Factors affecting distribution of vegetation types on abandoned cropland in the hilly-gullied Loess Plateau region of China

A study was conducted in the forest-steppe region of the Loess Plateau to provide insight into the factors affecting the process of vegetation establishment, and to provide recommendations for the selection of indigenous species in order to speed up the succession process and to allow the establishment of vegetation more resistant to soil erosion. Four distinctive vegetation types were identified, and their distribution was affected not only by the time since abandonment but also by other environmental factors, mainly soil water and total P in the upper soil layers. One of the vegetation types, dominated by Artemisia scoparia, formed the early successional stage after abandonment while the other three types formed later successional stages with their distribution determined by the soil water content and total P. It can be concluded that the selection of appropriate species for introduction to accelerate succession should be determined by the local conditions and especially the total P concentration and soil water content.

Can the study of natural vegetation succession assist in the control of soil erosion on abandoned croplands on the Loess Plateau, China?

In the Loess Plateau, China, arable cultivation of slope lands is common and associated with serious soil erosion. Planting trees or grass may control erosion, but planted species may consume more soil water and can threaten long-term ecosystem sustainability. Natural vegetation succession is an alternative ecological solution to restore degraded land, but there is a time cost, given that the establishment of natural vegetation, adequate to prevent soil erosion, is a longer process than planting. The aims of this study were to identify the environmental factors controlling the type of vegetation established on abandoned cropland and to identify candidate species that might be sown soon after abandonment to accelerate vegetation succession and establishment of natural vegetation to prevent soil erosion. A field survey of thirty-three 2 × 2–m plots was carried out in July 2003, recording age since abandonment, vegetation cover, and frequency of species together with major environmental and soil variables. Data were analyzed using correspondence analysis, classification tree analysis, and species response curves. Four vegetation types were identified and the data analysis confirmed the importance of time since abandonment, total P, and soil water in controlling the type of vegetation established. Among the dominant species in the three late-successional vegetation types, the most appropriate candidates for accelerating and directing vegetation succession were King Ranch bluestem (Bothriochloa ischaemum) and Lespedeza davurica (Leguminosae). These species possess combinations of the following characteristics: tolerance of low water and nutrient availability, fibrous root system and strong lateral vegetative spread, and a persistent seed bank.

Glacial-interglacial changes in dust deposition on the Chinese Loess Plateau

The Chinese Loess Plateau (CLP) contains an extensive record of aeolian deposition through multiple glacial–interglacial cycles. Independent chronologies based on pedostratigraphy, magnetic susceptibility, radiocarbon and luminescence dating were developed for 79 sites and used to estimate aeolian mass accumulation rates (MARs) for marine isotope stages 1–5. The regional median value of MAR for Stage 2 is 310 g/m2/yr compared to an estimate of 65 g/m2/yr for Stage 5. Estimated MARs from individual sites for Stage 2 are approximately 4.3 times greater than MARs for Stage 5 and 2.1 times greater than for Stage 1. MAR values at individual sites are consistently highest in the northwest and lowest in the southwest of the CLP during all marine isotope stages. MARs estimated on sections through loess terraces are consistently higher than MAR estimates at other sites, indicating that local recycling of loess material from exposed river valley deposits has been significant throughout the last 130 kyr. Although the spatial and temporal patterns in MAR are robust, there are uncertainties about the magnitude of these changes due to (a) lack of bulk density measurements and uncertainties in the chronologies for individual sites, (b) site and chronological biases in the sampling used to derive regional estimates, and (c) the unquantified nature of human impact on accumulation rates during the late Holocene. Nevertheless, the records from the CLP pose a number of challenges which could be addressed by numerical models of the palaeo-dust cycle.

Long-term response of the nematode community to elevated atmospheric CO2 in a temperate dry grassland soil

Long-term effects of the elevated atmospheric CO2 on biosphere have been in focus of research since the last few decades. In this experiment undisturbed soil monoliths of loess grassland were exposed to an elevated CO2 environment (two-times the ambient CO2 level) for a period of six years with the aid of the open top chamber method. Control without a chamber and CO2 elevation was applied as well. Elevated CO2 level had very little impact oil soil food web. It did not influence either root and microbial biomass or microbial and nematode community structure. The only significant response was that density of the bacterial feeder genus Heterocephalobus increased in the chamber with elevated CO2 concentration. Application of the open top chambers initiated more changes on nematodes than the elevated CO2 level. Open top chamber (OTC) method decreased nematode density (total and plant feeder as well) to less than half of the original level. Negative effect was found on the genus level in the case of fungal feeder Aphelenchoides, plant feeder Helicotylenchus and Paratylenchus. It is very likely that the significantly lower belowground root biomass and partly its decreased quality reflected by the increased C/N ratio are the main responsible factors for the lower density of the plant feeder nematodes in the plots of chambers. According to diversity profiles, MI and MI(2-15) parameters, nematode communities in the open top chambers (both on ambient and elevated CO2 level) seem to be more structured than those under normal circumstances six years after start of the experiment.

DIRTMAP: the geological record of dust.

Atmospheric dust is an important feedback in the climate system, potentially affecting the radiative balance and chemical composition of the atmosphere and providing nutrients to terrestrial and marine ecosystems. Yet the potential impact of dust on the climate system, both in the anthropogenically disturbed future and the naturally varying past, remains to be quantified. The geologic record of dust provides the opportunity to test earth system models designed to simulate dust. Records of dust can be obtained from ice cores, marine sediments, and terrestrial (loess) deposits. Although rarely unequivocal, these records document a variety of processes (source, transport and deposition) in the dust cycle, stored in each archive as changes in clay mineralogy, isotopes, grain size, and concentration of terrigenous materials. Although the extraction of information from each type of archive is slightly different, the basic controls on these dust indicators are the same. Changes in the dust flux and particle size might be controlled by a combination of (a) source area extent, (b) dust emission efficiency (wind speed) and atmospheric transport, (c) atmospheric residence time of dust, and/or (d) relative contributions of dry settling and rainout of dust. Similarly, changes in mineralogy reflect (a) source area mineralogy and weathering and (b) shifts in atmospheric transport. The combination of these geological data with process-based, forward-modelling schemes in global earth system models provides an excellent means of achieving a comprehensive picture of the global pattern of dust accumulation rates, their controlling mechanisms, and how those mechanisms may vary regionally. The Dust Indicators and Records of Terrestrial and MArine Palaeoenvironments (DIRTMAP) data base has been established to provide a global palaeoenvironmental data set that can be used to validate earth system model simulations of the dust cycle over the past 150,000 years.

Dust sources and deposition during the last glacial maximum and current climate: a comparison of model results with paleodata from ice cores and marine sediments

Mineral dust aerosols in the atmosphere have the potential to affect the global climate by influencing the radiative balance of the atmosphere and the supply of micronutrients to the ocean. Ice and marine sediment cores indicate that dust deposition from the atmosphere was at some locations 2–20 times greater during glacial periods, raising the possibility that mineral aerosols might have contributed to climate change on glacial-interglacial time scales. To address this question, we have used linked terrestrial biosphere, dust source, and atmospheric transport models to simulate the dust cycle in the atmosphere for current and last glacial maximum (LGM) climates. We obtain a 2.5-fold higher dust loading in the entire atmosphere and a twenty-fold higher loading in high latitudes, in LGM relative to present. Comparisons to a compilation of atmospheric dust deposition flux estimates for LGM and present in marine sediment and ice cores show that the simulated flux ratios are broadly in agreement with observations; differences suggest where further improvements in the simple dust model could be made. The simulated increase in high-latitude dustiness depends on the expansion of unvegetated areas, especially in the high latitudes and in central Asia, caused by a combination of increased aridity and low atmospheric [CO2]. The existence of these dust source areas at the LGM is supported by pollen data and loess distribution in the northern continents. These results point to a role for vegetation feedbacks, including climate effects and physiological effects of low [CO2], in modulating the atmospheric distribution of dust.