DETERMINATION OF THE CONTENTS OF CLAY-SIZED PARTICLES IN SOILS

Edwards and Bremner's (1967) technique of dispersing soils by Bonification to form stable suspensions for particle-size analysis without the addition of dispersant has been tested on a range of soils and a chalk. Carbonate soils or acid soils, containing a suite of minerals which normally form flocculated suspensions in water, yielded stable suspensions in water if the soil organic matter content was sufficiently high. With similar soils, low in organic matter, dispersant had to be added to stop partial flocculation of the test suspensions. Any gypsum present in soils has to be removed to prevent flocculation. For soils very strongly aggregated by organic matter, all the < 2 μm particles present can be released only by sonic vibration in dispersant rather than water. On four soils studied in detail, the maximum amounts of < 2 μm particles obtained by sonic vibration, vigorous shaking, and the I.S.S.S. method have been compared. On three of the four soils, the values were in close agreement.     

Losses of carbon and nitrogen with prolonged arable cropping from sandy soils of the South African Highveld

A knowledge of the kinetics of organic matter transformations in arable soils is important for managing them sustainably. Our aim in this study was to elucidate the effects of cropping period on pools of C and N in coarse‐textured savanna soils of the South African Highveld. Composite samples were taken from the top 20 cm of soils (Plinthustalfs) that have been cropped for lengths of time varying from 0 to 98 years in each of three different agro‐ecosystems in the Free State Province of South Africa. Thereafter, soil organic C and N concentrations were determined in the bulk soil (< 2 mm) as well as in the clay (< 2 μm), silt (2–20 μm), fine sand (20–250 μm), and coarse sand (250–2000 μm) separates. Long‐term cultivation of native grassland reduced soil C and N concentrations by 65 and 55%, respectively. Losses of soil organic matter occurred from all particle‐size separates, although rate loss constants increased as particle size increased. The concentrations of organic C reached equilibrium after 34 years for the bulk soil and after 55 years for clay‐size separates. Nevertheless, organic matter attached to silt continued to be lost as the cropping continued, probably due to wind erosion. Changes in soil properties thereby continued even after almost 100 years of cultivation.     

EFFECT OF ORGANIC CONSTITUENTS AND COMPLEXED METAL IONS ON AGGREGATE STABILITY OF SOME EAST ANGLIAN SOILS

Paired soils of high and low organic matter content from the East Anglian silts were used to determine the role of different organic constituents and complexed metal ions in relation to aggregate stability. Although leaching with periodate and borate caused some loss of stability for most of the soils, the changes were much less than those due to extraction of the soils with pyrophosphate or acetylacetone which were more effective in removing organic materials complexed with iron and aluminium. Amounts of iron and aluminium extracted were not well correlated with changes in stability. The results indicate that in these soils polysaccharides are less important to aggregate stability than organic matter bonded to the clay particles through association with aluminium or iron.     

Biologically Active Organic Matter in Soils of European Russia

Experimental and literature data on the contents and stocks of active organic matter in 200 soil samples from the forest-tundra, southern-taiga, deciduous-forest, forest-steppe, dry-steppe, semidesert, and subtropical zones have been generalized. Natural lands, agrocenoses, treatments of long-term field experiments (bare fallow, unfertilized and fertilized crop rotations, perennial plantations), and different layers of soil profile are presented. Sphagnum peat and humus–peat soil in the tundra and forest-tundra zones are characterized by a very high content of active organic matter (300–600 mg C/100 g). Among the zonal soils, the content of active organic matter increases from the medium (75–150 mg C/100 g) to the high (150–300 mg C/100 g) level when going from soddy-podzolic soil to gray forest and dark-gray forest soils and then to leached chernozem. In the series from typical chernozem to ordinary and southern chernozem and chestnut and brown semidesert soils, a decrease in the content of active organic matter to the low (35–75 mg C/100 g) and very low (<35 mg C/100 g) levels is observed. Acid brown forest soil in the subtropical zone is characterized by a medium supply with active organic matter. Most arable soils are mainly characterized by low or very low contents of active organic matter. In the upper layers of soils, active organic matter makes up 1.2–11.1% of total C_org. The profile distribution of active organic matter in the studied soils coincides with that of C_org: their contents appreciably decrease with depth, except for brown semidesert soil. The stocks of active organic matter vary from 0.4 to 5.4 t/ha in the layer of 0–20 cm and from 1.0 to 12.4/ha in the layer of 0–50 cm of different soil types.     

Dynamics of soil organic matter associated with particle-size fractions of water-stable aggregates

Stable macroaggregates (> 200 μm) of cultivated soils are reported in the literature to be richer in organic carbon, and in young organic carbon in particular, than microaggregates (< 200 μm). However, the nature of this additional carbon is not yet known. To determine it, we compared the composition of organic matter in stable macroaggregates with that in unstable ones. Macroaggregates 2–3 mm in diameter were separated from two silty cultivated soils from the Paris basin. They were slaked, and the primary particle composition of the resulting fractions was analysed. We used the natural abundance of ¹³C to quantify the amount of young carbon, derived from a maize crop, in the various size fractions. The stable macroaggregates were richer in total C and in young C (younger than 6 and 23 years respectively in the two soils studied) than the unstable ones. This young C comprised 50% particulate organic matter, 20% associated with silt and 30% with clay particles. We propose a schematic composition of aggregates in these soils in which stable aggregates are formed by the binding of microaggregates by additional young organic matter, predominantly plant debris. Young organic matter is preferentially incorporated and is responsible for aggregation, though it is eventually redistributed among aggregate classes through the destruction and re‐formation of the aggregates. We have developed a model to simulate this redistribution. The model shows that stable macroaggregates have a life of a few years, but that microaggregates may exist for decades. We suggest that the stabilization and de‐stabilization of macroaggregates in soils is linked to the incorporation and biodegradation of plant debris.     

连续3年施用生物有机肥对土壤有机质组分、 棉花养分吸收及产量的影响

采用盆栽试验,研究了连续3年施用生物有机肥对3种土壤有机质组分、 棉花养分吸收量及产量的影响。结果表明,连续施肥3年后,不同有机质含量土壤的有机质组分含量、 棉花养分吸收量及产量均较不施肥有不同程度的提高。3种土壤随着施肥量的增加,土壤有机质总量和活性有机质组分（活性有机质、 中活性有机质、 高活性有机质）增加,活性有机质在3年后的增加幅度高于有机质总量,说明连续施用生物有机肥可以改善土壤有机质质量。高等、 中等有机质含量的土壤施用生物有机肥2030 g/kg时养分吸收量最大; 低等有机质含量的土壤在施用生物有机肥40 g/kg时养分吸收量最高。高、 中、 低等有机质含量的土壤棉花产量分别在施用生物有机肥20、 20、 40 g/kg时最大,较不施肥增加了54.05%、 37.15%、 104.08%。通过相关分析表明,随着土壤的本底有机质含量由高到低,有机质组分、 棉花养分吸收量及产量之间的相关性则越好,养分吸收量和产量存在极显著相关。     

A re‐evaluation of the enriched labile soil organic matter fraction

Identifying ‘functional' pools of soil organic matter and understanding their response to tillage remains elusive. We have studied the effect of tillage on the enriched labile fraction, thought to derive from microbes and having an intermediate turnover time. Four soils, each under three regimes, long‐term arable use without tillage (NT), long‐term arable under conventional tillage (CT), and native vegetation (NV), were separated into four aggregate size classes. Particle size fractions of macro‐ (250–2000 μm) and microaggregates (53–250 μm) were isolated by sonication and sieving. Subsequently, densiometric and chemical analyses were made on fine‐silt‐sized (2–20 μm) particles to isolate and identify the enriched labile fraction. Across soils, the amounts of C and N in the particle size fractions were highly variable and were strongly influenced by mineralogy, specifically by the contents of Fe and Al oxides. This evidence indicates that the fractionation procedure cannot be standardized across soils. In one soil, C associated with fine‐silt‐sized particles derived from macroaggregates was 567 g C m^?2 under NV, 541 g C m^?2 under NT, and 135 g C m^?2 under CT, whereas C associated with fine‐silt‐sized particles derived from microaggregates was 552, 1018, 1302 g C m^?2 in NV, NT and CT, respectively. These and other data indicate that carbon associated with fine‐silt‐sized particles is not significantly affected by tillage. Its location is simply shifted from macroaggregates to microaggregates with increasing tillage intensity. Natural abundance ¹³C analyses indicated that the enriched labile fraction was the oldest fraction isolated from both macro‐ and microaggregates. We conclude that the enriched labile fraction is a ‘passive' pool of soil organic matter in the soil and is not derived from microbes nor sensitive to cultivation.     

Influence of fertilization and organic amendments on organic‐carbon fractions in Heilu soil on the loess plateau of China

The influence of fertilization on organic‐carbon fractions separated by density and particle size in Heilu soil (Calcic Kastanozems, FAO) was investigated in a 20‐year (1979–1999) long‐term experiment on the Loess Plateau of China. Compared to an unfertilized treatment, N application alone did not increase total organic carbon (TOC) and its fractions of density and particle size. However, the treatment of N + P fertilization significantly increased salty‐solution–soluble organic carbon (SSOC), microbial biomass C (MB‐C), and organic C associated with fine silt. When manure was applied alone and in combination with N and P fertilizer, the light fraction of organic C (LFOC), SSOC, and MB‐C were increased significantly, and the TOC was as high as that of a native Heilu soil. Organic C associated with different particle‐size fractions was also increased significantly, and the allocation of C among the fractions was altered: the proportions of C in sand (>50 μm), coarse‐silt (20–50 μm), and fine‐clay (<0.2 μm) fractions were increased whereas fine‐silt (2–20 μm) and coarse‐clay (0.2–2 μm) fractions were decreased. It is concluded that N fertilizer alone is not capable of restoring organic‐matter content in the Heilu soils of the Loess Plateau and that C‐containing material like manure and straw is necessary to produce significant increase in soil organic carbon in these soils.     

Beziehungen zwischen Klimafaktoren und C-, N-Pools in Partikelgrößen-Fraktionen zonaler Steppenböden Rußlands

Relationships between climatic factors and C, N pools in particle-size fractions of steppe soils, Russia Many soils of the Russian steppe are characterized by high soil organic matter contents and similar parent material. Thus, they are suitable for investigations of a climatic impact on C and N pools. We sampled 10 topsoils of the zonal Russian steppe at 0–10 and about 50–60 cm depth intervals. After particle-size fractionation into clay (<2 μm), silt (2–20 μm), fine sand (20–250 μm) organic C and N concentrations were determined in bulk soils and fractions. The results suggest that especially the older organic matter of the subsoil (in the silt fraction) is correlated with climatic factors. Topsoils show less evidence for climatic influences on C and N pools. As the ratio of mean annual precipitation to potential evaporation (=N/V) increases, C/N ratios decrease in all fractions and, thus, in the bulk subsoil. Obviously the degree of soil organic matter alteration was more pronounced in the order Greyzem (N/V = 1.0) > Chernozem, Phaeozem (N/V = 0.89) > Haplic Kastanozem (N/V = 0.6) > Calcic (N/V = 0.34), and Gypsic Kastanozem (N/V = 0.32). The organic carbon contents of the bulk subsoil are highest in the subsoil of the Chernozem and Phaeozem, and decrease with increasing N/V ratio (i.e., increasing heat input and dryness) to the Calcic Kastanozem. This is accompanied by an increasing enrichment of organic carbon in the silt fractions (r = ?0.99 for the correlation of the C enrichment in silt with N/V).     

THE EFFECT OF SOIL FACTORS ON THE EXTRACTION OF SOIL ORGANIC MATTER BY ANHYDROUS FORMIC ACID

Twenty-five soils, having a wide range of organic matter contents, were extracted with anhydrous formic acid containing 10 per cent acetylacetone, and the extracted material precipitated in two fractions with diisopropyl ether. Precipitates comprised from 5.1 to 51.1 per cent of the original soil organic matter, the proportion extracted tending to be greatest from acid soils of fairly high organic matter content and least from neutral or slightly alkaline soils of low organic matter content. Soil clay content appeared to have no effect on the efficiency of organic matter extraction, but was the most important soil factor governing the proportion of the total soil-N extracted. Amounts of N extracted ranged from 10.2 to 57.8 per cent of the original soil N content, extraction efficiency being greatest with soils of low clay content and low pH. There was evidence to suggest that soil clay afforded some protection to N compounds against extraction. The results indicate that formic acid/acetylacetone is most effective with soils in which much of the organic matter is only partly humified.     

Soil organic matter and its lignin component in surface horizons of salt-affected soils of the Argentinian Pampa

Salt-affected soils differ in their chemical properties to all other soils. Sodicity and salinity may affect the soil organic matter component of these soils. In a field experiment, we investigated organic matter decomposition in nonsaline nonsodic Aquic Argiudoll, a nonsaline sodic Typic Natraquoll, nonsaline nonsodic Petrocalcic Paleudoll and a saline sodic Typic Natralboll in the Pampa Deprimida, Argentina. The objectives were to identify the degree of stabilization of organic matter by association with mineral particles in these soils and to follow in particular the fate of lignin in these soils. We measured organic carbon, total nitrogen and the extent of lignin alteration with soil depth and in various particle size fractions. The salt-affected soils contained much less organic carbon and nitrogen in their mollic epipedons than the nonsaline nonsodic soils, and bioturbation into deeper layers was restrained. In the salt-affected soils most of the organic matter was in sand-sized particles. Retarded degradation of plant residues was indicated by the pattern of lignin-derived phenols, suggesting less alteration of lignin in the salt-affected soils than in the nonsaline nonsodic soils. We suggest that this results from the effects of high pH, high sodicity, and high salinity on the microorganisms and their enzymatic activities. The high pH and high concentrations of monovalent cations decreased formation of solid organo-mineral complexes. We conclude that in the salt-affected soils oxidatively altered organic compounds are susceptible to losses in dissolved or colloidal forms, because these compounds are not stabilized against leaching and mineralization by chemical bonding to soil minerals.     

Changes in Carbon following Forest Soil Transplants along an Altitudinal Gradient

Using a simple case study approach, this research tested the hypothesis that soil organic carbon (C) concentrations would decline when mineral soils from cool, nitrogen (N)–rich, high‐elevation (>1400 m) forests were transplanted to warmer, N‐poor, low‐elevation (～545 m) forests. Two short‐term (<5 year) experiments were performed in the Great Smoky Mountains National Park (Tenn./N.C.) in the southern Appalachian Mountains. In the first experiment, C concentrations in whole soils, particulate organic matter (POM), and mineral‐associated organic matter (MOM) declined significantly (P 0.001) when soils from a high elevation site (1H) were transplanted to a low‐elevation site (1L). In the second experiment, there was a significant (P  0.05) decline in POM C concentrations when high elevation soils (2H) were moved to a lower elevation (2L) as well as declines in whole soil C concentrations that were significant at P  0.10. In both cases, reciprocal transplants of low elevation soils to high elevations resulted in no detectable change in soil C concentrations. Warming of higher quality soil organic matter (whole soil C‐to‐N ratio <20) resulted in greater soil C loss. Consistent with prior predictions, the results suggest that a future warmer and drier climate may cause losses of forest soil C at high elevations in the southern Appalachian Mountains.     

Distribution of polycyclic aromatic hydrocarbons in particle‐size separates and density fractions of typical agricultural soils in the Yangtze River Delta,east China

Soil organic matter can be divided into different organic carbon (C) pools with different turnover rates. The organic pollutants in soils associated with these organic C pools may have different bioavailability and environmental risks during the decomposition of soil organic matter. We studied the distribution patterns of 15 USEPA priority polycyclic aromatic hydrocarbons (PAHs) in different particle‐size separates (clay, fine silt, coarse silt, fine sand and coarse sand) and density fractions (light and heavy fractions) of nine agricultural topsoils (0–20 cm depth) from a contaminated area in the Yangtze River Delta region of east China. There was a decreasing trend in PAH concentration in particle‐size separates with decreasing particle size. However, the different particle‐size separates had similar PAH composition. The concentration of PAHs in the light fraction ranged from 13 037 to 107 299 μg kg^?1, far higher than in the heavy fraction, which ranged from 222 to 298 μg kg^?1. Although the light fraction accounted for only 0.4–2.3% of the soils, it was associated with 31.5–69.5% of soil PAHs. The organic matter in coarse silt had the strongest capacity for enrichment with PAHs. Combining the distributions of PAHs and the turnover rates of organic matter in different soil fractions, the environmental risks of PAH‐polluted soils may be due mainly to the PAHs associated with sand and the light fraction.     

The effect of maize straw placement on mineralization of C and N in soil particle size fractions

Fundamental knowledge about the complex processes during the decomposition, mineralization and transfer of residue organic matter in soils is essential to assess risks of changes in agricultural practices. In a double tracer (¹³C, ¹⁵N) experiment the effect of maize straw on the mineralization dynamics and on the distribution of maize-derived organic matter within particle size fractions was investigated. Maize straw (a C₄ plant) labelled with ¹⁵N was added to soils (13.2 g dry matter kg^–1 soil) which previously had grown only C₃ plants, establishing two treatments: (i) soil mixed with maize straw (mixed), and (ii) soil with maize straw applied on the surface (surface). Samples were incubated in the laboratory at 14°C for 365 days. The size fractions (> 200 μm, 200–63 μm, 63–2 μm, 2–0.1 μm and < 0.1 μm), obtained after low-energy sonication (0.2 kJ g^–1), were separated by a combination of wet-sieving and centrifuging. The mineralization of maize C was similar in the two treatments after one year. However, decomposition of maize particulate organic matter (predominantly in the fraction > 200 μm) was significantly greater in the mixed treatment, and more C derived from the maize was associated with silt- and clay-sized particles. A two-component model fitted to the data yielded a rapidly mineralizable C pool (about 20% of total C) and a slowly mineralizable pool (about 80%). Generally, the size of the rapidly mineralizable C pool was rather small because inorganic N was rapidly immobilized after the addition of maize. However, the different mean half-lives of the C pools (rapidly decomposable mixed 0.035 years, and surface-applied 0.085 years; slowly decomposable mixed 0.96 years, and surface-applied 1.7 years) showed that mineralization was delayed when the straw was left on the surface. This seems to be because there is little contact between the soil microflora and plant residues. Evidently, the organic matter is more decomposed and protected within soil inorganic compounds when mixed into the soil than when applied on the soil surface, despite similar rates of mineralization.     

Effect of cultivation on chemical characteristics and respiratory activity of crusting soil from Mazowe (Zimbabwe)

Abstract

Clearing and cultivation in crusting soils from Mazowe (Zimbabwe) has lead to severe changes in most physico‐chemical characteristics related to the concentration and distribution patterns of plant nutrients and to the total amount of soil organic matter. Nevertheless, the concentration of the different humus fractions showed lower intensity changes, as did the mineralization rates of the organic matter. The most significant effects of cultivation on the soil chemical characteristics coincided with those considered to favor clay dispersion and crusting phenomena, including generalized desaturation of the exchange complex and losses of divalent ions with a potential bridging effect between soil particles. Concerning the soil organic matter, the humic acid tended to concentrate in the cultivated soils as a probable consequence of selective biodegradation of the other humic fractions. The composition and activity of soil humus suggest low‐performance organo‐mineral interactions: in these soils the active turnover of the plant wastes is not regulated by intense physico‐chemical interactions with the soil mineral fractions, or by physical encapsulation of organic particles. In consequence, the mineralization rates were relatively constant in the soils studied and unrelated to soil organic matter concentration. The results suggests that there is a possibility to revert the early degradation stages of these soils through a rational management of suitable amounts of crop wastes.     

Microaggregates in agricultural soils and their size distribution determined by X-ray attenuation

Understanding mechanisms of microaggregate formation in soils requires knowledge of their exact size distribution. With this in mind, we have used X‐ray attenuation to determine the size distributions of microaggregates and primary particles in the range 0.2–63 μm, with a resolution of 100 size increments. Ten arable and grassland soils with organic C contents ranging from 14.7 to 37.7 g kg^?1 were analysed. They were subjected to ultrasound at 52 J ml^?1 which destroyed most aggregates > 63 μm to give microaggregates in the size range 1–63 μm. The size distribution of microaggregates differed significantly from that of primary particles and was largely independent of their organic C content. Microaggregates were most abundant in 19 of the 100 size increments, contributing to 92% of the major peaks of the size distribution. These preferred increments differed from those of primary particles, but the order for the two was similar. Further analysis of the size distribution revealed a larger mean weight diameter of microaggregates, depending on the size distribution of primary particles. The results suggest a major effect of the size distribution of primary particles on microaggregation, whereas land use seems to have a negligible effect. The proportion of mechanically dispersible clay decreased with increasing C content and indicates structural stability at the microscale.     

ACCUMULATION OF CADMIUM AND ZINC FROM DIFFUSE IMMISSION ON ACID SANDY SOILS,AS A FUNCTION OF SOIL COMPOSITION

Sandy soils, in the border area of Belgium and the Netherlands (the Kempen region), are heavily contaminated by atmospheric deposition of cadmium and zinc from nearby smelters. Groundwater contamination by leaching from these low retention soils is subject of study. There are reports of high cadmium and zinc concentrations in groundwater in the area, but in most cases the direct sources are unknown. In an attempt to predict present or future risk of groundwater contamination by soil leaching, metal binding processes (retardation) were studied that are specific for these soil types under the existing acidifying conditions. From four fields nine contaminated profiles were sampled and analyzed for cadmium and zinc. Average concentrations of 131 μg g^-1 zinc and 1.6 μg g^-1 cadmium with maximum values of 2989 μg g^-1 respectively 16.3 μg g^-1 were found. In addition pH and contents of organic matter, aluminium, iron, and manganese were determined. The relative importance of these soil parameters for metal retardation is derived from the profiles. The data show that organic matter is the most important soil component for binding cadmium and zinc. Adsorption of cadmium and zinc on aluminium, iron and manganese (hydr) oxides appears to be of minor importance at low pH (<5.5).     

Organo-mineral associations in sandy acid forest soils: importance of specific surface area, iron oxides and micropores

Organo-mineral associations stabilize soil organic matter, though the mechanisms by which they do so are unclear. We used particle-size fractions < 6.3 μm of two soils to examine the importance of Fe oxides, short-range order Al silicates and the surface areas of minerals and micropores on the formation of organo-mineral associations. In the subsoil Fe oxides were most strongly statistically correlated with the mineral-bound organic carbon. We therefore assume that they are the most important substrates for the formation of organo-mineral associations. There is no indication that this is caused by physical protection of organic matter in their micropores (< 2 nm). In the Haplic Podzol, dithionite–citrate–bicarbonate-soluble short-range order Al silicates may also play a role. Fe oxide particles were calculated to offer specific surface areas of ∼ 200 m² g⁻¹ (goethite) and ∼ 800 m² g⁻¹ (ferrihydrite), corresponding to crystal diameters of only a few nm. We assume that the resulting large amount of oxide-specific reactive surface sites (conditionally charged hydroxyl groups) is responsible for their dominant role as sorbents. With maximum C loadings of 1.3 mg C per m² Fe oxide for the Dystric Cambisol and 1.1 mg C per m² Fe oxide + short-range order Al silicates for the Haplic Podzol the subsoils of both soils seem to have reached saturation with respect to organic matter sorption. In contrast to subsoil horizons, organo-mineral associations from topsoils contain much larger amounts of organic matter. Here a larger C loading on Fe oxides or a greater importance of other sorbents in addition to the oxides must be assumed.     

Organic matter in particle-size fractions from A and B horizons of a Haplic Alisol

The organic matter in soils may be stabilized by its interactions with minerals. We have studied such interactions in a Haplic Alisol under forest in which clay and organic matter have migrated from an eluvial A horizon to accumulate in an illuvial B horizon. We have tried to trace the fate of organic matter in these horizons (Ah and Bvt) by determining clay mineralogy, carbon and nitrogen content, hydrolysable amino acids, lignin signature by alkaline CuO oxidation and carbon species by ¹³C CPMAS NMR of bulk soils and particle‐size fractions. In both horizons, most of the organic matter was present in O–alkyl and methylene structures, each contributing one‐third to the bulk organic matter. In the Ah horizon the ratios of carbon‐to‐nitrogen, and yields for lignin and hydrolysable amino acids decreased as the particle‐size class decreased, but side‐chain oxidation of lignin compounds increased with decreasing particle size. In contrast to previous observations, the proportions of O–alkyl carbon increased as particle size decreased, constituting a major proportion of the organic carbon in the clay‐size fractions from both the Ah and Bvt horizons (≥ 38%), while proportions of methylene carbon decreased. Illite was the dominant mineral in the fraction ≤ 6 μm, whereas the mobile fine clay fraction (<0.2 μm) was rich in smectites – minerals with large surface areas. Our results support the hypothesis that potentially labile organic matter, such as O–alkyl carbon typically present in polysaccharides, may be stabilized against further degradation in organomineral complexes.     

The extent and significance of bioturbation on 137Cs distributions in upland soils

Differences between measured ¹³⁷Cs activity–depth profiles and idealised undisturbed profiles generated from an exponential model suggest that faunal turbation has redistributed ¹³⁷Cs in mineral and organic upland soils in southern Scotland. Bioturbation is also demonstrated by the vertical displacement of other inputs to the soils of known age (non-native tree pollen and spheroidal carbonaceous particles, SCPs). The causes and mechanisms of bioturbation were further investigated by soil micromorphology. Well-drained mineral soils with active populations of earthworms are the most bioturbated, showing near-complete homogenisation to depths of about 20 cm. Enchytraeids also seem to remobilise ¹³⁷Cs by the digestion of organic matter and may be the main cause of ¹³⁷Cs redistribution in organic-rich upland soils. Relative rates of mixing are evaluated by comparing ¹³⁷Cs depth profiles.