Effects of incorporated corn residues on glyphosate mineralization and sorption in soil

In modern agricultural systems employing conservation tillage practices, glyphosate is widely used as a preplant burndown herbicide in a wide range of crops. Conservation tillage systems are characterized by a significant presence of crop residues at the soil surface so that glyphosate is applied to a soil matrix rich in poorly decomposed crop residues. Incorporation of corn residues in the range from 0.5 to 4% caused different effects on mineralization and sorption of [14C]glyphosate in sandy and sandy loam soils. More specifically, low levels of incorporated corn residues did not affect or slightly stimulated herbicide mineralization in the sandy and sandy loam soils, respectively. In the sandy soil, incorporation of the highest level of corn residues (4%) caused a decrease in [14C]glyphosate mineralization. [14C]Glyphosate sorption on both soil types was reduced in samples receiving high amounts of incorporated corn residues.     

Microbial activity,community structure and potassium dynamics in rhizosphere soil of soybean plants treated with glyphosate

With the advent of glyphosate [N-(phosphonomethyl)glycine] tolerant crops, soils have now been receiving repeated applications of the herbicide for over 10 years in the Midwestern USA. There is evidence that long-term use of glyphosate can cause micronutrient deficiency but little is known about plant potassium (K) uptake interactions with glyphosate. The repeated use of glyphosate may create a selection pressure in soil microbial communities that could affect soil K dynamics and ultimately K availability for crops. Therefore, the objectives of this study were to characterize the effect of foliar glyphosate applied to GR (glyphosate resistant) soybeans on: (1) rhizosphere microbial community profiles using ester linked fatty acid methyl ester (EL-FAME) biomarkers, (2) exchangeable, non-exchangeable, and microbial K in the rhizosphere soil, and (3) concentrations of soybean leaf K. A greenhouse study was conducted in a 2 × 2 × 3 factorial design with two soil treatments (with or without long-term field applications of glyphosate), two plant treatments (presence and absence of soybean plants), and three rates of glyphosate treatments (0×, 1× at 0.87, and 2× at 1.74 kg ae ha^?1, the recommended field rate). After each glyphosate application, rhizosphere soils were sampled and analyzed for microbial community structure using ester linked fatty acid methyl ester biomarkers (EL-FAME), and exchangeable, plant tissue and microbial biomass K. Glyphosate application caused a significant decrease in the total microbial biomass in soybean rhizosphere soil that had no previous exposure to glyphosate, at 7 days after glyphosate application. However, no significant changes were observed in the overall microbial community structure. In conclusion, the glyphosate application lowered the total microbial biomass in the GR soybean rhizosphere soil that had no previous exposure to glyphosate, at 7 days after glyphosate application; caused no changes in the microbial community structure; and did not reduce the plant available K (soil exchangeable or plant tissue K).     

Film fully-mulched ridge-furrow cropping affects soil biochemical properties and maize nutrient uptake in a rainfed semi-arid environment

Abstract

This study evaluated the effects of plastic mulched ridge-furrow cropping on soil biochemical properties and maize (Zea mays L.) nutrient uptake in a semi-arid environment. Three treatments were evaluated from 2008 to 2010: no mulch (narrow ridges with crop seeded next to ridges), half mulch (as per no mulch, except narrow ridges were mulched), and full mulch (alternate narrow and wide ridges, all mulched with maize seeded in furrows). Compared to the no mulch treatment, full mulch increased maize grain yield by 50% in 2008 and 25% in 2010, but reduced yield by 21% in 2009 after low precipitation in early growth. Half mulch had a similar grain yield to no mulch in the three cropping years, suggesting half mulch is not an effective pattern for maize cropping in the area. Mulch treatments increased aboveground nitrogen (N) uptake by 21?34% and phosphorus (P) uptake by 21?42% in 2008, and by 16?32% and 14?29%, respectively, in 2010; but in 2009 mulching did not affect N uptake and decreased P uptake. Soil microbial biomass and activities of urease, β-glucosidase and phosphatase at the 0?15 cm depth were generally higher during vegetative growth but lower during reproductive growth under mulch treatments than no mulch. Mulching treatments increased carbon (C) loss of buried maize residues (marginally by 5?9%), and decreased light soil organic C (15?27%) and carbohydrate C (12?23%) concentrations and mineralizable C and N (8?36%) at harvest in the 0?20 cm depth compared with no mulch, indicating that mulching promotes mineralization and nutrient release in soil during cropping seasons. As a result of these biological changes, mineral N concentration under mulch was markedly increased after sowing in upper soil layers compared with no mulch. Therefore, our results suggest that mulched cropping stimulated soil microbial activity and N availability, and thus contributed to increasing maize grain yield and nutrient uptake compared with no mulch.     

Effects of addition of maize litter and earthworms on C mineralization and aggregate formation in single and mixed soils differing in soil organic carbon and clay content

C mineralization and aggregate stability directly depend upon organic matter and clay content, and both processes are influenced by the activity of microorganisms and soil fauna. However, quantitative data are scarce. To achieve a gradient in C and clay content, a topsoil was mixed with a subsoil. Single soils and the soil mixture were amended with 1.0 mg maize litter C g soil⁻¹ with and without endogeic earthworms (Aporrectodea caliginosa). The differently treated soils were incubated for 49 days at 15 °C and 40% water holding capacity. Cumulative C mineralization, microbial biomass, ergosterol content and aggregate fractions were investigated and litter derived C in bulk soil and aggregates were determined using isotope analyses. Results from the soil mixture were compared with the calculated mean values of the two single soils. Mixing of soil horizons differing in carbon and clay content stimulated C mineralization of added maize residues as well as of soil organic matter. Mixing also increased contents of macro-aggregate C and decreased contents of micro-aggregate C. Although A. caliginosa had a stimulating effect on C mineralization in all soils, decomposition of added litter by A. caliginosa was higher in the subsoil, whereas A. caliginosa decreased litter decomposition in the soil mixture and the topsoil. Litter derived C in macro-aggregates was higher with A. caliginosa than with litter only. In the C poor subsoil amended with litter, A. caliginosa stimulated the microbial community as indicated by the increase in microbial biomass. Furthermore, the decrease of ergosterol in the earthworm treated soils showed the influence of A. caliginosa on the microbial community, by reducing saprotrophic fungi. Overall, our data suggest both a decrease of saprotrophic fungi by selective grazing, burrowing and casting activity as well as a stimulation of the microbial community by A. caliginosa.     

Changes in soil microbial community structure by Allonychiurus kimi (Collembola) and their interactive effects on glyphosate degradation

Collembolans have been known to be involved in various soil ecosystem functions. However, the role of Collembola in organic contaminant degradation has not been sufficiently elucidated to assess its contribution. In this study, varying densities of Allonychiurus kimi (Lee, 1973) (0, 10, and 30 individuals per 30 g of soil) were introduced into glyphosate-contaminated soils (74.1 mg glyphosate kg⁻¹ soil). This study investigated changes in the microbial community and the residual glyphosate concentration in soils over incubation time to elucidate the effects of A. kimi on the glyphosate degradation through its influence on the microbial community. Furthermore, the investigation was conducted in soils collected in May and September 2018 to assess the contribution of A. kimi to glyphosate degradation in soils with varying microbial compositions and biomass. Autoclaved soil was used as a control to minimize the influence of indigenous soil microorganisms on glyphosate degradation. We hypothesize that as the initial density of A. kimi increases, the effects of A. kimi on the soil microbial community become pronounced, altering the degradation kinetics of glyphosate in the soil. The composition and biomass of the soil microorganisms were quantified using the phospholipid fatty acid (PLFA) method. Our study determined that the presence of A. kimi altered the microbial community structure by increasing the bacterial and total microbial, but not fungal, biomass. After seven days of treatment, the bacterial and total microbial biomass in the treatment with A. kimi were >2.0-fold and 1.5-fold greater, respectively, compared to those in the treatments without A. kimi. Specifically, the concentration of PLFA 18:1ω7c, i15:0, and 16:1ω7c was positively correlated with A. kimi density. The residual glyphosate concentration decreased exponentially over time as A. kimi density increased. At the end of the experiment, the remaining portions (%) of glyphosate in the May soil samples were 26.3, 20.1, and 6.2, with A. kimi densities of 0, 10, and 30 per vessel, respectively, and the portions in the September soil samples were 13.4, 12.7, and 2.2, respectively. The DT₅₀s (time required for 50 % degradation) decreased significantly with increasing A. kimi density, ranging from 6.8 to 10.1 days at an A. kimi density of 30 to 12.9–19.4 days without A. kimi. However, in the autoclaved soil, a similar effect was not apparent (i.e., DT₅₀s ranged from 23.3 to 27.4 days). Our study demonstrated that Collembola can enhance organic contaminant degradation in soils by altering the microbial community structure.     

Influence of mulch C/N ratio and decomposition stage on plant N uptake and N availability in soil with or without wheat straw

Mulches can improve soil properties, but little is known about nutrient availability in mulched soil that contains plant residues and the effect of mulching with manures. The aim of this study was to determine the effects of mulching with high or low C/N organic materials, in which low C/N materials differed in decomposability, and the presence of wheat straw in the soil on plant growth and N uptake, soil N availability and microbial biomass N within about four months after mulching. Three organic materials were used: mature wheat straw (W, C/N 80), young faba bean shoots (FB, C/N 7), and sheep manure (SM, C/N 8). There were eight treatments differing in amendment methods (mulching or mixing with W or both) and mulching materials (W, FB or SM). Treatments that were only mulched with W, FB or SM are referred to as m‐treatments. In m/s‐treatments, after W was mixed into the soil, W, FB or SM were placed on the soil surface as mulch. Two other treatments included an unamended control and soil mixed with W. Wheat was planted 0, 35 or 70 days after mulching (referred to as 0, 35, and 70 DAM) and grown for 35 days. Faba bean mulch increased shoot dry weight, shoot N uptake and available N compared to wheat or sheep manure mulch, particularly in the m‐treatments. Shoot dry weight was higher in m‐treatments than corresponding m/s‐treatments with the same mulch type. Shoot N uptake was higher in 70 DAM than in 0 DAM in all treatments and 0.3 to three‐fold higher in m‐treatments than the corresponding m/s‐treatments. Microbial biomass N was higher in 0 DAM than in 35 and 70 DAM in most treatments and up to two‐fold higher in m/s‐treatments than the corresponding m‐treatments. Available N in m/s‐treatments was two to six‐fold higher than m‐treatments in 0 DAM, but differed little in older mulch ages of W and SM. It can be concluded that compared to soil with only mulch, mixing of wheat straw into soil reduced plant growth and N uptake, particularly in the early stages of mulching (0 and 35 DAM). However, the presence of wheat in mulched soil may provide a longer lasting source of N for plants and reduce the risk of N leaching from rapidly decomposing low C/N mulch due to greater microbial biomass N uptake than only soil with mulch.     

Transformation of organic matter from maize residues into labile and humic fractions of three European soils as revealed by 13C distribution and CPMAS-NMR spectra

The dynamics of incorporation of fresh organic residues into the various fractions of soil organic matter have yet to be clarified in terms of chemical structures and mechanisms involved. We studied by ¹³C‐dilution analysis and CPMAS‐¹³C‐NMR spectroscopy the distribution of organic carbon from mixed or mulched maize residues into specific defined fractions such as carbohydrates and humic fractions isolated by selective extractants in a year‐long incubation of three European soils. The contents of carbohydrates in soil particle size fractions and relative δ¹³C values showed no retention of carbohydrates from maize but rather decomposition of those from native organic matter in the soil. By contrast, CPMAS‐¹³C‐NMR spectra of humic (HA) and fulvic acids (FA) extracted by alkaline solution generally indicated the transfer of maize C (mostly carbohydrates and peptides) into humic materials, whereas spectra of organic matter extracted with an acetone solution (HE) indicated solubilization of an aliphatic‐rich, hydrophobic fraction that seemed not to contain any C from maize. The abundance of ¹³C showed that all humic fractions behaved as a sink for C from maize residues but the FA fraction was related to the turnover of fresh organic matter more than the HA. Removal of hydrophobic components from incubated soils by acetone solution allowed a subsequent extraction of HA and, especially, FA still containing much C from maize. The combination of isotopic measurements and NMR spectra indicated that while hydrophilic compounds from maize were retained in HA and FA, hydrophobic components in the HE fraction had chemical features similar to those of humin. Our results show that the organic compounds released in soils by mineralization of fresh plant residues are stored mainly in the hydrophilic fraction of humic substances which are, in turn, stabilized against microbial degradation by the most hydrophobic humic matter. Our findings suggest that native soil humic substances contribute to the accumulation of new organic matter in soils.     

Changes in Soluble Manganese and Iron Concentrations of Tropical Wetland Soils as Influenced by Glyphosate Dosage

Glyphosate is largely used to control weeds in wetland soils of Brazil. We investigated changes in the chemistry of soluble manganese (Mn) and iron (Fe) in these soils as affected by glyphosate dosage. Triplicate samples of the A horizon of wetland soils with different organic-matter contents were incubated with deionized water (1:2) for 1, 3, and 30 days under flooding. Three different glyphosate doses (0, 0.048, and 0.096 g L^?1 m^?2) were spiked on the flooded water at the beginning of the incubation periods. After incubation, pH was measured and samples of the supernatant were collected for determination of Mn/Fe concentrations by atomic absorption. Glyphosate application impacted Mn but had no effect on pH and Fe. Soluble Mn concentrations decreased as glyphosate dosage increased for the high organic-matter soil after 3 days of incubation. It indicated that glyphosate application can change the chemistry of soil metals. The intensity of these changes depends on the glyphosate dosage, evolved metal, incubation time, and soil properties.     

Glyphosate degradation as a soil health indicator for heavy metal polluted soils

Glyphosate is a commonly used herbicide in grassland soils and microorganisms control its degradation. We introduce the concept of using the degradation rate as an indicator for ecosystem health. Testing this concept, we used soils with a long history of heavy metal pollution (Cu, Pb, and Zn). We hypothesized lower degradation rates in metal-polluted compared to less polluted soils. The degradation rates were measured by repeated measurements of the parent compound in spiked soil-water slurries incubated at 20 °C over 21 days. Average rates showed no differences comparing among soils. We observed a positive correlation between glyphosate degradation rates and soil metal pollution. Therefore, we concluded that the expected impact of the metals on the bacteria responsible for the herbicide degradation was not established. We discuss the potential influence on biological degradation rates of soil pH and adsorption and implications using the concept of the soil health indicator.     

Heterogeneous distribution may substantially decrease initial decomposition, long-term microbial growth and N-immobilization from high C-to-N ratio resources

The effect of heterogeneous resource distribution in soil has seldom been studied under controlled laboratory conditions, even though this type of distribution is the rule rather than the exception in both agricultural and undisturbed soil systems. We use distribution trials to test the effect of stratified distribution of chopped maize, sheep faeces derived from maize, and chopped rape stems compared with even distribution. In all treatments, CO₂ mineralization decreased initially in the stratified experiments and in the rape treatment less (39 versus 43%) of the added carbon was mineralized even after 202 days of incubation at 15°C. In both maize and rape residue treatments, we observed much less immobilization of N_min and less microbial growth in the stratified experiments. The data set for rape was sufficiently detailed to allow a model interpretation using a spreadsheet version of the soil organic matter module from DAISY, a soil‐plant‐atmosphere system model. This indicated that the observed differences between the stratified and evenly distributed experiments could be largely understood by assuming diffusion limitation of nitrate from the bulk soil to the residuesphere. The residuesphere is the part of soil that is immediately affected by the decomposition of residue, and was assumed to be included in the first 10 mm of soil surrounding the residues in the stratified experiments at all times. Only a very small part of the variation in CO₂ respiration, N_min and soil microbial biomass N (SMB‐N) could be explained by additionally assuming less substrate utilization efficiency of the less decomposable fraction of rape in the stratified experiment. We observed greater N mineralization in the evenly distributed experiment with faeces, and smaller concentrations of soil microbial biomass nitrogen extracted by chloroform fumigation. We assume that most of the additional SMB‐N determined in the faeces treatments was from microorganisms present in the faeces at the time they were added to soil, and that these were less susceptible to microbial predation in the stratified than in the distributed experiment. We conclude that the spatial distribution of decomposing litters in soil significantly affects the C and N dynamics. Diffusion limitation of available N in the active zone of decomposition was the main causal agent, which induced a decrease in microbial growth and substrate utilisation.     

Soil microbial activity is affected by Roundup WeatherMax and pesticides applied to cotton (Gossypium hirsutum)

Adoption of glyphosate-based weed control systems has led to increased use of the herbicide with continued use of additional pesticides. Combinations of pesticides may affect soil microbial activity differently than pesticides applied alone. Research was conducted to evaluate the influence of glyphosate-based cotton pest management systems on soil microbial activity. Soil was treated with commercial formulations of trifluralin, aldicarb, and mefenoxam + pentachloronitrobenzene (PCNB) with or without glyphosate (applied as Roundup WeatherMax). The soil microbial activity was measured by quantifying C and N mineralization. Soil microbial biomass was determined using the chloroform fumigation-incubation method. Soils treated with glyphosate alone exhibited greater cumulative C mineralization 30 days after treatment than all other treatments, which were similar to the untreated control. The addition of Roundup WeatherMax reduced C mineralization in soils treated with fluometuron, aldicarb, or mefenoxam + PCNB formulations. These results indicate that glyphosate-based herbicides alter the soil microbial response to other pesticides.     

Mulch effect on successive crop yields and soil carbon in Tonga

Intensification of crop production in Tonga has resulted in a move to mechanical soil preparation, often with a rotary hoe, and a consequent loss in both chemical physical fertility. An experiment was conducted on a clay loam soil (Typic Argiudoll) on the Forestry Vaini Research Station, Tongatapu Island, Tonga, to investigate the effect of a once‐off application of mulch on yield and quality of watermelon, maize and capsicum grown in rotation over a 1‐yr period. The treatments applied were a nonmulched control, transparent plastic and 200‐mm‐thick applications of locally available coconut sawdust, guinea grass and mature coconut fronds. The fresh fruit yield of watermelon in all the mulch treatments was 7.3–18.1% higher than in the nonmulched control. There was no significant effect of thick vegetative mulch on maize grain dry matter yield; however, the maize yield was significantly lower in the transparent plastic mulch than in the other treatments. The capsicum marketable fruit yield in the following crop was increased by 49–73% in all the vegetative mulch treatments compared to the non‐mulched control. The higher crop yields with the vegetative mulch were attributed to the measured lower soil temperatures and higher soil moisture in these treatments. There was no effect of mulch on soil total N (TN), but soil total C (TC), soil labile C (LC) and the carbon management index (CMI) were increased.     

Response of Mycorrhizal Infection to Glyphosate Applications and P Fertilization in Glyphosate-Tolerant Soybean,Maize, and Cotton

Glyphosate and phosphorus (P) fertilizer may alter arbuscular mycorrhizal (AM) fungal infection rates of glyphosate-tolerant cotton, maize, and soybean in low-P soil. Microbial biomass, water soluble P, Mehlich-3 P, and acid and alkaline phosphatase activities were not significantly impacted by glyphosate or P in the greenhouse. Phosphorus fertilization decreased mycorrhizal infection rates in cotton and maize and increased shoot biomass and shoot P in soybean in 2005, and decreased mycorrhizal infection in soybean and increased shoot biomass in cotton and maize and shoot P in all three crops in 2006. In pasteurized soil, glyphosate decreased percent mycorrhizal infection in maize, increased infection in cotton, and did not significantly affect infection in soybean. When soil was not pasteurized, glyphosate did not significantly alter mycorrhizal infection in any crop. The potential for glyphosate to alter AM fungal infection in glyphosate-tolerant plants may depend on whether soil microbial communities are compromised by other factors.     

Coffee mucilage impact on young coffee seedlings and soil microorganisms

A greenhouse pot experiment was carried out to assess the effects of fermented coffee mucilage applied as mulch together with maize leaves on the growth of young coffee plants of two different varieties and on soil microbial biomass indices. The coffee variety Catuai required 32% more water per g plant biomass than the variety Yellow Caturra, but had a 49% lower leaf area, 34% less shoot and 46% less root biomass. Maize and mucilage amendments did not affect leaf area, shoot and root yield, or the N concentration in shoot and root dry matter. The amendments always reduced the water use efficiency values, but this reduction was only significant in the maize+mucilage‐14 (= 14 g mucilage pot^?1) treatment. Soil pH significantly increased from 4.30 in the control to 4.63 in the maize+mucilage‐14 treatment. Microbial biomass C increased by 18.5 µg g^?1 soil, microbial biomass N by 3.1 µg g^?1 soil, and ergosterol by 0.21 µg g^?1 soil per g mucilage added pot^?1. The presence of mucilage significantly reduced the microbial biomass‐C/N ratio from a mean of 13.4 in the control and maize treatments to 9.3, without addition rate and coffee variety effects. The application of non‐composted mucilage is recommended in areas where drought leads to economic losses and in coffee plantations on low fertility soils like Oxisols, where Al toxicity is a major constraint.     

Decomposition of maize straw in three European soils as revealed by DRIFT spectra of soil particle fractions

Diffuse Reflectance Infrared Fourier Transform (DRIFT) technique was used to investigate decomposition of maize straw residues in particle-size fractions of three European soils in a 1-year incubation experiment that simulated both incorporation and mulching practices. The aliphatic bands in the 2920–2860 cm⁻¹ regions were used to derive decomposition curves and mineralization rates of maize residues. Evaluation of soil organic matter changes by DRIFT in particle-size fractions depended on soil texture and soil aggregate stability. Decomposition could be followed over the full incubation period only in three size-fractions of the silty German soil because of its intermediate aggregate stability. DRIFT spectra were not useful in the least stable sandy Danish soil and in the most stable clayey Italian soil. Decomposition rates derived from DRIFT results were similar to those obtained by isotopic techniques. DRIFT spectroscopy may represent a rapid and accurate method to follow fresh organic matter degradation directly in soil matrix or particle-size fractions in some soils provided that adequate soil dispersion is obtained.     

土壤水分和植物残体对紫色水稻土有机碳矿化的影响

采用为期62.d的实验室恒温(281)℃培养方法,研究了土壤水分和植物残体对紫色水稻土有机碳矿化的影响。结果表明,紫色水稻土有机碳矿化速率在培养30.d后基本达到稳定,好气条件下土壤有机碳累积矿化量高于淹水条件,且差异达到极显著水平。用一级动力学方程对植物残体的矿化速率进行拟合表明,好气条件下,植物残体的分解速率常数(k值)大小顺序为蚕豆秸秆玉米秸秆水稻秸秆,而淹水条件则为水稻秸秆蚕豆秸秆玉米秸秆。水分状况和植物残体化学组分的差异影响紫色水稻土中有机碳的动态变化,最终导致碳累积矿化量差异。     

Microbial degradation of fluometuron is influenced by roundup weatherMAX

Laboratory experiments were conducted to describe the influence of glyphosate and fluometuron on soil microbial activity and to determine the effect of glyphosate on fluometuron degradation in soil and by Rhizoctonia solani. Soil and liquid medium were amended with formulated fluometuron alone or with two rates of formulated glyphosate. The soil carbon mineralization was measured hourly for 33 days. The fluometuron remaining in the soil was quantified following 3, 6, 10, 15, 20, 30, and 40 days of incubation. The fluometuron remaining in medium and fungal biomass was measured after 1, 3, 6, 10, 15, and 20 days of incubation. The addition of glyphosate with fluometuron increased C-mineralization and increased the rate of fluometuron degradation relative to fluometuron applied alone. However, more fluometuron remained in the media and less fungal biomass was produced when glyphosate was included.     

Glyphosate Dissipation in Different Soils Under No-Till and Conventional Tillage

Glyphosate is the most used herbicide in Argentina, accounting for 62% of the commercialized pesticides on the market. It is used as a weed controller in no-till systems, and it is also applied to various genetically modified crops (e.g., soybean, corn, and cotton). Although it has a high solubility in water, it tends to adsorb and accumulate in agricultural soils. The main objectives of this work were to compare the dissipation of glyphosate and the accumulation of its metabolite aminomethylphosphonic acid (AMPA) over time in three soils from agricultural areas of Argentina under long-term management with no-till (NT) and conventional tillage (CT) practices. There were no differences in dissipation between NT and CT, indicating that the glyphosate-degrading microflora was not modified by the different tillage managements. Moreover, tillage practices did not alter the general soil properties; therefore, glyphosate bioavailability was not affected by NT or CT practice. Forty percent of the applied glyphosate was degraded within the first three days in all soils, indicating a fast initial dissipation rate. However, the dissipation rate considerably decreased over time, and the degradation kinetics followed a bi-exponential (or two-compartment) kinetic model. No differences were found between tillage practices. Dissipation was not related to the microbial activity measured as soil respiration. The fast decrease in the concentration of glyphosate at the beginning of the dissipation study was not reflected in an increase in the concentration of AMPA. The estimated half-lives for glyphosate ranged between 9 and 38 d. However, glyphosate bioavailability decreases over time, as it is strongly adsorbed to the soil matrix. This increases its residence time, which may lead to its accumulation in agricultural soils.     

Response of maize and soil microorganisms to decomposing poplar root residues after shallow or homogenous mixing into soil

During re‐conversion of short‐rotation poplar tree plantations back to arable land use, large amounts of tree residues must be incorporated into soil. A 90‐d pot experiment with and without N addition was carried out after mixing the same amounts of chaffed poplar root residues into the pots at 0–5 cm or at 0–20 cm depth. The objective was to investigate whether shallow mixing has positive effects on maize growth, reduces poplar root residue decomposition, and changes the microbial community structure towards fungi. Aboveground maize yield was strongly reduced after mixing of poplar root residues at 0–20 cm depth without N fertilization, but was not affected if mixed at 0–5 cm depth. Neither the mixing nor N fertilization had significant effects on root residue decomposition, estimated as recovered particulate organic matter. The total increase in microbial biomass C and biomass N was strongest after homogenous mixing of root residues at 0–20 cm, but remained unaffected by N fertilization. In contrast, the total amount of ergosterol remained unaffected by the mixing treatments, but responded positively to N fertilization. Shallow incorporation of poplar root residues did not affect the microbial biomass C/N ratio but disproportionately increased the fungal ergosterol to microbial biomass C ratio. Shallow incorporation of poplar root residues seems to reduce the demand for N fertilization of following crops, which should be further tested in field experiments.     

草甘膦的土壤酶效应研究

为了探讨当今世界使用量最大的除草剂——草甘膦的土壤环境效应,本文采用室内模拟方法,较为系统地研究了我国4类土壤：褐土、黄绵土、风沙土和红壤,共11个土样中4种主要酶类（脲酶、转化酶、磷酸酶以及脱氢酶）活性与草甘膦间的关系,计算并得到了能够表征土壤轻度污染的生态剂量值ED10。结果表明：非缓冲液法较好地反映了土壤酶的实际情况;草甘膦总体上激活土壤脲酶、转化酶和脱氢酶活性,最大增幅分别为190％、1372％和42％;抑制磷酸酶活性,最大幅度为35％;磷酸酶与草甘膦间为完．全抑制作用机理;激活脱氢酶活性揭示出草甘膦导致了土壤中微生物活性增强,从侧面反映出草甘膦是一种毒性较低的农药。计算获得4类供试土壤褐土、黄绵土、风沙土和红壤ED10值分别为168．3、438．5、35．1和141．4mg·kg^-1;在一定程度上用土壤酶活性比生物来表征土壤污染程度更敏感。土壤性质对草甘膦的毒性有重要影响。