Clay Enrichment Research Articles

The enrichments of organic matter and total nitrogen in sediment as affected by relavant factors

Serious soil erosion has already resulted in degradation of the Loess Plateau of China. Soil erosion is commonly accompanied by extensive soil nutrient loss. Because of enrichment processes, sediment nutrient content is often higher than that of natural soil. The objective of this study is to determine the enrichments of organic matter and total nitrogen in sediment in hilly and gully loess areas on the Loess Plateau of China. Measurements of enrichment ratios (ER) of organic matter (EROM) and total nitrogen (ERTN) in sediment as affected by rainfall, slope gradient, tillage, and fertilization were made in the field under natural rainfall conditions. The results showed that the enrichment of clay in sediment resulted in the enrichment of organic matter (OM) and total nitrogen (TN) in sediment. The averages of sediment clay ER, EROM and ERTN for the various slope gradients were 1.77, 2.09 and 1.61, respectively. The soil erosive module was negatively correlated with EROM and ERTN. Our results indicate that measures to reduce soil erosion, i.e. reducing rainfall erosivity, decreasing soil slope gradient, decreasing fertilizer use, and using level trenches, may increase EROM and ERTN. Both quantity and quality of sediment yield should be considered when implementing erosion control measures.

SOIL REFLECTANCE AS A TOOL FOR ASSESSING PHYSICAL CRUST ARRANGEMENT OF FOUR TYPICAL SOILS IN ISRAEL

Investigation of soil structural crust reflectance across the NIR-SWIR spectral region was conducted to account for the possibility of remotely sensed soil crust-related properties such as water infiltration. The raindrop energy disintegrates the soil aggregates and rearranges soil particles within the structural crust of the upper soil layer. The generation of structural crust affects the physical properties of the soil and can be identified by significant color changes on the soil’s surface. Spectral differences observed on the crusted soil are caused by the rearrangement of texture and clay minerals during raindrop events. In this study, four soils were subjected to increasing levels of raindrop energy in a rain simulator device with their reflectances measured concurrently. Two wavelengths representing particle size arrangement and clay content at 1.7 μm and 2.2 μm, respectively, were used to generate a dynamic chart of the crust formation. Soil mineralogy was found to be a major factor in the crust formation. In the clayey soils, two identical stages were identified: decreased albedo at 1.7 μm and an increase in the 2.2 μm absorption features. By employing this technique, a washing out of clay from the crust zone over the high raindrop energy was observed. However, in sandy soil, the predominant quartz particles created a different pattern. Because the coarse quartz particles reflect more energy, reverse reciprocal relationships with clay enrichment were found. Based on the relationship between raindrop energy and water infiltration rate (IR), a spectral model for predicting the IR was constructed. Three spectral domains were examined: reflectance (R), the first derivative of the reflectance (R’), and the ratio between reflectance of a given treatment and the reflectance of the non-crusted fog treated soil (R/Rfog). A significant relationship was discovered between the IR and the reflectance properties (R’ and ratio), suggesting clay content as the significant indicator of crust status. The results of this study show that soil reflectance can shed light on the structural crusting process. It is further suggested that this methodology may be used as well at high signal to noise ratios, employing airborne and space-borne hyperspectral sensors.

Lateral clay injection into normal faults

ABSTRACT The clay content of fault gouge is one of the main factors controlling transport and mechanical properties of a fault zone. This paper addresses the process of lateral clay injection into normal faults, which is one of the many processes contributing to the development of clay smear, and can lead to local enrichment of clay in a fault gouge. We combined field observations with geomechanical models to quantify the parameters leading to lateral clay injection into fault zones. Detailed field study shows that a releasing fault bend in a clay layer is required for clay injection to occur. The clay injection process is often associated with the formation of a branch in the fault and the development of a “squeezing block” which injects the clay into the fault zone. A simple analytical model predicts the onset of clay injection when C = σ'v (1 - sin ϕ) / (2 cos ϕ), where C is cohesion (MPa), σ'v is vertical stress (MPa) and ϕ (°) is friction angle. More detailed analysis using 2-D geomechanical finite element models is in good agreement with the analytical models and allows study of the system at higher fault throw. Results of sandbox models containing layers of an elastoplastic clay analogue also compare well with field observations and numerical models, and show the initiation of the releasing step and the evolution of the clay injection process with increasing fault throw. Using our results it is possible to predict the likelihood of lateral clay injection in the subsurface, in settings like the Gharif formation of the Haushi group of Central and South Oman or the Natih formation of North Oman. This requires an estimation of the mechanical properties of the clays at the time of faulting; data which can be obtained from wireline logs and cuttings. This approach to fault seal analysis emphasizes the mechanical aspects of the clay smear process, in addition to the kinematics which were considered in previous analyses. Its application should lead to improved prediction of fault seal processes in the subsurface.

Using radiometric data to identify æolian dust additions to topsoil of the Hillston district, western NSW

The identification and characterisation of æolian dust additions to Australian soil profiles has been attempted using a wide range of analytical techniques. Such techniques are often reliant on specialised equipment and operator expertise, but do not necessarily yield information on the spatial distribution of deposited dust across the landscape. An airborne geophysical technique which measures naturally occurring gamma-rays may now assist in the rapid assessment of dust accumulation sites at the catchment scale. This technique, known as gamma ray spectrometry or radiometrics, assesses the potassium (K), uranium (U), and thorium (Th) contents of the Earth's surface, and may potentially distinguish æolian sediments from the underlying rock or in situ developed soil profile by their differing K, U, and/or Th-bearing mineral suites. This paper summarises a study of this technique for identifying æolian dust in topsoils of the Hillston district in western New South Wales. Using airborne with follow-up ground-based radiometric data, a variable relationship was established between large K signatures and apparent topsoil dust accumulations, which were identified as containing appreciable feldspar and illite. The previously established correlation between topsoil clay content and eTh (equivalent Th) concentration was also observed in this region, indicating that the technique may allow for rapid identification of landform features with significant clay and silt enrichment, as often occurs with dust accession. Despite the ubiquity of æolian sediments in this region, the radiometric data were moderately effective in discriminating different dune systems and may therefore represent a useful initial technique for discerning likely areas of dust accumulation in this environment. However, in areas influenced by fluvial sediments, the radiometric signatures were not necessarily indicative of the topsoil dust content.

Natural and Anthropogenic Processes That Concentrate Mn in Rural and Urban Environments of the Lower Mississippi River Delta

This study evaluated natural processes and projected methylcyclopentadienyl manganese tricarbonyl fuel additives as sources of Mn accumulation in the environment. Data sets include fresh alluvium and sediments from the lower Mississippi River Delta and a soil metal survey of metropolitan New Orleans. The (1) railroad Mn, (2) industrial Mn, and (3) dynamic aquifer–stream transfer of Mn hypotheses were tested with the Mississippi River Delta data. Friction between Mn-rich steel wheels and rails contributes Mn ( P=0.017) to the environment, supporting (1). Sediment loads of Mn were similar ( P=0.77) upstream and downstream from the Louisiana industrial corridor, not supporting (2). The median Mn on the alluvium surface (59 mg/kg), in the aquifer (159 mg/kg), and in the riverbank aquifer discharge zone (513 mg/kg) support (3) as a mechanism for Mn enrichment of clay. The New Orleans soil metal survey data set shows a rural to urban increase of fourfold for Mn and three orders of magnitude for Pb. At 1999 U.S. highway fuel use, 8.3 mg of Mn per L would yield 5000 metric tons of Mn annually. If 13% of Mn were emitted, 650 tons of Mn would become aerosols annually, while 87% or 4350 tons would remain in engines. The 1999 toxic release inventory for Mn shows 370 tons as total emissions compared to the potential of 390 and 260 tons from vehicles, respectively, in urban and rural areas. A precautionary lesson from the use of Pb as a fuel additive is that the use of Mn as a fuel additive would be associated with an increased risk for neonates exceeding the estimated total tolerable daily intake of 2.1–16.5μg Mn (especially in urban inner city environments) because neonates lack fully functional hepatic clearance for Mn.

SW—Soil and Water: Linking Clay Enrichment and Sediment Delivery Processes

Recent research has directed attention to the size distribution of eroded material because of its influence on deposition mechanics and in carrying capacity of pollutant materials. At first, in this paper the relationship between aggregate breakdown mechanism and erosion processes is briefly reviewed. Then the link between the clay enrichment ratio and the sediment delivery ratio at morphological unit scale is investigated. For 129 soil samples well distributed over the Sicilian Sparacia basin, the values for the experimental clay enrichment ratio calculated by the measured ultimate grain-size distributions are compared with the theoretical clay enrichment ratio values obtained by a procedure based on effective grain-size distribution.The clay content measurements, carried out in sediment samples drawn out from the sedimentation tank located at the basin outlet during the period February 1996–February 1997, are used to calculate the coefficient appearing in the relationship applied to estimate the sediment delivery ratio of each morphological unit.A relationship for estimating this coefficient using sediment clay content, measured by a conventional ultimate particle-size distribution, is also proposed.Finally, a Monte Carlo technique and caesium-137 measurements carried out in the experimental basin are used for verifying the accuracy of the sediment delivery distributed model which is strongly dependent on the estimate of the coefficient appearing in the sediment delivery ratio relationship.

The selective removal of phosphorus from soil: is event size important?

Data from the Woburn Erosion Reference Experiment (Bedfordshire, UK) were used to test the hypothesis that losses of phosphorus (P) in small erosion events are as great as those in infrequent large events, and to examine the effect of storm characteristics on the selective enrichment of P in eroded sediment. For almost every plot event in the period 1988 to 1994, the clay-sized fraction of the sediment was enriched compared with the soil of the plots. There was more variation in clay enrichment for smaller erosion events than for larger ones. The clay and P contents of the sediment were strongly correlated (p < 0.01), and there was a wider range of P concentrations in the sediment derived from small events than in that from large events. However, individual events resulting in small soil losses (< 100 kg) did not account for greater P losses than larger events (> 100 kg). The greater frequency of smaller events, combined with the likelihood of higher P concentrations in the sediment, therefore accounted for a greater proportion of the P lost over the 6-yr period than the infrequent large events. Phosphorus concentrations generally increased with increasing peak discharge and decreased with increasing event duration. For the same return period, P losses were generally greater from plots cultivated up and down the slope than from those cultivated across the slope. Overall, our results suggest that small erosion events should be controlled to prevent P contamination of surface waters and that the most effective means of doing this are by the introduction of minimal tillage techniques and across-slope cultivations.

Mineralogia de solos com excesso de sódio no estado do Ceará

Foram estudados dois perfis de solos com problemas de sodicidade (Planossolo Solódico e Solonetz-Solodizado), no estado do Ceará, com o objetivo de conhecer a mineralogia das frações areia, silte e argila e os processos de alterações que ocorrem nos respectivos minerais. A areia e o silte grosso foram avaliados por microscopia de polarização e contraste de fase, enquanto o silte fino e a argila foram analisados por difração de raios-X. Utilizou-se, ainda, a microscopia eletrônica de varredura para obtenção de informações complementares sobre as transformações mineralógicas ocorridas nos horizontes dos dois perfis. Verificou-se que o Planossolo Solódico apresenta na fração argila quantidades mais ou menos equivalentes de caulinita e montmorilonita no horizonte 2Btn, o mesmo acontecendo com as proporções de caulinita e vermiculita no horizonte A. Nas outras frações, foram encontrados teores variáveis de quartzo, plagioclásios, hornblenda e biotita. Por sua vez, o Solonetz-Solodizado mostrou uma fração argila essencialmente caulinítica ao longo de todo o perfil e com quantidades variáveis de quartzo, feldspato e biotita nas outras frações. Em ambos os perfis, o enriquecimento de argila no Bt resultou essencialmente das alterações "In situ" dos minerais primários intemperizáveis contidos na rocha subjacente. A hipótese de alternância de processos de oxi-redução nos horizontes A e, ou, E, em razão da baixa permeabilidade do horizonte 2Btn, provocando um processo de ferrólise naqueles horizontes, é também uma alternativa de gênese que não pode ser descartada para explicar o alto contraste textural desses solos. O intemperismo da albita e de outros plagioclásios ricos em sódio, o clima semi-árido, e a drenagem impedida são os fatores condicionantes dos percentuais elevados de sódio, observados nos dois pedons. Também nos dois solos, os teores destoantes de quartzo, a geomorfologia da região e a presença de uma linha de pedras separando os horizontes A ou A + E do 2Btn revelam duplicidade de materiais originários.

Early Forest Soils and Their Role in Devonian Global Change

A paleosol in the Middle Devonian Aztec Siltstone of Victoria Land, Antarctica, is the most ancient known soil of well-drained forest ecosystems. Clay enrichment and chemical weathering of subsurface horizons in this and other Devonian forested paleosols culminate a long-term increase initiated during the Silurian. From Silurian into Devonian time, red clayey calcareous paleosols show a greater volume of roots and a concomitant decline in the density of animal burrows. These trends parallel the decline in atmospheric carbon dioxide determined from isotopic records of pedogenic carbonate in these same paleosols. The drawdown of carbon dioxide began well before the Devonian appearance of coals, large logs, and diverse terrestrial plants and animals, and it did not correlate with temporal variation in volcanic or metamorphic activity. The early Paleozoic greenhouse may have been curbed by the evolution of rhizospheres with an increased ratio of primary to secondary production and by more effective silicate weathering during Silurian time.

Pedogenesis on talus slopes, the Buckskin range, Nevada, USA

Soil development on steep mountainous topography generally leads to thin and undifferentiated pedons. On talus slopes of the Buckskin Mountains, a fault-blocked range of west-central Nevada, USA, thick, differentiated pedons occur. Talus occurs on the western front where block rotation has formed very steep slopes and consists of angular dacite cobbles and boulders. The study area is arid, averaging less than 20 cm of precipitation per year. Typical pedons consist of surface talus filled with vesicular, non-structured, sandy loam to loam material. This material overlies clay-textured, prismatic- and blocky-structured horizons, mostly rock-free that, in places, are over 1 m thick. Pedons are classified as fine, mixed, mesic Xeralfic Paleargids. Soil macro- and micromorphological features, soil mineralogy, and soil chemical and physical attributes suggest the following developmental sequence which began sometime before the middle Pleistocene. The talus serves as a baffle to collect eolian dust. Once eolian dust accumulates to a threshold thickness, primary minerals are transformed, in situ, mostly to a smectitic clay. As clay content increases, a second threshold level is reached at which time the horizon of clay enrichment has sufficient strength and shrink-swell potential to support and raft the talus upward. The exposed talus can then continue to trap eolian dust, the soil profile thickens, and the pedon is protected from erosion. A variation of this soil cumulation process may be partially responsible for textural-B horizon formation in avid and semiarid climates.

SOIL DEVELOPMENT ON LATE PLEISTOCENE MORAINES AT PINE CREEK, EAST-CENTRAL SIERRA NEVADA, CALIFORNIA

Late Pleistocene moraines at Pine Creek in the east-central Sierra Nevada Range are differentiated into three separate relative-age groups using soil morphological properties, with increasing soil development on progressively older deposits. Tiogaage soils (oxygen-isotope stage-2 equivalent) are weakly developed with A/AB/Bw/Cox profiles. Organic material extracted from Tioga-age till matrix has a conventional radiocarbon age of 19,570 ± 420 years B.P. Soils formed on Tahoe-age moraines (stage 4) show A/2BA/2Bw1/2Bwq2/2BC/2Cox horizonation with rubification to about 50 cm depth and slight clay enrichment. Organic matter from the Tahoe-age till matrix yields an infinite radiocarbon age (>38,000 years B.P.). Rovana moraines (stage 6) have moderate soil development, with A1/A2/2Bt1/2Bw2/2BC/2Cox profiles, conspicuous rubification down to about 103 cm, and significant clay accumulation in the Bt horizon. A soil-profile development index reflects the chronologic trend; however, lithologic and climatic variabil...

Clay Differentiation in Aridisols of Northern Mexico

AbstractAridisols with smectite B horizons are usually classified as Argids although the argillic characters may not always be present. In the Chihuahuan Desert of northern Mexico, some Aridisols have clayey horizons that do not seem to result from clay illuviation. Our purpose was to determine the origin of the clay enrichment and the mineralogical relationships between the parent materials and the B horizons. The study area is the Mapimi Playa, derived from a lutite lagoonal sediment parent material. A soil sequence from Orthids to Argids was identified. Four profiles were analyzed. The particle‐size distribution was determined by the sedimentation and centrifugation methods. The clay fractions were studied by x‐ray diffraction and peak decomposition, infrared spectroscopy, and chemical analyses. Soil water extracts were analyzed with an absorption spectrometer. From the Orthids to the Argids, clay enrichment in B horizons is controlled by the &lt;0.2‐µm clay size fraction and small amounts of palygorskite. The B horizons contain prominent smectite, which has more Mg than the lutite smectite (parental material). The existence of the AIMgOH infrared vibrations in the soil clay suggests the neogenesis of a smectite different from the lutite smectite. The composition of the soil water extract suggests that Mg‐smectite and palygorskite may form in Mapimi soil profiles. The Argids of Mapimi Reserve result principally from clay neogenesis under current climatic conditions.

LOESSIAL SOILS

We examined four traverses adjacent to narrow and wide segments of the Mississippi River Valley in Illinois for trends in loess thickness, particle-size distribution, and soil development. Where the valley is wide, loess thickness, mean particle size, and coarse silt content decrease rapidly with distance from the valley, and clay enrichment within the B horizon is increasingly evident away from the valley. Where the river valley is narrow, the adjacent soils exhibit different trends in loess thickness, mean particle size, clay enrichment, and coarse silt content along the transect. Within the study area, soils adjacent to narrow segments of the Mississippi River Valley are commonly classified as “well-developed” Argiudolls, while soils adjacent to wide valley segments are “weakly developed” Hapludolls. Local glaciofluvial sediments from the Mississippi River Valley appear to be a major source for loess located leeward of broad valley segments, but apparently are not a major source of loess for those uplands adjacent to narrow valley sections. The primary source of loess for these sites is considered to be from areas west of the Mississippi River Valley and loess deposited by variable winds.

Particle Size of Interrill‐eroded Sediments from Highly Weathered Soils

AbstractFifteen highly weathered Alfisols and Ultisols from Georgia were used in runoff/erosion pan studies to delineate sediment character istics of interrill‐eroding soils and their relationship to soil particle size. Simulated rainfall was applied to the pans at 11.2 cm h−1, and runoff and sediment parameters were measured at 5‐min intervals for 25 min. Runoff rates and sediment concentrations varied consistently over time for the soils used, with concentrations increasing until peaking at 10 to 15 min, then decreasing as crust formation caused surface consolidation and decreased detachability, despite continued high runoff rates for most of the soils. Sediment particle size became finer after the initial sampling period, and was dominated by silt‐sized particles, with lesser amounts of primary clay and sand. Silt‐sized particles in the sediments were related statistically to a measure of water‐dispersible (0.5‐h shaking) soil silt, whereas primary sediment clay was related to both water‐dispersible (36‐h shaking) and total soil clay. Dispersible and total soil clay were also correlated with sediment concentration and soil loss rate, emphasizing the role of clay and its dispersibility in erosion and crust formation on these soils. A quadratic equation using soil clay described sediment clay content, showing greater relative enrichment of primary clay in sediments from sandy vs. clayey soils. The high proportion of easily transportable sediment generated by interrill erosion on these soils poses a serious environmental problem in areas of active soil erosion.

Soil Genesis Associated with Periglacial Ice Wedge Casts, Southcentral Wyoming

AbstractThe development of ice and sand wedges (nonsorted polygon features) in Wyoming's high basins during the late‐Pleistocene glaciation allows determination of soil development on pre‐Wisconsin and late‐Wisconsin surfaces on the same landform. Pre‐Wisconsin soils on an old alluvial terrace in the Great Divide Basin (later hosts to ice wedges) exhibited clay translocation and accumulation followed by a period during which carbonate accumulated. Soils developed in the noncalcareous fill‐materials of the ice‐wedge casts show B horizon accumulation of clay and carbonate. The late‐Wisconsin soils developed in the ice wedge casts showed clay enrichment of 13 kg m−2 and CaCO3 enrichment of 113 kg m−2. The pre‐Wisconsin soil had CaCO3 enrichment of 250 kg m−2. Parent material stratification prevented determination of absolute clay enrichment in the host soil but its clay maximum is 33% compared to 15% in the late‐Wisconsin soil. The Bt horizons in the pre‐Wisconsin soils are also much thicker. Profile morphology indicates that the present semi‐arid environment probably developed immediately as the climate warmed at the end of the Wisconsin. Clay mica and kaolinite have become enriched in surface horizons and smectite in C horizons of both wedge and wedge‐host soils.

The self‐diffusion of sodium in soil: factors affecting the surface mobility

SUMMARYA study is presented of the self‐diffusion of sodium in a sub‐soil. Impedance factors (fs)associated with the surface phase have been derived, representing the mobility of sorbed sodium ions relative to mobility in an ideal solution. Effects of drying, of cationic com‐position and of enrichment with either native clay fraction or ‘pure’ clay minerals were investigated. The liquid phase impedance factor (fL) without added clay was 0.29: clay enrichment affected soil structure and hence fL. Values of fs were in the range 0.19–0.05, and were about one‐third of fL. The value off, decreased as clay content increased, being halved when 50% native clay or 10% clay mineral was added. In a sodium saturated soil fs was 0.06, less than half that in a calcium saturated soil.

Nitrogen in particle size fractions of soils incubated for five years with 15N‐ammonium and 14C‐hemicellulose

SUMMARYFour soils with a range of clay and silt contents were incubated for 5 a with 15N‐labelled (NH4)SO4 and 14C‐labelled hemicellulose and then fractionated according to particle size by ultrasonic dispersion and sedimentation. The distribution of labelled and native N between clay, silt and sand fractions was determined and elated to previous results on the C distributions.Between 29% and 48% of the added N was found in organic form. The 15N atom percentage excess decreased in the order: clay &gt; whole soil &gt; silt &gt; sand. For both clay and silt, the enrichment factor for labelled and native N decreased with increasing fraction weight. Clay enrichment was higher for labelled than for native N, the converse being true for silt. The distribution of whole soil labelled organic N was: clay 77–91%, silt 4–11%, and sand &lt;0.5%. Corresponding values for native N were 69–74%, 16–22%, and 1–2%, respectively.All soils had higher proportions of labelled than of native N in the clay, the converse was true for the silt. The C/N ratio of the native silt organic matter was higher and that of clay organic matter lower than whole soil C/N ratios. Differences between the C/N ratio distributions of native and labelled organic matter were small.The relative distribution of labelled N and C was very similar confirming that the turnover of C and N in soil organic matter is closely interrelated.

Chemical and Physical Enrichments of Sediment from Cropland

ABSTRACT LOSS of soil and plant nutrients resulting from the erosion process is a significant factor in decline of soil productivity. The erosion process is recognized as being selective in both physical and chemical properties. Smaller size particles, particularly clay and silt, adsorb and transport large quantities of nutrients due to their relatively greater specific surface areas. Selectivity of the erosion process is more pronounced for the more numerous but less severe hydrologic events since these events result in a disproportionate amount of smaller particles being transported from a site. Thus, nutrient enrichments are complex and tend to vary inversely with suspended sediment concentrations. We examined soil and nutrient losses from simulated rainstorms on texturally similar soils at three widely separated locations in the upper, mid and lower Mississippi River watershed. Physical characteristics of the sediment, including aggregate size distributions and primary particle size distributions, and selected chemical properties are discussed in relation to soil productivity. Enrichments in sediment of total Kjeldahl nitrogen, organic carbon, cation exchange capacity and available nitrogen are related to sediment characteristics and clay enrichment.

The distribution of native and labelled carbon between soil particle size fractions isolated from long‐term incubation experiments

SUMMARYFour soils with 6, 12, 23 and 46% clay were fractionated according to particle size after incubation for 5–6 years with 14C labelled straw, hemicellulose or glucose: 6–23% of the 14C was still present and the amount increased with increasing content of fine particles.clay fractions contained 66–84% of the 14C and the silt fractions accounted for 4–19%. &lt;2% was found in the sand fractions and 4–9% was water soluble. The distribution of the native C was: clay, 46–68%; silt, 20–31%; sand, 2–7%.The clay fractions had higher relative proportions of 14C than of native C, the reverse being true for the silt fractions. This distribution pattern was not directly related to soil clay content or to kind of organic amendment.The C enrichment factor of clay and silt fractions (per cent C in fraction/per cent C in whole soil) increased with decreasing fraction size for both native and 14C. However, clay enrichment factors were higher for 14C than for native C, whereas silt enrichment factors were lower.A soil (9% clay) that had been incubated in the field for 18 years with 14C labelled straw was also analysed. Labelled C content at sampling was 9% of the initial value. In contrast to the other soils the distribution of labelled and native C was similar in the clay and silt fractions, which contained 55% and 33% of the whole soil C, respectively.The results indicate that clay‐bound organic matter may be important in mediumterm organic matter turnover, whereas silt‐bound organic matter may participate in longer‐term organic matter cycling.

Enrichment of Transported Sediments with Organic Carbon, Nutrients, and Clay

AbstractMuch of the nutrient inflow into impoundments is the result of erosion of soil particles on the watershed, together with adsorbed nutrients and their movement from the upland watershed soils. As the detached particles move downstream, they undergo selective size segregation and the nutrients are subject to selective desorption and adsorption between the particles and the water. During transport an enrichment of clay sized particles within the system occurs as well as that of P, N, and organic C. The ratio of the concentration of any given component in the eroded material system to that in the contributing soils is defined as the enrichment ratio. The selectivity of the erosional and sediment delivery processes involved in the transfer of eroded materials from watersheds to lakes or reservoirs is assumed to be proportional to the time, or distance, of travel. This study compares the composition of the eroded material at the onset and at the termination of the erosion, or sedimentation, cycle. From this information insight into the mechanisms affecting the compositional changes along the erosional route may be obtained. Organic carbon, total nitrogen, and total phosphorus concentrations and clay contents in sediments from 41 impoundments or lakes were compared with the equivalent parameters in the soils of the contributing watersheds. The enrichment ratio of a given component (S/So, the concentration of the component in the eroded material divided by its concentration in the soil) was theoretically predicted to be: S/So = 1/So × L, where L is a limiting value representing a stable level of this component in the erosion sequence. The measured enrichment ratios for organic C and total N were in good agreement with the theoretical model, the stable levels being 24 and 1.7 g kg−1 for organic C and total N. No such agreement was found for P and clay enrichment ratios, possibly due to the effect of the variable flow conditions on the selective erosion of these components.