed less variation with median values from 3.2 to 5.3 (excluding marshy meadow). The different grassland types showed similar patterns of biomass allocation, with more than 70% BGB for all types. There is evidence of strong positive effects associated with mean annual precipitation (MAP) and negative effects associated with mean annual temperature (MAT) on AGB, BGB, and LB, although both factors have the opposite effect on R:S.

Simulating the vegetation-producing process in small watersheds in the Loess Plateau of China

Small watersheds are the basic composition unit of the Loess Plateau in China. An accurate estimation of vegetation net primary productivity (NPP) is of great significance for eco-benefit evaluation in small watershed management in this region. Here we describe the development and testing of a vegetation-producing process model (VPP) of a small watershed in the Loess Plateau. The model couples three modules: radiation adjustment; soil hydrological processes; and vegetation carbon assimilation. Model validation indicates that the VPP model can be used to estimate the NPP of small watersheds in the region. With the VPP model, we estimated the spatial NPP distributions in the Yangou watershed for 2007. The results show that in the Yangou watershed the NPP is relatively low, averaging 168 g C/(m 2 ·a). Trees and shrubs have a higher NPP than crops and grasses. The NPP is larger on the partly shaded and shaded slopes than on the partly sunny and sunny slopes. The NPP on the slopes increases gradually on 0-20° slopes and decreases slightly on slopes steeper than 20°. Our simulation indicates that the vegetation type is the most important factor in determining the NPP distribution in small watersheds in the Loess Plateau.

Prediction of aboveground grassland biomass on the Loess Plateau, China, using a random forest algorithm

Grasslands are an important component of terrestrial ecosystems that play a crucial role in the carbon cycle and climate change. In this study, we collected aboveground biomass (AGB) data from 223 grassland quadrats distributed across the Loess Plateau from 2011 to 2013 and predicted the spatial distribution of the grassland AGB at a 100-m resolution from both meteorological station and remote sensing data (TM and MODIS) using a Random Forest (RF) algorithm. The results showed that the predicted grassland AGB on the Loess Plateau decreased from east to west. Vegetation indexes were positively correlated with grassland AGB, and the normalized difference vegetation index (NDVI) acquired from TM data was the most important predictive factor. Tussock and shrub tussock had the highest AGB, and desert steppe had the lowest. Rainfall higher than 400 m might have benefitted the grassland AGB. Compared with those obtained for the bagging, mboost and the support vector machine (SVM) models, higher values for the mean Pearson coefficient (R) and the symmetric index of agreement (λ) were obtained for the RF model, indicating that this RF model could reasonably estimate the grassland AGB (65.01%) on the Loess Plateau.

Photosynthetic characteristics and resource utilization efficiency of maize (Zea mays L.) and millet (Setaria italica L.) in a semi-arid hilly loess region in China

Abstract Photosynthetic characteristics and resource utilization efficiency of maize (Zea mays L.) and millet (Setaria italica L.) are studied in a loess region of China. Limited sampling shows that maize has a higher light-saturated CO2 assimilation rate, apparent quantum yield and carboxylation efficiency than does millet in the middle (July) and late (September) growth stage. Compared with millet, maize has lower transpiration rate (T r ) and stomatal conductance (G s ) in July and higher T r and G s in September. This leads to higher leaf water use efficiency in maize in July, but it is lower than for millet in September. Mass-based leaf nitrogen concentrations (N mass) of both crops are similar in both growth stages. Leaf mass per area (LMA) of maize is the same as of millet in July but lower in September. These results indicate differences in photosynthetic capacity and resource use efficiencies between these crops.