Influence of Phosphate on Transformation and Plant Uptake of Cadmium in Cd-amended Soils

Phosphate was found to have neither influence on Cd transformation nor effect on plant Cd uptake in three Cd-amended upland soils.However,on submerged red earth,high phosphate dressing inhibited the transformation of Cd from exchangeable fraction to other lower-available ones.Cadmium uptake by rice plants increased simultaneously with increasing phosphate supply though plant resistance to Cd also increased at high phosphate level.Application of phosphate as an amendment for Cd-contaminated soil was therefore not recommended in view of the increasing influx of Cd into food chain especially on flooded soils.

Digital mapping of pesticides bioconcentration by integrating remote sensing techniques and plant uptake model

Although pesticides have been widely used worldwide to enhance crop yield and product quality,most pesticides are harmful to the environment and human health.Plants absorb pesticides mainly from air and soil.Therefore,the soil-plant pathway is essential for pesticide absorption.Bioconcentration factor(BCF)has extensively been applied to evaluate potential plant contamination by pesticides from soil.Hence,this study developed a simplified plant transpiration-based plant uptake model(PT-model)to estimate plant pesticides’BCF from soil based on plant transpiration.Remote sensing techniques were employed to generate spatiotemporal continuous plant transpiration via evapotranspiration.Pesticide BCF mapping was achieved by integrating PT-model with Moderate Resolution Imaging Spectroradiometer(MODIS)remotely sensed data.The results were compared with a verified model driven by relative humidity and air temperature(RA-model),which has been confirmed byfindings from previous studies.The estimated BCF was within the boundaries of the RA-model,indicating the simulation’s overall acceptability.In this study,the BCF temporal trend estimated by the proposed method agreed with the RA-model assimilating meteorology datasets,while the spatial distribution was partially inconsistent.Overall,the proposed method generates the spatiotemporal patterns of pesticide BCF with relatively consistent results supported by previous records andfindings.

A New Method to Determine Central Wavelength and Optimal Bandwidth for Predicting Plant Nitrogen Uptake in Winter Wheat

Plant nitrogen （N） uptake is a good indicator of crop N status. In this study, a new method was designed to determine the central wavelength, optimal bandwidth and vegetation indices for predicting plant N uptake （g N m-2） in winter wheat （Triticum aestivum L.）. The data were collected from the ground-based hyperspectral reflectance measurements in eight field experiments on winter wheat of different years, eco-sites, varieties, N rates, sowing dates, and densities. The plant N uptake index （PNUI） based on NDVI of 807 nm combined with 736 nm was selected as the optimal vegetation index, and a linear model was developed with R2 of 0.870 and RMSE of 1.546 g N m-2 for calibration, and R2 of 0.834, RMSE of 1.316 g N m-2, slope of 0.934, and intercept of 0.001 for validation. Then, the effect of the bandwidth of central wavelengths on model performance was determined based on the interaction between central wavelength and bandwidth expansion. The results indicated that the optimal bandwidth varies with the changes of the central wavelength and with the interaction between the two bands in one vegetation index. These findings are important for prediction and diagnosis of plant N uptake more precise and accurate in crop management.

Patterns and drivers of seasonal water sources for artificial sand-fixing plants in the northeastern Mu Us sandy land,Northwest China

Understanding plant water-use patterns is important for improving water-use efficiency and for sustainable vegetation restoration in arid and semi-arid regions. However, seasonal variations in water sources and their control by different sand-fixing plants in water-limited desert ecosystems remain poorly understood. In this study, stable isotopic ratios of hydrogen(δ^(2)H) and oxygen(δ^(18)O) in precipitation, soil water, groundwater, and xylem water were determined to document seasonal changes in water uptake by three representative plant species(Pinus sylvestris var. mongolica Litv., Amygdalus pedunculata Pall., and Salix psammophila) in the northeastern Mu Us sandy land, Northwest China. Based on the depth distribution and temporal variation of measured gravimetric soil water content(SWC), the soil water profile of the three species stands was divided into active(0.01 g g^(-1)< SWC < 0.08 g g^(-1), 20%< coefficient of variation(CV) < 45%), stable(0.02 g g^(-1)< SWC < 0.05 g g^(-1), CV < 20%), and moist(0.08 g g^(-1)< SWC < 0.20 g g^(-1), CV >45%) layers. Annually, P. sylvestris, A. pedunculata, and S. psammophila obtained most water from deep(59.2%±9.7%, moist layer and groundwater),intermediate(57.4%±9.8%, stable and moist layers), and shallow(54.4%±10.5%, active and stable layers) sources, respectively. Seasonally, the three plant species absorbed more than 60% of their total water uptake from the moist layer and groundwater in the early(June) dry season;then, they switched to the active and stable layers in the rainy season(July–September) for water resources(50.1%–62.5%). In the late(October–November) dry season, P. sylvestris(54.5%–66.2%) and A. pedunculata(52.9%–63.6%) mainly used water from stable and moist layers, whereas S. psammophila(52.6%–70.7%) still extracted water predominantly from active and stable layers. Variations in the soil water profile induced by seasonal fluctuations in precipitation and groundwater levels and discrepancies in plant phenology, root distribution, and water demand are the main factors affecting the seasonal water-use patterns of artificial sand-fixing plants. Our study addresses the issue of plant water uptake with knowledge of proportional source-water use and reveals important implications for future vegetation restoration and water management in the Mu Us sandy land and similar desert regions around the world.

Aluminum-Enhanced Proton Release Associated with Plasma Membrane H^+-Adenosine Triphosphatase Activity and Excess Cation Uptake in Tea (Camellia sinensis) Plant Roots

Cultivated tea(Camellia sinensis) plants acidify the rhizosphere, and Aluminum(Al) toxicity is recognized as a major limiting factor for plant growth in acidic soils. However, the mechanisms responsible for rhizosphere acidification associated with Al have not been fully elucidated. The present study examined the effect of Al on root-induced rhizosphere acidification, plasma membrane H^+-adenosine triphosphatase(H^+-ATPase) activity, and cation-anion balance in tea plant roots. The exudation of H^+from tea plant roots with or without Al treatment was visualized using an agar sheet with bromocresol purple. The H^+-ATPase activity of plasma membranes isolated from the roots was measured after hydrolysis using the two-phase partition system. The Al treatment strongly enhanced the exudation of H^+, and the acidification of tea plant roots by Al was closely associated with plasma membrane H^+-ATPase activity. The root plasma membrane H^+-ATPase activity increased with Al concentration. The Al content, amount of protons released, and H^+-ATPase activity were significantly higher in roots treated with Al than in those untreated. The results of the cation-anion balance in roots showed an excess of cations relative to anions, with the amount of excess cation uptake increasing with increasing Al concentrations. These suggest that Al-enhanced proton release is associated with plasma membrane H^+-ATPase activity and excess cation uptake. Findings of this study would provide insights into the contributing factors of soil acidification in tea plantations.

Removal of Pyrene from Contaminated Soils by White Clover

Phytoremediation has been used as an emerging technology for remediation of soil contamination with polycyclic aromatic hydrocarbons(PAHs),ubiquitous persistent environmental pollutants derived from natural and anthropogenic processes,in the last decade.In this study,a pot experiment was conducted to investigate the potential of phytoremediation of pyrene from spiked soils planted with white clover(Trifolium repens)in the greenhouse with a series of pyrene concentrations ranging from 4.22 to 365.38 mg kg-1.The results showed that growth of white clover on pyrenecontaminated soils was not affected.The removal of pyrene from the spiked soils planted with white clover was obviously higher than that from the unplanted soils.At the end of the experiment(60 d),the average removal ratio of pyrene in the spiked soils with white clover was 77%,which was 31%and 57%higher than those of the controls with or without micobes, respectively.Both roots and shoots of white clover took up pyrene from the spiked soils and pyrene uptake increased with the soil pyrene concentration.However,the plant-enhanced dissipation of soil pyrene may be the result of plant-promoted microbial degradation and direct uptake and accumulation of pyrene by white clover were only a small part of the pyrene dissipation.Bioconcentration factors of pyrene(BCFs,ratio of pyrene,on a dry weight basis,in the plant to that in the soil)tended to decrease with increase in the residual soil pyrene concentration.Therefore,removal of pyrene in the contaminated soils was feasible using white clove.

Migration of silver nanoparticles in an aquatic system with Eichhornia crassipes

The migration and dissolution of AgNPs in an aquatic system with plants was investigated.By using a hydroponic system with Eichhornia crassipes,the absorption and transportation processes of silver nanoparticles were investigated.The results show that AgNPs concentrations in the water phase declined with the increase in time,and the reduction degree was dependent on the initial concentrations of AgNPs.The silver concentrations in the roots(r=0.98,p<0.05),stems and leaves(r=1,p<0.001)were significantly positively correlated with the initial concentrations of AgNPs.Silver nanoparticles accumulated in plant roots more than stems and leaves.Compared with the addition of AgNO 3 at identical concentrations,lower removal rates of silver and plant uptake were observed in the AgNPs stress systems.A significant positive correlation was also found between the initial AgNPs concentrations and the removed amount of silver(r=0.99,p<0.001).For AgNPs,the primary removal mechanisms in these aquatic systems were agglomeration and sedimentation,while the absorption by plants had a relatively weak contribution.

Effects of Microbial Inoculants on Nutrient Availability and Rice Yield

Under field conditions, an experiment was conducted to study the effects of ammonification bacteria, potassium bacteria and phosphorus bacteria on nutrient availability in soil and yield of rice in the cold region of China and compared to the conventional fertilization. Results showed that DF1P2 treatment （ammonifiers 1.5× 108 cfu· m2, phosphorus bacteria 1.5× 108 cfu. m2, and potassium bacteria 1.5× 108 cfu· m2） increased available nutrient concentrations in soil, increased the concentrations of N, P, and K in plant organs and increased the rice yield and was the most significantly among all the treatments. This treatment could be recommended as the best suitable biological fertilizer application rate for the rice production in the cold region of China.

Biochar-induced changes in metal mobility and uptake by perennial plants in a ferralsol of Brazil’s Atlantic forest

Despite an abundance of short-term studies focusing on biochar’s effects on annual plants,the long-term effects of biochar on perennial plants and the effects of the biochar on the mobility and speciation changes of metals/metalloids not limited to main plant nutrients in soils are poorly constrained.This study reports on the amelioration a sloped orthic ferralsol by biochar from Tibouchina wood and the resulting effects on perennial crops and microbiota,including a comprehensive analysis of metals/metalloids speciation changes.Fields were amended with biochar and urine-amended biochar(2 kg/m^(2))and were planted with papaya,banana,and manioc.Soil and plant materials were analyzed using acid digestions,sequential extractions,and 16S rRNA gene sequencing.Biochar applications led to decreased soil acidity,shifted the cation exchange capacity from being Al-influenced to being Mg/K/Ca-dominated,and elevated the concentrations of Mg,K,Ca,Zn,and Ba in soils.The exchangeable/acid-soluble fraction of Ca,P,and S notably increased.The soil microbial biome became more species rich and diverse in the biochar-amended fields.Manioc benefited from biochar applications,demonstrating increased growth,which resulted in generally decreased concentrations of trace elements in most plant parts,however,with an increased total elemental uptake.Urine amendment contributed to higher concentrations of P,S,and K in soils,but did not further increase plant growth.Biochar was shown to be a promising soil amendment for agricultural use of orthic ferralsols of the Brazil’s Atlantic forest region,but the accumulation of potentially harmful metals needs to be considered.

Exopolysaccharide-Producing Plant Growth-Promoting Rhizobacteria Under Salinity Condition

Salt-tolerant plant growth-promoting rhizobacteria （PGPR） can play an important role in alleviating soil salinity stress during plant growth and bacterial exopolysaccharide （EPS） can also help to mitigate salinity stress by reducing the content of Na＋ available for plant uptake. In this study, native bacterial strains of wheat rhizosphere in soils of Varanasi, India, were screened to identify the EPS-producing salt-tolerant rhizobacteria with plant growth-promoting traits. The various rhizobacteria strains were isolated and identified using 16S rDNA sequencing. The plant growth-promoting effect of inoculation of seedlings with these bacterial strains was evaluated under soil salinity conditions in a pot experiment. Eleven bacterial strains which initially showed tolerance up to 80 g L-1 NaC1 also exhibited an EPS-producing potential. The results suggested that the isolated bacterial strains demonstrated some of the plant growth-promoting traits such as phosphate solubilizing ability and production of auxin, proline, reducing sugars, and total soluble sugars. Furthermore, the inoculated wheat plants had an increased biomass compared to the unoinoculated plants.

Predicting As,Cd and Pb uptake by rice and vegetables using field data from China

Plant uptake factor （PUF）, single-variable regression of natural log-transformed concentrations in rice grain/vegetables versus natural log-transformed concentrations in soil and multiple-variable regression with soil concentrations and pH, was derived, validated and compared based on the paired crop and soil data collected from studies regarding As, Cd and Pb contaminated croplands in China. Results showed that the median value of PUF did not present deterministic prediction. But after natural logarithm transformation, the PUF followed Gaussian distribution which could be useful in risk assessment. The single-variable regression models were significant for As, Cd and Pb uptake both by rice and vegetables; however, the standard errors of all the regressions were comparatively large. Soil pH as a variable was generally significant but it only contributed positively to model fit for Cd uptake. After model comparison and selection, the upper 95% prediction limits of the multiple regression model for Cd uptake by rice was recommended to calculate screening value of Cd for paddy soil based on the limit for Cd concentration in rice grain.

Concentrations of Heavy Metals in Suburban Horticultural Soils and Their Uptake by Artemisia selengensis

A total of 222 surface soil samples and 40 plant samples were collected to investigate the spatial distribution and possible sources of soil heavy metals and to know the uptake and translocation of heavy metals from roots to different plant parts in a representative vegetable production area in the Baguazhou Island, a suburb of Nanjing City, East China. The arithmetic mean values of total Cd, Cr,Cu, Ni, Pb, and Zn concentrations in the soils were 0.314, 133, 41.0, 58.0, 31.8, and 114 mg kg-1, respectively. All of these values were above the topsoil background values in the Nanjing area. Multivariate and geostatistical analyses showed that soil Cd contamination was derived mainly from agricultural practices. In contrast, Cu and Zn were derived mainly from soil parent materials and Pb from atmospheric deposition from highway gasoline stations. Artemisia selengensis, a locally important specialty vegetable, accumulated heavy metals primarily in the edible leaves. The general distribution of heavy metal concentrations in this plant species showed that the highest occurred in the leaves, intermediate in the stems and lowest in the roots. Cd had the highest concentration factor(root-to-soil ratio) and may pose increased health risks in the future to the local population through the consumption of contaminated vegetables.

Dissipation of polycyclic aromatic hydrocarbons and microbial activity in a field soil planted with perennial ryegrass

Dissipation and plant uptake of polycyclic aromatic hydrocarbons （PAHs） in contaminated agricul- tural soil planted with perennial ryegrass were investigated in a field experiment. After two seasons of grass cultivation the mean concentration of 12 PAHs in soil decreased by 23.4% compared with the initial soil. The 3-, 4-, 5-, and 6- ring PAHs were dissipated by 30.9%, 25.5%, 21.2%, and 16.3% from the soil, respectively. Ryegrass shoots accumulated about 280 ug.kg1, shoot dry matter biomass reached 2.48 x 104kg-ha1, and plant uptake accounted for about 0.99% of the decrease in PAHs in the soil. Significantly higher soil enzyme activities and microbial community functional diversity were observed in planted soil than that in the unplanted control. The results suggest that planting ryegrass may promote the dissipation of PAHs in long-term contaminated agricultural soil, and plant-promoted microbial degradation may be a main mechanism of phytoremediation.

Modelling sulphate-enhanced cadmium uptake by Zea mays from nutrient solution under conditions of constant free Cd^(2+) ion activity

A controlled hydroponic experiment was undertaken to investigate Cd uptake in relation to the activity of Cd species in solution other than the free ion （Cd^2＋） by maintaining a constant Cd^2＋ activity under variable SO4^2- and Cl^- concentrations exposed to maize （Zea mays vat. Cameron） plants. The objectives of these experiments were： （1） to distinguish and quantify the different uptake rates of free and inorganic-complexed Cd from nutrient solution, and （2） to model the uptake of Cd by maize with a Biotic Ligand Model （BLM） in a system which facilitates the close examination of root characteristics. Results of the current experiments suggest that, in addition to the free ion, CdSO4^0 complexes are important factors in determining Cd uptake in nutrient solution by maize plants. Higher nominal SO4^2- concentrations in solution generally resulted in a greater Cd accumulation by maize plants than predicted by the Cd^2＋ activity. A better integration of the complete dataset for the 3 harvest times （6, 9 and 11 days after treatment） was achieved by including consideration of both the duration of Cd exposure and especially the root surface area to express Cd uptake. Similarly, the fit of the BLM was also improved when taking into account exposure time and expressing uptake in terms of root morphological parameters.

Effect of pyrolysis temperature on Si release of alkali-enhanced Si-rich biochar and plant response

Recent studies have shown that silicon(Si)dissolution from biochar may be influenced by the pyrolysis temperature.In addition,the enhancement of biochar by treatment with alkali has been proposed to produce a Si source that can be used for environmentally friendly plant disease control.In this study,biochars from rice straw and rice husk pretreated with KOH,CaO and K_(2)CO_(3)and then pyrolyzed at 350,450 and 550°C were prepared to evaluate the effects of pyrolysis temperature on Si release and plant uptake from alkali-enhanced Si-rich biochar.Extractable Si and dissolution Si from the prepared biochars were assessed by different short-term chemical methods and long-term(30-day)release in dilute acid and neu-tral salt solutions,respectively,along with a rice potting experiment in greenhouse.For both rice straw-and husk-derived alkali-enhanced biochars(RS-10KB and HS-10K2B,respectively),increasing the pyrolysis temperature from 350 to 550°C generally had the highest extractable Si and increased Si content extracted by 5-day sodium carbonate and ammonium nitrate(5dSCAN)designated for fertilizer Si by 61-142%,whereas non-enhanced biochars had more extractable Si at 350°C.The alkali-enhanced biochars produced at 550°C pyrolysis temperature also released 82-172%and 27-79%more Si than that of 350°C produced biochar in unbuffered weak acid and neutral salt solutions,respectively,over 30 days.In addition,alkali-enhanced biochars,especially that derived from rice husk at 550°C facilitated 6-21%greater Si uptake by rice and 44-101%higher rice grain yields than lower temperature biochars,non-enhanced biochars,or conventional Si fertilizers(wollastonite and silicate calcium slag).Overall,this study demonstrated that 550°C is more efficient than lower pyrolysis temperature for preparing alkali-enhanced biochar to improve Si release for plant growth.

Arbuscular mycorrhizal fungi reduce ammonia emissions under different land-use types in agro-pastoral areas

Ammonia(NH3)emissions,the most important nitrogen(N)loss form,always induce a series of environmental problems such as increased frequency of regional haze pollution,accelerated N deposition,and N eutrophication.Arbuscular mycorrhizal(AM)fungi play key roles in N cycling.However,it is still unclear whether AM fungi can alleviate N losses by reducing NH3emissions.The potential mechanisms by which AM fungi reduce NH_(3)emissions in five land-use types(grazed grassland,mowed grassland,fenced grassland,artificial alfalfa grassland,and cropland)were explored in this study.Results showed that AM fungal inoculation significantly reduced NH3emissions,and the mycorrhizal responses of NH3emissions were determined by land-use type.Structural equation modeling(SEM)showed that AM fungi and land-use type directly affected NH_(3)emissions.In addition,the reduction in NH_(3)emissions was largely driven by the decline in soil NH_(4)^(+)-N and pH and the increases in abundances of ammonia-oxidizing archaea(AOA)amoA and bacteria(AOB)amoB genes,urease activity,and plant N uptake induced by AM fungal inoculation and land-use type.The present results highlight that reducing the negative influence of agricultural intensification caused by land-use type changes on AM fungi should be considered to reduce N losses in agriculture and grassland ecosystems.

Lead stabilization in a polluted calcareous soil using cost-effective biochar and zeolite amendments after spinach cultivation

Biochar has been shown to be an effective organic soil amendment for the in-situ immobilization of lead(Pb).Little research has been done on the effects of biochar and zeolite interactions on Pb immobilization in contaminated calcareous soils.Therefore,the aim of this study was to investigate the effects of different levels of zeolite(2%and 4%,weight:weight)and biochars(3%,weight:weight)prepared from green compost(GB),poultry manure(PB),and municipal waste(MB)on the content of Pb in spinach shoots and Pb stabilization in a greenhouse experiment conducted using a Pb-contaminated calcareous soil.Application of GB and PB significantly decreased the Pb mobility factor(MF)from 2.8%to 4.6%and the cumulative Pb release(diethylenetriamine pentaacetate(DTPA)extraction)from 16.8%to 20.1%in the calcareous soil,which were further enhanced by the combined addition of zeolite,with Pb MF reduction from 4.4%to 8.4%and cumulative Pb release reduction from 24.8%to 28.6%.The enhancement of soil Pb immobilization was attributed to the properties of the amendments,such as high pH,ash content,and phosphorus content(10.5^(-1)1.0,390-435 g kg^(-1),and 2.5-4.3 g kg^(-1),respectively).Results of the soil Pb sequential extraction and Pb release kinetics were highly and significantly correlated with spinach shoot Pb content.Soil chemical data showed that the application of MB with 4%zeolite was the most effective treatment for immobilizing Pb(8.4%reduction in Pb MF and 28.6%reduction in cumulative DTPA-extractable Pb),which is in agreement with the results of the spinach shoot Pb(89.0%reduction in Pb uptake).

A Meta-Analysis on Phenotypic Variation in Cadmium Accumulation of Rice and Wheat:Implications for Food Cadmium Risk Control

In some densely-populated countries, farmland has been widely cadmium (Cd) contaminated, and the utilization of the contaminated farmland for crop production is currently unavoidable. This necessitates the use of low-Cd crops (i.e., pollution-safe cultivars, the crop varieties with the ability to accumulate a low level of Cd in their edible parts when grown on polluted soil) in these areas and highlights the importance of knowledge on phenotypic variation in crop Cd accumulation for food Cd risk control. Studies on phenotypic variation in heavy metal accumulation started decades ago for a wide range of crops, and synthesis of the scattered experimental results in the literature is in need. We built a Low-Cd Crops Database based on literature research, and relevant meta-analysis was performed to quantitatively explore the phenotypic variation in Cd uptake and translocation of rice and wheat. Considerable variability existed among rice (median grain Cd bioconce nt ration factor (BCF) of 0.10) and wheat (median grain Cd BCF of 0.21) phenotypes in grain Cd accumulation, and this variability was labile to soil pH and the level of Cd stress. Wheat statistically had a higher root-to-shoot Cd-translocating ability than rice, highlighting potential food Cd risks and the importance of growing low-Cd wheat in slightly Cd-contaminated regions. Meanwhile, no correlations were detected among soil-to-root, root-to-shoot, and shoot-to-grain translocation factors, implying that Cd uptake and internal translocation in crops were probably controlled by different underlying gene tic mechanisms. Root-to-shoot Cd transport could be a favorable target trait for selecting and breeding low-Cd rice and wheat. In all, this review provides a comprehensive low-Cd crop list for remediation practice and a systematic meta-analysis inferring food Cd risks based on plant capacity for Cd accumulation and desired traits for low-Cd crop breeding.

Wheat-Exuded Organic Acids Influence Zinc Release from Calcareous Soils

Rhizosphere drives plant uptake of sparingly soluble soil zinc(Zn).An investigation with three experiments was conducted to study organic acid exudation by two contrasting wheat genotypes(Sehar-06 and Vatan),Zn fractions in 10 different calcareous soils from Punjab,Pakistan,and release of different soil Zn fractions by organic acids.The two genotypes differed significantly in biomass production and Zn accumulation under deficient and optimum Zn levels in nutrient solution.At a deficient Zn level,Sehar-06 released more maleic acid in the rhizosphere than Vatan.Ten soils used in the present study had very different physicochemical properties;their total Zn and Zn distribution among different fractions varied significantly.Zinc release behaviour was determined by extracting the soils with 0.005 mol L-1 citric acid or maleic acid.The parabolic diffusion model best described Zn release as a function of time.Parabolic diffusion model fitting indicated more maleic acid-driven than citric acid-driven soil Zn mobility from different fractions.Cumulative Zn release in six consecutive extractions during 24 h ranged from 1.85 to 13.58 mg kg-1 using maleic acid and from 0.37 to 11.84 mg kg-1 using citric acid.In the selected calcareous soils,the results of stepwise linear regression indicated significant release of Fe-Mn oxide-bounded soil Zn by maleic acid and its availability to the Zn-effcient genotype.Hence,release of maleic acid by plants roots played an important role in phytoavailability of Zn from calcareous soils.

Effects of Epigeic Earthworms on Decomposition of Wheat Straw and Nutrient Cycling in Agricultural Soils in a Reclaimed Salinity Area:A Microcosm Study

Earthworms, one of the most important macroinvertebrates in terrestrial ecosystems of temperate zones, exert important influ- ences on soil functions. A laboratory microcosm study was conducted to evaluate the influence of the earthworm Eisenia fetida on wheat straw decomposition and nutrient cycling in an agricultural soil in a reclaimed salinity area of the North China Plain. Each microcosm was simulated by thoroughly mixing wheat straw into the soil and incubated for 120 d with earthworms added at 3 different densities as treatments： control with no earthworms, regular density （RD） with two earthworms, and increased density （ID） with six earthworms. The results showed that there was no depletion of carbon and nitrogen pools in the presence of the earthworms. Basal soil respiration rates and metabolic quotient increased with the increase in earthworm density during the initial and middle part of the incubation period. In contrast, concentrations of microbial biomass carbon and microbial biomass quotient decreased in the presence of earthworms. Earthworm activity stimulated the transfer of microbial biomass carbon to dissolved organic carbon and could lead to a smaller, but more metabolically active microbial biomass. Concentrations of inorganic nitrogen and NO^-N increased significantly with the increase in earthworm density at the end of the incubation （P ~ 0.05）, resulting in a large pool of inorganic nitrogen available for plant uptake. Cumulative net nitrogen mineralization rates were three times higher in the ID treatment than the RD treatment.