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.     

The fate of catechol in soil as affected by earthworms and clay

The effect of endogeic earthworms (Octolasion tyrtaeum) and the availability of clay (Montmorillonite) on the mobilization and stabilization of uniformly ¹⁴C-labelled catechol mixed into arable and forest soil was investigated in a short- and a long-term microcosm experiment. By using arable and forest soil the effect of earthworms and clay in soils differing in the saturation of the mineral matrix with organic matter was investigated. In the short-term experiment microcosms were destructively sampled when the soil had been transformed into casts. In the long-term experiment earthworm casts produced during 7 days and non-processed soil were incubated for three further months. Production of CO₂ and ¹⁴CO₂ were measured at regular intervals. Accumulation of ¹⁴C in humic fractions (DOM, fulvic acids, humic acids and humin) of the casts and the non-processed soil and incorporation of ¹⁴C into earthworm tissue were determined.Incorporation of ¹⁴C into earthworm tissue was low, with 0.1 and 0.44% recovered in the short- and long-term experiment, respectively, suggesting that endogeic earthworms preferentially assimilate non-phenolic soil carbon. Cumulative production of CO₂-C was significantly increased in casts produced from the arable soil, but lower in casts produced from the forest soil; generally, the production of CO₂-C was higher in forest than in arable soil. Both soils differed in the pattern of ¹⁴CO₂-C production; initially it was higher in the forest soil than in the arable soil, whereas later the opposite was true. Octolasion tyrtaeum did not affect ¹⁴CO₂-C production in the forest soil, but increased it in the arable soil early in the experiment; clay counteracted this effect. Clay and O. tyrtaeum did not affect integration of ¹⁴C into humic fractions of the forest soil. In contrast, in the arable soil O. tyrtaeum increased the amount of ¹⁴C in the labile fractions, whereas clay increased it in the humin fraction.The results indicate that endogeic earthworms increase microbial activity and thus mineralization of phenolic compounds, whereas clay decreases it presumably by binding phenolic compounds to clay particles when passing through the earthworm gut. Endogeic earthworms and clay are only of minor importance for the fate of catechol in soils with high organic matter, clay and microbial biomass concentrations, but in contrast affect the fate of phenolic compounds in low clay soils.     

Inorganic and organic phosphorus pools in earthworm casts (Glossoscolecidae) and a Brazilian rainforest Oxisol

We compared differences in soil phosphorus fractions between large earthworm casts (Family Glossoscolecidae) and surrounding soils, i.e., Oxisols in 10 year-old upland agroforestry system (AGR), pasture (PAS), and secondary forest (SEC) in the Central Brazilian Amazon. AGR and PAS both received low-input fertilization and SEC received no fertilization. We found that earthworm casts had higher levels of organic hydroxide P than surrounding soils, whereas fertilization increased inorganic hydroxide P. Inorganic P was increased by fertilization, and organic P was increased by earthworm gut passage and/or selection of ingested materials, which increased available P (sum of resin and bicarbonate fractions) and moderately available P (sum of hydroxide and dilute acid fractions), and P fertilizer application and land-use increased available P. The use of a modified sequential P fractionation produced fewer differences between earthworm casts and soils than were expected. We suggest the use of a condensed extraction procedure with three fractions (Available P, Moderately Available P, and Resistant P) that provide an ecologically based understanding of the P availability in soil. Earthworm casts were estimated to constitute 41.0, 38.2, and 26.0 kg ha⁻¹ of total available P stocks (sum of resin and bicarbonate fractions) in the agroforestry system, pasture, and secondary forest, respectively.     

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.     

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.     

Effect of earthworm activity (Aporrectodea giardi) on atrazine adsorption and biodegradation

We investigated the influence of earthworm (Aporrectodea giardi) activity on soil properties and on atrazine (AT) adsorption and biodegradation by comparing a coarse‐textured smectite‐free wetland soil (Brittany, France) with the earthworm casts derived from the top horizon of this soil. Casts are characterized by lower pH, are enriched in organic carbon (OC) and clay content, have a larger cation exchange capacity, and a greater exchangeable Ca content. The clay mineralogy of the soil studied and casts is characterized by a muscovite–kaolinite–chlorite association. In addition, the clay fraction of the soil contains lepidocrocite (γ‐FeOOH), which was not found in the casts. Atrazine adsorption isotherms were reasonably well described by the Freundlich equation and were all non‐linear. The mean amounts of adsorbed AT for starting concentrations of 3–30 mg litre^?1 ranged from 8 to 34%, being largest in earthworm casts. Soil AT adsorption capacity was well correlated with OC content. Non‐decomposed organic matter present in the coarse size fractions and specific compounds present in earthworm casts (proteins, mono‐ and polysaccharides, polyphenols, sugars, lignin) and microbial and fungal biomass contribute to AT adsorption. Weak electrostatic (physical) sorption of AT on organic compounds and on mineral surfaces prevails. For casts, the formation of additional hydrophobic interactions between AT and SOM is proposed. We also studied AT biodegradation by the model bacterium Pseudomonas sp. strain ADP in the presence of soils or earthworm casts. An enhancement of the AT disappearance rate was observed in the presence of all the solid matrices tested compared with that obtained in an aqueous medium. The biodegradation rate was shown to be dependent not only on the OC content of the solid matrix, but mainly on its composition and structure.     

Location and chemical composition of stabilized organic carbon in topsoil and subsoil horizons of two acid forest soils

The ¹⁴C age of soil organic matter is known to increase with soil depth. Therefore, the aim of this study was to examine the stabilization of carbon compounds in the entire soil profile using particle size fractionation to distinguish SOM pools with different turnover rates. Samples were taken from a Dystric Cambisol and a Haplic Podzol under forest, which are representative soil types under humid climate conditions. The conceptual approach included the analyses of particle size fractions of all mineral soil horizons for elemental composition and chemical structure of the organic matter by ¹³C cross-polarization magic angle spinning nuclear magnetic resonance (CPMAS NMR) spectroscopy. The contribution of phenols and hydroxyalkanoic acids, which represent recalcitrant plant litter compounds, was analyzed after CuO oxidation.In the Dystric Cambisol, the highest carbon concentration as well as the highest percentage of total organic carbon are found in the <6.3 μm fractions of the B and C horizons. In the Haplic Podzol, carbon distribution among the particle size fractions of the Bh and Bvs horizons is influenced by the adsorption of dissolved organic matter. A relationship between the carbon enrichment in fractions <6.3 μm and the ¹⁴C activity of the bulk soil indicates that stabilization of SOM occurs in fine particle size fractions of both soils. ¹³C CPMAS NMR spectroscopy shows that a high concentration of alkyl carbon is present in the fine particle size fractions of the B horizons of the Dystric Cambisol. Decreasing contribution of O-alkyl and aromatic carbon with particle size as well as soil depth indicates that these compounds are not stabilized in the Dystric Cambisol. These results are in accordance with data obtained by wet chemical analyses showing that cutin/suberin-derived hydroxyalkanoic acids are preserved in the fine particle size fractions of the B horizons. The organic matter composition in particle size fractions of the top- and subsoil horizons of the Haplic Podzol shows that this soil is acting like a chromatographic system preserving insoluble alkyl carbon in the fine particle size fractions of the A horizon. Small molecules, most probably organic acids, dominate in the fine particle size fractions of the C horizons, where they are stabilized in clay-sized fractions most likely due to the interaction with the mineral phase. The characterization of lignin-derived phenols indicated, in accordance with the NMR measurements, that these compounds are not stabilized in the mineral soil horizons.     

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.     

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％～...     

The interactions between soil type and earthworm species determine the properties of earthworm casts

Earthworms are recognized to increase soil porosity, reorganize soil structure, and stimulate soil microflora and nutrient mineralization. The properties of earthworm casts should depend both on earthworm species or ecological group and on soil properties. Interactions between earthworm species and soil types have been suggested, but only poorly demonstrated. In order to better understand those interactions, two hypotheses led our study: (1) Soil type has a greater influence on cast properties than earthworm; (2) Earthworms from different species influence cast properties differently; (3) The intensity and direction of the impact of each earthworm species on cast properties vary with soil properties. Fifteen physical and chemical variables (N–NH₄⁺, N–NO₃⁻, total organic C and N, C/N ratio, CaCO₃, pH, P, K⁺, Mg²⁺, Mn²⁺, Na⁺, CEC, moisture, wettability) were measured in casts of three earthworm species (Lumbricus terrestris, Allolobophora chlorotica and Aporrectodea rosea) produced in three temperate soils. Univariate and multivariate analyses showed that earthworm species and soil types significantly impacted cast properties. pH, N_t, K and Mg contents were interactively altered by both factors. Multivariate analysis showed that a difference of soil type had a major impact on casts properties (62%) compared to the impact of a difference of earthworm species (10%). Cast properties were most impacted by L. terrestris, then by A. chlorotica and last by A. rosea. The response ratio (ratio of the properties of the casts to the properties of the bulk soil) was used to quantify the effect of earthworm species compared to the control soil. It showed a higher response of variables in casts in nutrient-rich soils, especially in casts of L. terrestris. The interactions between earthworm species and soil types on cast properties were discussed with regards to earthworm ecology, properties of the soil, and earthworm modifications of cast microflora.     

Characterization of hydrophobic acid fractions in water-soluble organic matter in Dystric Cambisol and in a stream in a small forested watershed: Seasonal and vertical variations in chemical properties

Abstract

Chemical properties of hydrophobic acid (HoA) fractions in water-soluble organic matter in soil and water are concerned with its interactions with mineral soil surfaces and organic pollutants. In 2004 we examined the seasonal and vertical changes in chemical properties of the HoA fraction in a Cambisol profile and compared these properties with those in the HoA fraction of an adjacent stream (aquatic humic substances) in a temperate forested watershed using high performance size exclusion chromatography (HPSEC) and ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy. The HoA fractions from Oi, Oe/Oa, A and B horizon soils in summer had lower O-alkyl C proportions than those recorded in samples in other seasons. The proportions of aromatic C in HoA fractions from A and B horizons were highest in summer. These seasonal variations were less significant than variations with soil depth. O-alkyl C proportions in HoA fractions decreased with increasing soil depth from the Oi to the A horizon. The HoA fractions from the B horizon showed a higher alkyl C proportion than samples from other horizons in winter and spring. These changes with soil depth from the Oi to A horizons might result from selective utilization of carbohydrate carbon by microorganisms, whereas those in the B horizon may result from sorption to mineral surfaces. The HoA fractions in the stream were similar in relative molecular weight, distribution of each type of proton and carbon species in HoA fractions from the B horizon, whereas stream HoA fractions collected in summer would be derived from organic horizons. This indicated that vertical changes in the chemical properties of HoA fractions in soil and pathways of water to the stream would largely affect the chemical properties of HoA fractions in the stream.     

How do earthworms influence organic matter quantity and quality in tropical soils?

Earthworms are important regulators of soil structure and soil organic matter (SOM) dynamics; however, quantifying their influence on SOM cycling in tropical ecosystems remains little studied. Simulated rainfall was used to disrupt casts produced by Amynthas khami and their surrounding soil (control) into a range of small sized aggregates (50-250, 250-500, 500-2000 and 2000-5000 μm). To gain insight into how earthworms influence SOM biogeochemical composition in the aggregates, we carried out elemental and stable isotope analysis, and analytical pyrolysis (Py GC/MS). We also characterized their lignin component after oxidation with cupric oxide (CuO).The C content of smaller size fractions (<500 μm) in the control soil was higher than in the larger fractions. Our study therefore suggests that the aggregate hierarchy concept, which is used to understand soil aggregates and SOM dynamics in temperate soils, may not be applicable to the tropical Acrisol studied here. Earthworms modified SOM organization in soil aggregates. Although the isotope analyses were useful for highlighting SOM enrichment in the earthworm casts, aggregate fractions could not be classified according to particle size. Molecular analyses were necessary to indicate that SOM in all size fractions of casts consisted of relatively undecomposed material. Protection of the most labile SOM structures occurred in the smallest aggregate size fraction (50-250 μm). Py GC/MS showed that earthworm casts and control aggregates <2000 μm could be clearly distinguished according to the molecular properties of their SOM. Aggregates larger than 2000 μm, however, were most probably composed of all fractions and were not different. As a consequence, our results indicate that studies to determine the impact of earthworms on SOM turnover in soil are spatially dependant on the scale of observation.     

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.     

Carbon and nitrogen dynamics in ageing earthworm casts in grasslands of the eastern plains of Colombia

 The effects of a large species of anecic earthworm, Martiodrilus carimaguensis Jiménez and Moreno, on soil C and N dynamics were investigated in a native savanna and a man-made pasture of the eastern plains of Colombia. We compared, across time (11 months), the total C, total N, NH⁺ ₄ and NO^– ₃ contents in the earthworm casts, the underlying soil and the adjacent soil. Additional sampling of root biomass and macrofauna was performed. In the two management systems, the total C and N contents were higher in casts (4.33–7.50%) than in the bulk soil (2.81–4.08%), showing that the earthworms selected food substrates with high organic contents. In general, C contents significantly increased during cast ageing (+100%), possibly because of CO₂ fixation processes, dead root accumulation and/or macrofaunal activities in casts. In fresh casts, NH⁺ ₄ levels were very high (294.20–233.98 μg g^–1 dry cast) when compared to the soil (26.96–73.95 μg g^–1 dry soil), due to the intense mineralisation processes that occurred during the transit of soil and organic matter through the earthworm gut. During the first week of cast ageing, NH⁺ ₄ levels sharply decreased, while NH^– ₃ levels showed successive peaks in the casts, the underlying soil and the adjacent soil. These results suggested the rapid production of NO^– ₃ by nitrification processes in the fresh casts, followed by diffusion to the nearby soil, first vertically, then horizontally. After 2 weeks of cast ageing, NH⁺ ₄ and NO^– ₃ levels only showed slight variations, likely because of organic matter protection in stable dry casts. The root biomass was higher (1.6–4.7 times) below the old earthworm casts. The ecological significance of these results is discussed. Received: 22 October 1998     

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.     

Sorption and desorption of diuron and norflurazon in Florida citrus soils

The potential for surface and groundwater contamination of soil applied herbicides is partly dependent on soil properties. Sorption and desorption of diuron and norflurazon were studied in seven soils representative of the southern citrus-belt of Florida using the batch-equilibrium technique. Sorption of herbicides was influenced by soil properties. Sorption coefficients (K _d) ranged from 0.84 to 3.26 mL g^?1 for diuron and 0.63 to 2.20 mL g^?1 for norflurazon indicating weak to moderate binding of herbicides to soil. For norflurazon, K _dwas significantly related to organic C content, soil pH, and cation exchange capacity. For diuron, absence of a significant relationship between K _dand selected soil properties suggests that the soil properties other than those studied may play a role in determining sorption on these soils. Desorption studies showed that higher amounts of diuron and norflurazon was desorbed by water than by 0.5 M CaCl₂. An inverse relationship was apparent between herbicides sorbed and that which was desorbed among the soils studied. The soil which exhibited higher sorption had lower desorption and the soil which exhibited lower sorption had higher desorption.     

Nitrogen distribution and 15N natural abundances in particle size fractions of a long‐term agricultural field experiment

The present study combined a physical fractionation procedure with the determination of the natural abundance of ¹⁵N to investigate the impact of organic manure and mineral fertilizer application, and fallow on changes of N associated with different soil particle size fractions. The long‐term field experiment was conducted since 1956 in Ultuna, Sweden, on an Eutric Cambisol. Nitrogen in bulk soil and in particle size fractions changed significantly since 1956. The N_t concentrations in bulk soil decreased in all treatments not receiving organic materials. Comparing the N contribution of particle‐size fractions to the total N amount revealed the following ranking: silt > clay > fine clay > fine sand > coarse sand. The relative contribution of N in silt sized particles significantly increased from low to high bulk soil N contents, whereas N in clay and fine clay fractions decreased. The C : N ratios of particle size fractions differed considerably more between treatments than C : N ratios in bulk soils. Generally, the C : N ratios decreased from coarse to fine fractions emphasizing the tendency of smaller fractions being more significant as N sink than as C_org sink. ¹⁵N abundances varied more between particle size fractions of single treatments than between bulk soil from differently treated plots. Within treatments we observed differences of up to 7.1 ‰ between particle size fractions. In most cases δ ¹⁵N values increased with decreasing particle sizes. This pattern on average was similar to changes in δ ¹³ C. Our results suggest that silt sized particles acted as medium‐term sink of introduced N and that ¹⁵N abundances in particle size fractions sensitively reflect changes in N status in response to soil management.     

Using earthworm avoidance behaviour to assess the toxicity of formulated herbicides and their active ingredients on natural soils

Background, aim, and scope

Soil quality has been threatened by intensive agricultural practises, namely those relying on the application of pesticides, such as herbicides. Among the non-target terrestrial organisms exposed to such scenarios, earthworms are key ecological receptors widely used in ecotoxicological studies. As such, this work aims to assess the effects of two herbicide active ingredients (a.i.)—sulcotrione and penoxsulam—and their respective commercial formulations—MIKADO® and VIPER® (referred as Mikado and Viper)—on the avoidance behaviour of Eisenia andrei. In an attempt to enhance the ecological relevance of the generated toxicity data, the avoidance tests were run with standard (LUFA 2.2; L) and natural soils (from corn and rice fields), as long as their habitat function did not constrain the earthworm behaviour.

Methodology

Earthworms were bred in the lab before test conductance. The natural soils used as substrates were collected before the cropping season on corn (C) and rice (R) fields, which are integrated in a wide area exploited for agriculture. Their physico-chemical characterization evolved the determination of pH (H₂O, KCl), conductivity, organic matter (OM) and clay/silt contents, and water-holding capacity (WHC). The avoidance tests intended to ascertain (1) the random distribution of earthworms in the natural soils C and R (dual-control tests), (2) the habitat function of natural soils against each other and against L soil, (3) the effect of active ingredients and formulated herbicides on E. andrei behaviour. Avoidance tests with the a.i.s were only performed in L soil. Data evaluation followed ISO (2005) guidelines.

Results

C and R soils presented higher OM (5.1% and 4.5%, respectively) and clay/silt (53.3 and 43.1, respectively) contents and WHC (107.2 and 109.9%, respectively) than L soil (4.1, 21.4 and 48.0%, correspondingly). Earthworms distributed randomly in dual-control tests, but preferred R soil significantly, relative to L or C soils. The LOEC and EC₅₀ values calculated for sulcotrione (>1,000.0 and 1,263.3 mg a.i. kg^-1, respectively) and Mikado (1,012.8 and 1,301.3 mg a.i. kg^-1, respectively) were much higher than those calculated for penoxsulam (100 and 80.6 mg a.i. kg^-1, respectively) or Viper (52.7 and 51.5 mg a.i. kg^-1, respectively), when L soil was used as substrate. Moreover, the habitat function of L soil contaminated with the formulated herbicide Viper was more constrained relative to that of the a.i. penoxsulam. Viper induced higher % avoidance on E. andrei exposed to the contaminated L soil compared to that under the R soil.

Discussion

The response of earthworms to R (attraction) and C (avoidance) soils could be related, not only to the quantity of OM content, but also to the quality of organic and inorganic fractions of soil, beyond other intrinsic properties of soils. Both Mikado and sulcotrione impacted the behaviour of E. andrei only slightly. This endpoint was more affected under penoxsulam or Viper exposures on L soil, being the latter-formulated product even more repellent for E. andrei than the a.i. The effect of adjuvants added to the commercial formulation of Viper, may have increased the toxicity of the a.i. Thereby, our results reinforce the need for a careful assessment of the impacts of formulated products. Furthermore, since there was a reduction in earthworm % avoidance under Viper exposures on the natural soil R, it was possible that pesticide bioavailability had been reduced by its sorption to OM and clay mineral sorption sites.

Conclusions

Though the standard L soil should be used for reproducibility and comparison means, other natural soils should be added to the assessment of chemicals, for sake of ecological relevance. Both herbicides induced avoidance behaviour on E. andrei, albeit stronger effects were denoted by penoxsulam and its respective formulated product, Viper. Overall, avoidance tests provided a sensitive, valuable and feasible response either to compare the habitat function of different standard and agricultural natural soils or to test the effect of herbicides.

Recommendations and perspectives

An effort should be made to enlarge the terrestrial ecotoxicological database as a way to fulfil the huge lack of information available for this ecosystem. In this context, additional research congregating a potential linkage between physiological activities sustaining the regular metabolism of earthworms and their avoidance behaviour or even their reproductive effects would be welcomed, especially in what regards formulated pesticides. Such approach would provide a robust and comprehensive understanding of chemical effects. Furthermore, it is encouraged that natural soils should be used to improve the reliability of chemical testing.     

Microbial populations,enzyme activities and nitrogen-phosphorus-potassium enrichment in earthworm casts and in the surrounding soil of a pineapple plantation

Summary Total populations of bacteria and fungi, dehydrogenase activity (as a measure of total potential microbial activity), and urease and phosphatase activities were determined in earthworm casts and surrounding laterite soils planted to pineapple. The casts contained higher microbial populations and enzyme activities than the soil. Except for fungal populations, statistically significant (P = 0.05) increases were found in all other parameters. Microbial populations and enzyme activities showed similar temporal trends with higher values in spring and summer and lower values in winter. The earthworm casts contained higher amounts of N, P, K and organic C than the soil (P = 0.05). Selective feeding by earthworms on organically rich substrates, which break down during passage through the gut, is likely to be responsible for the higher microbial populations and greater enzyme activity in the casts.     

Effects of organic manure and NPK fertilization on earthworm activity in an Oxisol

The effects of fertilization with N, P, K, and organic manure (alone or in combination) on earthworm populations, biomass, and casting activity were measured in a cultivated soil (organic C 1.5%, annual rainfall 2000–2300 mm). These applications of fertilizer caused significant increases in earthworm numbers, biomass, and casts. N alone or in combination with P and K also influenced these earthworm parameters significantly. The inorganic NPK fertilizer in combination with organic manure had a significantly greater effect on earthworm activities than NPK fertilizer alone, and therefore the addition of organic matter appears advisable in order to obtain maximum benefits from NPK fertilizer in this soil.