Effect of novaluron on microbial biomass,respiration, and fluorescein diacetate-hydrolyzing activity in tropical soils

The aim of this study was to assess the potential harmful effects of novaluron on soil microbiological parameters in clay loam alluvial soil (Typic udifluvent) and coastal saline soil (Typic endoaquept) under controlled laboratory tests. The applications of novaluron were made at or above the recommended rates, which includes field rate (FR), two times (2FR), and ten times (10FR) the FR. The laboratory incubation study was carried out at 60% of maximum water holding capacity of soils and at 30°C. Novaluron application rate even up to 10FR resulted in a short-lived and transitory toxic effect on soil microbial biomass C and fluorescein diacetate-hydrolyzing activity. Microbial metabolic quotient changed but for a short period. It can be concluded that novaluron had a transient and negligible harmful effect on the soil microbiological parameters studied at higher rates than those usually used in the field.     

Grazing intensity in subarctic tundra affects the temperature adaptation of soil microbial communities

Grazing by large ungulates, such as reindeer (Rangifer tarandus L.), in subarctic tundra exerts a considerable effect on the soil microclimate. Because of higher insulation by the aboveground vegetation in light versus heavily grazed areas, soil temperatures during the growing season are considerably higher under heavy grazing. Here, we hypothesized that these grazer-induced changes in soil microclimate affect the temperature sensitivity of soil microbial activity. To test this hypothesis, we conducted soil incubations at different temperatures (4 °C, 9 °C and 14 °C) for six weeks using soils from sites with contrasting long-term grazing intensities. Microbial respiration at low temperature (4 °C) was significantly higher in soils under light grazing than in soils under heavy grazing; however, grazing intensity did not affect respiration rates at 9 °C and 14 °C. In soils under light grazing, post-incubation β-glucosidase (BG) activity at 4 °C was higher in soils that had been incubated at 4 °C than in soils incubated at 14 °C, suggesting functional adaptation of the soil microbial community to low temperature. Similar adaptation was not detected in soils under heavy grazing. Ion Torrent sequencing of bacterial 16S rRNA genes showed major differences in the bacterial community composition in soils incubated at different temperatures. Overall, our results indicate that tundra soil microorganisms may be more cold-adapted under low than high grazing intensity. Due to this difference in temperature adaptation, the consequences of climate warming on soil microbial processes may be dependent on the grazing intensity.     

Effects of trifluralin on soil carbon mineralization at different temperature conditions

Trifluralin is a herbicide intensively used in Turkish cotton agriculture. The recommended field dose [(RFD), 480 g active ingredient l^?1], 2 × RFD, 4 × RFD and 6 × RFD of this herbicide were added to virgin (previously no trifluralin applied) and cotton field soils (previously trifluralin applied) from a district (Yumurtal?k, Adana) under Mediterranean climate conditions in order to determine their effects on soil microbial activity as measured by carbon mineralization at the different temperature conditions (20 °C, 25 °C and 30 °C). C mineralization of all samples was determined by the CO₂ respiration method over 30 days (20 °C, 25 °C and 30 °C at constant moist). The ratio (%) of carbon mineralization at all doses of cotton field soil at 30 °C was significantly higher than all other field dose–temperature combinations (P < 0.001). Based on these results, trifluralin is used as a carbon source by soil microorganisms. The herbicide trifluralin was degraded completely in the cotton field but a small fraction remained in the virgin field. This result can be explained by the cotton field soil having both more active microbial populations and more microorganisms adapted to the trifluralin applications than the virgin field.     

Kinetics of native and added carbon mineralization on incubating at different soil and moisture conditions in Typic Ustochrepts and Typic Halustalf

The carbon dynamics in soils is of great importance due to its links to the global carbon cycle. The prediction of the behavior of native soil organic carbon (SOC) and organic amendments via incubation studies and mathematical modeling may bridge the knowledge gap in understanding complex soil ecosystems. Three alkaline Typic Ustochrepts and one Typic Halustalf with sandy, loamy sand, and clay loam texture, varying in percent SOC of 0.2; S₁, 0.42; S₂, 0.67; S₃ and 0.82; S₄ soils, were amended with wheat straw (WS), WS + P, sesbania green manure (GM), and poultry manure (PM) on 0.5% C rate at field capacity (FC) and ponding (P) moisture levels and incubated at 35 °C for 1, 15, 30 and 45 d. Carbon mineralization was determined via the alkali titration method after 1, 5, 7 14, 21, and 28 d. The SOC and inorganic carbon contents were determined from dried up (50 °C) soil samples after 1, 15, 30, and 45 d of incubation. Carbon from residue mineralization was determined by subtracting the amount of CO₂-C evolved from control soils. The kinetic models; monocomponent first order, two-component first order, and modified Gompertz equations were fitted to the carbon mineralization data from native and added carbon. The SOC decomposition was dependent upon soil properties, and moisture, however, added C was relatively independent. The carbon from PM was immobilized in S₄. All the models fitted to the data predicted carbon mineralization in a similar range with few exceptions. The residues lead to the OC build-up in fine-textured soils having relatively high OC and cation exchange capacities. Whereas, fast degradation of applied OC in coarse-textured soils leads to faster mineralization and lower build-up from residues. The decline in CaCO₃ after incubation was higher at FC than in the P moisture regime.     

Effect of warming on the temperature dependence of soil respiration rate in arctic,temperate and tropical soils

We examined the response of the temperature coefficient (Q₁₀) for soil respiration rate to changes in environmental temperature through a laboratory incubation experiment. Soil samples were collected from three climatic areas: arctic (Svalbard, Norway), temperate (Tsukuba, Japan) and tropical (Pasoh, Malaysia). The arctic and temperate soils were incubated at 8 °C (control), 12 °C (4 °C warming) and 16 °C (8 °C warming) for 17 days. The tropical soil was incubated at 16 °C (8 °C cooling), 24 °C (control) and 32 °C (8 °C warming). Before and after the incubation experiment, the temperature dependence of soil microbial respiration was measured using an open-airflow method with IRGA by changing the temperature in a water bath. The initial Q₁₀ before the incubation experiment was larger in the soils from higher latitudes: 3.4 in the arctic soil, 2.9 in the temperate soil, and 2.1 in the tropical soil. The response of the microbial respiration rate to change in temperature differed among the three soil types. The temperature dependence of respiration rate in the arctic soil did not change in response to warming by 4 and 8 °C with a Q₁₀ of about 3. On the other hand, the Q₁₀ in the temperate soil decreased with increasing incubation temperature: from 2.8 in soils incubated at 8 °C to 2.5 at 12 °C and 2.0 at 16 °C. In the tropical soil, the Q₁₀ was not changed even by the 8 °C warming with a value of 2.1, whereas the Q₁₀ was increased from 2.1 to 2.7 by the 8 °C cooling. These results suggest that the response of microbial respiration to climatic warming may differ between soils from different latitudes.     

Soil respiration is not limited by reductions in microbial biomass during long-term soil incubations

Declining rates of soil respiration are reliably observed during long-term laboratory incubations. However, the cause of this decline is uncertain. We explored different controls on soil respiration to elucidate the drivers of respiration rate declines during long-term soil incubations. Following a long-term (707 day) incubation (30 °C) of soils from two sites (a cultivated and a forested plot at Kellogg Biological Station, Hickory Corners, MI, USA), soils were significantly depleted of both soil carbon and microbial biomass. To test the ability of these carbon- and biomass-depleted (“incubation-depleted”) soils to respire labile organic matter, we exposed soils to a second, 42 day incubation (30 °C) with and without an addition of plant residues. We controlled for soil carbon and microbial biomass depletion by incubating field fresh (“fresh”) soils with and without an amendment of wheat and corn residues. Although respiration was consistently higher in the fresh versus incubation-depleted soil (2 and 1.2 times higher in the fresh cultivated and fresh forested soil, respectively), the ability to respire substrate did not differ between the fresh and incubation-depleted soils. Further, at the completion of the 42 day incubation, levels of microbial biomass in the incubation-depleted soils remained unchanged, while levels of microbial biomass in the field-fresh soil declined to levels similar to that of the incubation-depleted soils. Extra-cellular enzyme pools in the incubation-depleted soils were sometimes slightly reduced and did not respond to addition of labile substrate and did not limit soil respiration. Our results support the idea that available soil organic matter, rather than a lack microbial biomass and extracellular enzymes, limits soil respiration over the course of long-term incubations. That decomposition of both wheat and corn straw residues did not change after major changes in the soil biomass during extended incubation supports the omission of biomass values from biogeochemical models.     

Augmenting soil water storage using uncharred switchgrass and pyrolyzed biochars

Biochar is an amendment that can augment soil water storage; however, its projected cost per ton could be financially limiting at field application scales. It may be more monetarily convenient if an alternate amendment was available that could deliver similar soil enhancements. We compared two switchgrass biochars pyrolyzed at 250 and 500 °C with raw switchgrass (uncharred) on moisture storage and bulk density changes in a Norfolk loamy sand (fine‐loamy, kaolinitic, thermic Typic Kandiudult). Amendments were mixed into triplicate pots at 20 g/kg along with untreated controls. Soils were laboratory incubated at 10% moisture content (w/w) for 118 days, and the pots were irrigated three times with 1.3 pore volumes of deionized water every 30 days. Soil bulk densities were recorded before each irrigation event. Assessment of alterations in soil water storage was examined through cumulative water evaporative losses from incubation day 0 to day 33 and by monitoring soil water contents for 13 consecutive days past each irrigation event. Rankings of soil water evaporative losses were as follows: uncharred switchgrass ≤ switchgrass (500 °C) ≤ switchgrass (250 °C) < control. After the first irrigation event, uncharred switchgrass amendment significantly increased moisture storage compared with soil treated with biochar and the control. While all amendments increased water storage relative to the control, uncharred switchgrass delivered equivalent, if not slightly better, moisture storage improvements compared with the two switchgrass biochars. Uncharred switchgrass would likely not be as effective over the long term (years to decades) as pyrolyzed biochars, due to greater degradation of uncharred material.     

Prior exposure to diurnal heating influences soil respiration and N availability upon rewetting

On sunny summer days, the top 10 cm of soil in southern Australia are heated to temperatures between 50 and 80 °C for a few hours a day, often for several successive days. These extreme temperature events are likely to have profound effects on the microbiota in these soils, but we do not know how this recurrent heat exposure influences microbial dynamics and associated nutrient cycling. In this study, an air-dry soil from southern Australia was exposed to one or two diurnal heating events with maximum temperature of 50 or 70 °C. The control was left at ambient temperature (Amb). All soils were rapidly rewet. Soil respiration was measured for 7 days after rewetting; microbial biomass C, available N and P were determined before rewetting and 1 and 7 days after rewetting. After heating and before rewetting compared to Amb, microbial biomass C (MBC) was 50–80% lower, but available P was 25% higher in heated soils. Available N differed little between Amb and heated soils. Rewetting resulted in a flush of respiration in Amb and soils heated once, but there was no respiration flush in soils heated twice. Cumulative respiration compared to Amb was about 10% higher in soils heated once and about 25% lower in soils heated twice. In Amb, MBC 1 day after rewetting was similar as before rewetting. But in heated soils, MBC increased from before rewetting to 1 day after rewetting about fourfold. Compared to Amb, available N 1 day after rewetting was 20–30% higher in soils heated to 70 °C. Seven days after rewetting, available N was 10% higher than Amb only in soils heated twice to 70 °C. It can be concluded that diurnal heating kills a large proportion of the microbial biomass and influences soil respiration and nutrient availability after rewetting of soils. The effect of heating depends on both maximum temperature and number of events.     

Effect of paclobutrazol on microbial biomass,respiration and cellulose decomposition in soil

Paclobutrazol is a plant growth regulator largely utilized in mango cultivation and usually applied directly to soil. The aim of this study was to examine the effect of paclobutrazol on soil microbial biomass, soil respiration and cellulose decomposition in Brazilian soils under laboratory conditions. Soil samples were collected from fields with and without a reported history of paclobutrazol application. A solution of paclobutrazol (8 mg of active ingredient kg^?1 of soil) was added to soils, which were then incubated at 28 °C for 30 days. Paclobutrazol decreased soil microbial biomass, soil respiration and cellulose decomposition in soil with and without a report of paclobutrazol application, while significant increase was observed in the respiratory quotient (qCO₂). Our results show that the soil microbiological attributes were negatively affected by paclobutrazol in short-term experiment.     

Changing microbial substrate use in Arctic tundra soils through a freeze-thaw cycle

Recent research has established that microbial processes in the arctic continue even when soils are frozen, and that cold-season processes can be important in the overall annual carbon and nitrogen cycles. Despite the importance of wintertime soil microbial processes, our understanding of their controls remains extremely poor. We particularly have a poor understanding of how microbial substrate use patterns change as soils freeze: do microbes use the same substrates as during the growing season, only slower, or do they switch to using different substrates? We used a ¹⁴C isotope equilibration technique to partition respiration between the actively turning over microbial biomass and products pool and the plant detritus pool in a range of Arctic tundra soils. Microbes showed a step-function shift in their metabolism as soils cool from +2 to +0.5 °C, roughly doubling the contribution of recycling of microbial C to total soil respiration. There was no additional shift in substrate use as soils underwent bulk soil freezing. The above-0 °C substrate shift is important because tundra soils spend a long time at or just below 0 °C as they are freezing in the early winter. The change in substrate use represents a shift from processing N-poor detritus to N-rich microbial products, causing N available for either plant uptake or leaching to be greatest when soils are near 0 °C. This may explain the observed patterns of growing season N immobilization vs. cold-season mineralization that appear common in Arctic tundra ecosystems.     

Effect of temperature on biochar priming effects and its stability in soils

Recent studies have shown both increased (positive priming) and decreased (negative priming) mineralisation of native soil organic carbon (SOC) with biochar addition. However, there is only limited understanding of biochar priming effects and its C mineralisation in contrasting soils at different temperatures, particularly over a longer period. To address this knowledge gap, two wood biochars (450 and 550 °C; δ¹³C −36.4‰) were incubated in four soils (Inceptisol, Entisol, Oxisol and Vertisol; δ¹³C −17.3 to −28.2‰) at 20, 40 and 60 °C in the laboratory. The proportions of biochar- and soil-derived CO₂–C were quantified using a two-pool C-isotopic model.Both biochars caused mainly positive priming of native SOC (up to +47 mg CO₂–C g⁻¹ SOC) in the Inceptisol and negative priming (up to −22 mg CO₂–C g⁻¹ SOC) in the other soils, which increased with increasing temperature from 20 to 40 °C. In general, positive or no priming occurred during the first few months, which remained positive in the Inceptisol, but shifted to negative priming with time in the other soils. The 550 °C biochar (cf. 450 °C) caused smaller positive priming in the Inceptisol or greater negative priming in the Entisol, Oxisol and Vertisol at 20 and 40 °C. At 60 °C, biochar caused positive priming of native SOC only in the first 6 months in the Inceptisol. Whereas, in the other soils, the native SOC mineralisation was increased (Entisol and Oxisol) and decreased (Vertisol) only after 6 months, relative to the control. At 20 °C, the mean residence time (MRT) of 450 °C and 550 °C biochars in the four soils ranged from 341 to 454 and 732−1061 years, respectively. At 40 and 60 °C, the MRT of both 450 °C biochar (25−134 years) and 550 °C biochar (93−451 years) decreased substantially across the four soils. Our results show that biochar causes positive priming in the clay-poor soil (Inceptisol) and negative priming in the clay-rich soils, particularly with biochar ageing at a higher incubation temperature (e.g. 40 °C) and for a high-temperature (550 °C) biochar. Furthermore, the 550 °C wood biochar has been shown to persist in soil over a century or more even at elevated temperatures (40 or 60 °C).     

Microbial biomass growth,following incorporation of biochars produced at 350 °C or 700 °C,in a silty-clay loam soil of high and low pH

Biochar has been widely proposed as a soil amendment, with reports of benefits to soil physical, chemical and biological properties. To quantify the changes in soil microbial biomass and to understand the mechanisms involved, two biochars were prepared at 350 °C (BC350) and 700 °C (BC700) from Miscanthus giganteus, a C4 plant, naturally enriched with ¹³C. The biochars were added to soils of about pH 4 and 8, which were both sampled from a soil pH gradient of the same soil type. Isotopic (¹³C) techniques were used to investigate biochar C availability to the biomass. Scanning Electron Microscopy (SEM) was used to observe the microbial colonization, and Attenuated Total Reflectance (ATR) to highlight structural changes at the surface of the biochars. After 90 days incubation, BC350 significantly increased the biomass C concentration relative to the controls in both the low (p < 0.05) and high pH soil (p < 0.01). It declined between day 90 and 180. The same trend occurred with soil microbial ATP. Overall, biomass C and ATP concentrations were closely correlated over all treatments (R² = 0.87). This indicates that neither the biomass C, nor ATP analyses were affected by the biochars, unless, of course, they were both affected in the same way, which is highly unlikely. About 20% of microbial biomass ¹³C was derived from BC350 after 90 days of incubation in both low and high pH soils. However, less than 2% of biomass ¹³C was derived from BC700 in the high pH soil, showing very low biological availability of BC700. After 90 days of incubation, microbial colonization in the charsphere (defined here as the interface between soil and biochar) was more pronounced with the BC350 in the low pH soil. This was consistent with the biomass C and ATP results. The microbial colonization following biochar addition in our study was mainly attributed to biochar C availability and its large surface area. There was a close linear relationship between ¹³CO₂ evolved and biomass ¹³C, suggesting that biochar mineralization is essentially a biological process. The interactions between non-living and living organic C forms, which are vital in terms of soil fertility and the global C cycle, may be favoured in the charsphere, which has unique properties, distinct from both the internal biochar and the bulk soil.     

Effect of crop residue biochar on soil acidity amelioration in strongly acidic tea garden soils

Strongly acidic soil (e.g. pH < 5.0) is detrimental to tea productivity and quality. Wheat, rice and peanut biochar produced at low temperature (max 300 °C) and differing in alkalinity content were incorporated into Xuan‐cheng (Ultisol; initial pH_{soil/water = 1/2.5} 4.12) and Ying‐tan soil (Ultisol; initial pH _{soil/water = 1/2.5} 4.75) at 10 and 20 g/kg (w/w) to quantify their liming effect and evaluate their effectiveness for acidity amelioration of tea garden soils. After a 65‐day incubation at 25 °C, biochar application significantly (P < 0.05) increased soil pH and exchangeable cations and reduced Al saturation of both tea soils. Association of H⁺ ions with biochar and decarboxylation processes was likely to be the main factor neutralizing soil acidity. Further, biochar application reduced acidity production from the N cycle. Significant (P < 0.05) increases in exchangeable cations and reductions in exchangeable acidity and Al saturation were observed as the rate of biochar increased, but there were no further effects on soil pH. The lack of change in soil pH at the higher biochar rate may be due to the displacement of exchangeable acidity and the high buffering capacity of biochar, thereby retarding a further liming effect. Hence, a significant linear correlation between reduced exchangeable acidity and alkalinity balance was found in biochar‐amended soils (P < 0.05). Low‐temperature biochar of crop residues is suggested as a potential amendment to ameliorate acidic tea garden soils.     

Effects of freeze–thaw cycles on microarthropods and nutrient availability in a sub-Arctic soil

It is predicted that Arctic regions may experience an increase in mean temperature in the future. This will affect the frequency of severe climatic events such as summer droughts and freeze–thaw cycles. In order to understand the impact of recurring freezing and thawing on soil organisms and their environment, intact plant–soil samples from the sub-Arctic were subjected to a series of such events. Springtail and mite species composition and abundance were monitored at intervals throughout the experiment. Furthermore, nutrient content and mobilisation in the soil and soil microbial biomass and nutrient content were examined.There was no conclusive evidence that recurring freeze–thaw events had a negative effect on the investigated soil faunal groups, and the treatment even seemed to stimulate the abundance of Acaridida. Respiration of soil subjected to 16 freeze–thaw cycles was low when kept at −2 °C and high when kept at +2 °C, indicating rapid response of microbial activity even after long exposure to low and fluctuating temperatures. Oribatida and Gamasida displayed a higher abundance in controls kept at −2 °C for up to 80 days, compared to controls at +2 °C and the freeze–thaw treatment. The Collembola were unaffected by the temperature treatments, but increased in abundance over time. The microbial C:N ratio increased after 40 days at −2 °C, indicating a higher degree of fungal dominance and lower tolerance of bacteria to constant freezing, but not to freeze–thaw. The decline in inorganic and microbial P during the experiment, and the proportionally stronger decrease of inorganic and microbial P than N in frozen soil compared to +2 °C soil, suggests that P is affected more than N mineralisation by freezing.     

Tebuconazole application decreases soil microbial biomass and activity

A short-term mesocosm experiment was conducted to ascertain the impact of tebuconazole on soil microbial communities. Tebuconazole was applied to soil samples with no previous pesticide history at three rates: 5, 50 and 500 mg kg⁻¹ DW soil. Soil sampling was carried out after 0, 7, 30, 60 and 90 days of incubation to determine tebuconazole concentration and microbial properties with potential as bioindicators of soil health [i.e., basal respiration, substrate-induced respiration, microbial biomass C, enzyme activities (urease, arylsulfatase, β-glucosidase, alkaline phosphatase, dehydrogenase), nitrification rate, and functional community profiling]. Tebuconazole degradation was accurately described by a bi-exponential model (degradation half-lives varied from 9 to 263 days depending on the concentration tested). Basal respiration, substrate-induced respiration, microbial biomass C and enzyme activities were inhibited by tebuconazole. Nitrification rate was also inhibited but only during the first 30 days. Different functional community profiles were observed depending on the tebuconazole concentration used. It was concluded that tebuconazole application decreases soil microbial biomass and activity.     

Response of soil microbial communities to the herbicide mesotrione: A dose-effect microcosm approach

Mesotrione is a new selective herbicide used for maize crops. The responses of microbial communities of a chernozem soil (Limagne basin, France) to pure or formulated (Callisto^®) mesotrione, applied at three different doses [one fold field rate (1 × FR), 10 × FR and 100 × FR], were studied using a laboratory microcosm approach. The effects were assessed on the prokaryotic cell abundance, the overall microbial activities (substrate-induced respiration (SIR) and dehydrogenase activity (DHA)) and the genetic structure of the bacterial and fungal communities (temporal temperature/denaturing gradient gel electrophoresis (TT/DGGE)). Mesotrione dissipation was similar whatever the formulation applied and the amounts dissipated were positively correlated to application rates. Several biodegradation products including the metabolites 4-methylsulfonyl-2-nitrobenzoic acid (MNBA) and 2-amino-4-methylsulfonylbenzoic acid (AMBA) were detected from day 42 post-treatment, in 10 × FR and 100 × FR treated soils. No response of the soil microbial communities was detected in soil spread with both the 1 × FR applications. Overall soil microbial activity was stimulated from day 6 by 10 × FR of Callisto^® and more strongly by 100 × FR of pure mesotrione and Callisto^®, whereas prokaryote abundance did not increase before day 95 in both the 100 × FR treatments. Genetic structural shifts recorded from day 42 in the bacterial and fungal communities were small and mainly attributable to variations in band intensity. Maximum dissimilarity of the bacterial and fungal genetic structures between control and 100 × FR treated soils did not exceed 12% and 28%, respectively. The general pattern was that more consistent effects occurred with increasing exposure times, especially in both the 100 × FR treated soils. These microbial responses could be due to the stimulation of (i) adapted mesotrione-degrading microorganisms and (ii) the activity of resistant heterotrophic microbial groups promoted by dead biomass from sensitive organisms. In addition, at 100 × FR doses, pure mesotrione seemed to induce stronger microbial responses than Callisto^®, formulation which contains adjuvants with potential side-effects on some microbial populations. This experimental approach indicated that pure mesotrione and Callisto^® affected soil microbial communities, but the effects were only detected at doses far exceeding the recommended field rates.     

Influence of storage on soil microbial biomass estimated by three biochemical procedures

The effects of 28 and 56 days' storage at 25°, 4° and ?20°C on the microbial biomass content of four soils from tussock grasslands were studied by three biochemical procedures. Two of the procedures involved measurement of CO₂ and mineral-N (Min-N) production by chloroform-fumigated and unfumigated soil, and consequent estimation of biomass C and Min-N flush respectively. In the third, adenosine 5'-triphosphate (ATP) content was determined.Patterns of CO₂ production were often influenced by storage treatment. The use of fixed incubation periods for estimating the CO₂ flush of fumigated soil and the steady rate of CO₂ production by unfumigated soil did, however, give biomass C estimates that were generally similar to those calculated from individually determined incubation periods for each treatment and soil.Biomass C values could change significantly at all storage temperatures, but generally least at ?20°C. Storage at ?20°C was also the most suitable for retaining ATP contents, whereas 4°C was best for values of Min-N flush. Values of Min-N flush after storage of soil at ?20°C decreased significantly in two of the soils but increased in another. No storage temperature was thus satisfactory for all three indices of microbial biomass. Generally, however, 4°C was adequate for short periods, and 25°C the least suitable.     

Nitrogen Mineralization Potential as Influenced by Microbial Biomass,Cotton Residues and Temperature

Integrating information on nitrogen (N) mineralization potentials into a fertilization plan could lead to improved N use efficiency. A controlled incubation mineralization study examined microbial biomass dynamics and N mineralization rates for two soils receiving 56 and 168 kg N ha^?1 in a Panoche clay loam (Typic Haplocambid) and a Wasco sandy loam (Typic Torriorthent), incubated with and without cotton (Gossypium hirsutum L.) residues at 10 and 25°C for 203 days. Microbial biomass activity determined from mineralized carbon dioxide (CO₂) was higher in the sandy loam than in clay loam independent of incubation temperature, cotton residue addition and N treatment. In the absence of added cotton residue, N mineralization rates were higher in the sandy loam. Residue additions increased N immobilization in both soils, but were greater in clay loam. Microbial biomass and mineralization were significantly affected by soil type, residue addition and temperature but not by N level.     

Degradation of the herbicide sulfentrazone in a Brazilian Typic Hapludox soil

The herbicide sulfentrazone is classified as highly mobile and persistent and this study aimed to examine degradation of this compound on a Typic Hapludox soil that is representative of regions where sulfentrazone is used in Brazil. Soil samples were supplemented with sulfentrazone (0.7 μg active ingredient (a.i.) g^?1 soil), and maintained at 27 °C. Soil moisture was corrected to 30%, 70%, or 100% water-holding capacity (WHC) and maintained constant until the end of the experimental period. Soils without added herbicide were used as controls. Aliquots were taken after 14, 30, 60, 120, 180, and 255 days of incubation for quantitative analysis of sulfentrazone residues by gas chromatography. Another experiment was conducted in soil samples, with and without the herbicide, at different temperatures (15, 30, and 40 °C), with moisture kept constant at 70% of WHC. The sulfentrazone residues were quantified by gas chromatography after 14, 30, 60, and 120 days of incubation. Sulfentrazone degradation was not affected by soil moisture. A significant effect was observed for the temperature factor after 120 days on herbicide degradation, which was higher at 30 °C. A half-life of 146.5 days was recorded. It was observed that the herbicide stimulated growth of actinomycetes, whereas bacterial and fungal growth was not affected. The microorganisms selected as potential sulfentrazone degraders were Rhizobium radiobacter, Ralstonia pickettii, Methylobacterium radiotolerans, Cladosporium sp., Eupenicillium sp., and Paecilomyces sp.     

Demethylation of Dimethylarsinic Acid and Arsenobetaine in Different Organic Soils

Methylation and demethylation of arsenic may change substantially the toxicity and mobility of arsenic in soils. Little is known about demethylation of organic arsenic species in organic soils. We incubated dimethylarsinic acid (DMA) and arsenobetaine (AsB) in soils and aqueous soil extracts from a forest floor and fen, in order to investigate demethylation processes. Incubations were conducted at 5°C in the dark under oxic or anoxic conditions. Arsenobetaine demethylated rapidly in all soil extracts with half-lives of 3.6–12 days, estimated from first order kinetic. Demethylation of DMA was relatively slow with half-lives of 187 and 46 days in the forest floor extracts and oxic fen extracts, respectively. In comparison, DMA was stable for 100 days in anoxic fen extracts. The apparent half-lives were much shorter in soils for DMA (1.3–12.6 days) and AsB (0.5–1.9 days) than in soil extracts, suggesting also irreversible AsB and DMA adsorption to soils beside demethylation. An unknown arsenic species and DMA were detected as metabolites of AsB demethylation. The results indicate rapid demethylation of AsB probably via the pathway AsB → Dimethylarsenoylacetate → DMA, followed up by slow demethylation of DMA → monomethylarsonic acid → inorganic As species.