Red Paddy Soil Research Articles

Effect of moisture condition on the immobilization of Cd in red paddy soil using passivators

ABSTRACTIn this study, the immobilization of cadmium (Cd) was evaluated in red paddy soil amended with four different passivators (biochar, lime (CaO), calcium dihydrogen phosphate (Ca(H2PO4)2) and zeolite (modified with NaOH)) for three moisture regimes (70% water-holding capacity (WHC), continuous flooding (CF) and wetting-drying cycle (WDC)). The results showed that: the pH of the soil was obviously influenced by the moisture regimes and decreased in the general order of CF > WDC > 70% WHC, and the addition of lime and Ca(H2PO4)2 could obviously enhance and reduce the values to some extent, respectively. Flooding condition could enhance soil pH and increase the content of free Fe-oxides, amorphous Fe-oxides and water-soluble organic/inorganic carbon in paddy soil. The efficiency in decreasing the exchangeable Cd of different moisture regimes followed the order: CF > 70%WHC > WDC. CF combined with lime was the most effective treatment for reducing exchangeable Cd. The transformation of Cd fractions mainly occurred between the acid-extractable and reducible fractions in the immobilization experiments. The most effective passivator is Ca(H2PO4)2, and CF is the most favorable regime in promoting the transformation of Cd in red paddy soil from acid-extractable to reducible fraction.

Ammonia-oxidizing archaea are more sensitive than ammonia-oxidizing bacteria to long-term application of green manure in red paddy soil

Growing winter green manure is an effective method for improving nitrogen (N) management in paddy fields to enhance rice production. Ammonia oxidization is a key process in N cycling in these soils, but effects of green manuring on ammonia oxidizers have not been fully addressed for red paddy soils. The objective of this study was to investigate long-term impacts of winter green manure on abundance and diversity of ammonia oxidizers in rice paddy soils of Southern China. The field experiment established in 1982 included four treatments: rice-rice-winter fallow (RRF), rice-rice-ryegrass (RRG), rice–rice–rape (RRP) and rice–rice–milk vetch (RRV). The abundance and diversity of amoA genes from ammonia-oxidizing archaea (AOA) and bacteria (AOB) were quantified using quantitative PCR and 454 pyrosequencing, respectively. The AOA were more abundant than AOB in red paddy soils, with ratios of AOA to AOB from 36 to 1686. Long-term application of milk vetch increased the abundance of both AOA and AOB after green manures were incorporated. Some of the relative abundances of most abundant operational taxonomic units (OTUs) of AOA increased and others decreased after application of green manures, while most abundant OTUs of AOB remained unaffected. Redundancy analysis (RDA) found a clear separation between milk vetch and winter fallow, indicating that the community structure of AOA was influenced by application of milk vetch. Phylogenetic analysis showed that the most dominant OTUs of AOA and AOB were affiliated with Nitrososphaera and Nitrosospira, respectively. In conclusion, in red paddy soil, long-term application of milk vetch increased the abundances of AOA and AOB after incorporated. Moreover, long-term application of green manures had more profound influences on AOA community than on AOB in red paddy soils.

Effects of long-term application of different green manures on ferric iron reduction in a red paddy soil in Southern China

Dissimilatory Fe(III) reduction is an important process in the geochemical cycle of iron in anoxic environment. As the main products of dissimilatory iron reduction, the Fe(II) species accumulation could indicate the reduction ability. The effects of different green manures on Fe(III) reduction in paddy soil were explored based on a 31-year rice-rice-winter green manure cropping experiment. Four treatments were involved, i.e., rice-rice-milk vetch (RRV), rice-rice-rape (RRP), rice-rice-rye-grass (RRG) and rice-rice-winter fallow (RRF). Soils were sampled at flowering stage of milk vetch and rape (S1), before transplantation (S2), at tillering (S3), jointing (S4), and mature (S5) stages of the early rice, and after the harvest of the late rice (S6). The contents of TFeHCl (HCl-extractable total Fe), Fe(II)HCl (HCl-extractable Fe(II) species) and Fe(III)HCl (HCl-extractable Fe(III) species) were measured. The correlations among those Fe species with selected soil environmental factors and the dynamic characteristics of Fe(II)HCl accumulation were investigated. The results showed that TFeHCl in RRF was significantly higher than those in the green manure treatments at most of the sampling stages. Fe(II)HCl increased rapidly after the incorporation of green manures in all treatments and kept rising with the growth of early rice. Fe(II)HCl in RRG was quite different from those in other treatments, i.e., it reached the highest at the S2 stage, then increased slowly and became the lowest one at the S4 and S5 stages. Fe(III)HCl showed oppositely, and Fe(II)HCl/Fe(III)HCl performed similarly to Fe(II)HCl. The maximum accumulation potential of Fe(II)HCl was significantly higher in RRF, while the highest maximum reaction rate of Fe(II)HCl accumulation appeared in RRG. Significant correlations were found between the indexes of Fe(II)HCl accumulation and soil pH, oxidation-reduction potential (Eh) and total organic acids, respectively. In together, we found that long-term application of green manures decreased the TFeHCl in red paddy soils, but promoted the ability of Fe(III) reduction, especially the ryegrass; Fe(II)HCl increased along with the growth of rice and was affected by soil conditions and environmental factors, especially the water and redox ability.

As contamination alters rhizosphere microbial community composition with soil type dependency during the rice growing season

Rhizosphere microbes play a cardinal role in transformation and crop uptake of arsenic (As), thereby relieving or intensifying the risk of As contamination in the food webs. How rhizosphere microbiomes respond to As contamination in different paddy soils and rice growth stages is still unclear. Here, we conducted a rice pot experiment to address the effects of rice developmental stage and As contamination on the rhizosphere microbial communities in two contrast paddy soils, a yellow clayey paddy soil (YP, pH 5.1, soil organic matter 20.8 g/kg) and red paddy soil (RP, pH 6.2, soil organic matter 46.1 g/kg). The rhizosphere microbial communities were investigated using phospholipid fatty acids analysis at tillering, panicle initiation, and maturity stages. The results showed that rice growing in YP soil accumulated 2-10 times higher contents of As in root than that in RP soil. There was a significant effect of rice growing stage, independent of soil types and As treatment, on rhizosphere microbial community composition in both YP and RP soils as depicted by canonical correspondence analysis. As contamination significantly altered rhizosphere microbial community composition only in YP soil, which showed the soil type dependency of the As contamination effect. In RP soil, the higher content of soil organic matter reduced the impact of As contamination. Soil pH explained more percentage of variation in microbial community composition than soil DOC and DON did. These influences of soil physiochemical properties on heavy metal available and rhizosphere microbial community may lay the foundation for exploration of bioremediation potential.

Cadmium availability in rice paddy fields from a mining area: The effects of soil properties highlighting iron fractions and pH value

Cadmium (Cd) availability can be significantly affected by soil properties. The effect of pH value on Cd availability has been confirmed. Paddy soils in South China generally contain high contents of iron (Fe). Thus, it is hypothesized that Fe fractions, in addition to pH value, may play an important role in the Cd bioavailability in paddy soil and this requires further investigation. In this study, 73 paired soil and rice plant samples were collected from paddy fields those were contaminated by acid mine drainage containing Cd. The contents of Fe in the amorphous and DCB-extractable Fe oxides were significantly and negatively correlated with the Cd content in rice grain or straw (excluding DCB-extractable Fe vs Cd in straw). In addition, the concentration of HCl-extractable Fe(II) derived from Fe(III) reduction was positively correlated with the Cd content in rice grain or straw. These results suggest that soil Fe redox could affect the availability of Cd in rice plant. Contribution assessment of soil properties to Cd accumulation in rice grain based on random forest (RF) and stochastic gradient boosting (SGB) showed that pH value should be the most important factor and the content of Fe in the amorphous Fe oxides should be the second most important factor in affecting Cd content in rice grain. Overall, compared with the studies from temperate regions, such as Europe and northern China, Fe oxide exhibited its unique role in the bioavailability of Cd in the reddish paddy soil from our study area. The exploration of practical remediation strategies for Cd from the perspective of Fe oxide may be promising.

Long-term rice-rice-green manure rotation changing the microbial communities in typical red paddy soil in South China

On the basis of a long-term (30 years) field experiment that involved four rotation systems, rice-rice-winter fallow (RRF), rice-rice-ryegrass (RRG), rice-rice-rape (RRP), and rice-rice-milk vetch (RRV), this study described the effects of green manure on the microbial communities in the red paddy soils using 454 pyrosequencing for the 16S rRNA gene. The Chao1 richness and non-parametric Shannon's index increased in all soil samples that received green manure treatments. The communities’ structures with the green manure applications were significantly dissimilar from that under the winter fallow. Using Metastats tests, many genera in the RRG, RRP and RRV soils were significantly different from those in the RRF soil, including a number of genera that functioned in the nitrogen and sulfur cycles. Analyses of the genera with these functions revealed the shifts in microbial ecosystem functions after long-term green manuring. Changes in the microbial communities increased the ammonium supply and decreased the soil acidification in green-manure-amended soils. Together, these data suggested powerful effects of green manure on both the microbial communities and the biogeochemical cycle driven by the shifts in bacterial functional groups.

Speciation and Availability of Heavy Metals On Serpentinized Paddy Soil and Paddy Tissue

There has been increasing concern in recent years regarding toxic elements transfer in the soil-plant system. Soils developed on serpentinitic rocks have serious limitations for agriculture. They have high levels of magnesium (Mg) and heavy metals such as nickel (Ni), chromium (Cr), cobalt (Co), and manganese (Mn) and are deficient in some essential macronutrients. The continuous accumulation of serpentine soil downward in the Ranau Valley paddy field brings by excessive erosion and high rainfalls through the main irrigation systems and overflowing flood streams. The presence of serpentine soil contamination is clearly evidenced by oily water and reddish paddy soil. Therefore the basic objective of this work is to examine the availability and mobility of certain elements by assessing the physicochemical properties of paddy soil and the soil-plant interaction as induced by the effect of serpentine. In this study the transfer of abundant metals (Ni, Cr, Co, Fe, Al and Mn) was investigated from a paddy field soils to different parts of Oryza sativa L. (cultivar YTM Sarawak Merah) tissue (root and shoot) through modified metal speciation procedures; that are available and residual fractions. The analyses in nutrient content and heavy metals accumulation in the paddy soil and tissues are reportedat the full crop stage through the translocation factor (TF) and bioaccumulation factor (BAF) for a better understanding of the soil-plant system. The heavy metals in the area where potentially bioavailable since the soils are acidic (pH 5.60 – 6.15) which suggested a medium nutrient availability, have a high silt (33.21-69.02), lower clay (13.55-46.98) and sand (7.41-33.44) contents and higher metal concentrations associated with the residual fractions.

Aging effect on the leaching behavior of heavy metals (Cu, Zn, and Cd) in red paddy soil.

Aging effect can influence the fractions distribution and mobility of metals after they are added into soil. In this study, incubation and soil column experiments under simulated acid rain condition were conducted to evaluate aging effect on the leaching characteristic of Cu, Zn, and Cd in artificial polluted red paddy soil. Our results showed that aging effect reduced metal contents in exchangeable and HoAc soluble fractions. Power function was the most excellent to describe the variation of exchangeable fraction, while pseudo first- and second-order functions were more successful to describe the leaching characteristic of metals from soil columns. The leaching amount of the metals from the polluted soil only accounted for a small part of their total content in soil, and the leachability of Cu was the weakest. Both the exchangeable and HoAc soluble fraction were available as indicators to evaluate the leachability of metals in red paddy soil. The shorter time the soil was contaminated, the more amounts of metals released from the soil. The reduction of exchangeable fraction caused by aging effect was the main reason for the decrease of metal mobility in soil.

改变施肥管理后不同肥力稻田土壤CO2的排放特征

Paddy soils show significant potential of carbon sequestration. The soil organic carbon( SOC) content of red paddy soils have been reported to be steady after 30 years' cultivation and it varies with different fertilization practice. In this study,three red paddy soils,which cultivated under different organic fertilization treatments in a 30 years fertilizer experiment in Changsha, China, was adjusted to the following seven treatments without compromising the original experiment: the original high organic fertilization treatment( HOM),the high organic fertilization treatment changed from the original normal organic fertilization treatment( N-H),the original normal organic fertilization treatment( NOM),the normal organic fertilization treatment changed from chemical fertilization( C-N),the chemical fertilization treatment changed from high organic fertilization treatment( H-C),the chemical fertilization treatment changed from normal organic fertilization treatment( N-C),the original chemical fertilization treatment( CF). CO2 flux of the three original and fourchanged treatments was measured to study the effects of the following-up fertilization reforming on the CO2 flux in red paddy soils with different fertilities in 2012—2013. The results clearly showed that the following-up changing of fertilization had obvious impacts on the dynamics of CO2 flux. The CO2 flux in the soils under long-term organic fertilization treatments decreased after the adjustment from organic fertilization to chemical fertilization. However,the CO2 flux in the soils under long-term chemical fertilization treatments increased remarkably after the adjustment from chemical fertilization to organic fertilization. The results also indicated that both organic fertilizer and soil organic carbon( SOC) had important impacts on CO2 flux. The amount of organic carbon inputted by organic fertilizers had a significantly positive relationship with the total amount of annual CO2-C flux( r = 0.9015**,n = 21),and the inherent SOC content( x) also had a positive relationship with the total amount of annual CO2-C( y)( y = 10. 962x- 68. 86,R2= 0. 7507,n = 9,P 0. 01) in the paddy soils received chemical in 2012. The fertilization reforming from organic fertilization to chemical fertilization would lead to the loss of SOC in paddy soils due to its mineralization,and the SOC loss increased with the rising of inherent SOC content. The SOC content in the paddy soils under long-term organic fertilization would be consistent with that in the paddy soils under long-term chemical fertilization after the fertilization reforming from organic to chemical fertilization for a certain time. The fertilization reforming from chemical to organic fertilization,or from low organic to high organic fertilization would lead to the SOC accumulation in paddy soils,and the accumulating rate of SOC had a positive relationship with the amount of the inputted organic material. Under same organic fertilization treatment,the apparent decomposition rate of organic material applied in the soils with high SOC content was larger than that in the soils with low SOC content,which would result in lower SOC accumulation. So,the paddy soils with different SOC content would have the same SOC level when they received same organic fertilization management for a certain time. As a conclusion,the sustained organic fertilization in red paddy soils with high or low SOC content is essential to maintain or improve the soil organic carbon content in Southern China.

Influence of removal of organic matter and iron and manganese oxides on cadmium adsorption by red paddy soil aggregates

Different soil components in various aggregates were selectively removed for evaluating their influence on Cd adsorption.

Iron Reduction Coupled to Reductive Dechlorination in Red Soil

Iron reduction plays an important role in the reductive transformation of organochlorine pesticides in red paddy soils. This interaction between iron reduction and organochlorine pesticides (OCPs) in red soils in south China is particularly important because of the high abundance and reactivity of iron within unique man-made paddy ecosystems. However, the relationships between iron reduction and reductive dechlorination and the geochemical constraints of these relationships are not fully understood. In this comprehensive review, we summarized current understanding of iron reduction, reductive dechlorination, the relationships between them, and their interactions with the reduction of nitrate, sulfate, Cu(II), and humic substances in red soils. Recent studies showed that iron reduction and dechlorination occur simultaneously in soils and that iron reduction could either stimulate or inhibit dechlorination. Meanwhile, sulfate and Cu(II) reduction can stimulate or inhibit iron reduction and dechlorination. Nitrate reduction can be coupled to iron reduction, but it inhibits dechlorination. Increasing evidence showed that humic substances can enhance the rates of both iron reduction and dechlorination by accelerating electron transfer. However, there is insufficient information in the literature for delineating the effects of several rising environmental problems (e.g., heavy metal pollution, deficiencies in phosphorus and aluminum) on iron reduction, OCP transformation, and the related microbial activities. Future studies are necessary because such information may be key for sustainable development of agriculture and pollution control in red soils.

Effects of Long-Term Winter Planted Green Manure on Distribution and Storage of Organic Carbon and Nitrogen in Water-Stable Aggregates of Reddish Paddy Soil Under a Double-Rice Cropping System

In agricultural systems, maintenance of soil organic matter has long been recognized as a strategy to reduce soil degradation. Manure amendments and green manures are management practices that can increase some nutrient contents and improve soil aggregation. We investigated the effects of 28 yr of winter planted green manure on soil aggregate-size distribution and aggregate-associated carbon (C) and nitrogen (N). The study was a randomized completed block design with three replicates. The treatments included rice-rice-fallow, rice-rice-rape, rice-rice-Chinese milk vetch and rice-rice-ryegrass. The experiment was established in 1982 on a silty light clayey paddy soil derived from Quaternary red clay (classified as Fe-Accumuli-Stagnic Anthrosols) with continuous early and late rice. In 2009, soil samples were collected (0-15 cm depth) from the field treatment plots and separated into water-stable aggregates of different sizes (i.e., >5, 2-5, 1-2, 0.5-1, 0.25-0.5 and <0.25 mm) by wet sieving. The long-term winter planted green manure significantly increased total C and N, and the formation of the 2-5-mm water-stable aggregate fraction. Compared with rice-rice-rape, rice-rice-Chinese milk vetch and rice-rice-ryegrass, the rice-rice-fallow significantly reduced 2-5-mm water-stable aggregates, with a significant redistribution of aggregates into micro-aggregates. Long-term winter planted green manure obviously improved C/N ratio and macro-aggregate-associated C and N. The highest contribution to soil fertility was from macro-aggregates of 2-5 mm in most cases.

Potassium Mobilization and Transformation in Red Paddy Soil as Affected by Rice

A rhizobox experiment was conducted to study the effect of growing rice (Oryza sativa L.) on the movement and transformation of K in a red paddy soil. Results showed that K uptake by rice reduced the concentrations of soil water‐soluble K (Sol‐K), exchangeable K (Ex‐K), and non‐exchangeable K (Nonex‐K) in the root zone. Soil Sol‐K in non‐root‐zone compartment close to the root zone (0–6 cm) migrated toward the root zone, and soil Ex‐K (0–4 cm) and soil Nonex‐K (0–3 cm) was released into soil solution. As the rice growth progressed, soil Sol‐K, Ex‐K, and Nonex‐K in the root zone and soil Sol‐K in non‐root‐zone compartment close to the root zone (0–6 cm) continued to decrease. Decreases of soil Ex‐K and Nonex‐K in non‐root‐zone compartment extended from the root to 6 and 5 cm into the bulk soil, respectively. At the late grain‐filling stage, soil Sol‐K concentration in the root zone and non‐root‐zone compartment (0–6 cm) declined, and then remained stable, whereas Sol‐K in the non‐root zone (6–7 cm) kept diffusing toward the root zone. Soil Ex‐K and Nonex‐K in the root zone and non‐root‐zone compartment continued to reduce, and the decrease distance of soil Nonex‐K was extended to 6 cm. Within the whole season, soil Ex‐K and Nonex‐K were the main forms of K available to the plants, followed by Sol‐K. The information obtained in this study indicated that K fertilizer should be added to the place around the root zone.

Abundance and Community Composition of Ammonia-Oxidizers in Paddy Soil at Different Nitrogen Fertilizer Rates

Ammonia oxidation, the first and rate-limiting step of nitrification, is carried out by both ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA). However, the relative importance of AOB and AOA to nitrification in terrestrial ecosystems is not well understood. The aim of this study was to investigate the effect of the nitrogen input amount on abundance and community composition of AOB and AOA in red paddy soil. Soil samples of 10-20 cm (root layer soil) and 0-5 cm (surface soil) depths were taken from a red paddy. Rice in the paddy was fertilized with different rates of N as urea of N1 (75 kg N ha−1 yr−1), N2 (150 kg N ha−1 yr−1), N3 (225 kg N ha−1 yr−1) and CK (without fertilizers) in 2009, 2010 and 2011. Abundance and community composition of ammonia oxidizers was analyzed by real-time PCR and denaturing gradient gel electrophoresis (DGGE) based on amoA (the unit A of ammonia monooxygenase) gene. Archaeal amoA copies in N3 and N2 were significantly (P<0.05) higher than those in CK and N1 in root layer soil or in surface soil under tillering and heading stages of rice, while the enhancement in bacterial amoA gene copies with increasing of N fertilizer rates only took on in root layer soil. N availability and soil NO3−-N content increased but soil NH4+-N content didn't change with increasing of N fertilizer rates. Otherwise, the copy numbers of archaeal amoA gene were higher (P<0.05) than those of bacterial amoA gene in root lary soil or in surface soil. Redundancy discriminate analysis based on DGGE bands showed that there were no obvious differs in composition of AOA or AOB communities in the field among different N fertilizer rates. Results of this study suggested that the abundance of ammonia-oxidizers had active response to N fertilizer rates and the response of AOA was more obvious than that of AOB. Similarity in the community composition of AOA or AOB among different N fertilizer rates indicate that the community composition of ammonia-oxidizers was relatively stable in the paddy soil at least in short term for three years.

Early responses of vegetation and soil organic carbon to waterlogging and winter wildfire on abandoned red paddy soils

Aims Large tracts of land have been abandoned since last century and the trend continues. Our objectives were to seek an effective solution for facilitating plant diversity on abandoned red paddy soils at an early stage, and to investigate the dynamics of soil organic carbon in relation to changes in vegetation. Methods The experiment consisted of four treatments, including control(no disturbance from human activities), waterlogging, winter wildfire, and a combination of waterlogging and winter wildfire. Measurements were made in each treatment within quadrate sampling method. In each quadrate, the maximum plant height, density, coverage, and composition were surveyed and recorded by species. The above and belowground biomass and soil organic carbon content were determined with the methods of harvesting, soil coring, and K2Cr2O7 calefaction, respectively. Important findings The results showed that waterlogging and winter wildfire significantly altered the vegetation richness, evenness, diversity, and species composition on the abandoned red paddy soils at the early stage. Paspalum paspaloides and Murdannia triquetra were the dominant plant species when the abandoned paddies had suffered waterlogging for the first few years, and Microstegium vimineum was the dominant species on sites experienced winter wildfire. Bidens frondosa was a co-dominant species in all the treatments. Compared with the control, the belowground biomass at the depth of 0–20 cm was increased by 49.84%, 73.34%, and 28.94%, respec-tively, in the waterlogging treatment in the year 2011, 2012 and 2013, and the aboveground biomass was increased by 25.74%, 64.30%, and 50.24%, respectively, in the winter wildfire treatment. The roots collected from the upper 5 cm soil layer accounted for 66.50%–80.34% of the total from the 0–20 cm layer. Winter wildfire resulted in greater values in the Shannon-Wiener diversity index of the vegetation. Different from the uptrend before abandonment, the soil organic carbon in the treatments of control, waterlogging, winter wildfire, and a combination of waterlogging and winter wildfire was reduced by 11.16%, 18.99%, 9.17%, and 19.12%, respectively, after six years of abandonment, and it was significantly lower in the waterlogging treatment(p 0.05). Plant species richness, evenness, diversity, composition, biomass, and soil organic carbon all had strong relationships with waterlogging and winter wildfire on abandoned red paddy soils(p 0.05).

Adsorption characteristics of Cu and Zn onto various size fractions of aggregates from red paddy soil

Soil aggregate is the basic structure unit of soils and the ability of various size fractions are different in the aspect of adsorbing and transferring heavy metals in the environment. In this study, bulk soil from red paddy field was partitioned into four aggregate-size fractions and their adsorption characteristics for Cu and Zn were studied. Our results showed that: Pseudo-second order model was more successful to fit the adsorption process in the kinetic experiments and the isothermal experiments data can be described well with the Freundlich model as a whole. Due to higher contents in organic matter, CEC and free iron oxide, the <0.002mm fraction was found to have the highest initial sorption rate and maximum adsorption capacity. The adsorption amount of metals increased as the increasing of pH and the percentage of adsorbed metal susceptible to desorption into 0.01M NaNO3 was greater for Zn than for Cu, while their variation trends were quite opposite. More specific adsorption sites in the <0.002mm fraction lead to more desorption amount for this particle size of soil at low pH condition. After 60 days of incubation, Cu and Zn were observed to enrich in the clay-size aggregates with fractions more stable than other particles.

Tenax extraction for exploring rate-limiting factors in methyl-β-cyclodextrin enhanced anaerobic biodegradation of PAHs under denitrifying conditions in a red paddy soil

The effectiveness of anaerobic bioremediation systems for PAH-contaminated soil may be constrained by low contaminants bioaccessibility due to limited aqueous solubility and lack of suitable electron acceptors. Information on what is the rate-limiting factor in bioremediation process is of vital importance in the decision in what measures can be taken to assist the biodegradation efficacy. In the present study, four different microcosms were set to study the effect of methyl-β-cyclodextrin (MCD) and nitrate addition (N) on PAHs biodegradation under anaerobic conditions in a red paddy soil. Meanwhile, sequential Tenax extraction combined with a first-three-compartment model was employed to evaluate the rate-limiting factors in MCD enhanced anaerobic biodegradation of PAHs. Microcosms with both 1% (w/w) MCD and 20mM N addition produced maximum biodegradation of total PAHs of up to 61.7%. It appears rate-limiting factors vary with microcosms: low activity of degrading microorganisms is the vital rate-limiting factor for control and MCD addition treatments (CK and M treatments); and lack of bioaccessible PAHs is the main rate-limiting factor for nitrate addition treatments (N and MN treatments). These results have practical implications for site risk assessment and cleanup strategies.

Long-Term Effect of Fertilizer and Rice Straw on Mineral Composition and Potassium Adsorption in a Reddish Paddy Soil

Increasing K+ adsorption can be an effective alternative in building an available K pool in soils to optimize crop recovery and minimize losses into the environment. We hypothesized that long-term fertilization might change K+ adsorption because of changes in the chemical and mineralogical properties of a rice (Oryza sativa L.). The aims of this study were (i) to determine clay minerals in paddy soil clay size fractions using X-ray diffraction methods and a numerical diagram-decomposition method; (ii) to measure K+ adsorption isotherms before and after H2O2 oxidation of organic matter, and (iii) to investigate whether K+ adsorption is correlated with changes in soil chemical and mineral properties. The 30-yr long-term fertilization treatments caused little change in soil organic C (SOC) but a large variation in soil mineral composition. The whole-clay fraction (<5 μm) corresponded more to the fertilization treatment than the fine-clay fraction (<1 μm) in terms of percentage of illite peak area. The total percentage of vermiculite-chlorite peak area was significantly negatively correlated with the total percentage of illite peak area in the <5 μm soil particles (R=-0.946, P<0.0006). Different fertilization treatments gave significantly different results in K+ adsorption. The SOC oxidation test showed positive effects of SOC on K+ adsorption at lower K+ concentration (⩽120 mg L−1) and negative effects at higher K+ concentration (240 mg L−1). The K+ adsorption by soil clay minerals after SOC oxidization accounted for 60–158% of that by unoxidized soils, suggesting a more important role of soil minerals than SOC on K+ adsorption. The K+ adsorption potential was significantly correlated to the amount of poorly crystallized illite present (R=0.879, P=0.012). The availability of adsorbed K+ for plant growth needs further study.

Effects of different concentrations of nitrogen and soil moistures on paddy soil nitrification and microbial characteristics

为了明确不同氮素浓度和水分对土壤硝化作用和微生物特性的影响, 特别是高氮素浓度下的响应特异性, 以红壤水稻土为供试土壤, 设置4个硫铵用量水平[0(CK)、120 mg(N)·kg^-1(A1)、600 mg(N)·kg^-1(A2)、 1 200 mg(N)·kg^-1(A3)], 调节土壤水分为饱和持水量(WHC)的40%、60%和80%, 研究了短期内不同氮素浓度和不同水分条件下土壤硝化作用、微生物生物量碳和微生物功能多样性的变化。结果表明: 在40%、60%和80%WHC水分条件时, 硫铵A2、A3浓度处理土壤硝化率和硝化速率普遍较低, 硫铵A1浓度处理硝化率和硝化速率随土壤含水量的升高而升高; 同含水量时随硫铵用量的升高而显著降低。在40%、60%和80%WHC水分条件时, 微生物生物量碳随硫铵浓度的升高而降低; 同浓度硫铵用量水平时, 微生物生物量碳的变化基本表现为: 60%WHC>80% WHC>40%WHC。分析发现不同水分和硫铵处理之间存在交互作用。BIOLOG分析显示: 不同氮素浓度和不同水分处理, 60%WHC下A1处理的平均吸光值(AWCD)和Shannon、Simpson、McIntosh指数最大, 其次为60%WHC的硫铵CK处理, 而不同水分下硫铵A2、A3处理, 其AWCD值和Shannon、Simpson、McIntosh多样性指数都较低, 进一步说明过量施肥导致微生物活性降低。不同氮素浓度和水分条件下土壤微生物和生化性状不同, 过量施用化肥后将有可能造成土壤微生物性状和生化功能衰减。

Nitrogen Uptake of Flue-Cured Tobacco in Typical Types of Soil in Southwest China

The aims of this study were to ascertain the difference of nitrogen uptake and verify the absorption law of the nitrogen from soil and fertilizer for tobacco grown in southwest China with red soil, yellow soil, and paddy soil by 15N tracer. The results showed a significant positive correlation between N accumulation amount and yield in flue-cured tobacco, and a suitable nitrogen demand of 60–100 kg ha-1 for tobacco in southwest zone. Absorption dynamics of nitrogen were different for flue-cured tobacco planted in various types of soil. For rate of nitrogen absorption before topping, the highest was for the tobacco planted in red soil, followed by that in paddy soil and the lowest was for that in yellow soil. N uptake peaks of tobacco planted in red soil, paddy soil and yellow soil were peaked in 7 weeks, 9 weeks, and 11 weeks after transplanting respectively. The absorption changes of soil nitrogen and fertilizer nitrogen in tobacco all showed a single peak curve, and the peak of the absorption curves of fertilizer nitrogen was earlier than soil nitrogen. Uptake rate of soil nitrogen in flue-cured tobacco was gradually higher than fertilizer nitrogen after starting prosperous growth and transferred to the stage of soil nitrogen-dominated absorption. The accumulation ratios of soil nitrogen and fertilizer nitrogen in tobacco before topping accounted 59.8% and 72.1% respectively. The proportions of N accumulation before topping were 87.8%, 47.3%, and 49.2% for tobacco grown in red soil, yellow soil and paddy soil respectively. In conclusion, the fertilizer N demand for tobacco should be estimated with N requirement and soil N supply before topping under proper N demand according to nitrogen absorption law.