Phospholipid fatty acid profiles and xylanase activity in particle size fractions of forest soil and casts of Lumbricus terrestris L. (Oligochaeta,Lumbricidae)

Earthworms are key agents in organic matter decomposition, as they remove surface plant litter material and mix it with mineral soil. Plant litter material is comminuted in the gizzard of anecic earthworms and this is enhanced if sand particles are available. We hypothesize that this comminution of soil and litter will result in changes in the distribution of soil organic matter and soil microorganisms in the different particle-size fractions. We investigated soil organic matter content, xylanase- and microbial activity and community structure in bulk soil and particle size fractions of Lumbricus terrestris L. casts and in soil with and without the addition of beech litter.Earthworm gut passage did not affect the particle-size distribution but the content of soil organic matter was decreased in the fine sand fraction in treatments without litter (−6.80%) and increased in treatments with litter (+33.23%). The soil organic matter content of the clay fraction tended to be higher in earthworm casts. Xylanase activity was at a maximum in the fine sand fraction, lower in the coarse sand fraction and at a similar minimum in the silt- and clay-sized fraction. In the coarse sand fraction of the cast and litter treatments xylanase activity was increased by 39.1% and 124.8%, respectively. In the silt-sized fraction of casts the addition of litter increased xylanase activity (+58.6%) whereas, in casts without litter it was decreased (−36.25%). In the particle size fractions of casts, the content of bacterial PLFAs was decreased in the fine sand fraction and tended to be decreased in the clay fraction compared to the respective fractions in soil. In the silt fraction the fungal-to-bacterial PLFA ratio was higher in casts than in soil.We conclude that earthworms stabilize soil organic matter in cast aggregates predominantly by increasing the soil organic matter content in the clay fraction where it becomes protected against microbial attack. Organic matter in the coarse and fine sand fractions is decomposed primarily by fungi; xylanase is very active in these sand fractions and incorporation of litter into these fractions by the earthworms increased fungal biomass. Comminution of litter during passage through the earthworm gut increased the biomass and activity of fungi also in the silt fraction. The use of PLFA profiles in combination with other quantitative microbial methods improves the understanding of stabilizing and mobilizing processes in earthworm casts.     

Organic‐carbon fractions in an agricultural topsoil assessed by the determination of the soil mineral surface area

According to recent conceptual models, the organic carbon (OC) of soils can be divided into OC fractions of increasing stability from labile free OC to resistant OC associated with the soil mineral phase. In this study, we present a method for quantifying two OC fractions based on soil aggregate–size fractionation and the N₂ gas–adsorption method. For this purpose, we analyzed soil material of the plow layer of a Haplic Chernozem subjected to different fertilizer treatments (no fertilizer, mineral fertilizer, mineral and organic fertilizer). The total organic‐C concentration (TOC) and the clay content of the different size fractions were determined as well as the specific surface area (SSA_mineral) and the sample pore volume after thermal oxidation (OC‐free). The TOC of the different soil‐aggregate fractions was linearly related to SSA_mineral. Clay‐associated OC and nonassociated OC fractions of the different soil samples were quantified using two methods based on the OC surface loading at the clay fraction. The application of organic fertilizer increased the amount of nonassociated OC but hardly affected the concentration of clay‐associated OC. This finding agrees with previous studies on C dynamics in soils and indicates a finite capacity of soil materials to sequester OC. Even without any addition of organic fertilizer, the mineral phase of the analyzed soil material appears to be C‐saturated.     

Effects of mineral surface iron on the CPMAS 13C-NMR spectroscopic detection of organic matter from soil fractions in an agricultural topsoil with different amendments

The decrease of NMR visibility of the C signal in soil samples due to the association between organic carbon (OC) and the topsoil mineral surface was investigated. CPMAS ¹³C‐NMR spectra were obtained for soil particle‐size fractions (< 2 μm, 2–20 μm, > 20 μm) and bulk soils from an agricultural topsoil (Chernozem) that had received three different amendments (no fertilization, mineral fertilization (NPK), mineral (NPK) and organic (cattle manure) fertilizations) at Bad Lauchstädt, Germany. The soil organic carbon content of the three soils depended on the degree of soil fertilization. There was no constant relationship between the total NMR signal intensity and the total amount of organic carbon (TOC) for all size fractions. Indeed, a key role played in the C signal intensity by the paramagnetic ferric ion from the clay content in soil fractions and bulk soils was confirmed. Thus, we describe the variations of C signal intensity by taking into account the distribution of clay‐associated OC and non‐associated OC pools. Depending on the amendment, the C signal visibility was weakened by a factor of 2–4 for the clay‐associated OC. This estimation was rendered possible by combining mineral specific surface area (SSA) measurements with the N₂ gas adsorption method (BET method) and determination of TOC and iron concentrations. This approach contributes to the quantitative evaluation of the CPMAS ¹³C‐NMR detection.     

Short- and long-term effects of the endogeic earthworm Millsonia anomala (Omodeo) (Megascolecidæ, Oligochæta) of tropical savannas,on soil organic matter

Summary This study aimed to establish the effects of Millsonia anomala, a tropical geophagous earthworm common in the humid savannas of Lamto (Ivory Coast), on soil organic matter dynamics over different time scales under laboratory conditions. The texture of casts produced by the worms fed on a shrub savanna soil was not significantly different from that of the soil, which showed that M. anomala ingested soil particles without selection. Physical fractionation of soil organic matter showed that the coarse organic fraction (250–2000 m) was depleted by 25–30% in fresh casts compared to the control noningested soil; this was mainly due to a fragmentation of coarse organic debris. Incubation of casts and a 2-mm sieved control soil under laboratory conditions for more than 1 year showed that the C mineralisation rate was almost four times lower in the casts (3% year^-1) than in the control soil (11% year^-1). We therefore concluded that on a long time scale M. anomala populations may significantly reduce the decomposition rate of soil organic matter in Lamto savannas.     

Relative importance of sorption versus aggregation for organic matter storage in subsoil horizons of two contrasting soils

Soil organic matter (OM) stabilization by the mineral phase can take place through sorption and aggregation. In this study we examined both of these processes, (i) organic carbon (OC) sorption onto clay‐sized particles and (ii) OC occlusion in silt‐size aggregates, with the objective of evaluating their relative importance in OM storage and stabilization in soil. We studied two loamy soil profiles (Haplic Luvisol and Plinthic Cambisol) currently under agricultural use down to a depth of 2 m. Our approach was based on two parallel fractionation methods using different dispersion intensities; these methods isolated a free clay fraction (non‐occluded) and a clay fraction occluded within water‐stable silt‐size aggregates. The two clay fractions were analysed for their C content and ¹⁴C activity. The proportion of sorbed OC was estimated as OC loss after hydrofluoric acid (HF) demineralization. Our results showed an important contribution to SOM stabilization by occlusion of OC into silt‐size aggregates with depth through both soil profiles. In the Haplic Luvisol, OC associated with clay and located in silt‐size aggregates accounted for 34–64% of the total soil OC, whereas in the Plinthic Cambisol this occluded material represented 34–40% of total OC. In the Haplic Luvisol, more OC was located in silt‐size aggregates than was sorbed onto clay‐size minerals, suggesting that silt‐size aggregation plays a dominant role in OC storage in this soil. In the Plinthic Cambisol, the abundance of sorbed OC increased with depth and contributed more to the stored C than that associated with silt‐size aggregates. Radiocarbon dating of both clay fractions (either occluded within silt‐size aggregates or not) suggests, in the case of the Plinthic Cambisol, a preferential stabilization of OC within silt‐size aggregates.     

Earthworm effects on selected physical and chemical properties of soil aggregates

Summary Some physical and chemical properties of 1-to 2-mm aggregates obtained from casts and the burrow-wall material of the earthworm species Lumbricus terrestris, Aporrectodea longa, and Aporrectodea caliginosa were determined in order to show the effects of earthworms on the stabilization of soil aggregates. The results were compared with those of the natural soil from the Ap horizon of a Parabraunerde (Luvisol, FAO). Both the tensile strength and the water stability of aggregates from casts and burrow-wall material were reduced compared with those of the natural aggregates but were increased compared with those of remoulded aggregates. These results indicate that to a great extent existing bonds are destroyed by earthworm ingestion. Nevertheless, earthworm activities are advantageous for the stabilization of reformed aggregates. The coarse sand fraction is reduced by selective ingestion by earthworms. The organic C content is increased by 4.1–21.0% for burrow-wall material and by 21.2–43.0% for casts. The carbonate content of aggregates from casts and burrow-wall material of L. terrestris was reduced by more than 50%, while that of A. longa showed no noticeable changes and that of A. caliginosa was increased by more than 60%. The total content of polysaccharides was increased by 35–87% for casts and by 33–46% for the burrow-wall material of all earthworm species. The most frequently detected monosaccharides were glucose, galactose, and glucosamine. L. terrestris appeared to have the strongest effect on the interparticle bonding of the reformed aggregates, measured both as tensile strength and water stability, followed by A. longa and A. caliginosa.     

Earthworm casts form stable aggregates in physically degraded soils

Topsoils affected by surface mining suffer severe physical degradation and lose most of their earthworm populations. After mining, replaced soils are planted to grassland and managed to improve soil structure. Earthworm inoculation into selected restored areas produced populations similar to those of undisturbed soils within 3 years. Soil properties in inoculated areas were compared with those of controls to evaluate the contribution of casts to bulk soil aggregation, and soil organic matter and root content responses to earthworm activity. Crumb porosity and coarse particle content were measured in water-stable macro-aggregates and earthworm casts to establish whether aggregates were formed by earthworms. Over a 5- to 6-year period, inoculation increased stable aggregation (>2 μm, >60 μm and >3 mm), even at 0- to 5-cm depth where it reduced soil organic matter content. Productivity and root content were also increased by inoculation; roots and organic matter were re-distributed to greater depth. Crumb porosity decreased with casts > aggregates (inoculated plots) > aggregates (control plots). Coarse particle content increased with casts < aggregates (inoculated plots) < aggregates (control plots). Coarse particle and porosity data were consistent with much of the newly aggregated soil being processed and formed by earthworms as casts. Whilst levels of soil organic matter were often closely associated with percentage stable aggregation, root content showed weaker associations. Aggregation percentage was most closely associated with abundance of Aporrectodea longa, although at particular depths significant correlations were also obtained for Aporrectodea caliginosa and Lumbricus terrestris. Results suggest that earthworms, rather than plant roots, initiate aggregation in severely degraded grassland soils.     

Soil organic matter distribution as influenced by enchytraeid and earthworm activity

Loam and sandy soils, and the earthworm casts produced with ¹⁴C-labelled plant material in both soils, were incubated in airtight glass vessels with and without enchytraeids to evaluate the effects of soil fauna on the distribution and fragmentation of organic matter. After 1, 3, and 6 weeks, the amount of C mineralised was determined in soils and earthworm casts, and the soil was fractionated into particulate organic matter (POM), the most active pool of soil organic matter, after complete physical dispersion in water. The percentage weight of fine fractions (0-50 µm) was 67.4% in the loam soil. Sand (coarse, i.e. 150-2,000 µm and fine 50-150 µm) represented 87.2% of total weight in sandy soil, while the percentages of C (PC) were 23.2% in coarse POM (2,000-150 µm) and 11.9% in fine POM (150-50 µm). These percentages were higher than those in loam soil, i.e. 3.4% (coarse POM) and 5.4% (fine POM). The PC in coarse POM (9.50%) and fine POM (16.4%) remained higher in casts from sandy soil than in casts from loam soil (4.7% in coarse and 14.3% in fine POM). The highest percentages of ¹⁴C-labelled leaves were found in fine fractions, 55.9% in casts from loam soil and 48.8% in casts from sandy soil. The C mineralisation of the added plant material was higher in casts from the sandy soil (20.3%) than from the loam soil (13.5%). Enchytraeids enhanced C mineralisation in the bulk sandy soil, but did not affect the mineralisation of added plant material in either soil. The main enchytraeid effect was enhancement of the humification process in the bulk sandy soil, the casts from this soil, and the bulk loam soil.     

Effect of biosolid amendment on enzyme activities in earthworm (Lumbricus terrestris) casts

We studied the effect of amendment of sewage sludge biosolids on enzyme activity in soil and earthworm (Lumbricus terrestris) casts. Enzyme activities and contents of nutrients and organic matter of surrounding soil were compared with the corresponding properties of earthworm casts. This short time experiment was conducted at 20 ± 0.5 °C in the laboratory, simulating field conditions of biosolid treatments. In general, all of doses of biosolid treatments influenced the enzyme activity and contents of nutrients and organic matter of earthworm casts and surrounding soil. Enzyme activity such as urease (UA), alkaline phosphatase (APA), and arylsulfatase (ASA) and the contents of organic matter and nutrients N and P in earthworm casts and surrounding soil increased with increasing biosolid application. Without biosolid additions, enzyme activities in cast of L. terrestris exceeded those in the soil. In contrast, when biosolid was added, DHA in casts was lower than the soil. Activities of UA and APA were consistently higher in L. terrestris casts than in soil of all biosolid treatments. Biosolid amendments generally increased ASA at low doses, but at higher doses, ASA decreased. In general, organic matter and contents of N and P were higher in surface casts of L. terrestris and soils than in the control soil. Activities of UA, APA, the contents of organic carbon and nutrients N and P in soil and casts showed positive correlations. On the contrary, ASA and DHA were negatively correlated with the contents of organic matter and nutrients.     

Estimation of clay content from easily measurable water content of air‐dried soil

Soil texture is one of the main factors controlling soil organic carbon (SOC) storage. Accurate soil‐texture analysis is costly and time‐consuming. Therefore, the clay content is frequently not determined within the scope of regional and plot‐scale studies with high sample numbers. Yet it is well known that the clay content strongly affects soil water content. The objective of our study was to evaluate if the clay content can be estimated by a simple and fast measure like the water content of air‐dried soil. The soil samples used for this study originated from four different European regions (Hainich‐Dün, Germany; Schwäbische Alb, Germany; Hesse, France; Bugac, Hungary) and were collected from topsoils and subsoils in forests, grasslands, and croplands. Clay content, water content of air‐dried soil, and SOC content were measured. Clay content was determined either by the Pipette method or by the Sedigraph method. The water content of air‐dried soil samples ranged from 2.8 g kg^–1 to 63.3 g kg^–1 and the corresponding clay contents from 60.0 g kg^–1 to 815.7 g kg^–1. A significant linear relationship was found between clay content and water content. The scaled mean absolute error (SMAE) of the clay estimation from the water content of air‐dried soil was 20% for the dataset using the Pipette method and 28% for the Sedigraph method. The estimation of the clay content was more accurate in fine‐textured than in coarse‐textured soils. In this study, organic‐C content played a subordinate role next to the clay content in explaining the variance of the water content. The water retention of coarse‐textured soils was more sensitive to the amount of organic C than that of fine‐textured soils. The results indicate that in our study the water content of air‐dried soil samples was a good quantitative proxy of clay contents, especially useful for fine‐textured soils.     

Microbiological Properties in Earthworm Cast and Surrounding Soil Amended with Various Organic Wastes

Abstract

Changes produced in the microbiological properties of earthworm Lumbricus terrestris casts and surrounding soil by the addition of various organic wastes such as wheat straw (WS), tea production waste (TEW), tobacco production waste (TOW), cow manure (CM), and hazelnut husk (HH) were evaluated in an incubation experiment. Twenty‐one days after organic waste treatment, analyses of microbial biomass (Cmic), basal soil respiration (BSR), metabolic quotient (qCO₂), and enzyme activities (dehydrogenase, catalase, β‐glucosidase, urease, alkaline phosphatase, and arylsulphatase) were carried out on collected cast and soil samples. Addition of organic wastes to the soil increased values of Cmic, BSR, and enzyme activities in soil and earthworm casts, indicating activation by microorganisms. Except for catalase activity, these values of microbiological parameters in casts were higher than in surrounding soil at all waste treatments and control. The addition of organic wastes caused a rapid and significant increase in organic carbon, total nitrogen, and microbiological properties in both soils; this increase was especially noticeable in soils treated with TEW.     

Liming upland grassland: the effects on earthworm communities and the chemical characteristics of carbon in casts

Different earthworm species have different tolerances of acid soil conditions, and the application of lime to upland grassland to improve the grazing quality may therefore alter the size and diversity of the earthworm community. Altering soil properties may also affect the chemical characteristics of organic C in earthworm casts. We surveyed the earthworm community of an upland grassland in southern Scotland at the outset of annual lime applications, and after 3 years, and used ¹³C nuclear magnetic resonance (NMR) spectroscopy to assess the distribution of C between different functional groups in the organic matter. In addition, soil was incubated for 8 weeks with several earthworm species in the presence or absence of lime, and the earthworm casts were subsequently analysed by ¹³C NMR spectroscopy. Liming did not significantly affect earthworm abundance or species diversity, but it did affect the chemical composition of the casts. Casts from earthworms incubated in unlimed soil had greater ratios of alkyl‐C to O?alkyl‐C, indicative of more decomposed, recalcitrant C, and spectra from litter‐feeding species had the greatest intensities of O?alkyl‐C signals. In limed soil, the largest O?alkyl‐C signal intensities were not restricted to litter‐feeding species, indicating an increase in the quality of organic matter ingested by geophagous species.     

Changes in potassium availability and other soil properties due to soil ingestion by earthworms

An incubation experiment was conducted to study the changes that occur in potassium availability and other soil properties with ingestion of soil by earthworms. Two soils were used. Raumai soil with high non-exchangeable K and Milson soil with low non-exchangeable K were incubated with two species of earthworm, Aporrectodea caliginosa and Lumbricus rubellus, for 8 weeks. The casts and soil samples were analysed for exchangeable K, Ca, Mg, Na, and H, pH, organic C, and texture. The results indicated that in Raumai soil, the exchangeable K levels of the casts of both earthworm species were significantly higher than for the control soil, the effect being more marked for L. rubellus than for A. caliginosa. In Milson soil, the exchangeable K levels were significantly lower in the casts of both types of earthworm than in the control soil. The nitric acid-extractable K of the soil and casts was not markedly different for either soil type, but available non-exchangeable K values were significantly higher for the casts of L. rubellus from Milson soil than for the noningested Milson soil. In Raumai soil, the exchangeable Ca was higher in the casts of L. rubellus, exchangeable Mg and H were reduced, and exchangeable Na did not change markedly in the cast compared to the control soil. For Milson soil, the casts contained lower exchangeable Ca and H but higher Na and Mg than the control. The casts of both species of earthworm had significantly higher pH values for both soil types. There was no marked difference in the organic C content of the control soil and cast samples for Milson but a reduction in the casts of A. caliginosa for the Raumai soil. Finer fractions increased in the casts of both earthworm species in both soil types.     

Droplet infiltration and organic matter composition of intact crack and biopore surfaces from clay‐illuvial horizons

The organic matter (OM) in biopore walls and aggregate coatings may be important for sorption of reactive solutes and water as well as for solute mass exchange between the soil matrix and the preferential flow (PF) domains in structured soil. Structural surfaces are coated by illuvial clay‐organic material and by OM of different origin, e.g., earthworm casts and root residues. The objectives were to verify the effect of OM on wettability and infiltration of intact structural surfaces in clay‐illuvial horizons (Bt) of Luvisols and to investigate the relevance of the mm‐scale distribution of OM composition on the water and solute transfer. Intact aggregate surfaces and biopore walls were prepared from Bt horizons of Luvisols developed from Loess and glacial till. The mm‐scale spatial distribution of OM composition was scanned using diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The ratio between alkyl and carboxyl functional groups in OM was used as potential wettability index (PWI) of the OM. The infiltration dynamics of water and ethanol droplets were determined measuring contact angles (CA) and water drop penetration times (WDPT). At intact surfaces of earthworm burrows and coated cracks of the Loess‐Bt, the potential wettability of the OM was significantly reduced compared to the uncoated matrix. These data corresponded to increased WDPT, indicating a mm‐scaled sub‐critical water repellency. The relation was highly linear for earthworm burrows and crack coatings from the Loess‐Bt with WDPT > 2.5 s. Other surfaces of the Loess‐Bt and most surfaces of the till‐derived Bt were not found to be repellent. At these surfaces, no relations between the potential wettability of the OM and the actual wettability of the surface were found. The results suggest that water absorption at intact surface structures, i.e., mass exchange between PF paths and soil matrix, can be locally affected by a mm‐scale OM distribution if OM is of increased content and is enriched in alkyl functional groups. For such surfaces, the relation between potential and actual wettability provides the possibility to evaluate the mm‐scale spatial distribution of wettability and sorption and mass exchange from DRIFT spectroscopic scanning.     

Soil organic matter fractionation as a tool for predicting nitrogen mineralization in silty arable soils

After decades of searching for a practical method to estimate the N mineralization capacity of soil, there is still no consistent methodology. Indeed it is important to have practical methods to estimate soil nitrogen release for plant uptake and that should be appropriate, less time consuming, and cost effective for farmers. We fractionated soil organic matter (SOM) to assess different fractions of SOM as predictors for net N mineralization measured from repacked (disturbed) and intact (undisturbed) soil cores in 14 weeks of laboratory incubations. A soil set consisting of surface soil from 18 cereal and root‐cropped arable fields was physically fractionated into coarse and fine free particulate OM (coarse fPOM and fine fPOM), intra‐microaggregate particulate OM (iPOM) and silt and clay sized OM. The silt and clay sized OM was further chemically fractionated by oxidation with 6% NaOCl to isolate an oxidation‐resistant OM fraction, followed by extraction of mineral bound OM with 10% HF (HF‐res OM). Stepwise multiple linear regression yielded a significant relationship between the annual N mineralization (kg N/ha) from undisturbed soil and coarse fPOM N (kg N/ha), silt and clay N (kg N/ha) and its C:N ratio (R² = 0.80; P < 0.01). The relative annual N mineralization (% of soil N) from disturbed soils was related to coarse fPOM N, HF‐res OC (% of soil organic carbon) and its C:N ratio (R² = 0.83; P < 0.01). Physical fractions of SOM were thus found to be the most useful predictors for estimating the annual N mineralization rate of undisturbed soils. However, the bioavailability of physical fractions was changed due to the disturbance of soil. For disturbed soils, a presumed stable chemical SOM fraction was found to be a relevant predictor indicating that this fraction still contains bio‐available N. The latter prompted a revision in our reasoning behind selective oxidation and extraction as tools for characterizing soil organic N quality with respect to N availability. Nonetheless, the present study also underscores the potential of a combined physical and chemical fractionation procedure for isolating and quantifying N fractions which preferentially contribute to bulk soil N mineralization. The N content or C:N ratio of such fractions may be used to predict N mineralization in arable soils.     

Soil structure and earthworm activity in a marine silt loam under pasture versus arable land

Agricultural management influences soil organic matter (SOM) and earthworm activity which interact with soil structure. We aimed to describe the change in earthworm activity and related soil (micro)structure and SOM in a loamy Eutrodept as affected by permanent pasture (PP) and conventional arable (CA). Thin sections were studied and biogenic calcite spheroids, worm casts, infillings and groundmass coatings were quantified. In both soils, sedimentary stratification was absent up till 50 cm depth, and equal amounts of biogenic calcite spheroids were counted, suggesting similar earthworm activity in the past. Currently the percentage volume of optically recognizable biologically influenced groundmass was 54% in PP and 10% in CA. The Ah of PP had an organic matter content of 66 mg kg^-1 whereas the content was 22 mg kg^-1 in the Ah of the CA soil. Low earthworm activity in the CA soil has led to the formation of a physicogenetic soil structure with mainly angular blocky aggregates, fissure and angular blocky microstructures. The percentage volume of unsorted coatings counted in thin sections was 5%, indicating soil structure deterioration. SOM was evenly distributed through the groundmass. In contrast, high earthworm activity in the PP soil has caused a biogenic structure consisting of granular and subangular blocky aggregates and spongy and granular microstructures with abundant channels. SOM was incorporated as fine (10-100 µm) particulate organic matter in worm casts and infillings and intimately mixed with clay material. Such encapsulated SOM indicates the presence of microaggregates within biogenic macroaggregates, in which SOM may be physically protected against rapid decomposition.     

蚯蚓粪施用量对黄土区典型土壤团聚体及其有机碳分布的影响

为探究蚯蚓粪施用量对黄土区典型土壤团聚体及其结合碳的影响,该研究采用土柱培养试验,研究了黄绵土(CS)、黑垆土(DS)和风沙土(AS)团聚体和有机碳的数量及稳定性对不同蚯蚓粪施用量(0、1％、3％和5％)的响应.结果表明,施用蚯蚓粪后粒径不大于0.25 mm团聚体含量在CS和DS中降低了9.2％～24.7％和7.0％～...     

Adsorption of the Bacillus thuringiensis toxin (Cry1Ab) on Na‐montmorillonite and on the clay fractions of different soils

The adsorption of the toxin from Bacillus thuringiensis (Bt‐toxin), which is synthesized in genetically modified maize, on sterilized Na‐montmorillonite and on H₂O₂‐treated and untreated clay fractions of three soils from different sites were studied. All adsorption isotherms can be described by a linear isotherm. Although all clay fractions from the different soils show nearly the same mineralogical composition, we found different affinities ranging from k = 47.7 to k = 366.7 of the adsorbates for the Bt‐toxin. The H₂O₂‐treated clay fractions show no correlation between the adsorption affinity and the amount of soil organic matter. On the other hand, there is a correlation between the content of organic carbon and the adsorption affinity of the untreated clay fractions. This can be explained by the fact that due to the coatings of soil organic matter on aggregates, the Bt‐toxin polymers are not able to adsorb within the clay aggregates.     

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.     

Long‐term effects of cropped vs. fallow and fertilizer amendments on soil organic matter I. Organic carbon

A long‐term field experiment, conducted since 1962 in Gumpenstein (Austria) on a Dystric Cambisol, was used for the present investigation. We combined a physical fractionation procedure with the determination of natural abundance of ¹³C and FT‐IR spectroscopy to study the influence of fertilizer amendments (organic manure and mineral fertilizers) and management practices (fallow vs. cropped) on changes in organic carbon (OC) associated with different particle‐size fractions. The OC content in bulk soil decreased or was not affected by slurry+straw, PK, and NPK treatments in both fallow and cropped plots after 28 and 38 yr of treatment. However, OC in plots receiving organic manures increased depending on the quality of the organic manures applied. The ranking among the different treatments under both fallow and cropped plots was: animal manure (liquid) > animal manure (solid) > cattle slurry = slurry+straw = PK = NPK. Results showed that the two types of management practices, fallow (non‐tilled) vs. cropped (tilled) had effects on OC concentrations. Comparing the OC contribution of particle‐size fractions to the total OC amount revealed the following ranking: silt > clay > fine sand > coarse sand except in the plots receiving solid or liquid animal manure. Size fractions within treatments showed larger variations of ¹³C abundances than bulk samples between treatments. The natural abundances of ¹³C increased especially in cropped (and tilled) plots. It was shown by cluster analysis that FT‐IR spectra differentiated between the different treatments originating from different land management practices. The present study revealed that below‐ground C deposition by agricultural plants can hardly compensate the C losses due to tillage.