Southern Chinese Loess Plateau Research Articles

Dynamics of Carbon and Water Fluxes over Cropland and Agroforest Ecosystems on the Southern Chinese Loess Plateau

Studies on the spatiotemporal dynamics in ecosystem carbon and water exchanges are essential in predicting the effects of climate change on regional carbon and energy budgets. Using the eddy covariance technique, carbon and water fluxes were observed in a typical winter wheat ecosystem (WWE) and an agroforest ecosystem (AFE) in the southern Loess Plateau from 2004 to 2010. The seasonal and inter-annual variability in gross primary productivity (GPP), net ecosystem exchange (NEE), evapotranspiration (ET), and water use efficiency (WUE) were examined and the main influencing factors were identified using the Pearson correlation. The results indicate that the seasonal GPP and NEE showed a bimodal distribution in WWE, while this was unimodal in AFE. The sinusoidal function did well in the characterization of seasonal ET dynamics for both ecosystems, with the determination coefficients being 0.85 and 0.94, respectively. In WWE and AFE, the annual mean GPP were 724.33 and 723.08 g C m−2 a−1, respectively, and the corresponding ET were 392.22 and 410.02 mm a−1. However, the difference in NEE between the two ecosystems was obvious, NEE were −446.28 and −549.08 g C m−2 a−1, respectively, showing a stronger carbon sink in AFE. There were strong coupling relationships between the GPP and ET of both ecosystems; the overall slopes were 1.71 and 1.69, respectively. The seasonal trend of WUE was bimodal in WWE, with peak values of 3.94 and 3.65 g C kg−1 H2O, occurring in November and April, respectively. However, the monthly WUE in AFE had one single peak of 4.07 g C kg−1 H2O in January. Photosynthetically active radiation (PAR) and soil temperature (Ts) were most positively correlated with GPP, net radiation (Rn) and Ts were the major factors influencing ET, while vapor pressure deficit (VPD) and soil water content (SWC) were the major influencing factors for WUE. These results provide observational support for regional carbon neutrality simulations.

Winter determines the annual:seasonal temperature change of Asia temperate monsoon region since mid-Holocene

The evolution of global temperature during Holocene is crucial for understanding the mechanisms of climate change and future climate trends. However, there is a discrepancy between the temperature trends reconstructed from proxy and model, which is attributed to the seasonal biases and the appearance of the Holocene thermal maximum was dominated by the temperature of the warm season. In this paper, fossil charcoal assemblages from the southern Chinese Loess Plateau were analysed to provide quantitative mean annual temperatures (MAT) and four-season temperature records during 6.55–3.45 cal ka BP. The results indicate that both the seasonal temperatures and MAT during the middle Holocene reached warm peaks between 5.55 and 5.35 cal ka BP. The similar trend between the winter temperature (WIT) and MAT denotes the highest contribution of WIT changes to the MAT in the study area since the mid-Holocene.

Formation and evolution of a loess sinkhole in the southern Chinese Loess Plateau

Sinkholes are formed by the collapse of the overlying strata or the subsidence of the ground from cracks or holes in the underlying strata, which are widely developed in the karst and loess regions over southern and northern China. These sinkholes have been explored intensively to infer their climatic and ecological significances, as well as the management of geological disasters. In this study we investigated the Sanyuan (SY) sinkhole in the southern Chinese Loess Plateau to understand how it was formed and evolved. Two 30-m cores were retrieved from the SY sinkhole and a detailed chronological framework was established by applying radionuclides dating for the top 1-m sediments as well as 14C dating for the two 30-m cores. These dating results show that SY sinkhole was formed 23.7 m below the sinkhole surface at ca. 6.0 kyr BP (before 1950 CE). Changes in lithology, sedimentation rate, magnetic susceptibility, lightness, and mean grain-size of these two cores reveal that the sinkhole has undergone two-stage evolution. The first stage was characterized by a relatively slow deposition of loess sediments during 6.0–1.9 kyr BP, and the second stage was a rapid accumulating period since 1.9 kyr BP likely affected by human activities. Our results suggest that SY sinkhole can well document the combined influence of climate change and human activities, offering an ideal archive to decipher the natural and anthropogenic imprints since the mid-Holocene.

Apple trees can extract soil water from both deep layers and neighboring cropland in the tableland region of Chinese Loess Plateau

It is of great significance to explore soil water distribution and consumptions of the agroforestry systems in water-deficient areas. In recent decades, continuous development of apple orchards in the southern Chinese Loess Plateau has formed the orchard-cropland mosaic vegetation pattern. In this study, soil water content (SWC) in two series were measured, (1) 0–21 m soil profiles of five apple orchards with different stand ages (5, 10, 15, 20 and 24 years) and one cropland; (2) 0–6 m soil profiles at different sites within the orchard-cropland border zone (with apple orchard aged 10 and 24 years old, denoted as AC10 and AC24, respectively). Results showed that (1) the water consumption depth of apple orchards increased in an “S”-shaped pattern with stand age and reached 20 m after 20 years, while the average SWC within 3–21 m layer decreased and the soil water deficits increased at a rate of 5.8 mm m−1 yr−1; and (2) the average SWC in 3–10 m layer of 15, 20 and 24-year-old orchards accounted for 69.3%, 64.2% and 57.9% of the field capacity, respectively, these proportions came to 71.2%, 67.4% and 61.1% in the 3–21 m layer, respectively; and (3) AC24 had greater influence on SWC of adjacent cropland than AC10. The soil water storage of 2–6 m layer in AC24 ranged from 713 mm to 879 mm within 7 m away from the orchard, which were significantly lower than those of 7–15 m away from the orchard (933–1042 mm). Overall, the apple tree root system can absorb soil water from a depth of 20 m in the vertical direction and up to 7 m from the adjacent cropland in the horizontal direction. Therefore, a reasonable orchard-cropland mosaic pattern should be arranged in regional agricultural planning, so as to promote the optimized utilization of regional soil water resources, and to coordinate the development of fruit industry and food security, in which the width of a cropland can’t be < 14 m.

Plutonium-based radiometric dating of rapidly accumulated sediments in the Sanyuan sinkhole, southern Chinese Loess Plateau

Radionuclides, such as 210Pb, 137Cs and 239,240Pu, have been widely used for dating recent sediments in terrestrial and marine environments, while 129I, as an important artificial radionuclide in the environment, is also a potential tracer for sediment dating and environmental process studies. However, they were not always successfully applied to sediment dating because of their different sources, half-lives, environmental behaviour and measurement techniques. The dating applicability of these nuclides in a sedimentary environment with rapid accumulation on land was investigated for sinkhole sediment from the southern Chinese Loess Plateau. Our results showed that 210Pb and 137Cs could not be adequately used for dating the sediments due to the difficulties in accurately measuring 137Cs and excess 210Pb (210Pbex) signals caused by the dilution effect of rapid accumulation. 129I is not an ideal dating tracer because of its multisource feature causing no remarkable peak value in the sediment cores. The depth distribution of 239,240Pu in the sediment core showed a single peak corresponding to its maximum fallout in 1963 from the atmospheric nuclear weapons test, suggesting that Pu isotopes have significant advantages in dating recent sediments. The sensitive inductively coupled plasma–mass spectrometry (ICP-MS) measurement technique enables the determination of very low levels of 239Pu and 240Pu and makes 239,240Pu a suitable tracer for dating the rapidly accumulated sediment. Based on the 239,240Pu mass balance equation estimation and field observations, we proposed the water-eroded input from soil surrounding the sinkhole as another vital source of the sediments in addition to the aeolian contribution.

Investigating the vertical distribution of dissolved organic matter in 5-m soil profiles in farmland and typical woodland on the southern loess plateau

With the implementation of the ‘Grain-for-Green’ program on the Chinese Loess Plateau (CLP), drought-tolerant deep-rooted plants have been increasingly introduced to the northwest in China. However, the vertical features of dissolved organic matter (DOM) in deep soil profiles on CLP during the ‘Grain-for-Green’ program is still not well understood. In the study, ultraviolet–visible (UV–Vis) spectroscopy and three-dimensional fluorescence excitation-emission matrices (3D-EEMs) with parallel factor analysis (PARAFAC) were used to characterize DOM in 5-m profile of farmland and forestland (Pinus tabulaeformis and Robinia pseudoacacia) in the southern CLP. The results demonstrated that the average dissolved organic carbon (DOC) content of the surface layer of farmland (119.3 mg kg−1 soil) was lower than that of forestland (Pinus tabulaeformis 175.5 mg kg−1 soil; Robinia pseudoacacacia 166.4 mg kg−1 soil). The DOC content gradually decreased with increasing soil depth and reached stability after 2 m depth. Three substances, including tryptophan-like substances (C1) and two humic acid-like substances (C2, C3), were detected from all samples. Tryptophan-like substances (C1) significantly increased with soil depth while humic acid-like substances (C2, C3) significantly decreased particularly in farmland. The humic acid-like content of surface soils (Robinia pseudoacacia) was relatively higher, but the difference between the two vegetation soils was not significant. The freshness index (β/α) values of DOM as well as biological index (BIX) values were significantly higher in farmland than that in forestland, and the humification index (HIX) values were lower than in forestland soils, indicating that the change of soil DOM in farmland was more active than that in forestland and more dependent on local terrestrial sources. These results could contribute to a better understanding of the vertical distribution and features of soil DOM during the ‘Grain-for-Green’ program of CLP.

Decoupled Chinese Loess Plateau Dust Deposition and Asian Aridification at Millennial and Tens of Millennial Timescales

AbstractIt is generally assumed that increased dust accumulation rate (DAR) on the Chinese Loess Plateau (CLP) is associated with intensified inland Asian aridification. However, the timescales and area that any such association operates over is unclear. In this study, we demonstrate a lack of correlation between the southern CLP loess DAR variations and aridification over the past 130 ka, which does not support a direct link between high DAR and intensified inland Asian aridification over millennial to tens of millennial timescales. Instead, we propose that loess DAR variation is more likely determined by wet‐dry cycles and associated loose sediment production and availability via fluvial and glacial processes.

Genesis of Loess Particles on the Chinese Loess Plateau

AbstractThe sources of the sediments in distinct areas of the Chinese Loess Plateau (CLP) are still disputed, starting with the particle formation in the original desert areas, as well as the neighboring river systems. The formation of loess particles in the southern CLP was investigated using Scanning Electron Microscopy (SEM) and combined material modal composite analysis. Here, we determined the mineralogy in the Duanjiapo (DJP) section on the southernmost CLP and the last glacial‐interglacial samples on the central and north CLP. The results reveal that the samples plot along the N‐S transect, which is very likely the result of differential weathering. The SEM observations of the internal textures of soil aggregates from DJP suggest that precipitation mediated dissolution led to the formation of the fine particle fraction (5–10 μm). The most dissolution was observed for the hornblende particles. The further evolution of these smaller particles followed a decrease (2.6–0 Ma) in the grain size up section. Our record of the changes in and trend of the hornblende concentrations suggests an imbalance in the budget of the hornblende influx via sub‐aerosol wind transport versus the mineral dissolution, which provides robust evidence for a sediment provenance in the deserts north and west of CLP. Understanding the sediment source is critical to trace Asian monsoon evolution. The hornblende record suggests a coeval change in the trends of the summer monsoon and winter monsoon, and this contemporaneous change may not support the recently proposed Pacific Walker Circulation mechanism in the interpretation of Asian monsoon.

East Asian monsoon precipitation decrease during Plio-Pleistocene transition revealed by changes in the chemical weathering intensity of Red Clay and loess-paleosol

East Asian Monsoon variations during the Plio-Pleistocene transition have attracted much scientific interest, but they are still not well understood. We analysed the trace and major element concentrations of the Red Clay and loess-paleosol sequence of the XSC section, on the southern Chinese Loess Plateau (CLP), over the time interval of 4–2 Ma. From the results of geochemical and mineralogical analysis, we conclude that the Na/K ratio can be used to reflect the weathering intensity of plagioclase. Grain- size effects on the Na/K ratio were minimized based on the analysis of multiple size fractions. The Na/K ratios show similar trends between different grain-size fractions during the Pliocene-Pleistocene transition. Additionally, the Na/K ratio for various Holocene eolian sediments is well correlated with the regional mean annual precipitation (MAP). Based on this observation, we relate the Na/K ratio to East Asian Summer Monsoon (EASM) precipitation and propose that the EASM is the primary control of the post-depositional weathering intensity. The Na/K record of the EASM precipitation, as inferred from the Na/K record of the XSC section, progressively weakened and increased in amplitude between 4 and 2 Ma, indicating that the climate on the monsoonal margin evolved from warm and moist during the Late Pliocene to dry and cold during the Early Pleistocene. Our results support the hypothesis that global cooling was responsible for driving the decrease in EASM-derived rainfall via the migration of the intertropical convergence zone (ITCZ) and the associated monsoon precipitation belt during the Plio-Pleistocene transition.

Microscale evidence for and formation mechanisms of shear-strength anisotropy of a loess-paleosol sequence since the late Early Pleistocene: The case study of the Xiushidu profile, Southern Chinese loess Plateau

The shear-strength anisotropy of loess-paleosol sequences (LPS) provides vital information for engineering and construction activities and mitigation against geological disasters. The shear-strength anisotropy of undisturbed and disturbed samples from a profile from Xiushidu village, Jingyang County, in the Chinese Loess Plateau, and the formation mechanisms of such anisotropy were studied through direct shear tests and the microstructural analysis of their characteristics. Our results suggest that the shear strength of loess layers L1, L2, L3, L4, and L5 along the vertical direction was greater than that along the horizontal direction, under different values of axial compression. Under the same vertical pressure, the shear strength along the vertical direction was also generally greater for the layers above the seventh paleosol layer (S6), while the opposite result appears in the layers below S6. For the disturbed samples, their anisotropic characteristics are the same as for the undisturbed samples. The microstructure characteristics, including skeleton particles, contact relations, pore forms, cementation degree, and pore area ratio (PAR), explicated the reasons for the anisotropy of shear strength. Moreover, the influence of PAR, porosity (n), and dry density (ρd) on the shear strength in the vertical direction are greater than that in the horizontal direction, and the influence of water content is opposite to that of PAR, n, and ρd. Gravity, van der Waals forces, and hydrogeochemical reactions play important roles in the shear-strength anisotropy of the LPS. Therefore, research on the anisotropy of shear strength contributes to increasing our understanding of such layers, in preventing and mitigating against potential geological disasters and in implementing more appropriate engineering construction techniques.

Formation and Evolution of a Loess Sinkhole in the Southern Chinese Loess Plateau

Formation and Evolution of a Loess Sinkhole in the Southern Chinese Loess Plateau

Changes in Paleovegetation and Climate Seasonality in Central China Over Last Two Glacial Cycles: A Stable Isotope Perspective From Land Snails

AbstractModern investigations have shown that oxygen and carbon isotopes of land snail shells are useful indicators of climate and vegetation in monsoonal regions. However, stable isotope studies on snail fossil shells have seldom been done, and the reliability of those indicators needs further verification. Moreover, intrashell stable isotope analysis of individual snails is rather scarce, and seasonal variation in the glacial‐interglacial monsoonal climate remains unclear. Therefore, we performed δ18O and δ13C analyses on fossil shells of cold‐aridiphilous Cathaica pulveratrix and subhumidiphilous Metodontia yantaiensis from the loess section over the last two glacial cycles at the Beiyao site on the southern Chinese Loess Plateau. The δ18O values of fossil shells reflected monsoonal rainfall amounts and more rainfall during MIS3 and MIS7 than during MIS4 and MIS6. Meanwhile, the δ13C values of fossil shells indicated the relative abundance of C3/C4 plants and more C4 biomass during MIS3 and MIS7 than during MIS4 and MIS6. The δ18O and δ13C values of the two species from the same horizon are significantly different, reflecting differences in their growing season and/or physiological habits. Intrashell variations in stable isotopes showed that climatic seasonality was much stronger during the glacial periods than during the interglacials. Our findings provide an environmental background for explaining past human activities at the Beiyao site. The investigation of stone artifacts showed that ancient human activities were relatively strong during MIS3 and MIS7. During these stages, the warm and humid climate with smaller seasonal contrast was favorable for the regional expansion of human activities.

Decoupling of the detrital linkage between proximal dunefields and early and middle Pleistocene accumulation in the Chinese Loess Plateau: evidence from the Badain Jaran and Tengger sandy deserts

The dunefields upwind of the Chinese Loess Plateau (CLP) are widely regarded as the major source of the sediments of the CLP. To test this inference, we conducted a source tracing study using U–Pb age spectra of detrital zircons from the central Badain Jaran Desert (BJD). The results reveal that since 1.1 Ma the sediments in the central BJD have originated predominantly from the Gobi-Altay Mountains (GAMs), with a dominant Northern Tibetan Plateau (NTP) provenance at ∼0.7 Ma and at the present. Together with a previous study of the neighboring Tengger Desert (TD), our results support the conclusion that during 2.35–0.62 Ma the central parts of the proximal dunefields of the BJD and TD acted as reservoirs of sediments deflated mainly from strata with a GAMs-origin, with the addition of NTP-derived sediments after 0.36 Ma. Comparison of the original provenance areas of sediments in the central parts of the BJD and TD with the loess deposits in the central and southern CLP indicates that the proximal dunefields of the BJD and TD were not the dominant sources of dust transported to the CLP during early to middle Quaternary. Based on our results, combined with evidence from previous studies, we propose that NTP-derived sediments dominated accumulation in the CLP for most of the Quaternary, while during 2.35–0.62 Ma the proximal dunefields, including the BJD and TD, acted only as reservoirs of sediments deflated and eroded predominantly from the GAMs. Essentially, these sandy deserts functioned as transfer stations, which, after ∼0.36 Ma, received increased amounts of NTP-derived sediments and subsequently supplied silt to downwind areas, including the CLP.

Anisotropic microstructure of loess-paleosol sequence and its significance for engineering and paleoclimate: A case study from Xiushidu (XSD) profile, southern Chinese Loess Plateau

Abstract Due to the lack of a deep understanding of the anisotropic microstructure of a loess-paleosol sequence and its significance for engineering and paleoclimate, this study describes a micromorphological investigation of a loess-paleosol sequence (including layers of L0-S9) with an approximate 56.4 m thickness on the southern edge of the Chinese Loess Plateau. The results demonstrate that the microstructure of the loess changes significantly with the burial depth. The coarse particles gradually evolve into aggregates, flocculates, and even coagula. Correspondingly, the main contact relation changes from point-contact to face-cementation, with pore forms changing from space pores to intergranular pores, and the cementation degree from poor to moderate. However, the changes in the paleosol microstructure with the burial depth are slight. The cementation degree of skeleton particles in the paleosol is high, and paleosol's structure is denser. The loess and paleosol are the marks of a dry-cold climate and a warm-humid climate, respectively, and are used in the reconstruction of the paleoclimate. The grain-size composition of the loess-paleosol also verifies this conclusion. The various pore area ratios decrease with the burial depth, especially the macropore area ratio. In the loess layer, these ratios in the horizontal section are larger than those in the vertical section, but it is the opposite in the paleosol layer. The anisotropic engineering properties are interpreted by this anisotropic microstructure of the loess-paleosol, which is controlled by the sedimentation and pedogenesis in different paleoclimates.

Insights Into the Significance of the Chinense Loess Plateau for Preserving Biodiversity From the Phylogeography of Speranskia tuberculata (Euphorbiaceae).

The significance of the Chinese Loess Plateau (CLP) in maintaining biodiversity for northern China has rarely been shown, as previous phylogeographic studies are mostly woody species and they have revealed that Quaternary refugia are mainly located in mountain regions. We selected a drought-enduring endemic herb, Speranskia tuberculata (Euphorbiaceae), to determine its glacial refugia and postglacial demographic history. To this end, we sampled 423 individuals from 38 populations covering its entire geographic distribution. Three chloroplast DNA (cpDNA) fragments, two low-copy nuclear genes, and six nuclear microsatellites (nSSRs) were used and supplemented with ecological niche modeling (ENM) to infer the phylogeographic history of this species. Populations with private haplotypes and high haplotype diversity of cpDNA are mainly located in the CLP or scattered around northeastern China and the coastal region. Spatial expansion, detected using a neutrality test and mismatch distribution, may have resulted in a widely distributed ancestral cpDNA haplotype, especially outside of the CLP. For nuclear DNA, private haplotypes are also distributed mainly in the CLP. In nSSRs, STRUCTURE clustering identified two genetic clusters, which are distributed in the west (western cluster) and east (eastern cluster), respectively. Many populations belonged, with little to no admixture, to the western cluster while (hardly) pure populations of the eastern cluster were barely found. Genetic differentiation is significantly correlated with geographic distance, although genetic diversity is uniformly distributed. ENM suggests that the distribution of S. tuberculata has recently expanded northwards from the southern CLP, whereas it has experienced habitat loss in the south. Thus, S. tuberculata populations probably survived the last glacial maximum (LGM) in the southern CLP and experienced post-glacial expansion. Wind-dispersed pollen could bring the majority of genotypes to the front during spatial expansion, resulting in uniformly distributed genetic diversity. Based on evidence from molecular data and vegetation and climate changes since the LGM, we conclude that drought-enduring species, especially herbaceous species, are likely to have persisted in the CLP during the LGM and to have experienced expansion to other regions in northern China.

Impacts of land-use conversions on the water cycle in a typical watershed in the southern Chinese Loess Plateau

Land use change is one of the dominant driving factors of hydrological change at the watershed scale. Thus, understanding the hydrological responses to land use changes can facilitate development of sustainable water resource management. The land use of the Wei River (the largest tributary of the Yellow River) Basin (WRB) has changed greatly due to the large-scale ecological restoration program in the Chinese Loess Plateau (e.g., grain-for-green program), causing dramatic impacts on the water cycle. This study was to simulate the impacts of land use and land cover change (LUCC) on the key hydrological components, using the Soil and Water Assessment Tool (SWAT). We investigated the spatiotemporal changes of LUCC from 1980 to 2010 in terms of four sub-regions (i.e., three subbasins and one mid-downstream area) and three landforms (i.e., mountain, hill, and plain), respectively. Then, we quantified the spatial heterogeneity of hydrological responses to land use change scenarios. Our LUCC analysis showed that cropland declined by about 0.8%, from 58,776 km2 in 1980 to 57,519 km2 in 2010, whereas the forest and grassland areas correspondingly increased 552 km2 and 16 km2, respectively. The urban area changed more dramatically in the middle and lower reaches of the Wei River owing to the faster socioeconomic development in this region than the rest areas. In mountain area, the main types of land use were forest and grassland, and the main transformation was from cropland to forest land and grassland. The area of cropland in plain could be over 50%, and it was mainly converted to urban land. The hydrological simulation indicated that LUCC from 1980 to 2010 caused 5.3% of decrease in the water yield and 6.2% increase in soil water content, but there was nearly no change in evapotranspiration (ET). Scenarios about slopping land use conversion (SLC) program showed that the conversion of cropland to grassland or forest (i.e., grain-for-green) resulted in negative effects on soil water content and water yield, with a greater effect by the reforestation. In addition, we found that the change of ET was clear in areas where cropland with slope > 15° was converted to grassland or forest, suggesting slope is also an important factor for hydrological responses to LUCC. This study can provide valuable decision support for land use planning and water resources protection in the WRB.

An Attempt to Recover a Paleomonsoon Signal in the Chinese Loess Plateau Using Anisotropy of Magnetic Susceptibility

The paleowind direction in the Chinese Loess Plateau (CLP) was studied using anisotropy of magnetic susceptibility (AMS) data. Magnetic fabrics were examined for the last glacial loess (L1) and last inter-glacial paleosol (S1) in the southern and central CLP along a present day precipitation zone of 600 mm per year. Statistical analyses of the orientations of AMS ellipsoids indicate that strong rainwater events may cause random distributions for the AMS azimuth of maximum axis and no deviation from the vertical plane for the average minimum axis. As high rainfall can distort paleowind analyses, it is challenging to conduct paleowind studies on the southern CLP - a topography of high precipitation density. Future studies could focus on the western and northern CLP, where there is less rainfall, making it more suitable for paleowind analysis by AMS.

Cascading response of flora and terrestrial mollusks to last deglacial warming

The cascading impacts of climate-driven vegetation change on terrestrial invertebrates via trophic and habitat routes is a matter of outstanding concern. However, the combined responses of plant-animal associations in terrestrial ecosystems have seldom been reported on geological timescales, which raises the important questions of how frequently and to what extent such multi-trophic shifts in taxa have occurred during previous warming scenarios. Here we use a combination of mollusk, palynological, and climatic proxy evidence preserved within loess deposits in the southern Chinese Loess Plateau (CLP) to demonstrate how long-term changes in hydroclimatic dynamics during the past 26 kyr were reflected in the subsequent ecological changes. Specifically, there was a shift from a predominantly cold-tolerant mollusk fauna in a dry-steppe environment under cold-dry conditions during the last glacial, to a predominantly thermal-mesophilous fauna in a meadow-steppe environment under a warm temperate climate in the early-middle Holocene. Warmth-adapted mollusk species such as Opeas striatissimum and Punctum orphana were found to have been associated with several deciduous tree taxa (e.g., Corylus, Quercus, and Pterocarya) in both the Holocene record and in the modern ecosystem (based on surveys of 356 surface-soil plots). This suggests there is a clear ecological interdependency between specific vegetation types and mollusk assemblages. As such, the long-term vegetation-mollusk fossil records highlight the indirect impacts of climate change on terrestrial invertebrate turnover via plant-derived food structure and habitat routes, with major implications for understanding how these ecosystems are likely to respond to future climate changes.

Mid-Holocene soil water and vegetation in the Xi’an area of the southern Chinese Loess Plateau

This paper investigates mid-Holocene palaeo-vegetation and soil water in the Xi’an area of the southern Chinese Loess Plateau (CLP). A well-dated (8.5–6.0 ka BP and 5.0–3.1 ka BP) loess-palaeosol (unit S0) was logged, sampled and analyzed at four sites in the Xi'an area of the southern CLP, from which we report soil micromorphological (type and occurrence of root pores), particle size and geochemical data (CaCO3 and Sr concentration) to: (i) infer changes in soil water content, (ii) infer the limits of gravity water and the types of hydrologic cycle, and (iii) reconstruct the palaeovegetation. Numerous woody plant root holes were identified (2–4 mm diameter) within the illuvial horizon underlying palaeosol S0, the morphology of which is consistent with those produced by woody plants. The clay-rich argillic horizon of palaeosol S0 was found to contain high concentrations of kaolinite, which in concert with the geochemical indices, indicates that soil gravity water attained a depth of 2.2–2.8 m, whilst soil water content was ~20–25%. This implies that pedogenic processes at this site were more intense during the mid- compared to late Holocene loessic soils elsewhere on the CLP, and ultimately attests to relatively wet climatic conditions (e.g. up to 800 mm yr−1 rainfall), the hydrologic cycle type of precipitation-soil water-groundwater, and the vegetation of deciduous broad-leaved forest. We propose that combined analyses of root-channel morphology and mobile element geochemistry can be used to infer changes in palaeo-hydrology and ultimately, the strength of the Asian Summer Monsoon.

Carbon dioxide fluxes in a farmland ecosystem of the southern Chinese Loess Plateau measured using a chamber-based method.

BackgroundFarmland accounts for a relatively large fraction of the world’s vegetation cover, and the quantification of carbon fluxes over farmland is critical for understanding regional carbon budgets. The carbon cycle of farmland ecosystems has become a focus of global research in the field of carbon dynamics and cycling. The objectives of this study are to monitor the temporal variation in the net ecosystem exchange (NEE) and soil respiration in a spring maize (Zea mays L.) farmland ecosystem of the southern Loess Plateau of China.MethodsA fully automated temperature-controlled flux chamber system was adopted in this study. The system contained nine chambers for CO2 flux measurements, and three treatments were conducted: with and without maize plants in the chamber, as well as a bare field. Observations were conducted from June to September 2011. This time period covers the seedling, jointing, heading, grain filling, and ripening stages of spring maize. Other factors, such as air temperature (Ta), soil temperature (Ts), soil water content (SWC), photosynthetically active radiation (PAR), and precipitation (P), were simultaneously monitored.ResultsThere was observed diurnal variation in the NEE of the maize ecosystem (NEE-maize). A short “noon break” occurred when the PAR intensity was at its maximum, while soil respiration rates had curves with a single peak. During the overall maize growth season, the total NEE-maize was –68.61 g C m−2, and the soil respiration from the maize field (SR-maize) and bare field (SR-bare field) were 245.69 g C m−2 and 114.08 g C m−2, respectively. The temperature sensitivity of soil respiration in the maize field exceeded that in the bare field. Significant negative correlations were found between the NEE, PAR, and temperature (all p-values < 0.01), with both Ta and PAR being the primary factors that affected the CO2 fluxes, collectively contributing 61.7%, 37.2%, and 56.8% to the NEE-maize, SR-maize, and SR-bare field, respectively. It was therefore concluded that both meteorological factors and farming practices have an important impact on the carbon balance process in corn farmland ecosystems. However, it is necessary to conduct long-term observational studies, in order to get a better understanding of the driving mechanism.