Loess-derived Soils Research Articles

Prediction of spatial patterns of collapsed pipes in loess-derived soils in a temperate humid climate using logistic regression

Soil piping (tunnel erosion) has been recognised as an important erosion process in collapsible loess-derived soils of temperate humid climates, which can cause collapse of the topsoil and formation of discontinuous gullies. Information about the spatial patterns of collapsed pipes and regional models describing these patterns is still limited. Therefore, this study aims at better understanding the factors controlling the spatial distribution and predicting pipe collapse. A dataset with parcels suffering from collapsed pipes (n=560) and parcels without collapsed pipes was obtained through a regional survey in a 236km² study area in the Flemish Ardennes (Belgium). Logistic regression was applied to find the best model describing the relationship between the presence/absence of a collapsed pipe and a set of independent explanatory variables (i.e. slope gradient, drainage area, distance-to-thalweg, curvature, aspect, soil type and lithology). Special attention was paid to the selection procedure of the grid cells without collapsed pipes. Apart from the first piping susceptibility map created by logistic regression modelling, a second map was made based on topographical thresholds of slope gradient and upslope drainage area. The logistic regression model allowed identification of the most important factors controlling pipe collapse. Pipes are much more likely to occur when a topographical threshold depending on both slope gradient and upslope area is exceeded in zones with a sufficient water supply (due to topographical convergence and/or the presence of a clay-rich lithology). On the other hand, the use of slope-area thresholds only results in reasonable predictions of piping susceptibility, with minimum information.

Magnesium-isotope fractionation during low-Mg calcite precipitation in a limestone cave – Field study and experiments

The chemical and isotopic composition of speleothem calcite and particularly that of stalagmites and flowstones is increasingly exploited as an archive of past environmental change in continental settings. Despite intensive research, including modelling and novel approaches, speleothem data remain difficult to interpret. A possible way foreword is to apply a multi-proxy approach including non-conventional isotope systems. For the first time, we here present a complete analytical dataset of magnesium isotopes (δ 26Mg) from a monitored cave in NW Germany (Bunker Cave). The data set includes δ 26Mg values of loess-derived soil above the cave (−1.0 ± 0.5‰), soil water (−1.2 ± 0.5‰), the carbonate hostrock (−3.8 ± 0.5‰), dripwater in the cave (−1.8 ± 0.2‰), speleothem low-Mg calcite (stalactites, stalagmites; −4.3 ± 0.6‰), cave loam (−0.6 ± 0.1‰) and runoff water (−1.8 ± 0.1‰) in the cave, respectively. Magnesium-isotope fractionation processes during weathering and interaction between soil cover, hostrock and solute-bearing soil water are non-trivial and depend on a number of variables including solution residence times, dissolution rates, adsorption effects and potential neo-formation of solids in the regolith and the carbonate aquifer. Apparent Mg-isotope fractionation between dripwater and speleothem low-Mg calcite is about 1000ln α Mg-cc-Mg(aq) = −2.4‰. A similar Mg-isotope fractionation (1000ln α Mg-cc-Mg(aq) ≈ −2.1‰) is obtained by abiogenic precipitation experiments carried out at aqueous Mg/Ca ratios and temperatures close to cave conditions. Accordingly, 26Mg discrimination during low-Mg calcite formation in caves is highly related to inorganic fractionation effects, which may comprise dehydration of Mg 2+ prior to incorporation into calcite, surface entrapment of light isotopes and reaction kinetics. Relevance of kinetics is supported by a significant negative correlation of Mg-isotope fractionation with the precipitation rate for inorganic precipitation experiments.

Spatial variability of soil horizon depth in natural loess-derived soils

Soil variability across landscapes is well known and results from the combination of geomorphologic and pedogenetic processes. Despite its importance, little quantitative information exists on the horizontal and vertical variation of soil profiles, especially in natural landscapes that are unaffected by soil erosion. This study aims at measuring the variation in soil horizon depth in loess-derived soils and relating it to surface characteristics. Several terrain variables and the variable soil type, as derived from existing soil maps, are considered in the prediction model. In total 399 augerings of up to 8.7 m deep were made in natural forest areas in Central Belgium, where soil profiles were not affected by anthropogenic soil erosion. The variability of 5 different soils horizons was evaluated: eluvial (E) and illuvial clay (Bt) horizons, transition horizon (BC), decalcified loess material (C1) and undisturbed calcareous loess material (C2). All horizons exhibited significant variability. The top two soil horizons could not be linked significantly to any of the predictor variable. For the lower three soil horizons some weak, yet significant relations were found with the predictor variables slope gradient, plan curvature, wetness index, landform and soil type. Geostatistical analysis indicated a lack of spatial dependence for all horizons, except for the upper eluvial E horizon. This high spatial randomness resulted in poor predictions models for the depth of these soil horizons, with model efficiencies ranging between − 0.14 and 0.08, while for the E horizon, a simple ordinary kriging model provided a model efficiency of 0.43.

Factors controlling the spatial distribution of soil piping erosion on loess-derived soils: A case study from central Belgium

Collapsible loess-derived soils are prone to soil piping erosion, where enlargement of macropores may lead to a subsurface pipe network and eventually to soil collapse and gully development. This study aims at understanding the main factors controlling spatial patterns of piping in loess-derived soils under a temperate climate. To map the spatial distribution of piping and identify the environmental controls on its distribution, a regional survey was carried out in a 236 km 2 study area in the Flemish Ardennes (Belgium). Orthophotos taken at optimal field conditions (winter) were analyzed to detect piping in open landscapes and ground thruthing was systematically done through field surveys. In total, 137 parcels having 560 collapsed pipes were mapped. Dimensions of the sinkholes and local slope gradient were measured in the field and topographical variables were derived from LiDAR data. Land use plays an important role as 97% of the sites with piping are found under pasture. The probability of piping increases rapidly on hillslopes with gradients exceeding 8% and with a concave profile and plan curvature, enhancing subsurface flow concentration. The zones with soil profiles on shallow loess over a relatively thin layer of homogeneous blue massive clays (Aalbeke Member) are most prone to piping. Soil characteristics are of less importance to explain piping occurrence. Furthermore, the topographical threshold line indicating the critical slope gradient for a given contributing drainage area was determined. This threshold line (negative power relation) is similar to the threshold line for shallow gully initiation.

Role of deforestation and hillslope position on soil quality attributes of loess-derived soils in Golestan province, Iran

Conversion of natural land resources into croplands, which is triggered by the rapid population growth, causes serious soil degradation. A loess hillslope located in eastern Golestan province of Iran was selected to study the role of deforestation and slope position on soil quality attributes. Surface (0–30 cm) and subsurface (30–60 cm) soil samples were taken from five slope positions (summit, SU, shoulder, SH, backslope, BS, footslope, FS and toeslope, TS) of forest (FO) and adjacent deforested cultivated land (DEF) in a factorial trial with completely randomized design. Ten pedons were also investigated and undisturbed soil samples were taken from different horizons for micromorphological studies. The texture of the original loess is silt loam. The soil textural class varies from silty clay loam in FO to silt loam in DEF, mainly due to the loss of finer particles as a result of soil erosion followed by deforestation and long-term cultivation. Mean weight diameter (MWD) of aggregates decreased following deforestation (0.88 mm compared to 1.49 mm in FO), as a result of considerable losses of organic carbon (OC) and breakdown of aggregates. Bulk density (BD) increased and soil infiltration rate decreased by about 50% in DEF. Reduction of annual organic matter input to soil as a result of deforestation and also rapid oxidation of organic matter in DEF were responsible for a significant decrease (>70%) in OC and total nitrogen (TN). Soil microbial respiration (SMR) also decreased significantly, following deforestation. Carbon, and N contents and population of fungi were significantly higher in all hillslope positions of the FO than the DEF. Changes in soil quality attributes were not significant in different slope positions of FO, which might be related to the stability of forest landscape. Effect of different slope positions on soil quality attributes was more pronounced in the DEF. The SH and BS were found as the most susceptible positions to erosion in DEF. Soils of the FO were mainly classified as Alfisols and Mollisols with evidences for clay illuviation compared to the weakly developed Inceptisols formed in the DEF. Micromorphological investigations revealed that the FO soils had strong granular and crumb microstructure with a high porosity indicating the presence of high amount of organic matter. The high microbial and faunal activity was confirmed through the presence of excremental pedofeatures in the topsoil of the FO. Lower organic matter and consequently microbial activity in the topsoil of the DEF have resulted in the massive microstructure with little porosity.

Mineralogy and morphological properties of buried polygenetic paleosols formed in late quaternary sediments on upland landscapes of the central plains, USA

East central Kansas is largely comprised of alternating, level beds of Permian shale and limestone of the Central Plains, USA. Polygenetic upland soils of east central Kansas have been formed though multiple and likely different sets of soil forming factors. Upland soils in this region have a complex genesis, often contain one or more paleosols, and form in multiple parent materials including loess, locally reworked loess or colluvium, and residuum. The depth to bedrock rarely exceeds 2 m. Upland hillslope soils commonly contain one or more paleosols, and can be observed on a variety of hillslope positions on the landscape. Generally, the lower paleosols are recognized by strongly expressed structure, thick continuous clay coatings on all faces of peds, and strong reddish color with either 7.5YR or 5YR hues. Soil textures of the paleosols often feel less clayey than the overlying horizons in field determinations. At first, this was attributed to a decrease in clay content, stickiness, and plasticity. However, subsequent laboratory characterization revealed that the clay content was usually highest in the lower paleosol horizons, and that the clay mineralogy of the modern soil was dominated by smectite, while the paleosols contained a mixed suite of minerals. Therefore, the perceived decrease in clay content was caused by a change in clay mineralogy, a feature that can be exploited in future field descriptions in order to more accurately distinguish between stratigraphic units in these thin, welded polygenetic soils. The age of the paleosols sampled in this study were typical for the late Quaternary-aged Severance formation, clustering in two age ranges, which were ≈ 19,000 to 20,000 and ≈ 22,500 to 27,700 uncalibrated 14C yr BP. The results from this study illustrate that although they might be thin, truncated, and welded, late Quaternary-aged loess-derived soils and paleosols occur in regular, predictable patterns on many upland hillslopes in the Bluestem Hills Major Land Resource Area, and this region should be included in future regional investigations of the Central Plains.

The Plow and Agricultural Sustainability

Settled agriculture began about 10 to 13 millennia ago in the valleys of the Nile, Tigris, Euphrates, and other rivers. Prehistoric tools developed included a short-handled forked branch that evolved into a hoe, and a long-armed hockey-shaped stick with a curved handle called an ard. The ard, which eventually evolved into a plow, won religious sanctions in many ancient cultures such as those of Greece, China, and India. The plow-based agriculture flourished in alluvial and loess-derived soils. These soils are generally coarse-textured, less cohesive, devoid of stones and suitable for growing closely spaced cereals such as wheat (Triticum aestivum). In contrast, hoe-based agriculture dominated in regions where widely spaced roots and tuberous crops, such as cassava (Manihoc esculenta) and yam (Dioscorea spp.), were grown on sloping lands. Mechanization of plowing with the tractor during the early part of 20th century vastly expanded the land area under cultivation. However, plowing also caused erosion and exacerbated environmental problems of non-point source pollution, emission of greenhouse gases (GHGs) and loss of biodiversity. The environmental movement, begun in 1950s and 1960s, led to the development of no-till (NT) farming. After approximately 50 years of its introduction, NT farming is practiced only on about 6% of the global cropland area, mostly in North and South America, and in Australia. Adoption of NT farming by resource-poor farmers of South Asia and sub-Saharan Africa is constrained by removal of crop residues for fodder and animal dung as residential fuel, lack of appropriate seed drill which can sow in an unplowed soil covered by crop residue mulch, and non-availability or prohibitively high cost of herbicides. While use of the plow is unsustainable on erodible soils, its use is essential until alternative sources of residential fuels are available to rural households, efficient seed drills are developed by village blacksmiths, and herbicides become economical and effective against perennial weeds or other viable options of weed control are available.

Three Missouri Alfisols Impacted by Aeolian Deposition of Lead-Zinc-Cadmium-Silver-Indium Bearing Mine Tailings

Modern mining technologies promote the nation's gross domestic product and reduce environmental impacts; however, previous mining activities in Missouri have a legacy of soil contamination. This study focuses on Pb-Cd-Zn-Ag-In soil contamination resulting from a closed Pb-Zn mine at St. Joe State Park in Missouri. Metal contamination is the result of ancestral smelting operations and presently exposed tailings. Three soil profiles located adjacent to a closed mine-smelting operation and two reference pedons located outside of the study area were examined to determine the concentrations of Pb, Cd, Zn, Ag, and In. Aqua regia digestion and selective chemical extractions were employed to assess metal availability. The near-surface soil horizons show dramatically greater Pb, Cd, Zn, Ag, and In concentrations than deeper soil horizons or the reference loess-derived soil profiles. Surface Pb concentrations in the study area range from 224 to 589 mg Pb kg − 1 , whereas the surface Cd concentrations range from 0.3 to 3.1 mg Cd kg− 1. Pyrophosphate and EDTA (Ethylenediaminetetraacetic Acid) extractions demonstrate that a small to substantial portion of the total Pb and Cd pool is potentially available. Water soluble metal concentrations infer that the risk of metal leaching from the soil is minimal.

The effect of slope gradient on plant cover index (C) of faba bean crops grown on loess soil

The effect of slope gradient on plant cover index (C) of faba bean crops grown on loess soil. In a period from 2002 to 2005, a single-element fi eld experiment was conducted at the Experimental Station in Mydlniki near Cracow. The fi eld top soil was a brown loess-derived soil. The main objective of the experiment was to determine the effect of slope gradient on the plant cover index C of small bean crops. The impact of two slope gradients (9% and 16%) on the level of surface soil losses was studied, as was, in the experiment, the amount of surface soils exported from fallowed plots and from the plots grown with faba beans. The experiment on the impacted 22.13 × 1.87 m plots was four times repeated in each gradient zone. It was stated that the plant cover index C was 0.187 in the case of faba beans grown and cropped on the slopes showing a 16% gradient, and, it was 0.154 on the slopes with a 9% gradient. The impact of the total area of plant top ends expressed by an index called LAI (Leaf Area Index) on the value of index C is described by exponential functions, i.e. in the case of slope gradient being 16%: y = 0.6081e–0.6016x, and the slope gradient of 9%: y = 0.5447e–0.6341x.

Seasonal variations in soil erosion resistance during concentrated flow for a loess-derived soil under two contrasting tillage practices

Soil erodibilty during concentrated flow ( K c) and critical flow shear stress ( τ cr), both reflecting the soil's resistance to erosion by concentrated runoff, are important input parameters in many physically-based soil erosion models. Field data on the spatial and temporal variability of these parameters is limited but crucial for accurate prediction of soil loss by rill or gully erosion. In this study, the temporal variations in K c and τ cr for a winter wheat field on a silt loam soil under three different tillage practices (conventional ploughing, CP; shallow non-inversion tillage, ST; deep non-inversion tillage, DT) in the Belgian Loess Belt were monitored during one growing season. Undisturbed topsoil samples (0.003 m 3) were taken every three weeks and subjected to five different flow shear stresses ( τ = 4–45 Pa) in a laboratory flume to simulate soil detachment by concentrated flow. To explain the observed variation, relevant soil and environmental parameters were measured at the time of sampling. Results indicated that after two years of conservation tillage, K c(CP) > K c(DT) > K c(ST). K c values can be up to 10 times smaller for ST compared to CP but differences strongly vary over time, with an increasing difference with decreasing soil moisture content. The beneficial effects of no-tillage are not reflected in τ cr. K c values vary from 0.006 to 0.05 sm −1 for CP and from 0.0008 to 0.01 sm −1 for ST over time. Temporal variations in K c can be mainly explained by variations in soil moisture content but consolidation effects, root growth, residue decomposition and the presence of microbiotic soil crusts as well play a role. τ cr values increase with increasing soil shear strength but K c seems more appropriate to represent the temporal variability in soil erosion resistance during concentrated flow. The large intra-seasonal variations in K c, which are shown to be at least equally important as differences between different soil types reported in literature, demonstrate the importance of incorporating temporal variability in soil erosion resistance when modelling soil erosion by concentrated flow.

Clay illuviation and calcium carbonate accumulation along a precipitation gradient in Kansas

One of the significant features of loess-derived soils in Kansas is the occurrence of clay-rich subsurface horizons above a layer enriched with pedogenic carbonates. In order to examine the extent of clay increase and pedogenic carbonate enrichment in a precipitation gradient, ten soil profiles from three different precipitation regions were studied using micromorphological and mineralogical techniques. The precipitation gradient was divided into three groups: 400–550 mm, 550–750 mm, and 750–1100 mm regions. The objectives were to (1) understand the cause of clay orientation in clay-rich horizons (2) investigate the reasons for the clay increase, and (3) observe the interaction of clay and pedogenic carbonate accumulation features along a precipitation gradient in Kansas. Although clay films were identified in the field for soils in the 400–550 mm regions, illuvial clay films were not observed in thin section analysis. The clay accumulations mostly occurred as grain coatings. The rest of the clay accumulations observed were very thin, striated, and mostly associated with voids. The argillic horizons had a granostriated b-fabric, which indicates stress orientation of micromass caused by high shrink–swell activity. Thick and continuous illuvial coatings were observed in the buried horizons of paleosols. In the other two regions where precipitation exceeds 550 mm, illuvial clay coatings with strong orientation were observed along with thin and striated stress-oriented clay. Both types of clay orientations exceeded 1% of the cross-sectional area for the thin section. Although illuvial clay features and pedogenic carbonates were observed in all soils at approximately the same depth, complete obliteration of clay coatings was not observed in these horizons. In-situ weathering of biotite was one of the reasons for the clay increase in all soil profiles. In all soils studied, the clay increase and cause of clay orientation cannot be attributed to a single genetic process or event. Both illuviation and shrink–swell activity were involved in the orientation of clay. Although orientation of clay and pedogenic carbonates were observed in all soils at approximately the same depth, the decomposition of clay coatings was not observed in these horizons.

Holocene alluvial sediment storage in a small river catchment in the loess area of central Belgium

Soil erosion and sediment deposition widely affect landscape development, particularly in erosion-prone areas with loess-derived soils. Nevertheless, until now, few attempts were made to quantify soil losses and sediment storage over long (centennial or millennial) timescales. In this study, the Holocene alluvial sediment storage in a small river catchment (52 km 2) of the Belgian loess belt is estimated, and a preliminary sediment budget for the catchment is presented. In the valley of the Nethen River (c. 13 km long), a detailed survey of the alluvial sediment archive was conducted. Hand augerings and percussion drillings were made along cross-valley transects at 12 locations in the catchment. AMS 14C dating of peat samples provided a temporal framework for the interpretation of the cores. Results show that the thickness of Holocene sediment deposits in the Nethen valley is 4 to 6 m, which corresponds to a total clastic sediment mass of ∼14 × 10 6 t stored in the valley bottom. Three alluvial units could be distinguished and associated with deposition phases from 9600 to 2900 B.C., 2900 B.C. to A.D. 1000 and A.D. 1000 to present. In contrast to the older sediments (units 1 and 2), deposits from the last 1000 year (unit 3) contain little organic matter. They are seldom intercalated with peat layers, and devoid of tufa. Unit 3 reaches a thickness of c. 2 m, thereby representing 50% of the Holocene sediment mass stored in the alluvial plain. The mean sedimentation rate in the alluvial plain for this last phase is ∼26 t ha − 1 a − 1 , which is about tenfold larger than the sedimentation rates calculated for the older Holocene sediment units. Sediment supply towards the alluvial plain has therefore increased tremendously since Medieval times. These results are in contrast to dating results obtained for colluvial sediments in a nearby dry valley within the catchment of the Nethen, where soil erosion and sediment deposition started in the early Iron Age and was already substantial during the Roman Age. This means that there is a time lag of about one millennium between the onset of high sedimentation rates in the upstream area and high deposition rates in the alluvial plain. This is probably caused by a change in coupling (sediment connectivity) between the plateau, slopes, and rivers. As soil erosion proceeds, first the dry zero-order valleys in the catchment act as sediment traps, and only after these are filled sediment reaches the floodplains. The preliminary sediment budget for the Nethen catchment illustrates that 50% of the sediment that was eroded during the Holocene was stored in colluvial deposits, which are mainly located on footslopes and in dry valley bottoms. Another 29% of the sediment mass is stored in the alluvial plain.

Genesis and micromorphology of loess-derived soils from central Kansas

The genesis and micromorphology of three Harney soils from different precipitation regions (from 540 mm to 715 mm) (fine, smectitic, mesic Typic Argiustolls) in the Smoky Hills of central Kansas were investigated. The objectives were to (1) examine the morphological, chemical, physical and mineralogical characteristics of Harney soils formed in loess; (2) determine the clay mineral distribution with depth and the origin of the clay minerals present; and (3) investigate the relationship between the clay mineralogy and other soil properties such as soil plasmic fabric, COLE values and fine clay/total clay ratios. Mineralogical and micromorphological techniques were used to evaluate the characteristics of the loess-derived soils. The first pedon was formed in 88 cm of Bignell loess over Peoria loess and the other two pedons were formed from Peoria loess. The chemical properties were similar for the pedons studied. Differences were observed in physical properties, especially in particle size distribution, oven-dry bulk density and coefficient of linear extensibility values. Although the soils were mapped in the same soil series, the geomorphic positions of the pedons and the nature of the parent material affected the characteristics of the soils. Smectite was the predominant clay mineral, especially in the fine clay fraction, regardless of the location in the precipitation gradient. The dominance of smectite increased in the C-horizons. This implies a detrital source of smectite in the B-horizons formed in both Bignell and Peoria loess units. The presence of randomly interstratified mica-smectite and the micromorphological observations of weathering biotite indicate that weathering also plays an important role in the mineralogy of Harney soils. The high content of clay mica in the surface horizons was caused by dust fall in the study area. Thick and continuous argillans were observed when FC/TC and COLE values were low and crystalline smectite was present. In the lower part of the soil profiles, the plasmic fabric was mostly ma-skelsepic (granostriated b-fabric) and smectite was more crystalline as indicated by sharper X-ray diffraction peaks.

Changes in soil properties and the availability of soil micronutrients after 18 years of cropping and fertilization

Micronutrient deficiencies are common in many parts of China's Loess Plateau. The objective of this experiment was to study the effects of long-term cropping and fertilization practices on soil properties and micronutrient availability in this region. The field plot experiment began in 1984. It included five cropping systems and four fertilizer treatments. In September 2002, soil samples were collected and soil pH, organic matter content, available P, and CaCO 3 were measured. Total and available Zn, Cu, Mn, and Fe were also determined. The relationship between soil properties and available micronutrients was determined by correlation and path analysis. After 18 years, soil pH and CaCO 3 levels were lower in the cropped and fertilized treatments compared to the fallow treatment. In contrast, soil organic matter and available P levels were higher in cropped compared to fallow treatments. A comparison of unfertilized treatments indicated that available Zn and Cu levels in cropped treatments were lower compared to the fallow treatment, probably due to the removal of these micronutrients from the system through crop uptake and harvest. In contrast, available Mn and Fe levels were higher in cropped treatments compared to the fallow treatment. The impacts of fertilization on available micronutrients varied with cropping systems. Generally, available Zn and Fe were higher in fertilized compared to unfertilized treatments, but available Cu was not significantly influenced by fertilization. Fertilization tended to increase available Mn in continuous wheat and maize, but reduced available Mn in continuous clover and the crop–legume rotation. The total (plant available + unavailable) micronutrient contents were lower in the four cropped-treatments compared to the fallow treatment. The addition of manure or P fertilizer increased total Zn, Fe, and Mn, but had no significant effect on total Cu. The results of correlation analysis and path analysis indicated that soil organic matter exerts a significant and direct effect on the availability of Zn, Mn, and Fe, but has little influence on available Cu. The effects of available P, CaCO 3, and pH on micronutrient availability were indirect, passing through soil organic matter. The results of this study suggest that long-term cropping and fertilization altered several important soil properties and increased the plant available micronutrient content of this loess-derived soil.

Loss of soil organic matter upon ploughing under a loess soil after several years of conservation tillage

Restricting tillage to shallow-mixing operations as conducted in many conservation tillage systems occasionally causes problems such as compaction of the formerly ploughed layer or severe weed and slug infestation. To overcome these constraints, one moulboard ploughing operation is carried out and, subsequently, cultivation is shifted back to non-inverting shallow-mixing tillage. This study investigated the effect of one single ploughing action conducted after several years of conservation tillage on the amount of organic matter in the soil profile. It was hypothesized that soil organic carbon (SOC) and soil nitrogen (SN) stocks decrease very rapidly during the first few months after ploughing. The study was carried out in temperate climate on three adjacent fields with loess-derived soil (Stagnic Luvisol) located in southern Lower Saxony, Germany. The crop rotation included sugar beet ( Beta vulgaris L.), winter wheat ( Triticum aestivum L.), winter barley ( Hordeum vulgare L.), and white mustard ( Sinapis alba L.) as a catch crop. Conservation tillage started in 1992 on all fields and consisted of 30 cm deep non-inverting tillage combined with shallow-mixing (10 cm deep) operations. Mouldboard ploughing (30 cm deep) was conducted once in each field in August 1998–2000 prior to sowing of mustard. Thereafter, shallow conservation tillage (10 cm) was established. Soil samples were taken once before and at three dates (1–6 months, 7–9 months, 1.5–2.5 years) after ploughing and masses of SOC and SN were determined. After 7–9 years of conservation tillage, SOC and SN were concentrated in the top 10 cm layer of the soil. One ploughing operation resulted in a substantial loss of organic matter. At 0–30 cm soil depth, losses of 0.26 kg m −2 SOC and 0.046 kg m −2 SN occurred within 1–6 months after ploughing, accounting for 4 and 7% of the total initial masses, respectively. At the soil layer of 0–45 cm, these early losses were much lower (2% and 5% of the initial masses of SOC and SN per unit of soil area, respectively), caused by an increase of masses in the 30–45 cm depth. Further losses of SOC and SN from the 0–30 cm layer were small, while from the lower layer a second pronounced loss of organic matter occurred. After 1.5–2.5 years after ploughing, losses from the 0–45 cm depth accounted for 6% and 10% of the initial total mass of SOC and SN, respectively. In temperate climate, one ploughing operation after several years of conservation tillage obviously causes a partial loss of organic matter within a few weeks after ploughing.

Characteristics, controlling factors and importance of deep gullies under cropland on loess-derived soils

Several studies describe the formation and importance of shallow ephemeral gullies under cropland (depth<0.8 m). Some of these gullies may develop into channels with a depth of more than 0.8 m (up to 4 m). Despite their spectacular nature, these deep gullies have not yet been studied in detail. Therefore, the objectives of this study are to analyze the characteristics and the controlling factors of these deep gullies, as well as their importance in terms of sediment production. Comparison of a dataset with 28 deep gullies, formed in the period 1985–2003 and 123 shallow ephemeral gullies formed in the period 1994–1999 in central Belgium indicates that the deep gullies have clearly different morphological characteristics compared to shallow ephemeral gullies. Several factors were analyzed to understand the formation of deep gullies. Plotting runoff contributing area versus slope of the soil surface at the gully head indicates that the topographical threshold for deep gully formation is significantly larger compared to ephemeral gully formation. Deep gullies form on short, steep valley sides and their position is strongly affected by the presence of linear landscape elements. All deep gullies incised on landscape positions with a very erodible soil horizon at shallow depth. Analysis of causative rainfall showed no significant differences in incision thresholds between rills and shallow ephemeral gullies on the one hand and deep gullies on the other hand. The relation between area-specific sediment yield in central Belgium and drainage area, indicates that the development of deep gullies contributes significantly to the sediment output of small rural catchments and causes peaks in the mean area-specific sediment yield that are up to a factor of three higher compared with catchments where shallow ephemeral gullying occurs.

On the characteristics and origin of closed depressions in loess-derived soils in Europe—a case study from central Belgium

A closed depression (CD) is a landform where the hillslopes encircle a common sediment depository, and the sediment eroded from the surrounding hillslopes is trapped in the system. Despite the fact that closed depressions (CDs) have been regularly reported in the literature, little research on their origin exists. Therefore, a case study was made of the morphological characteristics and genesis of CDs in central Belgium that are still visible in the landscape. Through field surveys, 26 CDs were found in a 43-km 2 study area (southwest of Leuven). Most CDs were situated on a plateau, far from villages but close to important rural roads. The morphological features of the CDs (i.e., diameter, area, maximum depth and steepest slope) were measured. The results show that there were significant differences between the morphology of the CDs under forest and under cropland; CDs under cropland were larger, shallower and had gentler slopes compared to those under forest. Based on these morphological features, a hypothetical evolution scheme of CDs was formulated. Through a detailed literature search, eight out of 26 CDs were identified as having an anthropogenic origin; they were former quarries (mines) for calcareous sandstone or iron sandstone. In four other CDs, augerings were made, and detailed studies of their soil profiles within and in the vicinity of CDs indicated that they are quarries for calcareous loess. In one of the four CDs, with a catchment area of 0.588 ha, the soil erosion rate was calculated, using soil sequences phosphate content and charcoal analysis. Since 1950 AD, the mean annual soil loss due to water and tillage erosion within the catchment of the CD ranged between 5.5 and 9.8 ton/ha or 0.37–0.65 mm. Based on 14C-dating, it was calculated that mean annual soil loss since 430 AD amounted to 2.1 ton/ha or 0.14 mm.

Erosional impact on organic matter content and productivity of selected Iowa soils

From 1983 to 1989 an erosion-productivity study was conducted in Iowa. The primary objective of this project was to determine the effects of accelerated erosion and its interactions with other variables on soil productivity, as measured by corn yields of loess and till-derived soils in Iowa. Multiple regression techniques were used to relate corn yields to erosion and selected soil, climatic, and weather variables. Organic matter content of the soils was significantly correlated with erosion phase. Yields of till-derived soils decreased more with increasing degree of erosion than loess-derived soils. The conclusions are based on data collected at 569 sites located in 44 Iowa counties.

Medium-term evolution of a gully developed in a loess-derived soil

Field surveys in the Belgian loess belt revealed the presence in many forested areas of large, permanent gully systems, most of which are currently inactive. In cultivated areas, such gullies can only be observed in cross-sectional soil profiles through hollows, as virtually all such large gullies are currently infilled with colluvium. Little is known about the spatial distribution, initiation and temporal evolution of these large, permanent gully systems on loess-derived soils. Therefore, the medium-term evolution of a gully initiated in a cultivated area on loess-derived soils southwest of Leuven (Belgium) in May–June 1986, was studied over 13 years. Two intense rainfall events created this (ephemeral) gully, which was not erased by subsequent tillage. Between June 1986 and the December 1999, eight field surveys were conducted to measure gully dimensions. During two surveys, topographic indices (e.g., slope and drainage area) were also measured. Daily rainfall for the measuring period were obtained from a rainfall station located some 10 km southwest of the gully. Analysis of rainfall data showed that no extreme rainfall event was required to initiate such large (permanent) gullies, as observed in forested areas and through cross-sectional profiles in cultivated fields in the Belgian loess belt. Return periods of the event that caused the gully varied between <1 year and 25 years, depending on the assumptions used for defining event rain intensity. Once established, length, surface area and volume of the studied gully evolved with time, cumulative rainfall or cumulative runoff, following a negative exponential relation. This accords with observations reported for gullies in Australia and the USA. This study shows that a degressive increase of gully extension, can be largely explained by the evolution of a “slope–drainage area” factor ( S× A, which is proportional to stream power) with time. While gully length and gully surface area asymptotically evolve towards a final value, gully volume decreased at a given point in time. From this, it is inferred that sediment deposition will potentially infill the gully to such an extent that the farmer can drive across it. From this moment on, the combined effect of water and tillage erosion in the gully drainage area, will lead towards rapid infilling. This expected evolution of a gully in cultivated fields accords with observations of large infilled gully systems in cultivated areas in eastern Belgium. The permanent gullies observed under forest are attributed to the fact that after severe gully erosion, this area was reforested or abandoned. Therefore, the sediment source was cut off and the gully was not filled in by sediment deposition.

Rates of nitrogen application required to achieve maximum energy efficiency for various crops: results of a long-term experiment

A long-term experiment with four rates of mineral nitrogen (N) application (averaged across all the crops in a crop rotation: 0, 50, 100, 150 kg ha −1 per year) was conducted on a fertile loess-derived soil in central Germany. The objectives of this study were to (i) determine the rates of mineral nitrogen N application required for maximum net energy output (energy output minus energy input), maximum energy output/input ratio, and minimum energy intensity (energy input per unit grain equivalent) for various crops in a realistic crop rotation (potatoes [ Solanum tuberosum L.], winter wheat [ Triticum aestivum L.], winter barley [ Hordeum vulgare L.], sugar beets [ Beta vulgaris L.], spring barley [ Hordeum vulgare L.]): (ii) identify long-term trends (from 1968 to 2000) in the rates of mineral N application necessary to achieve the most efficient use of energy in the production of winter wheat and (iii) assess the effects of changing the system boundaries and the energy equivalents assigned to selected inputs on the energy balance by means of a sensitivity analysis. In the last two crop rotations (1989–1993 and 1994–1999), the amount of N fertilizer required to maximize net energy output of the main products (cereal grains, beet roots) increased in the order sugar beets–winter wheat–winter barley. At optimum N fertilization, the net energy output increased in the order winter barley–winter wheat–sugar beets. Averaged across the two rotations, the N fertilizer demand for a maximum output/input ratio and minimum energy intensity increased in the order sugar beets–winter wheat–winter barley. There was no clear-cut time trend in the rate of N application required to maximize grain yield and net energy output of wheat; maximum grain yield, maximum net energy output, and output/input ratio increased significantly with time, whereas the minimum energy intensity decreased over the experimental period. For all the crops, the rate of N application required for the maximum net energy output was much higher than that required for the maximum output/input ratio and minimum energy intensity.