Using glomalin as an indicator for arbuscular mycorrhizal hyphal growth: an example from a tropical rain forest soil

Glomalin concentrations of extra-radical arbuscular mycorrhizal (AM) hyphae were estimated by deploying hyphal in-growth cores containing glomalin-free sand in field soils in a tropical forest and in pot cultures. In field soils, glomalin was 0.044±0.013 μg m⁻¹ hyphae. In pot cultures glomalin concentrations were lower (range 0.0068-0.036 μg m⁻¹), and varied significantly among species. Using this technique, preliminary estimates of extraradical AM hyphal production on Inceptisols were 1.91 Mg ha⁻¹yr⁻¹ and on Oxisol were 1.47 Mg ha⁻¹ yr⁻¹, but they could range between 0.9-5.7 Mg ha⁻¹ yr⁻¹. These rates of hyphal production are approximately 10% (range 5-33%) of estimated above ground primary production of the forest.     

Influence of earthworm activity on aggregate-associated carbon and nitrogen dynamics differs with agroecosystem management

Earthworms are known to be important regulators of soil structure and soil organic matter (SOM) dynamics, however, quantifying their influence on carbon (C) and nitrogen (N) stabilization in agroecosystems remains a pertinent task. We manipulated population densities of the earthworm Aporrectodea rosea in three maize-tomato cropping systems [conventional (i.e., mineral fertilizer), organic (i.e., composted manure and legume cover crop), and an intermediate low-input system (i.e., alternating years of legume cover crop and mineral fertilizer)] to examine their influence on C and N incorporation into soil aggregates. Two treatments, no-earthworm versus the addition of five A. rosea adults, were established in paired microcosms using electro-shocking. A ¹³C and ¹⁵N labeled cover crop was incorporated into the soil of the organic and low-input systems, while ¹⁵N mineral fertilizer was applied in the conventional system. Soil samples were collected during the growing season and wet-sieved to obtain three aggregate size classes: macroaggregates (>250 μm), microaggregates (53-250 μm) and silt and clay fraction (<53 μm). Macroaggregates were further separated into coarse particulate organic matter (cPOM), microaggregates and the silt and clay fraction. Total C, ¹³C, total N and ¹⁵N were measured for all fractions and the bulk soil. Significant earthworm influences were restricted to the low-input and conventional systems on the final sampling date. In the low-input system, earthworms increased the incorporation of new C into microaggregates within macroaggregates by 35% (2.8 g m⁻² increase; P=0.03), compared to the no-earthworm treatment. Within this same cropping system, earthworms increased new N in the cPOM and the silt and clay fractions within macroaggregates, by 49% (0.21 g m⁻²; P<0.01) and 38% (0.19 g m⁻²; P=0.02), respectively. In the conventional system, earthworms appeared to decrease the incorporation of new N into free microaggregates and macroaggregates by 49% (1.38 g m⁻²; P=0.04) and 41% (0.51 g m⁻²; P=0.057), respectively. These results indicate that earthworms can play an important role in C and N dynamics and that agroecosystem management greatly influences the magnitude and direction of their effect.     

Earthworm-mycorrhiza interaction on Cd uptake and growth of ryegrass

The effects of inoculation of earthworms and arbuscular mycorrhiza separately, and in combination, on Cd uptake and growth of ryegrass were studied in soils contaminated with 0, 5, 10, 20 mg of Cd kg⁻¹ soil. Both earthworms and mycorrhiza were able to survive in all the treatments with added Cd. Earthworm activity significantly increased mycorrhizal infection rate of root and ryegrass shoot biomass. Earthworm activity decreased soil pH by about 0.2 units, and enhanced root Cd concentration and ryegrass Cd uptake. Mycorrhiza inoculation increased shoot and root Cd concentration substantially, and at the highest dosage of 20 mg Cd kg⁻¹ decreased biomass of ryegrass. Inoculation of both earthworms and mycorrhiza increased ryegrass shoot Cd uptake at low Cd concentrations (5 and 10 mg Cd kg⁻¹ soil), when compared with inoculation of earthworms or mycorrhiza alone. In conclusion, earthworm, mycorrhiza and their interaction may have a potential role in elevating phytoextraction efficiency in low to medium level metal contaminated soil.     

Toxicological effects of TiO2 and ZnO nanoparticles in soil on earthworm Eisenia fetida

Nanoparticles (NPs) of TiO₂ and ZnO are receiving increasing attention due to their widespread applications. To evaluate their toxicities to the earthworm Eisenia fetida (Savigny, 1826) in soil, artificial soil systems containing distilled water, 0.1, 0.5, 1.0 or 5.0 g kg⁻¹ of NPs were prepared and earthworms were exposed for 7 days. Contents of Zn and Ti in earthworm, activities of antioxidant enzymes, DNA damage to earthworm, activity of cellulase and damage to mitochondria of gut cells were investigated after acute toxicity test. The results from response of the antioxidant system combined with DNA damage endpoint (comet assay) indicated that TiO₂ and ZnO NPs could induce significant damage to earthworms when doses were greater than 1.0 g kg⁻¹. We found that Ti and Zn, especially Zn, were bioaccumulated, and that mitochondria were damaged at the highest dose in soil (5.0 g kg⁻¹). The activity of cellulase was significantly inhibited when organisms were exposed to 5.0 g kg⁻¹ of ZnO NPs. Our study demonstrates that both TiO₂ and ZnO NPs exert harmful effects to E. fetida when their levels are higher than 1.0 g kg⁻¹ in soil and that toxicity of ZnO NPs was higher than TiO₂.     

Heavy metal accumulation by two earthworm species and its relationship to total and DTPA-extractable metals in soils

The concentrations of Zn, Cd, Pb and Cu in earthworm tissues were compared with the total and DTPA-extractable contents of these heavy metals in contaminated soils. Samples were taken from a pasture polluted by waste from a metallurgic industry over 70 y ago. Three individuals of Aporrectodea caliginosa and Lumbricus rubellus and soil samples were collected at six points along a gradient of increasing pollution. Total metal contents of earthworms, soil, and metals extracted by DTPA from the soil were measured. Total heavy metal contents of the soils ranged from 165.7 to 1231.7 mg Zn kg⁻¹, 2.7 to 5.2 mg Cd kg⁻¹, 45.8 to 465.5 mg Pb kg⁻¹ and 30.0 to 107.5 mg Cu kg⁻¹. Their correlations with metals extracted by DTPA were highly significant. Contents of the metals in earthworm tissues were higher in A. caliginosa than in L. rubellus, with values ranging from 556 to 3381 mg Zn kg⁻¹, 11.6 to 102.9 mg Cd kg⁻¹, 1.9 to 182.8 mg Pb kg⁻¹ and 17.9 to 35.9 mg Cu kg⁻¹ in A. caliginosa, and from 667.9 to 2645 mg Zn kg⁻¹, 7.7 to 26.3 mg Cd kg⁻¹, 0.5 to 37.9 mg Pb kg⁻¹ and 16.0 to 37.6 mg Cu kg⁻¹ in L. rubellus, respectively. Correlations between body loads in earthworms with either total or DTPA-extractable contents of soil metals were significant, except for Cd in L. rubellus and Cu in A. caliginosa. Considering its simple analytical procedure, DTPA-extractable fraction may be preferable to total metal content as a predictor of bio-concentrations of heavy metals in earthworms. Biota-to-Soil Accumulation Factor (BSAF) of these four metals are Cd>Zn>Cu>Pb, with range of mean values between: Cd (6.18-17.02), Zn (1.95-7.91), Cu (0.27-0.89) and Pb (0.08-0.38) in A. caliginosa, and Cd (3.64-6.34), Zn (1.5-6.35), Cu (0.29-0.87) and Pb (0.04-0.13) in L. rubellus. The BSAF of Ca, Fe and Mn are Ca>Mn>Fe, with mean values of: Ca (0.46-1.31), Mn (0.041-0.111), Fe (0.017-0.07) in A. caliginosa and Ca (0.98-2.13), Mn (0.14-0.23), Fe (0.019-0.048) in L. rubellus, respectively. Results of principal component analysis showed that the two earthworm species differ in the pattern of metal bioaccumulation which is related to their ecological roles in contaminated soils.     

Effects of earthworms on decomposition and metal availability in contaminated soil: Microcosm studies of populations with different exposure histories

Population-specific differences in the responses of earthworms to simultaneous exposure to Cu and Zn were studied in microcosm experiments. Two populations of Aporrectodea caliginosa tuberculata (Eisen) with different metal exposure histories were chosen for the studies. Microcosms were prepared containing either uncontaminated soil or soils with low or high combined Cu/Zn -concentrations (79/139 or 178/311 mg kg⁻¹ dry mass of soil, respectively). Earthworms from each population were introduced to the microcosm treatments with some microcosms serving as controls without earthworms. One series of microcosms was destructively sampled after 16 weeks incubation in a climate chamber. Survival, growth, reproduction and decomposition by earthworms in each treatment were measured. An additional microcosm series was sampled for soil and earthworm measurements at four weeks intervals to determine temporal changes in the availability of metals in the soils and their accumulation into earthworms. Cu and Zn were sequentially extracted from the soil samples of both microcosm series to estimate mobility and availability of the metals in the soil. Earthworms with long-term exposure history to metal-contaminated soil seemed to tolerate higher soil metal concentrations than earthworms without earlier exposure. Both earthworms and metals affected soil respiration (CO₂ production) and nitrogen mineralization. In addition, earthworms seemed to decrease the mobility and bioavailability of metals in the soil through their burrowing activity.     

Long-term soil organic carbon dynamics in a subhumid tropical climate: C data in mixed C3/C4 cropping and modeling with ROTHC

Scanty information on long-term soil organic carbon (SOC) dynamics hampers validation of SOC models in the tropics. We observed SOC content changes in a 16-year continuously cropped agroforestry experiment in Ibadan, south-western Nigeria. SOC levels declined in all treatments. The decline was most pronounced in the no-tree control treatments with continuous maize and cowpea cropping, where SOC levels dropped from the initial 15.4 to 7.3-8.0 Mg C ha⁻¹ in the 0-12 cm topsoil in 16 years. In the two continuously cropped alley cropping (AC) systems, one with Leucaena leucocephala and one with Senna siamea trees, SOC levels dropped to 10.7-13.2 Mg C ha⁻¹. Compared to the no-tree control treatments, an annual application of an additional 8.5 Mg ha⁻¹ (dry matter) of plant residues, mainly tree prunings, led to an extra 3.5 Mg C ha⁻¹ (∼0.2% C) in the 0-12 cm top soil after 11 years, and 4.1 Mg C ha⁻¹ after 16 years. The addition of NPK fertilizer had little effect on the quantities of above-ground plant residues returned to the soil, and there was no evidence that the fertilizer affected the rate of SOC decomposition. The fact that both C₃ and C₄ plants returned organic matter to the soil in all cropping systems, but in contrasting proportions, led to clear contrasts in the ¹³C abundance in the SOC. This ¹³C information, together with the measured SOC contents, was used to test the ROTHC model. Decomposition was very fast, illustrated by the fact that we had to double all decomposition rate constants in the model in order to simulate the measured contrasts in SOC contents and δ¹³C between the AC treatments and the no-tree controls. We hypothesized (1) that the pruning materials from the legume trees and/or the extra rhizodeposition from the tree roots in the AC treatments accelerated the decomposition of the SOC present at the start of the experiment (true C-priming), and/or (2) that the physical protection of microbial biomass and metabolites by the clay fraction on this site, having a sandy top soil in which clay minerals are mainly of the 1:1 type, is lower than assumed by the model.     

Effects of earthworm density on growth, development, and reproduction in Lumbricus rubellus (Hoffm.) and possible consequences for the intrinsic rate of population increase

In this paper, the influence of earthworm density is assessed on the life-history parameters: growth, development, reproduction, and survival of Lumbricus rubellus (Hoffm.). Density ranges from two to nine earthworms in 1-l containers, corresponding to field densities of 300-1350 earthworms m⁻². Earthworms were kept under optimal laboratory conditions, with a surplus of food. The results show that at high earthworm density, individual growth is retarded, maturation delayed and cocoon production decreased, even when food is optimal. The integration of these results into a Dynamic Energy Budget model suggests that the food intake of individual earthworms is lower at the higher tested densities. This lower food intake is most probably driven by competition for space at the higher densities. The lower food intake results in a lower intrinsic rate of population increase which is negative at the highest tested density.     

Lead tolerance in Aporrectodea rosea earthworms from a clay pigeon shooting site

Lead tolerance in individuals of the earthworm species Aporrectodea rosea collected from a clay pigeon shooting site was investigated. Lead concentrations in the shooting site soil and the un-shot control site were 6410±2250 and 296±98 mg_Pb kg⁻¹ dry weight, respectively. Of these concentrations 1050±240 and 12±9 mg_Pb kg⁻¹ dry weight were suggested to be available, using ammonium acetate (1 M), respectively. With respect to earthworm body burdens of lead the shooting site earthworms had a body burden of 6.1±1.2 mg_Pb g⁻¹ dry weight while the uncontaminated site earthworms had almost a 1000-times lower body burden of 7.1±9.0 μg_Pb g⁻¹ dry weight. Lead tolerance was assessed in uncontaminated soil that had been augmented with lead, using lead nitrate solutions, to obtain lead concentrations in soil of 0.5, 5 and 50 mg_Pb kg⁻¹ dry weight. Earthworms were exposed for 28 days during which time a semi-qualitative assessment was made of their condition. Results showed no decrease in condition in the shooting site earthworms with increasing exposure time or concentration. In contrast, earthworms collected from an uncontaminated site showed a significant (p<0.05) decrease in condition when exposed to lead concentrations above, and including, a concentration of 5 mg kg⁻¹ dry weight soil. These results suggested lead tolerance in the shooting site earthworms.     

Earthworms change the distribution and availability of phosphorous in organic substrates

In laboratory controlled soil microcosms, the distribution and availability of phosphorous (P) were determined in the surface-casts and the burrows-linings of the anecic earthworm L. terrestris and were compared with non-ingested soil. To simulate more realistic earthworm community conditions, a combination of L. terrestris plus the endogeic A. caliginosa was tested. For a 2-month period, the earthworms were given two organic food substrates: rye-grass littered onto the soil surface and sewage sludge mixed with soil. The following treatments were designed: (i) soil alone (S), (ii) soil and sewage sludge (SS), soil and rye-grass litter (SL), and (iv) soil, litter and sludge (SSL). Analyses were performed for P contents (total, available and organic), organic matter content (organic carbon, C_org and total nitrogen, N_tot) and the two acid and alkaline phosphatase activities (AcPA and AkPA). Earthworms enhanced AcPA and were also responsible for additional AkPA in soil. The two AcPA and AkPA increased not only in surface-casts but also in burrows-linings that paralleled with the decrease of organic P in SL and SSL treatments. The stimulation of AcPA began quickly and declined rapidly in casts (from 19 to 8 μmol phenol g⁻¹ dry wt h⁻¹, respectively at week 2 and 8 in the SL treatment) but it was initiated later and maintained at a high level for longer in burrows (more than 10 μmol phenol g⁻¹ dry wt h⁻¹ at week 8 in the SL treatment). Significant positive correlations were found between the AkPA activities and N_tot contents (r=0.95, p=0.001) and to a lesser extend with C_org contents (r=0.76, p=0.05) in casts from the SL treatment, while AcPA significantly correlated with N_tot (r=0.91, p=0.004) but not with C_org (r=0.72, p=0.06). P availability was always highest in casts. However, the available P contents decreased sharply over time in casts and were still low in burrow-linings, suggesting that a large part of inorganic P produced was rapidly immobilized for the microbial growth. Total P content was unchanged except in the SL treatment in which it increased in casts and burrows (ca. 725 μg g⁻¹, at week 4). Organic P was first the highest in casts and then decreased over time (from 168 at week 1 to 140 μg g⁻¹ at week 8 in the SL treatment). This study illustrates that earthworms facilitate P transfer downward increasing a P patchy distribution in the soil, and significantly change the biogeochemical status of P (availability, organic phosphorous pool, AcPA activities) in certain hot spots such as casts and burrow-linings.     

Enzyme activities and microbial biomass in coastal soils of India

Soil salinity is a serious problem for agriculture in coastal regions, wherein salinity is temporal in nature. We studied the effect of salinity, in summer, monsoon and winter seasons, on microbial biomass carbon (MBC) and enzyme activities (EAs) of the salt-affected soils of the coastal region of the Bay of Bengal, Sundarbans, India. The average pH of soils collected from different sites, during different seasons varied from 4.8 to 7.8. The average organic C (OC) and total N (TN) content of the soils ranged between 5.2-14.1 and 0.6-1.4 g kg⁻¹, respectively. The electrical conductivity of the saturation extract (ECe) of soils, averaged over season, varied from 2.2 to 16.3 dSm⁻¹. The ECe of the soils increased five fold during the summer season (13.8 dSm⁻¹) than the monsoon season (2.7 dSm⁻¹). The major cation and anion detected were Na⁺ and Cl⁻, respectively. Seasonality exerted considerable effects on MBC and soil EAs, with the lowest values recorded during the summer season. The activities of β-glucosidase, urease, acid phosphatase and alkaline phosphatase were similar during the winter and monsoon season. The dehydrogenase activity of soils was higher in monsoon than in winter. Average MBC, dehydrogenase, β-glucosidase, urease, acid phosphatase and alkaline phosphatase activities of the saline soils ranged from 125 to 346 mg kg⁻¹ oven dry soil, 6-9.9 mg triphenyl formazan (TPF) kg⁻¹ oven dry soil h⁻¹, 18-53 mg p-nitro phenol (PNP) kg⁻¹ oven dry soil h⁻¹, 38-86 mg urea hydrolyzed kg⁻¹ oven dry soil h⁻¹, 213-584 mg PNP kg⁻¹ oven dry soil h⁻¹ and 176-362 mg PNP g⁻¹ oven dry soil h⁻¹, respectively. The same for the non-saline soils were 274-446 mg kg⁻¹ oven dry soil, 8.8-14.4 mg TPF kg⁻¹ oven dry soil h⁻¹, 41-80 mg PNP kg⁻¹ oven dry soil h⁻¹, 89-134 mg urea hydrolyzed kg⁻¹ oven dry soil h⁻¹, 219-287 mg PNP kg⁻¹ oven dry soil h⁻¹ and 407-417 mg PNP kg⁻¹ oven dry soil h⁻¹, respectively. About 48%, 82%, 48%, 63%, 40% and 48% variation in MBC, dehydrogenase activity, β-glucosidase activity, urease activity, acid phosphatase activity and alkaline phosphatase activity, respectively, could be explained by the variation in ECe of saline soils. Suppression of EAs of the coastal soils during summer due to salinity rise is of immense agronomic significance and needs suitable interventions for sustainable crop production.     

Earthworms and rate of breakdown of soybean and maize residues in soil

A field survey was conducted to determine the numbers and biomass of earthworms in soils receiving different tillage and cropping treatments, and to investigate in a greenhouse study the effect of earthworms on the rate of breakdown of soybean (Glycine max) and maize (Zea mays L.) residues. The numbers and biomass of earthworms under continuous soybeans were greater than those present under maize, possibly due to the adverse effects of insecticide and anhydrous ammonia fertilizer used with maize. No-tillage doubled the population of earthworms under soybeans, when compared with ploughing. Numbers (141 m^?2) and biomass (26.5 g m^?2) of earthworms under no-till soybeans were still much lower than the numbers (1298 m^?2) and biomass (224.5 g m^?2) under pasture receiving heavy applications of animal manure. Using 16-1 pots in the greenhouse, the effect of 0, 15 (250m^?2) and 30 (500m^?2) earthworms (Lumbricus rubellus) pot^?1 on the rate of breakdown during 54 days of 50 g of soybean or maize residues in the Raub silt loam (Aquic Argiudoll) was studied. At 36 days, 60% of the soybean residues were recovered from pots to which no earthworms had been added, whereas in the presence of earthworms, only 34% of the soybean residues remained. In the absence of earthworms, 85% of the maize residues were recovered at 36 days, compared with only 52% in the presence of earthworms. At 36 days, 48% of the original maize residues added were still > 2 mm in length in the absence of earthworms, whereas only 26% were > 2 mm in length in the presence of earthworms. Earthworms also increased the aggregate stability of the Raub soil, when determined on moist (19–22% w/w) samples, but had no effect on soil water retention at ?33 and ?1500 kPa. The possible implications of greater earthworm activity on increasing residue incorporation and breakdown and subsequent effects on soil temperatures under no-till maize production are also discussed.     

Life cycle of Pontoscolex corethrurus () in tropical artificial soil

Pontoscolex corethrurus (Müller) is a widespread, peregrine earthworm species of the Glossoscolecidae family, native to the Neotropics. This endogeic earthworm shows wide tolerance to environmental variations, being found in many different habitats and soil types throughout the tropics and sub-tropics. To evaluate the life cycle of this species in tropical artificial soil (TAS), a substrate used in ecotoxicological tests, and the influence of food availability and humidity on its growth, earthworms were incubated individually under laboratory conditions. The life cycle was evaluated in four treatments with 24 earthworms (replicates) each: TAS (120 g) without additional food (TAS0), TAS with 5 g of horse manure (TAS5), TAS with 10 g of manure (TAS10) and TAS with 25% greater soil moisture with 5 g of manure (TAS5H). Food was provided every 14 d and the containers maintained at room temperature (20 ± 4 °C). Cocoons collected in the field were placed individually in the different treatments and the hatching date, growth, development, and reproduction followed for 50 weeks after hatching. New cocoons hatched at 34 d, adulthood was reached at 8 months, and cocoon deposition began 3 months later, so that its life cycle was completed in 12 months. By the 13th week all juveniles in TAS0 had died due to insufficient food resources, while in the other treatments, all the animals had matured by 44 weeks, with significantly higher biomass and length in TAS10 (0.72 g, 5.4 cm), compared with TAS5 and TAS5H (0.59–0.61 g, 5.1–5.4 cm). Growth and weight gain were positively related to food availability, and no negative effect of excess moisture was observed on their growth. With additional food TAS showed no limitations for the development of the species, and it can be used for ecotoxicological tests involving acute toxicity (mortality) and avoidance, although its potential use in chronic tests (reproduction) is more limited. The length of its life cycle and other biological characteristics associated with this parthenogenetic species create some challenges that must be dealt with before this species can be recommended for wider use as a standard ecotoxicological test species.     

Micro-scale CO2 and CH4 dynamics in a peat soil during a water fluctuation and sulfate pulse

We investigated the response of CO₂ and CH₄ production to a water table fluctuation and a SO₄²⁻ pulse in a bog mesocosm. Net gas production rates in the mesocosm were calculated from concentration data by diffusive mass-balances. Incubation experiments were used to quantify the effect of SO₄²⁻ addition and the distribution of potential CO₂ and CH₄ production rates. Flooding of unsaturated peat resulted in rapid depletion of O₂ and complex patterns of net CH₄, CO₂, and H₂S production. Methane production began locally and without a time lag at rates of 3-4 nmol cm⁻³ d⁻¹ deeper in the peat. Similar rates were determined after a time lag of 10-60 days in the surface layers, whereas rates at lower depths declined. Net CO₂ production was largest immediately after the water table position was altered (100-300 nmol cm⁻³ d⁻¹) and declined to −50-50 nmol cm⁻³ d⁻¹ after a few weeks. SO₄²⁻ addition (500 mM) significantly increased potential CH₄ production rates in the surface layer from an average of 132-201 nmol cm⁻³ d⁻¹ and reduced it below from an average of 418-256 nmol cm⁻³ d⁻¹. Our results suggest that deeper in the peat (40-70 cm) under in situ conditions, methanogenic populations are less impaired by unsaturated conditions than in the surface layers, and that at these depths after flooding the substrate availability for CH₄ and DIC production is significantly enhanced. They also suggest that methanogenic and SO₄²⁻-reducing activity were non-competitive in the surface layer, which might explain contradictory findings from field studies.     

Humic acids buffer the effects of urea on soil ammonia oxidizers and potential nitrification

Humic acids (HAs) play an important role in the global nitrogen cycle by influencing the distribution, bioavailability, and ultimate fate of organic nitrogen. Ammonium oxidation by autotrophic ammonia-oxidizing bacteria (AOB) is a key process in ecosystems and is limited, in part, by the availability of NH₄⁺. We evaluated the impact of HAs on soil AOB in microcosms by applying urea (1.0%, equal to 10 mg urea/g soil) with 0.1% bHA (biodegraded lignite humic acids, equal to 1 mg/g soil), 0.1% cHA (crude lignite humic acids) or no amendment. AOB population size, ammonium and nitrate concentrations were monitored for 12 weeks after urea and HA application. AOB densities (quantified by real-time PCR targeting the amoA) in the Urea treatments increased about ten-fold (the final abundance: 5.02 × 10⁷ copies (g of dry soil)⁻¹) after one week of incubation and decreased to the initial density after 12 weeks incubation; the population size of total bacteria (quantified by real-time PCR with a universal bacterial probe) decreased from 1.12 × 10¹⁰ to 2.59 × 10⁹ copies (g of dry soil)⁻¹ at week one and fluctuated back to the initial copy number at week 12. In the Urea + bHA and Urea + cHA treatments, the AOB densities were 4 and 6 times higher, respectively, than the initial density of approximately 5.07 × 10⁶ copies (g of dry soil)⁻¹ at week 1 and did not change much up to week 4; the total bacteria density changed little over time. The AOB and total bacteria density of the controls changed little during the 12 weeks of incubation. The microbial community composition of the Urea treatment, based on T-RFLP using CCA (canonical correspondence analysis) and pCCA (partial CCA) analysis, was clearly different from those of other treatments, and suggested that lignite HAs buffered the change in diversity and quantity of total bacteria caused by the application of urea to the soil. We hypothesize that HAs can inhibit the change in microbial community composition and numbers, as well as AOB population size by reducing the hydrolysis rate from urea to ammonium in soils amended with urea.     

Variation in oxalic acid production and mycelial compatibility within field populations of Sclerotinia sclerotiorum

This study was aimed at detecting mycelial compatibility groups and variations in oxalic acid production in Sclerotinia sclerotiorum. For this purpose, 121 isolates of this plant pathogen recovered from lettuce, soybean and sunflower field crops, and grouped in 46 MCGs were tested for their ability to release oxalic acid and other organic acids to the medium. Oxalic acid production on liquid media was measured spectrophotometrically and release of organic acids was estimated by isolate abilities to discolour solid media amended with bromophenol blue. There were significant differences among MCGs in both oxalic acid and organic acids releasing, ranging the mean production of oxalic acid between 18 and 110 μg oxalic acid mg⁻¹ dry wt. When isolates were grouped by their hosts, those obtained from soybean presented the highest release of oxalic acid (71 μg oxalic acid mg⁻¹ dry wt), while those from sunflower showed the highest release of other acids to the medium. Solid medium discoloration was not correlated with oxalic acid concentration in liquid medium (Spearman R=−0.085; P=0.126).     

Can a mixed stand of N2-fixing and non-fixing plants restrict N2O emissions with increasing CO2 concentration?

Initial effects of elevated atmospheric CO₂ concentration on N₂O fluxes and biomass production of timothy/red clover were studied in the laboratory. The experimental design consisted of two levels of atmospheric CO₂ (ca. 360 and 720 μmol CO₂ mol⁻¹) and two N fertilisation levels (5 and 10 g N m⁻²). There was a total of 36 mesocosms comprising sandy loam soil, which were equally distributed in four thermo-controlled greenhouses. In two of the greenhouses, the CO₂ concentration was kept at ambient concentration and in the other two at doubled concentration. Forage was harvested and the plants fertilised three times during the basic experiment, followed by harvest, a fertilisation with the double amount of nitrogen and rise of water level. Under elevated CO₂, harvestable and total aboveground dry biomass production of a mixed Trifolium/Phleum stand was increased at both N treatments compared to ambient CO₂. The N₂O flux rates under ambient CO₂ were significantly higher at both N treatments during the early growth of mixed Phleum/Trifolium mesocosms compared to the N₂O flux rate under elevated CO₂. However, when the conditions were favourable for denitrification at the end of the experiment, i.e. N availability and soil moisture were high enough, the elevated CO₂ concentration enhanced the N₂O efflux.     

DNA damage and effects on antioxidative enzymes in earthworm (Eisenia foetida) induced by atrazine

To evaluate atrazine (2-chloro-4-ethylamino-6-isopropylamino-1, 3, 5-triazine) ecotoxicology in soil, the effect of atrazine on the activity of antioxidative enzymes (superoxide dismutase, SOD; catalase, CAT; and guaiacol peroxidase, POD) and DNA damage induced by atrazine were investigated in earthworms. Atrazine was added to artificial soil at rates of 0, 2.5, 5 and 10 mg per kg of soil. Earthworm tissues exposed to each treatment were collected on the 7th, 14th, 21st, and 28th day of the treatment. Compared to the controls, the CAT activity was stimulated at 2.5 mg kg⁻¹ treatment except on the 14th day, and inhibited at 5, 10 mg kg⁻¹ atrazine except 5 mg kg⁻¹ on the 28th day and 10 mg kg⁻¹ on the 21st day; the overall SOD activity was inhibited, while the POD activities were stimulated by all atrazine concentrations in 28 days. The olive tail moments of single-cell gel electrophoresis of coelomocytes, as an indication of DNA damage, were increased after treatment with different doses of atrazine on the 7th, 14th, 21st, and 28th day, and significant differences were found compared to the controls. In conclusion, atrazine induces oxidative stress and DNA damage on earthworms, and the adverse effects may be the important mechanisms of its toxicity to earthworms.     

Competition between invasive earthworms (Amynthas corticis, Megascolecidae) and native North American millipedes (Pseudopolydesmus erasus, Polydesmidae): Effects on carbon cycling and soil structure

Invasive earthworms can have significant impacts on C dynamics through their feeding, burrowing, and casting activities, including the protection of C in microaggregates and alteration of soil respiration. European earthworm invasion is known to affect soil micro- and mesofauna, but little is known about impacts of invasive earthworms on other soil macrofauna. Asian earthworms (Amynthas spp.) are increasingly being reported in the southern Appalachian Mountains in southeastern North America. This region is home to a diverse assemblage of native millipedes, many of which share niches with earthworm species. This situation indicates potential for earthworm-millipede competition in areas subject to Amynthas invasion.In a laboratory microcosm experiment, we used two ¹³C enriched food sources (red oak, Quercus rubra, and eastern hemlock, Tsuga canadensis) to assess food preferences of millipedes (Pseudopolydesmus erasus), to determine the effects of millipedes and earthworms (Amynthas corticis) on soil structure, and to ascertain the nature and extent of the interactions between earthworms and millipedes. Millipedes consumed both litter species and preferred red oak litter over eastern hemlock litter. Mortality and growth of millipedes were not affected by earthworm presence during the course of the experiment, but millipedes assimilated much less litter-derived C when earthworms were present.Fauna and litter treatments had significant effects on soil respiration. Millipedes alone reduced CO₂ efflux from microcosms relative to no fauna controls, whereas earthworms alone and together with millipedes increased respiration, relative to the no fauna treatment. CO₂ derived from fresh litter was repressed by the presence of macrofauna. The presence of red oak litter increased CO₂ efflux considerably, compared to hemlock litter treatments.Millipedes, earthworms, and both together reduced particulate organic matter. Additionally, earthworms created significant shifts in soil aggregates from the 2000-250 and 250-53 μm fractions to the >2000 μm size class. Earthworm-induced soil aggregation was lessened in the 0-2 cm layer in the presence of millipedes. Earthworms translocated litter-derived C to soil throughout the microcosm.Our results suggest that invasion of ecosystems by A. corticis in the southern Appalachian Mountains is unlikely to be limited by litter species and these earthworms are likely to compete directly for food resources with native millipedes. Widespread invasion could cause a net loss of C due to increased respiration rates, but this may be offset by C protected in water-stable soil aggregates.     

Fluxes of CO2, CH4, and N2O in an alpine meadow affected by yak excreta on the Qinghai-Tibetan plateau during summer grazing periods

To assess the impacts of yak excreta patches on greenhouse gas (GHG) fluxes in the alpine meadow of the Qinghai-Tibetan plateau, methane (CH₄), carbon dioxide (CO₂), and nitrous oxide (N₂O) fluxes were measured for the first time from experimental excreta patches placed on the meadow during the summer grazing seasons in 2005 and 2006. Dung patches were CH₄ sources (average 586 μg m⁻² h⁻¹ in 2005 and 199 μg m⁻² h⁻¹ in 2006) during the investigation period of two years, while urine patches (average −31 μg m⁻² h⁻¹ in 2005 and −33 μg m⁻² h⁻¹ in 2006) and control plots (average −28 μg m⁻² h⁻¹ in 2005 and −30 μg m⁻² h⁻¹ in 2006) consumed CH₄. The cumulative CO₂ emission for dung patches was about 36-50% higher than control plots during the experimental period in 2005 and 2006. The cumulative N₂O emissions for both urine and dung patches were 2.1-3.7 and 1.8-3.5 times greater than control plots in 2005 and 2006, respectively. Soil water-filled pore space (WFPS) explained 35% and 36% of CH₄ flux variation for urine patches and control plots, respectively. Soil temperature explained 40-75% of temporal variation of CO₂ emissions for all treatments. Temporal N₂O flux variation in urine patches (34%), dung patches (48%), and control (56%) plots was mainly driven by the simultaneous effect of soil temperature and WFPS. Although yak excreta patches significantly affected GHG fluxes, their contributions to the whole grazing alpine meadow in terms of CO₂ equivalents are limited under the moderate grazing intensity (1.45 yak ha⁻¹). However, the contributions of excreta patches to N₂O emissions are not negligible when estimating N₂O emissions in the grazing meadow. In this study, the N₂O emission factor of yak excreta patches varied with year (about 0.9-1.0%, and 0.1-0.2% in 2005 and 2006, respectively), which was lower than IPCC default value of 2%.