Histic Gleysol Research Articles

Turf-bearing topsoils on the central Tibetan Plateau, China: Pedology, botany, geochronology

Abstract Vast areas on the Tibetan Plateau are covered by alpine sedge mats consisting of different species of the genus Kobresia. These mats have topsoil horizons rich in rhizogenic organic matter which creates turfs. As the turfs have recently been affected by a complex destruction process, knowledge concerning their soil properties, age and pedogenesis are needed. In the core area of Kobresia pygmaea mats around Nagqu (central Tibetan Plateau, c. 4500 m a.s.l.), four profiles were subjected to pedological, paleobotanical and geochronological analyses concentrating on soil properties, phytogenic composition and dating of the turf. The turf of both dry K. pygmaea sites and wet Kobresia schoenoides sites is characterised by an enrichment of living (dominant portion) and dead root biomass. In terms of humus forms, K. pygmaea turfs can be classified as Rhizomulls mainly developed from Cambisols. Wet-site K. schoenoides turfs, however, can be classified as Rhizo-Hydromors developed from Histic Gleysols. At the dry sites studied, the turnover of soil organic matter is controlled by a non-permafrost cold thermal regime. Below-ground remains from sedges are the most frequent macroremains in the turf. Only a few pollen types of vascular plants occur, predominantly originating from sedges and grasses. Large amounts of microscopic charcoal (indeterminate) are present. Macroremains and pollen extracted from the turfs predominantly have negative AMS 14C ages, giving evidence of a modern turf genesis. Bulk-soil datings from the lowermost part of the turfs have a Late Holocene age comprising the last c. 2000 years. The development of K. pygmaea turfs was most probably caused by an anthropo(zoo)-genetically initiated growth of sedge mats replacing former grass-dominated vegetation (‘steppe’). Thus the turfs result from the transformation of pre-existing topsoils comprising a secondary penetration and accumulation of roots. K. schoenoides turfs, however, are characterised by a combined process of peat formation and penetration/accumulation of roots probably representing a (quasi) natural wetland vegetation.

Modelling nitrous oxide emission from water-logged soils of a spruce forest ecosystem using the biogeochemical model Wetland-DNDC

During the last decades, decision makers and policy have increasingly demanded for regional and national inventories of greenhouse gas emission, such as nitrous oxide (N2O), to develop appropriate strategies and mitigation options. A potential way to derive large-scale estimates of N2O emission is the use of process-based models, such as PnET-N-DNDC or Wetland-DNDC. While PnET-N-DNDC has been effectively applied for various upland forest ecosystems, the Wetland-DNDC model has not yet been validated with regard to N2O emission. We calibrated and validated the Wetland-DNDC model on the basis of a 4-year field data set of two water-logged soils (Humic Gleysol and Histic Gleysol) of a spruce forest ecosystem. Model calibration by means of the Levenberg–Marquardt algorithm considerably improved the model performance for the period of calibration (2001–2002). The error variance was reduced by up to a factor of two and the modelling efficiency was increased from −1.24 to −0.15 (Humic Gleysol) and from −0.42 to 0.1 (Histic Gleysol). However, the model performance for the period of validation (2003–2004) and particularly for the extreme dry period in summer 2003 was not fully satisfying, notably with regard to the temporal pattern of the N2O emission.

Pore Shrinkage Dependency of Inorganic and Organic Soils on Wetting and Drying Cycles

Cycles of wetting and drying (WD) occur naturally in soils and affect the pore structure through altered hydraulic stresses. Two organic‐rich soils, a Eutric Histosol and a Histic Gleysol, and two inorganic soils, a Calcic Gleysol and a Dystric Gleysol, ranging in texture and microstructure, were investigated. Undisturbed soil samples were predried to either −100 kPa water potential by using a ceramic plate or to 30°C by using an oven and then resaturated for one or three WD cycles. In addition, different combinations defined by the intensity, frequency, and sequence of WD cycles were analyzed. Soil structure was altered significantly if the intensity of drying was severe at 30°C, while drying to −100 kPa had only a small effect. The frequency and sequence of WD cycles did not alter the structure and shrinkage behavior significantly. Compared with the initial pore volume, intense WD cycles decreased it by 23.6 to 60.1% in the two organic‐rich soils, whereas it increased by 1.5 to 4.8% in the silty Calcic Gleysol and by 3.6 to 15.1% in the clayey Dystric Gleysol. Both organic‐rich soils showed more shrinkage but less swelling than did the two inorganic soils. Intense WD cycles altered the water potential vs. void ratio curves of the two organic‐rich soils more gradually, while steeper patterns were observed for the two inorganic soils. This study shows that the changes in soil structure and pore shrinkage depend mostly on the maximum intensity of previous WD cycles.

Anisotropic shrinkage and swelling of some organic and inorganic soils

SummaryShrinkage and swelling affect hysteretic changes of soil volume as a function of water content. The processes complicate the precise description of changes to soil structure during cycles of wetting and drying, particularly the interpretation for pore structure from water retention characteristics. We investigated soil shrinkage and swelling curves and the hysteresis of void ratio as a function of moisture ratio of four soils (Eutric Histosol, Histic Gleysol, Calcic Gleysol and Dystric Gleysol), and illustrated the anisotropy of soil shrinkage and swelling in two dimensions. We used a model we had developed previously to fit and to define the endpoints of different shrinkage or swelling ranges. The two organic‐rich soils showed only structural and proportional shrinkages, whereas the two inorganic Gleysols covered all four shrinkage ranges. Of all the shrinkage ranges, proportional shrinkage contributed to 60.2–96.8% of total decrease in volume. A typical soil swelling curve was composed of two distinct parts, which were defined as virgin swelling and residual swelling with separation of the wet‐side maximum curvature. The virgin swelling accounted for 81.2–91.9% of the total increase in soil volume. We found pronounced hysteresis during shrinkage and swelling. A single cycle of wetting and drying lost 10–78% of the soil volume. Shrinkage of the organic‐rich soils was significantly greater than that of inorganic Gleysols. The geometry factor, depicting the relationship between the vertical and horizontal deformation, increased from 1.0 to 3.2–4.0 during drying but decreased from 4.6–21.7 to 3.5–5.6 during wetting. The dependence of water content on the geometry indicates anisotropic shrinkage and swelling of the soils. Our results improve the understanding of the alteration of soil structure during shrinkage and swelling and can be also applied to predict hydraulic properties more accurately.

Conceptualization of runoff processes using a geographical information system and tracers in a nested mesoscale catchment

AbstractTracer investigations were combined with a geographical information system (GIS) analysis of the 31 km2 Girnock catchment (Cairngorm Mountains, Scotland) in order to understand hydrological functioning by identifying dominant runoff sources and estimating mean residence times. The catchment has a complex geology, soil cover and topography. Gran alkalinity was used to demonstrate that catchment geology has a dominant influence on baseflow chemistry, but flow paths originating in acidic horizons in the upper soil profiles controlled stormflow alkalinity. Chemically based hydrograph separations at the catchment scale indicated that ∼30% of annual runoff was derived from groundwater sources. Similar contributions (23–36%) were estimated for virtually all major sub‐basins. δ18O of precipitation (mean: − 9·4‰; range: − 16·1 to − 5·0‰) and stream waters (mean: − 9·1‰; range: − 11·6 to − 7·4‰) were used to assess mean catchment and sub‐basin residence times, which were in the order ∼4–6 months. GIS analysis showed that these tracer‐based diagnostic features of catchment functioning were consistent with the landscape organization of the catchment. Soil and HOST (Hydrology of Soil Type) maps indicated that the catchment and individual sub‐basins were dominated by hydrologically responsive soils, such as peats (Histosol), peaty gleys (Histic Gleysols) and rankers (Umbric Leptosols and Histosols). Soil cover (in combination with a topographic index) predicted extensive areas of saturation that probably expand during hydrological events, thus providing a high degree of hydrological connectivity between catchment hillslopes and stream channel network. This was validated by aerial photographic interpretation and groundtruthing. These characteristics of hydrological functioning (i.e. dominance of responsive hydrological pathways and short residence times) dictate that the catchment is sensitive to land use change impacts on the quality and quantity of streamflows. It is suggested that such conceptualization of hydrological functioning using tracer‐validated GIS analysis can play an important role in the sustainable management of river basins. Copyright © 2006 John Wiley & Sons, Ltd.

A Lateglacial palaeosol cover in the Altdarss area, southern Baltic Sea coast (northeast Germany): investigations on pedology, geochronology and botany

AbstractA new site with Lateglacial palaeosols covered by 0.8 - 2.4 m thick aeolian sands is presented. The buried soils were subjected to multidisciplinary analyses (pedology, micromorphology, geochronology, dendrology, palynology, macrofossils). The buried soil cover comprises a catena from relatively dry (’Nano’-Podzol, Arenosol) via moist (Histic Gleysol, Gleysol) to wet conditions (Histosol). Dry soils are similar to the so-called Usselo soil, as described from sites in NW Europe and central Poland. The buried soil surface covers ca. 3.4 km2. Pollen analyses date this surface into the late Allerød. Due to a possible contamination by younger carbon, radiocarbon dates are too young. OSL dates indicate that the covering by aeolian sands most probably occurred during the Younger Dryas. Botanical analyses enables the reconstruction of a vegetation pattern typical for the late Allerød. Large wooden remains of pine and birch were recorded.

N2O emissions of a mature Norway spruce (Picea abies) stand in the Black Forest (southwest Germany) as differentiated by the soil pattern

Nitrous oxide is a greenhouse gas contributing to stratospheric ozone depletion with major sources that derive from soils. Most measurements were performed at well‐aerated or anaerobic sites, excluding common hydromorphic soils (e.g., Gleysols). The main thesis of this study was that the intermediate aeration of such soils promotes N2O emissions. The investigated catena “Wildmooswald,” which is little more than 200 m long, includes common temperate forest soils (n = 7) of three aeration categories (Cambisols, Gleysols, and Histosol). The influence of fluctuating water tables of the Gleysols led to distinctively higher N2O release compared to the Cambisol within the same mature Norway spruce ecosystem. Even a stable “hot spot” of N2O emissions was detected at a Histosol, where artificial drainage created a redox environment comparable to the Gleysols. Ranking the soils according to emission rate results in fairly regular doubling steps; for example, mean annual flux rates in kg N ha−1 yr−1 for the 2.5 years of measurements were −0.1, 0.4, 1.0, 1.9/1.9, 3.2, and 6.4 for Fibric Histosol, Chromic Cambisol, Endoskeletic Cambisol, Histic Gleysol 1/2, Humic Gleysol, and Sapric Histosol, respectively. The annual emission rates (0.93 kg N ha−1 yr−1) of the predominant Cambisols (63.4% of the area) are comparable to other studies, but only 34.3% of intermediately aerated soils led to a redoubling of the average to 1.86 kg N ha−1 yr−1, which is clearly higher than previously presented from temperate forests. Therefore N2O releases of soils are underestimated if soils having intermediate aeration conditions are left unconsidered.

CONCENTRATIONS AND PROPERTIES OF DISSOLVED ORGANIC MATTER IN FOREST SOILS AS AFFECTED BY THE REDOX REGIME

Changes in concentrations and properties of dissolved organic matter (DOM) caused by oxygen deficiency are poorly understood. We estimated the influence of redox conditions on DOM dynamics in the field, sampling soil solutions at different depths of three soils (Humic and Histic Gleysol, Chromic Cambisol) along a soil catena in the cool-humid Black Forest (Germany) over a period of 2 years. We measured dissolved organic carbon (DOC) and determined the specific absorbance at 280 nm and two humification indices derived from fluorescence spectra to describe the aromaticity and complexity of DOM. Redox potential (Eh) was monitored continuously in situ. In the forest floor, DOC concentrations ranged independent of soil organic matter content and redox regime between 40 and 60 mg C L−1. DOC concentrations in all soils decreased with depth, accompanied by a decrease in DOM aromaticity and complexity. In the mineral subsoil, DOC concentrations, aromaticity, and DOM complexity were smallest in the aerobic soil (Chromic Cambisol; Eh > 500 mV) and largest in the most anaerobic soil (Histic Gleysol; Eh < 100 mV). Large DOM retention in the aerobic soil could be related to high contents of Fe oxides, highlighting their importance for DOM adsorption. Despite significantly reduced DOM retention under anaerobic conditions, it remains relatively large because the main DOM adsorbents changed from Fe oxides under oxic conditions to clay minerals, which were about 100 times more abundant under anaerobic conditions than Fe oxides. We found indications that biodegradation of DOM contributes more to DOM retention under anaerobic conditions, and we conclude that large DOM fluxes from anaerobic forest soils are the result of limited DOM adsorption in the subsoil rather than large DOM release from the topsoil.