Pedogenic Changes Research Articles

Taxonomy and genesis of soils in contrasting ecosystems of Southern Western Ghats, India

Clay minerals formed during soil formation are imprints of ecosystem functioning and pedogenic processes in landscapes. The study evaluated the variations in taxonomy, soil weathering and pedogenic changes in contrasting forest soils in the Southern Western Ghats, India. Three different forest systems with comparable soil-forming factors vis-a-vis evergreen, moist deciduous and a man-made system (rubber plantation) were evaluated for physical, chemical, morphological, and pedogenic characteristics of the related soils. Soil profiles were dug up to 1.50 m in the selected ecosystems and horizon-wise soil samples were collected. All the pedons were classified in the soil order of Ultisols and subclassified as Ustic Palehumult (evergreen forest), Ustic Haplohumult (moist deciduous forest) and Typic Haplustult (rubber plantation), respectively. The soil pH was found to be acidic in all the ecosystems ranging from 4.6 to 5.8. Chlorite, hydroxyl interlayered vermiculite (HIV), mica, kaolinite, feldspars, gibbsite and quartz dominated the mineral assemblages in the clay fractions of soils of natural forest systems. The losses of SiO2 in the non-clay fraction and corresponding enrichment of the component in the clay-sized fraction in all horizons indicated an active continuous weathering process in all the systems of this region. Clay accumulation by way of formation and translocation was highest in the lower horizons of the soils of evergreen (Bt2 at a depth range of 28–57 cm) and moist deciduous (Bt3C at a depth range of 23–150 cm) forest, whereas it was highest at Bt1 at a depth range of 15–28 cm in the plantation soil. The depth variations in the clay formations in natural and manmade forest systems provide insights into ecosystems regulated profile development under natural and man-made systems in the humid tropics, a vital information gap hitherto in soil pedogenesis of the humid tropics. Such pedogenic assessments along with the factors responsible for such modifications would provide important inputs in developing soil health management strategies in the humid tropics.

Soil formation and mineralogical changes on basaltic lava vs scoria along a hydroclimatic gradient on Santa Cruz Island, Galápagos

Climate and parent material type are important soil forming factors. On Santa Cruz Island, Galápagos, the climate shows a pronounced altitudinal zonation from arid lowlands to humid highlands, and the volcanic parent materials, although chemically rather uniform (basaltic), show pronounced differences in porosity, including dense lava flows and very vesicular scoria deposits. In this study, the effect of these two driving factors on pedogenesis was investigated. Our results show that pH and effective cation exchange capacity (CECeff) decreased, but potential cation exchange capacity (CECpot), phosphate retention, and Alo + ½Feo as well as Alp/Alo increased with increasing elevation regardless of parent material type. Conversely, topsoil bulk density (BD) and organic carbon (OC) content did not show a clear altitudinal pattern on lava but decreased (BD) or increased (OC) with increasing elevation on scoria. Clay mineralogy changed from minor (2:1-type) mixed layer phases in the low-elevation arid zone to vermiculitic (lava) or kaolinitic mineralogy (scoria) at mid-elevation (humid), whereas short-range-order (SRO) minerals (allophane and ferrihydrite) dominated in the colloidal fraction of both lava- and scoria-derived soils under very humid conditions at high elevation. The sequence of soil types developed on lava was, with increasing elevation, Cambisol → Phaeozem → Andosol, while on scoria it was Regosol → Umbrisol → Andosol. Our results show that climate is the overriding factor controlling the major mineralogical and pedogenic changes in the studied environment; however, the higher porosity of the scoria parent materials considerably accelerated weathering and pedogenic processes compared to the denser lavas. Our study highlights the importance of the parent material’s physical nature in modulating rates of chemical weathering and pedogenesis.

Response of microbial communities and their metabolic functions to calcareous succession process

Soil chronosequence is of great important in studying rates and directions of soil evolution, which can provide valuable information for the validation of soil genesis theory. However, the variation of microbial composition and structure in a calcareous soil chronosequence in karst region of southwest China is not clear. To reveal the response of microbial communities and their metabolic functions to calcareous succession process, a chronosequence of four calcareous soils (black calcareous soil, brown calcareous soil, yellow calcareous soil and red calcareous soil) with a depth of 0–20 cm from tropical monsoon rainforests of Guangxi Nonggang National Nature Reserve, southwest China was collected to analyze the soil physichemical and microbial properties. The results showed that the overall soil nutrient contents decreased along calcareous soil chronosequences and all calcareous soils were nitrogen (N) limitation. And, there were significant differences in the structure of microbial communities in calcareous soil chronosequences. To accommodate N-restriction, fungal community shifted from pathotroph to symbiotroph trophic pattern and Ectomycorrhizal fungi (ECM) emerged. ECM competing with free-living decomposers for N will slow soil carbon (C) cycling and increase soil C storage. Penicillium and Gaiella, the keystone genera, were related to phosphorus (P) cycle closely. Taken together, the occurrence of these microorganisms emphasizes the importance for C, N and P cycle in calcareous chronosequence soils and thus contributes to the ongoing worldwide endeavor to characterize their function for investigating the rate and direction of calcareous pedogenic changes.

Separating provenance and palaeoclimatic signals from particle size and geochemistry of loess-palaeosol sequences using log-ratio transformation: Central European loess belt, Czech Republic

Loess-palaeosol sequences represent the most complete terrestrial archives of Quaternary climatic cyclicality. Particle size and geochemistry are widely used proxy data in palaeoclimatologic analysis of loess-palaeosol sequences. The palaeoclimatologic signals hidden in the texture and chemical composition of the Czech loess-palaeosol sequences, which are part of the European loess belt, are modulated by temporal changes in the interplay of oceanic and continental macroclimates and by the diverse bedrock geology of the Bohemian Massif.Innovative tools of compositional data analysis, including log-ratio transformation and scalar-on-function regression, can substantially enhance the information value of large granulometric and geochemical datasets, when compared to classical statistics of raw data. Particle size distribution and bulk-rock geochemistry of 389 and 542 samples, respectively, from four Czech loess-palaeosol sequence sections representing the last glacial-interglacial cycle were analysed in this study. Centered log-ratio transformation was applied to the key elemental proxies and their spatial (between sections) and stratigraphic (within section) distribution. Centered log-ratio transformed densities of key elements were then plotted against particle size distribution to assess the particle size control on element concentrations.Nearly all loess-palaeosol sequence samples exhibit a bimodal particle size distribution with medium/coarse silt and coarse clay fraction representing the main modes. The dominant silt fraction is completely allogenic. The clay component is partly allogenic, transported by high altitudes air stream, and partly authigenic (neoformed in situ by pedogenic processes). However there is only a minor variation in particle size between the sections, the centered log-ratio transformed density functions reveal that the particle size control on the distribution of major and trace elements is highly site-specific. The provenance signal is recorded especially in coarser-grained fractions transported for a short distance from the source area by near-surface wind. The proportion of the authigenic clay fraction, the alteration of feldspars, micas and low alkali contents indicated by high Rb/K ratio and high values of Rb/Sr and Sr/Ca ratios, which indicate the intensity of carbonate leaching, reflect the intensity of chemical weathering, which is highest in the podzols. Precipitation rates were likely the main microclimatic factor that controlled the compositional differences. The loess-palaeosol sequences in areas with higher present-day annual precipitation show higher contents of clay minerals, higher degree of loessification (cementation) and subsequent pedogenic changes.

Pedoenvironmental indicators of soil in Western Amazonia, Brazil

This study aimed to ascertain what soil attributes may be an indicator of natural and anthropic pedogenic changes in top grassland (TG), low grassland (LG), ecotone zone (EZ), and forest (F) in Western Amazonia. Twenty samples of the diagnostic horizons were collected from the studied pedoenvironments: TG (A 0.0-14 and Bi 0.30-0.66 m); LG (A 0.0-0.15 and Cg1 0.27-0.80 m); EZ (A 0.0-0.15 and Bi 0.32-0.50 m); and forest (F) (A1 0.0-0.15 and Bt1 0.67-0.10 m). Sand, silt, clay, particle density (PD),soil bulk density (BD),saturated soil hydraulic conductivity, calcium (Ca), magnesium (Mg), exchangeable aluminum (Al3+), potassium (K), sodium (Na), phosphorus (P), pH in water and KCl, and organic matter (OM) contents were calculated and submitted to variance analysis. LG is more related to OM and clay than SHC value. These same attributes characterized TG. EZ is highlighted by its higher values and relationship with Al and K. It was concluded that for TG, LG, EZ, and F top soil layers, 28% of natural and anthropic changes can be identified based on sand, clay, pH in KCl, and OM data; on the other hand, subsurface 23% of determinations can be attributed to pH in H2O, Al, Na, and sand.

Visible and Near-Infrared Reflectance Spectroscopy Analysis of a Coastal Soil Chronosequence

The soil chronosequence is a useful method for investigating pedological theories. Soil chemical, physical and mineralogical properties in chronosequences change over time and exhibit systematic and time-dependent trends, which can be used to analyze the rates and directions of pedogenic changes. The potential of soil spectroscopy as an emerging, rapid and cost-effective technique for predicting soil properties has been widely accepted and has motivated the application of spectroscopic techniques to the analysis of soil chronosequence. We present a soil chronosequence derived from 1000-year-old calcareous marine sediments and examine changes in six soil properties over this period. We evaluated the utility of a soil spectroscopic method to detect soil property changes and to predict the pedogenic properties and soil ages of the chronosequence. The results show that some soil pedogenic processes, such as soil organic matter accumulation, CaCO3 leaching and clay migration, can be identified in the millennium chronosequence. Power chronofunctions are formulated for soil organic matter (SOM) and Logarithmic chronofunctions are fitted for clay, CaCO3 and pH. These pedogenic processes are identified in the reflectance intensity and absorption features of soil spectroscopy, and pedogenic properties can be calibrated via soil reflectance spectroscopy. Profile ages can also be predicted via pseudo multi-depth spectra of soil profiles, and soil spectral curves for 0–30 cm generated the best prediction results (RPD = 1.85). We conclude that soil properties, changing due to weathering and soil formation, act as a bridge linking spectroscopy and weathering levels/pedogenic processes. The results imply that applying spectroscopy techniques to chronosequence study and mapping the degree of soil development in certain areas should be possible.

Paleoenvironmental history of the Middle Dnieper Area from the Dnieper to Weichselian Glaciation: A case study of the Maksymivka loess profile

The paper presents the results of interdisciplinary studies on environmental characteristics of the Middle and Upper Pleistocene (MIS 8-2) of central Ukraine based on an example of the new research site at Maksymivka. The tested profile is situated on the left bank of the Dnieper River on the western edge of the Dnieper Plain. It is a representative section of the cliff of the east lakeshore of the Kremenchuk Reservoir, located in the border zone both in longitudinal and latitudinal terms. It is located in the southern part of the Dnieper terraced plateau on the border of two varieties of forest-steppes with an admixture of meadow steppes, defined as left-bank-Dnieper and Dniester-Dnieper. To the south of the site extends a typical steppe zone. The transient position of the research site is a good point to start to trace the history of the volatility of the vegetation cover and the shift of the landscape-vegetation boundaries according to the climatic fluctuations. To reconstruct environmental changes in the analysed time interval the litho-, pedo- and palaeobotanical methods were used. To determine the chronology of changes a number of parallel datings of the loess-paleosol sequence were conducted: TL (Gdańsk, Kyiv, Lublin), OSL (Gliwce).The profile is comprised of glacial and loess parts. The bottom section is built of glacigenic sediments of the marginal zone of the Dnieper Glaciation. The overlying loess-soil sequence of the last two glacial–interglacial cycles underwent detailed analyses. This section is composed of multiple soil levels, whose participation in the entire thickness of the layer dominates over primary loess. The studies have confirmed that they are lower stratigraphic rank soils of the Vytachiv (vt) unit (=MIS 3), and two complex higher rank units: Pryluky (pl; MIS 5) and Kaydaky (kd; MIS 7). For the distinguished litho- and pedostratigraphic units, paleoenvironmental characteristics were developed. The stratigraphic rank of these units was confirmed by dating. The results of the TL and OSL dating of the samples show good agreement between the laboratories, particularly in relation to the last cycle. However, the OSL dates for the penultimate cycle are significantly underestimated in relation to the TL dates, which in turn relate well to the global time framework. The reasons of the OSL dates underestimation might be connected with the post-sedimentation pedogenic changes.

Soil formation and mineralogy of a Rhodic Luvisol — insights from magnetic and geochemical studies

Relict terra rossa soil from the most south-eastern part of Bulgaria, characterized by transitional Mediterranean climate, has been comprehensively studied by integrating magnetic, geochemical and spectroscopic methods to reveal the origin, pedogenic processes and phases in soil development of this particular soil type. The red colored Rhodic Luvisol is developed on metamorphosed Triassic limestones. Magnetic methods, which include thermomagnetic analysis of susceptibility, isothermal remanent magnetization (IRM) acquisition and thermal demagnetization, IRM component analysis, hysteresis measurements, low-temperature (down to 10K) IRM behavior, anhysteretic remanence and frequency dependent susceptibility, indicate the presence of three major magnetic phases — maghemite, hematite (Hm) and goethite (Gt). Hematite and goethite are identified also by diffuse reflectance spectroscopy (DRS). Depth variations of the ratio Hm/(Hm+Gt), deduced from the DRS spectra show higher hematite content in the upper soil horizons (A+B), while goethite's contribution is enriched in the lowermost part of the profile. A similar ratio, based on the established magnetic proxies for hematite and goethite, was constructed and its variations were compared with the DRS data. The magnetic proxy for Hm/(Hm+Gt) reflects the variations in the remanence-carrying mineral fraction of hematite and goethite and the obtained difference with the DRS data are ascribed to the presence of the paramagnetic (or superparamagnetic) goethite in the A and illuvial Bt1 and Bt2 horizons. The low ratio Feo/Fed between dithionite (Fed)- and oxalate (Feo)-extractable iron, and the large proportion of extractable iron with respect to total iron (Fed/Fetot) indicate an advanced degree of weathering. Depth variations of magnetic parameters and ratios (χlf, χfd, S-ratio) suggest magnetic enhancement with SP–SD maghemite grains, accompanied by magnetically stable magnetic carriers in the soil, while the parent material is magnetically depleted. Based on grain-size variations of the pedogenic maghemite phase, deduced from a χARM vs. χfd plot, and variations in the content of hematite and goethite in depth, in the uppermost part of the profile, aging of the pedogenic iron oxides is assumed (grain growth from SP to SD and larger), while in the bottom part of Bt1 and the Bt2, changes only in the concentration of maghemite are inferred. The presence of goethite in different parts of the profile and the mineralogy of Fe–Mn nodules are linked to more recent (Holocene) pedogenic changes in the profile. Analyses of trace and rare earth element content and magnetic data suggest possible contributions of aeolian dust flux from Sahara during the soil formation.

Robust assessment of moderate heavy metal contamination levels in floodplain sediments: A case study on the Jizera River, Czech Republic

Enrichment factors for Cr, Cu, Ni, Pb and Zn in floodplain fines from the middle and the lower courses of the Jizera River (a tributary of the Elbe River in the Czech Republic) were evaluated to compare the original contamination profiles with post-depositional and pedogenic changes. Background concentrations of heavy metals were assessed from uncontaminated sediments (soils) in the study area that belong to the same sedimentary facies and were not affected by reductimorphic processes. Facies assignment is accessible by geophysical imaging combined with core analysis. Sediments from point bars and channel banks in direct contact with riverine water are more heavily polluted than overbank fines from the distal floodplain. The point pollution source, a century-old battery and car production facility in the city of Mladá Boleslav, has certainly been responsible for Ni and Cr pollution, contributed substantially to Cu and Pb pollution, and had a less significant effect on the Zn enrichment factor. Although the use of soil enrichment factors has been criticized, these factors help to manage hydraulic sorting and recognition of post-depositional migration in soil profiles of floodplain sediments. When moderate pollution (enrichment factor about 1.5 for Cu, Pb and Zn) is found, background concentrations must be carefully evaluated and natural enrichment must be taken into account. Studies of such small enrichment factors contribute to the understanding of the dispersal and fates of pollutants in floodplains.

Bacterial communities in soil mimic patterns of vegetative succession and ecosystem climax but are resilient to change between seasons

Organism succession during ecosystem development has been researched for aboveground plant communities, however, the associated patterns of change in below-ground microbial communities are less described. In 2008, a study was initiated along a developmental sand-dune soil chronosequence bordering northern Lake Michigan near Wilderness Park (WP). It was hypothesized that soil bacterial communities would follow a pattern of change that is associated with soil, plant, and ecosystem development. This study included 5 replicate sites along 9 soils (n = 45) ranging in age from ∼105 to 4010 years since deposition. Soil bacterial community composition and diversity were studied using bacterial tag-encoded FLX amplicon pyrosequencing of the 16S rRNA gene. Bray–Curtis ordination indicated that bacterial community assembly changed along the developmental soil and plant gradient. The changes were not affected by seasonal differences, despite likely differences in plant root C (e.g. exudates), temperature, and water availability in soil. Soil base cations (Ca, Mg) and pH declined, showing log-linear correlations with soil age (r ∼ 0.83, 0.84 and 0.81; P < 0.01). Bacterial diversity (Simpson's 1/D) declined rapidly during the initial stages of soil development (∼105–450 y) and thereafter (>450 y) did not change. Turnover of plant taxa was also more rapid early during ecosystem development and correlated with bacterial community structural change (P < 0.000001; r = 0.56). It is hypothesized that plants help to drive pedogenic change during early (<450 y) soil development (e.g. pH decline, cation leaching) which drive selection of soil bacterial communities. In mature soils (∼450–4000 y), resilient and stable soil bacterial community structures developed, mimicking steady-state climax communities that were observed during latter stages of primary plant succession. These relationships point to possible feedbacks between plant and bacterial communities during ecosystem development.

Understanding ecosystem retrogression

Over time scales of thousands to millions of years, and in the absence of rejuvenating disturbances that initiate primary or early secondary succession, ecosystem properties such as net primary productivity, decomposition, and rates of nutrient cycling undergo substantial declines termed ecosystem retrogression. Retrogression results from the depletion or reduction in the availability of nutrients, and can only be reversed through rejuvenating disturbance that resets the system; this differs from age‐related declines in forest productivity that are driven by shorter‐term depression of nutrient availability and plant ecophysiological process rates that occur during succession. Here we review and synthesize the findings from studies of long‐term chronosequences that include retrogressive stages for systems spanning the boreal, temperate, and subtropical zones. Ecosystem retrogression has been described by ecologists, biogeochemists, geologists, and pedologists, each of which has developed somewhat independent conceptual frameworks; our review seeks to unify this literature in order to better understand the causes and consequences of retrogression. Studies of retrogression have improved our knowledge of how long‐term pedogenic changes drive shorter‐term biological processes, as well as the consequences of these changes for ecosystem development. Our synthesis also reveals that similar patterns of retrogression (involving reduced soil fertility, predictable shifts in organismic traits, and ecological processes) occur in systems with vastly different climatic regimes, geologic substrates, and vegetation types, even though the timescales and mechanisms driving retrogression may vary greatly among sites. Studies on retrogression also provide evidence that in many regions, high biomass or “climax” forests are often transient, and do not persist indefinitely in the absence of rejuvenating disturbance. Finally, our review highlights that studies on retrogressive chronosequences in contrasting regions provide unparalleled opportunities for developing general principles about the long‐term feedbacks between biological communities and pedogenic processes, and how these control ecosystem development.

Understanding ecosystem retrogression

Over time scales of thousands to millions of years, and in the absence of rejuvenating disturbances that initiate primary or early secondary succession, ecosystem properties such as net primary productivity, decomposition, and rates of nutrient cycling undergo substantial declines termed ecosystem retrogression. Retrogression results from the depletion or reduction in the availability of nutrients, and can only be reversed through rejuvenating disturbance that resets the system; this differs from age-related declines in forest productivity that are driven by shorter-term depression of nutrient availability and plant ecophysiological process rates that occur during succession. Here we review and synthesize the findings from studies of long-term chronosequences that include retrogressive stages for systems spanning the boreal, temperate, and subtropical zones. Ecosystem retrogression has been described by ecologists, biogeochemists, geologists, and pedologists, each of which has developed somewhat independent conceptual frameworks; our review seeks to unify this literature in order to better understand the causes and consequences of retrogression. Studies of retrogression have improved our knowledge of how long-term pedogenic changes drive shorter-term biological processes, as well as the consequences of these changes for ecosystem development. Our synthesis also reveals that similar patterns of retrogression (involving reduced soil fertility, predictable shifts in organismic traits, and ecological processes) occur in systems with vastly different climatic regimes, geologic substrates, and vegetation types, even though the timescales and mechanisms driving retrogression may vary greatly among sites. Studies on retrogression also provide evidence that in many regions, high biomass or “climax” forests are often transient, and do not persist indefinitely in the absence of rejuvenating disturbance. Finally, our review highlights that studies on retrogressive chronosequences in contrasting regions provide unparalleled opportunities for developing general principles about the long-term feedbacks between biological communities and pedogenic processes, and how these control ecosystem development.

Phosphorus and soil development: Does the Walker and Syers model apply to semiarid ecosystems?

The Walker and Syers model of phosphorus (P) transformations during pedogenesis is widely accepted for the development of humid ecosystems, but long-term P dynamics of more arid ecosystems remain poorly understood. We tested the Walker and Syers model in semiarid piñon-juniper woodlands by measuring soil P fractions under tree canopies and in intercanopy spaces along a well-constrained, approximately 3000 ka (1 ka = 1000 years) volcanic substrate age gradient in northern Arizona, USA. The various pools of soil P behaved largely as predicted; total soil P and primary mineral P declined consistently with substrate age, labile inorganic P increased early in soil development and then declined at later stages, and organic phosphorus increased consistently across the chronosequence. Within each site, soils under tree canopies tended to have higher concentrations of labile and intermediately available P fractions compared to intercanopy soils. However, the degree of spatial heterogeneity conferred by tree islands was moderated by the stage of soil development. In contrast, tree islands had no influence on within-site distribution of more recalcitrant soil P pools, which appear to be controlled solely by the stage of pedogenesis. Coincident with declines in total P, primary mineral P, and labile inorganic P, we found that phosphatase enzyme activity increased with substrate age; a result consistent with greater ecosystem-level P demand on older, more highly weathered substrates. Our results suggest that, compared to humid climates, reduced inputs of water, energy, and acidity to semiarid ecosystems slow the rate of change in P fractions during pedogenesis, but the overall pattern remains consistent with the Walker and Syers model. Furthermore, our data imply that pedogenic change may be an important factor controlling the spatial distribution of labile P pools in semiarid ecosystems. Taken together, these data should both broaden and unify terrestrial ecosystem development theory.

VARIATION OF THE LITHIUM-BARIUM RATIO IN THE HOLOCENE LOESS-PALEOSOL PROFILES IN THE SOUTH OF THE CHINESE LOESS PLATEAU

Lithium (Li) and barium (Ba) distributions in the Laoguantai and Liwan loess-paleosol profiles in the south of the Chinese Loess Plateau show distinct variations caused by pedogenesis. Mean Li contents in paleosols are higher than those in the upper or subjacent loess layer. Paleosol units at the two sites, respectively, contain 5% and 18% more Li than that the loess units. Conversely, mean Ba values in paleosol units are noticeably lower than in loess units. Paleosol units contain 33% and 46% less Ba than loess units at the Laoguantai and Liwan sites, respectively. Comparison of Li/Ba ratios shows that Li varied slightly throughout the weathering profiles, but was enriched in the paleosol layers. The Ba distribution showed greater variation throughout the weathering profiles and has a larger impact on the Li/Ba ratios. This indicates that Ba is more actively lost during pedogenesis, whereas Li is relatively stable and accumulates as a pedogenic product. Variations in the Li/Ba ratios in the weathering profiles reflect pedogenic changes after dust deposition. Accordingly, the Li/Ba ratio is a reasonably good proxy for reconstructing the climate change and pedogenesis history contained in the loess-paleosol sequences. The Li/Ba ratios reveal that pedogenic intensity underwent polyphase changes during the Holocene in the eastern Guanzhong Basin of South Central China, likely driven by shifts in the East Asian monsoon.

Climatic and anthropogenic impacts on soil formation in the semiarid loess tablelands in the middle reaches of the Yellow River, China

Research into human interactions in past global changes at a landscape scale offers a common unit for integrating diverse evidence and allows the changes to be traced through time. Holocene pedogenic change and climatic and anthropogenic factors on a loess tableland in the semi-arid region of the middle reaches of the Yellow River drainage basin were investigated. Several Holocene loess-soil profiles were sampled and analysed. The results show that on the gentle loess ground with a surface gradient changing from 3°30′ to ca. 1°, natural sheet erosion and redeposition occurred in responding to climatic amelioration and, especially, to increased precipitation in the early Holocene. There was no obvious sheet erosion and redeposition recorded during the formation of Ustic Luvisols in the mid-Holocene Climatic Optimum. When the pre-historical people of the Taosi Culture occupied the loess tableland and cleared land for settlement and arable cultivation between 4400 and 3900 a B.P., a global event of rapid climatic change (4200–3800 a B.P.) hit the area. Both climatic instability and human disturbance has resulted in strong overland flows causing intensive sheet erosion on the upper part of the slope, redeposition and episodic flooding on the lower part of the slope and bottomland. The climatic instability, human disturbance, erosion and redeposition have resulted in a pedogenic regression over the tableland. On the ground outside the settlement, Ustic Luvisol formation was interrupted temporarily. Dust accumulation and pedogenic modification to it resumed soon when the settlement was deserted at ca. 3900 a B.P. On the ground within the extent of the settlement, the mid-Holocene Ustic Luvisol has been truncated permanently because of human occupation. In the late Holocene from 3100 onward, erosion and redeposition accelerated clearly due to the increasingly intensified arable cultivation, widespread and dense settlement and climatic decline. The construction of settlement, road system, terraced fields from the pre-historical time has redirected the runoff and resulted in gully erosion which has the loess tableland dissected.

Holocene pedogenic change and the emergence and decline of rain-fed cereal agriculture on the Chinese Loess Plateau

Multi-disciplinary investigations were carried out in a Holocene eolian loess-soil profile in the centre of the Chinese Loess Plateau at the transition between the arid Mongolian steppe and the semi-arid mixed forest, and also the transitional zone between nomadism and the rain-fed cereal agriculture in history. The cultural remains of an earliest rain-fed cereal agriculture, and also the remains of a Bronze Age nomadic community identified in the profile indicate that the regional emergence and decline of the rain-fed cereal agriculture was closely connected with the change in dust accumulation and soil formation. The climatic proxies derived from the profile show that monsoonal climatic variation, especially precipitation behind the pedogenic change, was ultimately responsible for the cultural and land-use change over the Loess Plateau. Increased precipitation brought on by southeast monsoon, conditioned the domestication and cultivation of millets in the southern part of the region during 8000–7000 a BP at the beginning of the Holocene climatic optimum. Sufficient precipitation and the well-developed fertile soils facilitated the expansion of millets cultivation to the northern Loess Plateau and the southern Mongolian Plateau between 7000 a BP and 3600 a BP. Reduced precipitation and deteriorated land-use conditions because of intensified northwest monsoon at the end of the Holocene climatic optimum caused a regional desertion by the arable farming communities and the invasion of the nomads in the northern part of the Loess Plateau from ca. 3600 a BP during the Bronze Age. Monsoonal climatic change and the resultant pedogenic change dramatically affected the cultural development over the Loess Plateau during the Holocene.

Pedogenic evolution of paddy soils in different soil landscapes

Paddy soils are Hydragric Anthrosols and are an important soil resource for food production. They are widely distributed in China and Asia. Soils that can be used for paddy cultivation vary considerably but basically are grouped into three kinds of landscape, i.e., well-drained sloping uplands, alluvial plains with groundwater fluctuation, and poorly drained polder areas with a near-surface water table. Typical soil chronosequences were studied to clarify the pedogenic changes after wetland rice cultivation, especially in relation to management practices. All of them are in important rice production regions of China. The study focussed on redox condition and iron oxides dynamics. On alluvial plains, the soil conditions range from continuously reducing controlled by high groundwater to alternating reduction and oxidation because of artificial submergence and groundwater fluctuations. Artificially induced leaching losses, previously prevented by a high groundwater table, are partly compensated during paddy cultivation by the rejuvenation process of alluvial deposition. Soils on an older delta, which no longer receives river-sediments, are more strongly developed and showed a decrease of DCB-extractable iron in surface horizon of young paddy soils and more profoundly throughout the profile in old paddy soils. In the polder area, the apparent lowering of groundwater table by deposition of sediment has changed the soil hydrology and increased redox potential. With the better soil aeration, crystalline iron oxides and their ratio to total iron increase with cultivation age. On well-drained terraces, the change of soil moisture regime is overwhelming. Previously well-aerated upland soils are periodically reduced, which accelerates leaching and loss of iron oxides from surface horizons, and probably clay decrease by lateral movement and ferrolysis processes superimposed on the difference of parent soils. An iron-enriched subsurface horizon has formed rapidly and it is a diagnostic feature of this type of paddy soil.

The chemistry of pedogenic thresholds

Pedogenesis can be slow or fast depending on the internal chemical response to environmental forcing factors. When a shift in the external environment does not produce any pedogenic change even though one is expected, the soil is said to be in a state of pedogenic inertia. In contrast, soil properties sometimes change suddenly and irreversibly in a threshold response to external stimuli or internal change in soil processes. Significant progress has been made in understanding the thermodynamics and kinetics of soil-property change. Even in the open soil system, the direction of change can be determined from measures of disequilibrium. Favorable reactions may proceed in parallel, but the most prevalent and rapid ones have the greatest impact on product formation. Simultaneous acid–base, ion exchange, redox and mineral-transformation reactions interact to determine the direction and rate of change. The nature of the governing reactions is such that soils are well buffered to pH change in the alkaline and strongly acid regions but far less so in the neutral to slightly acid zones. Organic matter inputs may drive oxidation–reduction processes through a stepwise consumption of electron acceptors (thereby producing thresholds) but disequilibrium among redox couples and regeneration of redox buffer capacity may attenuate this response. Synthesis of secondary minerals, ranging from carbonates and smectites to kaolinite and oxides, forms a basis for many of the reported cases of pedogenic inertia and thresholds. Mineralogical change tends to occur in a serial, irreversible fashion that, under favorable environmental conditions, can lead to large accumulations of specific minerals whose crystallinity evolves over time. These accumulations and associated “ripening” processes can channel soil processes along existing pathways or they can force thresholds by causing changes in water flux and kinetic pathways.

Soil chronosequences, soil development, and soil evolution: a critical review

Soils chronosequences are valuable tools for investigating rates and directions of soil and landscape evolution. Post-incisive chronosequences are the most common type of chronosequence. They are found in many landscapes, including sand dunes, glacial moraines, landslide scars, old pasture, burnt landscape patches, old mining areas, lava flows, alluvial fans, floodplains, river terraces, and marine terraces. They register pedogenic change over time-scales ranging from years to millions of years. Soil chronosequences help in testing rival theories of pedogenesis. Traditional soil formation theory sees a soil developing progressively under the influence of the environmental state factors until it is in equilibrium with prevailing environmental conditions. This developmental view of pedogenesis is supported by the classic soil chronosequence studies. A new evolutionary view of pedogenesis, which was prompted by the omnipresent inconstancy of environmental conditions and the notions of multidirectional changes and multiple steady states (as predicted by non-linear dynamics), proposes that environmental inconstancy and non-linear behaviour in soil-landscapes lead to soil evolution, rather than to soil development. Soils `evolve' through continual creation and destruction at all scales, and may progress, stay the same, or retrogress, depending on the environmental circumstances. Some recent soil and vegetation chronosequence investigations support an evolutionary view of pedogenesis. It is concluded that soil chronosequences are still potent instruments for pedological investigations and that they have a starring role to play in the testing of pedological theories.

The role of time versus climate in the formation of deep soils of the Apennine fringe of the Po Valley, Italy

Two very deep, red soils on a loess-mantled fluvial terrace of the Trebbia River in northern Italy were sampled and analyzed to determine their history and pedologic significance. Magnetostratigraphy confirms that the upper part of the terrace gravel and all of the 4-m-thick loess cover is normally magnetized and therefore less than about 780,000 yr. Artifacts collected from the oldest loess suggest an age of about 400,000 yr for the inception of loess deposition in this area. The soil horizons in loess dominantly have argillic or albic character, with redoximorphic features that include mottles, glossic tongues, and zones of ferromanganese concretions. Soil colors become progressively redder with depth in the loess cover and are most red, a 2.5 YR color hue, in the top of the terrace gravel. The upper horizons in gravel have abundant ghosts of completely weathered marl and sandstone. Rubification and weathering of stones diminishes with depth to fresh, slightly carbonate enriched parent material at 12 m. The heavy mineral composition of the sand fraction suggests minor discontinuities in loess-derived horizons. Abundance of opaque heavy minerals and picotite, which are resistant to weathering, increases from the soil surface to the base of the loess, is highest in the upper part of the terrace gravel, and decreases from there to the C horizon in gravel. Depth functions of major elements to Si in the medium silt fraction (10–30 μm) indicate a downward increase in degree of decalcification and of dissolution of primary minerals. Micromorphology adds detail to the interpretation of these complex soils: yellow complex ferriargillans are present in almost all horizons; they are rare at the top of the sequence in loess and become progressively more abundant with depth, reaching a maximum in the upper 3 m of the gravel and then decreasing with further depth to 12 m. We interpret this peak in abundance of yellow cutans to result from long continued clay illuviation to this depth from a succession of eluvial zones in the aggrading loessial sequence above. Distinctive red, hematite-rich argillans are abundant only in the upper horizons in the gravel. The red argillans relate to initial development of the soil in gravel, because they are included as pedorelicts in the deepest loess layer, where they are embedded in the yellow argillans. Stratigraphic and pedologic evidence indicates that these soils began forming in terrace gravel for perhaps 300,000 yr during the early middle Pleistocene. From this relict core these soils continued developing, both downward by continuing deep decalcification and argilluviation and upward as a sequence of wet Alfisols developed in a succession of loess layers that began aggrading about 400,000 yr ago. A paleoclimatic record can be perceived weakly in these soils, mainly by thick horizons of FeMn nodules that mark boundaries between episodes of loess deposition that took place during glacial cold phases. Paleofeatures from one unit to another are similar in kind and intensity, indicating a degree of continuity of a basic set of pedogenic processes, principally declcification, argilluviation, and redoximorphism. These soils can be regarded as Vetusols because a single dominant set of pedogenic features are represented in all parts of these soils and deeper parts of these aggrading super profiles have never been isolated from continuing pedogenic change to the present.