The effects of co-utilizing green manure and rice straw on soil aggregates and soil carbon stability in a paddy soil in southern China

The co-utilization of green manure (GM) and rice straw (RS) in paddy fields has been widely applied as an effective practice in southern China.However,its effects on soil aggregate and soil organic carbon (SOC) stability remain unclear.In the present study,the effect of GM,RS,and co-utilization of GM and RS on particle size distribution of soil aggregates and SOC density fractions were measured in a field experiment.The experiment included six treatments,i.e.,winter fallow (WF) without RS return (Ctrl),WF with 50%RS return (1/2RS),WF with 100%RS return (RS),GM without RS return (GM),GM with 50%RS return (GM1/2RS) and GM with 100%RS return (GMRS).The results showed that the proportion of small macro-aggregates (0.25–2 mm) and the mean weight diameter (MWD) of aggregates in the GMRS treatment was greater (by 18.9 and 3.41%,respectively) than in the RS treatment,while the proportion of silt+clay particles (<0.053 mm) was lower (by 14.4%).The concentration of SOC in microaggregates (0.053–0.25 mm)and silt+clay particles was higher in the GMRS treatment than in GM and RS treatments individually.The concentration and proportion of free light organic carbon (fLOC) in aggregates of various particle sizes and bulk soil was greater in the GMRS treatment than the RS treatment,whereas the concentration and proportion of mineral-associated organic carbon in small macroaggregates,microaggregates,and bulk was lower in the GMRS treatment than in the RS treatment.The proportion of intra-aggregate particulate organic carbon (iPOC) was greater in the GMRS treatment than in GM treatment.The GMRS treatment had strong positive effects on iPOC in small macroaggregates,suggesting that SOC was transferred from fLOC to iPOC.In conclusion,co-utilizing green manure and rice straw cultivated the SOC pool by increasing the concentration of fLOC and improved soil carbon stability by promoting the sequestration of organic carbon in iPOC as a form of physical protection.

Research and Exploration on Ideological and Political Theories Teaching in the Course of Soil Science Experiment:Taking Pingdingshan University as an Example

Experimental course is an important link of ideological and political education in colleges and universities under the new situation,and plays an important role in the construction of ideological and political theories teaching in the course.Taking Pingdingshan University as an example,by introducing the real soil cases in Pingdingshan area,the ideological and political elements of the course are integrated into the teaching links of laboratory safety training,self-study before class,experimental teaching implementation,evaluation and improvement.By implementing the concept of"All-round Education"in experimental teaching,we are committed to realizing the comprehensive coverage of value shaping,knowledge inheritance and ability training,and explore and practice the reform of ideological and political theories teaching in the course of Soil Science Experiment.

Effects of Straw Covering and Different Types of Potassium Fertilizer on Salinity Accumulation in Surface Layer of Tobacco-planted Paddy Soil

[Objective] This study aimed to investigation the effects of straw covering and different types of potassium fertilizer on salinity accumulation in surface layer of tobacco-planted paddy soil in southern China. [Method] Tobacco variety ‘Yunyan87’ was used as the experimental material to investigate the effects of salinity accumulation in surface layer of tobacco-planted paddy soil on the growth and development of flue-cured tobacco using different types of potassium fertilizer and mulching cultivation methods. [Result] The results showed that K＋ , Ca2＋ , SO42and NO3-were the major salt ions in topsoil at different growth stages of flue-cured tobacco, Na ＋ and Mg2＋ contents were also relatively high at vigorous growth stage, indicating that these salt ions were easily accumulated in surface layer of soil; to be specific, the absolute increase of salt ion concentration showed a decreasing order of K＋ SO42- NO3-Ca2＋ Mg2＋ Na＋ Cl-, while the relative increase of salt ion concentration showed a decreasing order of Ca2＋ K＋ Na＋ NO3-SO42-Mg2＋ Cl-. At 60 d posttransplanting, total salt content in topsoil reached the minimum of 359.1 mg/kg in Treatment 2, total salt content in topsoil reached the maximum of 536.1 mg/kg in Treatment 5 （CK）, which was significantly higher than that in other treatments. At 90 d post-transplanting, no significant difference was observed in total salt content among various treatments. At harvesting period, total salt content in topsoil reached the maximum of 3 278.4 mg/kg in Treatment 1, which was significantly higher than that in other treatments. Topsoil pH showed no significant differences among various treatments at three different periods, ranging from 5.39 to 5.59. Straw covering could effectively reduce salt content in topsoil, accelerate vigorous growth of tobacco, shorten vigorous growth period and increase plant height, leaf number and lead area; at vigorous growth stage, root vitality and root volume of tobacco were improved, but the yield and output value were relatively low. Major agronomic traits and yield of tobacco showed no significant difference among various treatments. Output value of tobacco reached the maximum of 24 196.8 yuan/hm2 in Treatment 3, which was significantly higher than that in other treatments. [Conclusion] Appropriate types and proportions of potassium fertilizer and straw covering can effectively reduce the total salt content in tobacco-planted paddy soil and increase the effective supply amount of K＋ , Ca2＋ , SO42-and NO3-, thereby promoting and improving the root vitality of tobacco, which is conducive to the growth and development of tobacco and will eventually enhance the yield, quality and economic benefits of flue-cured tobacco.

Effects of Long-Term Winter Planted Green Manure on Physical Properties of Reddish Paddy Soil Under a Double-Rice Cropping System

Soil physical properties are important indicators of the potential for agricultural production.Our objective was to evaluate the effects of long-term inputs of green manures on physical properties of a reddish paddy soil（Fe-Typic Hapli-Stagnic Anthrosols） under a double cropping system.The common cropping pattern before the study was early-late rice-fallow（winter）.The field treatments included rice-rice-fallow（R-R-WF）,rice-rice-rape（R-R-RP）,rice-rice-Chinese milk vetch（RR-MV）,and rice-rice-ryegrass（R-R-RG）.The rape,Chinese milk vetch and ryegrass were all incorporated as green manures 15 d before early rice transplanting during the following year.The soil bulk density in all green manure treatments was significantly reduced compared with the winter fallow treatment.Soil porosity with green manure applications was significantly higher than that under the winter fallow.The green manure treatments had higher 0.25-5 mm water stable aggregates and aggregates stabilities in the plow layer（0-15 cm depth） compared with the fallow treatment.The mean weight diameter（MWD） and normalized mean weight diameter（NMWD） of aggregates in the green manure treatment were larger than that with the winter fallow.Soil given green manure retained both a higher water holding capacity in the plow layer soil,and a larger volume of moisture at all matric potentials（-10,-33 and-100 kPa）.We conclude that the management of double-rice fields in southern central China should be encouraged to use green manures along with chemical fertilizers to increase SOC content,improve soil physical properties and soil fertility.

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 aggregateassociated 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.

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 diagramdecomposition method; （ii） to measure K＋ adsorption isotherms before and after H202 oxidation of organic matter, and （iii） to investigate whether K＋ adsorption is correlated with changes in soil chemical and mineral properties. The 30-yr longterm fertilization treatments caused little change in soil organic C （SOC） but a large variation in soil mineral composition. The whole-clay fraction （〈5 Jam） corresponded more to the fertilization treatment than the fine-clay fraction （〈1 gin） in terms of percentage of illite peak area. The total percentage of vermiculite-chlorite peak area was significantly negatively correlated with the total percentage ofillite peak area in the 〈5 lam 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-0 and negative effects at higher K＋ concentration （240 mg L-l）. 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.

Soil Quality Assessment of Acid Sulfate Paddy Soils with Different Productivities in Guangdong Province,China

Land conversion is considered an effective measure to ensure national food security in China, but little information is available on the quality of low productivity soils, in particular those in acid sulfate soil regions. In our study, acid sulfate paddy soils were divided into soils with high, medium and low levels based on local rice productivity, and 60 soil samples were collected for analysis. Twenty soil variables including physical, chemical and biochemical properties were determined. Those variables that were significantly different between the high, medium and low productivity soils were selected for principal component analysis, and microbial biomass carbon （MBC）, total nitrogen （TN）, available silicon （ASi）, pH and available zinc （AZn） were retained in the minimum data set （MDS）. After scoring the MDS variables, they were integrated to calculate a soil quality index （SQI）, and the high, medium and low productivity paddy soils received mean SQI scores of 0.95, 0.83 and 0.60, respectively. Low productivity paddy soils showed worse soil quality, and a large discrepancy was observed between the low and high productivity paddy soils. Lower MBC, TN, ASi, pH and available K （AK） were considered as the primary limiting factors. Additionally, all the soil samples collected were rich in available P and AZn, but deficient in AK and ASi. The results suggest that soil AK and ASi deficiencies were the main limiting factors for all the studied acid sulfate paddy soil regions. The application of K and Si on a national basis and other sustainable management approaches are suggested to improve rice productivity, especially for low productivity paddy soils. Our results indicated that there is a large potential for increasing productivity and producing more cereals in acid sulfate paddy soil regions.

Effects of long-term green manure application on the content and structure of dissolved organic matter in red paddy soil

Dissolved organic matter(DOM) plays important roles in soil biogeochemistry activity and nutrients transportation in soils, but studies regarding the long-term effects of green manures on the content and structure of DOM in red paddy soil have not been reported yet. A long-term green manure experiment established in 1982 was utilized to test the DOM contents in different treatments, and the spectral characteristics of DOM were investigated by using ultraviolet-visible(UV-Vis) spectrometry and Fourier transform infrared(FTIR) spectrometry. The experiment included four cropping systems: ricerice-milk vetch(RRV), rice-rice-rape(RRP), rice-rice-ryegrass(RRG) and rice-rice-winter fallow(RRF), among them, milk vetch, rape, and ryegrass are popular winter green manure species in southern China. The results showed that the content of dissolved organic carbon(DOC), which is widely used to estimate the concentration of DOM, was significantly promoted after the incorporation of green manures compared with the other sampling stages. The contents of aromatic groups and the degree of humification of DOM increased in RRV and RRP, suggesting more complex compositions of the soil DOM after long-term application of milk vetch and rape. The contents of phenol, alcohol and carboxylic acid group at the mature stage of early rice were significantly higher than those at the stage of after green manures turned over, especially for the RRV treatment. The absorption ratio of FTIR indicated that winter plantation of rape increased the aromatic-C/aliphatic-C ratio, and ryegrass increased the aromatic-C/carboxyl-C ratio. In conclusion, long-term planting of milk vetch and rape as green manures increased the degree of aromaticity, humification and average molecular weight of DOM, and made the DOM more stable in red paddy soil.

Tillage Effect on Organic Carbon in a Purple Paddy Soil

The distribution and storage of soil organic carbon (SOC) based on a long-term experiment with various tillage systems were studied in a paddy soil derived from purple soil in Chongqing, China. Organic carbon storage in the 0-20 and 0-40 cm soil layers under different tillage systems were in an order: ridge tillage with rice-rape rotation (RT-rr) > conventional tillage with rice only (CT-r) > ridge tillage with rice only (RT-r) > conventional tillage with rice-rape rotation (CT-rr). The RT-rr system had significantly higher levels of soil organic carbon in the 0-40 cm topsoil, while the proportion of the total remaining organic carbon in the total soil organic carbon in the 0-10 cm layer was greatest in the RT-rr system. This was the reason why the RT-rr system enhanced soil organic carbon storage. These showed that tillage system type was crucial for carbon storage. Carbon levels in soil humus and crop-yield results showed that the RT-rr system enhanced soil fertility and crop productivity. Adoption of this tillage system would be beneficial both for environmental protection and economic development.

Short-Term Influence of Herbicide Quinclorac on Enzyme Activities in Flooded Paddy Soils

The influence of quinclorac (3,7-dichloroquinoline-8-carboxylic acid) on enzyme activities in flooded paddy soils was assessed under laboratory conditions. The enzymes differed markedly in their response to quinclorac. Quinclorac inhibited proteinase, hydrogen peroxidase, phosphorylase, and urease activities.The higher the concentration of quinclorac applied, the more significant the inhibition to these observed activities with a longer time required to recover to the level of the control. However, soils supplemented with quinclorac were nonpersistent for proteinase, phosphorylase and urease as opposed to soils without quinclorac. Dehydrogenase activity was also sensitive to quinclorac. Three soil samples with concentrations of quinclorac higher than 1 μg g-1 soil declined to less than 20% of that in the control. However, the highest dehydrogenase activity (up to 3.28-fold) was detected in soils with 2 μg g-1 soil quinclorac on the 25th day after treatment. Quinclorac had a relatively mild effect on saccharase activity at the concentrations used in this experiment and a stimulatory one on soil respiration when added to soil at normal field concentrations.Nonetheless it was inhibited at higher concentrations in paddy soils. Quinclorac is still relatively safe to the soil ecosystem when applied at a normal concentration (0.67 μg g-1 dried soil) but may have some effects on soil enzymes at higher concentrations.

Changes in Organic Carbon and Nutrient Contents of Highly Productive Paddy Soils in Yujiang County of Jiangxi Province,China and Their Environmental Application

Paddy field is an important land use in subtropical China. Development of high soil fertility and productivity is the management goal of paddy field, Fertilization and management practices have not only influenced the status of organic matter and nutrients in the soil but also affected the environmental quality. This article investigates the contents of organic carbon and the nutrients, and the change over the last 20 years in highly productive paddy soils and their environmental application. Field soils were sampled and the analytical results were compared with the corresponding values in the Second Soil Survey in Yujiang County of Jiangxi Province, China. The results showed that surface soils at a depth of 0-10 cm in highly productive paddy fields in Yujiang County of Jiangxi Province had contents of organic carbon （20.2 ±3.88） g kg^-1, total nitrogen （2.09±0.55） g kg^-1, and available phosphorus （42.7 ±32.7） mg kg^-1, respectively, which were all at very rich levels. Over the last 20 years, the organic carbon pool of the highly productive paddy soils reached a steady state. Total N and available P significantly increased, whereas available K changed a little. The amount and percentage of P immobilization in the surface soil （0-10 cm） of highly productive paddy fields were （142.7 ~ 41.1） mg kg-~ and （36.2~ 10.4）% of added P, and CEC （7.93 ~ 1.32） cmol kg-~. These two parameters were not higher than the mean values of paddy soils and upland red soils in the areas. Results also showed that fertilizer P in highly productive paddy soils had a high mobility and was prone to move toward a water body, which is the main source of nutrients causing eutrophication. Because of a weak K-fixing capacity, the available K content was not high in highly productive paddy soils. This suggests that attention should be paid to the K balance and the increase of soil K pool.

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-ryegrass （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 TFeHa （HCI-extractable total Fe）, Fe（Ⅱ）HCI （HCI-extractable Fe（Ⅱ） species） and Fe（Ⅲ）HCI （HCI- extractable Fe（Ⅲ） species） were measured. The correlations among those Fe species with selected soil environmental factors and the dynamic characteristics of Fe（Ⅱ）HCI accumulation were investigated. The results showed that TFeHc~ in RRF was significantly higher than those in the green manure treatments at most of the sampling stages. Fe（II）Ha increased rapidly after the incorporation of green manures in all treatments and kept rising with the growth of early rice. Fe（Ⅱ）Ha 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）Ha showed oppositely, and Fe（Ⅱ）HCI/Fe（Ⅲ）HCI performed similarly to Fe（Ⅱ）HCI The maximum accumulation potential of Fe（Ⅱ）HCI was significantly higher in RRF, while the highest maximum reaction rate of Fe（Ⅱ）Ha accumulation appeared in RRG. Significant correlations were found between the indexes of Fe（Ⅱ）HCI 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 TFeHa in red paddy soils, but promoted the ability of Fe（lll） reduction, especially the ryegrass; Fe（Ⅱ）Ha increased along with the growth of rice and was affected by soil conditions and environmental factors, especially the water and redox ability.

Influences of Quinclorac on Culturable Microorganisms and Soil Respiration in Flooded Paddy Soil

Objective To investigate the potential effects of herbicide quinclorac (3,7-dichloro-8-quinoline-carboxylic) on the culturable microorganisms in flooded paddy soil. Methods Total soil aerobic bacteria, actinomycetes and fungi were counted by a 10-fold serial dilution plate technique. Numbers of anaerobic fermentative bacteria (AFB), denitrifying bacteria (DNB) and hydrogen-producing acetogenic bacteria (HPAB) were numerated by three-tube anaerobic most-probable-number (MPN) methods with anaerobic liquid enrichment media. The number of methanogenic bacteria (MB) and nitrogen-fixing bacteria (NFB) was determined by the rolling tube method in triplicate. Soil respiration was monitored by a 102G-type gas chromatography with a stainless steel column filled with GDX-104 and a thermal conductivity detector. Results Quinclorac concentration was an important factor affecting the populations of various culturable microorganisms. There were some significant differences in the aerobic heterotrophic bacteria. AFB and DNB between soils were supplemented with quinclorac and non-quinclorac at the early stage of incubation, but none of them was persistent. The number of fungi and DNB was increased in soil samples treated by lower than 1.33μg·g-1 dried soil, while the CFU of fungi and HPAB was inhibited in soil samples treated by higher than 1.33μg·g-1 dried soil. The population of actinomycete declined in negative proportion to the concentrations of quinclorac applied after 4 days. However, application of quinclorac greatly stimulated the growth of AFB and NFB. MB was more sensitive to quinclorac than the others, and the three soil samples with concentrations higher than 1 μg·g-1 dried soil declined significantly to less than 40% of that in the control, but the number of samples with lower concentrations of quinclorac was nearly equal to that in the control at the end of experiments. Conclusion Quinclorac is safe to the soil microorganisms when applied at normal concentrations (0.67μg·g-1).

Adsorption and Desorption Characteristics of Cadmium and Lead in Typical Paddy Soils of Jiangxi Province and Its Environmental Risk Assessment

[Objective] This study aimed to investigate the adsorption and desorption characteristics of cadmium and lead in typical paddy soils of Jiangxi Province. [Method] Gleyed paddy soil and waterloggogenic paddy soil were collected from Jiangxi Province and used as experimental materials to investigate single and com- petitive adsorption and desorption behaviors of cadmium and lead by batch equilib- rium method. The environmental risk of the presence of cadmium and lead in paddy soils was assessed using distribution coefficients. [Result] Under equal ratio condi- tions, the adsorption capacity of lead by two types of paddy soils was higher than that of cadmium, and the adsorption rate in waterloggogenic paddy soil was higher than that in gleyed paddy soil. The desorption capacity of cadmium by two types of paddy soils was higher than that of lead, and the desorption rate in gleyed paddy soil was higher than that in waterloggogenic paddy soil. Under competitive condi- tions, the adsorption capacity of cadmium and lead by paddy soils was significantly reduced compared with single ion system, while the desorption rate was remarkably improved. The potential environmental risk of cadmium contamination was greater than that of lead in paddy soils. Moreover, environmental risks of cadmium and lead were reduced with the increase of pH, which increased significantly under the coex- istence state. [Conclusion] In the coexistence of cadmium and lead, cadmium con- tamination should be controlled and avoided compared with lead contamination in paddy soils.

Land Management Effects on Biogeochemical Functioning of Salt-Affected Paddy Soils

Most lowlands in Northeast Thailand(Isaan region)are cultivated with rice and large areas are affected by salinity, which drastically limits rice production.A field experiment was conducted during the 2003 rainy season to explore the interactions between salinity and land management in two fields representative of two farming practices:an intensively managed plot with organic inputs and efficient water management,and one without organic matter addition.Field measurements,including pH,Eh,electrical conductivity(EC),and soil solution chemistry,were performed at three depths, with a particular focus on Fe dynamics,inside and outside saline patches. High reducing conditions appeared after flooding particularly in plots receiving organic matter and reduction processes leading to oxide reduction and to the release of Fe and,to a lesser extend,Mn to the soil solution.Oxide reduction led to the consumption of H^+ and the more the Fe reduction was,the higher the pH was,up to 6.5.Formation of hydroxy-green rust were likely to be at the origin of the pH stabilization.In the absence of organic amendments,high salinity prevented the establishment of the reduction processes and pH value remained around 4.Even under high reduction conditions,the Fe concentrations in the soil solution were below commonly observed toxic values and the amended plot had better rice production yield.

Effect of Different Fertilizer Treatments on Quantity of Soil Microbes and Structure of Ammonium Oxidizing Bacterial Community in a Calcareous Purple Paddy Soil

The quantity of soil microbes and the structure of ammonium oxidizing bacterial （AOB） community were analyzed using the dilution plate counting and most probable number method （MPN）, and denaturing gradient gel electrophoresis （DGGE）, respectively. Fertilizer application tended to increase the number of soil microbes and alter the AOB community compared to the control with no fertilizer application （CK）. Among the eight fertilizer treatments, soil samples from the treatments of mineral fertilizers （e.g., N, P, K） in combination with farmyard manure （M） had greater number.s of soil microbes and more complex structure of AOB community than those receiving mineral fertilizers alone. The principal component analyses （PCA） for ammonium oxidizing bacterial community structure showed that the eight fertilizer treatments could be divided into two PCA groups （PCA1 and PCA2）. For the soil sampled after rice harvest, PCA1 included NP, NM, NPM and NPKM fertilizer treatments, while PCA2 was consisted of CK, N, M and NPK fertilizer treatments. For soil samples collected after wheat harvest, PCA1 was consisted of M, NM, NPM and NPKM fertilizer treatments, while PCA2 was composed of CK, N, NP and NPK fertilizer treatments. For a given rotation, the richness of AOB community in PCA1 was greater than that in PCA2. In addition, AOB community structure was more complex in the soil after rice harvest than that after wheat harvest. The results indicated that different fertilizer treatments resulted in substantial changes of soil microbe number and AOB community. Furthermore, mineral fertilizers （N, NP, NPK） combined with farmyard manure were effective for increasing the quantity of soil microbes, enriching AOB community, and improving the soil biofertility.

Modeling Methane Emission from Rice Paddy Soils:Ⅱ.Model Validation and Application

A simulation model developed by the authors (Huang et al., 1999) was validated against independent field measurements of methane emission from rice paddy soils in Texas of USA, Tuzu Of China and Vercelli of Italy.A simplified version of the simulation model was further validated against methane emission measurements from various regions of the world, including italy, China, Indonesia, Philippines and the United States. Model validation suggested that the seasonal variation of methane emission was mainly regulated by rice growth and development and that methane emission could be predicted from rice net productivity, cultivar character, soil texture and temperature, and organic matter amendments. Model simulations in general agreed with the observations. The comparison between computed and measured methane emission resulted in correlation coefficients r2 values from 0.450 to 0.952, significant at 0.01-0.001 probability level.On the basis of available information on rice cultivated area, growth duration, grain yield, soil texture and temperature, methane emission from rice paddy soils of China's Mainland was estimated for 28 rice cultivated provinces/municipal cities by employing the validated model. The calculated daily methane emission rates, on a provincial scale, ranged from 0.12 to 0.71 g m-2 with an average of 0.26 g m-2. A total amount of 7.92 Tg CH4 per year, ranging from 5.89 to 11.17 Tg year-1, was estimated to be released from Chinese rice paddy soils. Of the total, 45% was emitted from the single-rice growing season, and 19% and 36% were from the early-rice and the late-rice growing seasons, respectively. Approximately 70% of the total was emitted in the region located at latitude between 25°and 32°N. The emissions from rice fields in Sichuan and Hunan provinces were calculated to be 2.34 Tg year-1, accounting for approximately 30% of the total.

Influence of Biochar on Nitrogen Use Efficiency and Root Morphology of Rice-Seedling in Two Contrasting Paddy Soils

Biochar may affect the root morphology and nitrogen(N)use efficiency(NUE)of rice at seedling stage,which has not been clearly verified until now.To clarify it,we conducted a pot experiment regarding to two soil types(Hydragric Anthrosol and Haplic Acrisol),two biochar application rates(0.5 wt%and 1.5 wt%)and two rice varieties(common rice var.Xiushui134 and hybrid super rice var.Zhongkejiayou12-6)meanwhile.Seedling NUE of common rice Xiuhui134 was significantly increased(p<0.05)by 78.2%in Hydragric Anthrosol and by 91.4%in Haplic Acrisol following biochar addition with 1.5 wt%.However,biochar addition exerted no influence on seedling NUE of super rice Zhongkejiayou12-6 in both soils.Overall,0.09–0.10 units higher soil pH and 105–116%higher soil NH_(4)^(+)-N were observed in Xiushui134 growing two soils with 1.5 wt%biochar.In addition,improved root morphology(including longer root length,larger root surface area,bigger root volume,and more root tips)contributed to the higher seedling NUE of Xiushui134 in two soils.The soil pH and NH_(4)^(+)-N content,also the root morphology were influenced by biochar,which though could not thoroughly explained the NUE of Zhongkejiayou12-6.In conclusion,biochar application to paddy soil changed soil pH and NH_(4)^(+)-N content,root growth,and the consequent seedling NUE of rice,which effects are relative with rice cultivar,biochar addition rate,and soil type.

Modeling Methane Emission from Rice Paddy Soils:Ⅰ.Model Development

With an understanding of the processes of methane production, oxidation and emission, a semi-empirical model, focused on the contributions of rice plants to the processes and also the influence of environmental factors, was developed to predict methane emission from rice paddy soils. In the present model, the amount of methane transported from the soil to the atmosphere was determined by the rates of CH4 production and an emitted fraction. The rates of CH4 production in irrigated rice soils were computed from the availability of methanogenic substrates that are primarily derived from rice plaaes and added organic matter and the influence of soil texture, soil redox potential and temperature. The fraction of methane emitted was assumed to be modulated by the rice plants and declines with rice growth and development. TO make it applicable to a wider area with limited data sets, a simplified version of the model was also derived to predict methane emission in a more practical manner.

Bacterial Communities in a Buried Ancient Paddy Soil from the Neolithic Age

An ancient irrigated paddy soil from the Neolithic age was excavated at Chuodunshan Site in the Yangtze River Delta, close to Suzhou, China. The soil organic matter (SOM) content in the prehistoric rice soil is comparable to the average SOM content of present rice soils in this region, but it is about 5 times higher than that in the parent materials. As possible biomarkers to indicate the presence of the prehistoric paddy soil, the bacterial communities were investigated using the techniques of aerobic and anaerobic oligotrophic bacteria enumeration, Biolog analysis, and polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE). The results showed that in the buried soil layers, the prehistoric paddy soil had the largest number of aerobic and anaerobic oligotrophic bacteria, up to 6.12 and 5.86 log cfu g-1 dry soil, respectively. The prehistoric paddy soil displayed better carbon utilization potential and higher functional diversity compared to the parent materials and a prehistoric loess layer. The Shannon index and richness based on DGGE profiles of bacterial 16S rRNA genes were higher in prehistoric paddy soil than those in the prehistoric loess soil. It might be concluded that the prehistoric irrigated rice cultivation accumulated the SOM in plowed soil layer, and thus increased soil bacterial populations, metabolic activity, functional diversity and genetic diversity. Bacterial communities might be considered as the sensitive indicators of the presence of the prehistoric paddy soil in China's Yangtze River Delta.