Topsoil organic carbon mineralization and CO_2 evolution of three paddy soils from South China and the temperature dependence

Carbon mineralization and its response to climatic warming have been receiving global attention for the last decade. Although the virtual influence of temperature effect is still in great debate, little is known on the mineralization of organic carbon （SOC） of paddy soils of China under warming. SOC mineralization of three major types of China＇s paddy soils is studied through laboratory incubation for 114 d under soil moisture regime of 70% water holding capacity at 20℃ and 25℃ respectively. The carbon that mineralized as CO2 evolved was measured every day in the first 32 d and every two days in the following days. Carbon mineralized during the 114 d incubation ranged from 3.51 to 9.22 mg CO2-C/gC at 20℃ and from 4.24 to 11.35 mg CO2-C/gC at 25℃ respectively; and a mineralizable C pool in the range of 0.24 to 0.59 gC/kg, varying with different soils. The whole course of C mineralization in the 114 d incubation could be divided into three stages of varying rates, representing the three subpools of the total mineralizable C： very actively mineralized C at 1-23 d, actively tnineralized C at 24--74 d and a slowly mineralized pool with low and more or less stabilized C mineralization rate at 75-114 d. The calculated Q10 values ranged from 1.0 to 2.4, varying with the soil types and N status. Neither the total SOC pool nor the labile C pool could account for the total mineralization potential of the soils studied, despite a well correlation of labile C with the shortly and actively mineralized C, which were shown in sensitive response to soil warming. However, the portion of microbial C pool and the soil C/N ratio controlled the C mineralization and the temperature dependence. Therefore, C sequestration may not result in an increase of C mineralization proportionally. The relative control of C bioavailability and microbial metabolic activity on C mineralization with respect to stabilization of sequestered C in the paddy soils of China is to be further studied.

Organic carbon stratification and size distribution of three typical paddy soils from Taihu Lake region,China

Developing realistic soil carbon （C） sequestration strategies for China＇s sustainable agriculture relies on accurate estimates of the amount, retention and turnover rates of C stored in paddy soils. Available C estimates to date are predominantly for the tilled and flood-irrigated surface topsoil （ca. 30 cm）. Such estimates cannot be used to extrapolate to soil depths of 100 cm since soil organic carbon （SOC） generally shows a sharp decrease with depth. In this research, composite soil samples were collected at several depths to 100 cm from three representative paddy soils in the Taihu Lake region, China. Soil organic carbon distribution in the profiles and in aggregate-size fractions was determined. Results showed that while SOC decreased exponentially with depth to 100 cm, a substantial proportion of the total SOC （30%-40%） is stored below the 30 cm depth. In the carbon-enriched paddy topsoils, SOC was found to accumulate preferentially in the 2-0.25 and 0.25-0.02 mm aggregate size fractions. δ^13C analysis of the coarse micro-aggregate fraction showed that the high degree of C stratification in the paddy topsoil was in agreement with the occurrence of lighter δ^1313C in the upper 30 cm depth. These results suggest that SOC stratification within profiles varies with different pedogenetical types of paddy soils with regards to clay and iron oxyhydrates distributions. Sand-sized fractions of aggregates in paddy soil systems may play a very important role in carbon sequestration and turnover, dissimilar to other studied agricultural systems.

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.

Phospholipid fatty acid patterns of microbial communities in paddy soil under different fertilizer treatments

The microbial communities under irrigated rice cropping with different fertilizer treatments, including control （CK）, PK, NK, NP, NPK fertilization, were investigated using phospholipid fatty acid （PLFA） profile method. The results of this study revealed that the fertilizer practice had an impact on the community structure of specific microbial groups. The principal components analysis （PCA） showed that proportion of the actinomycete PLFAs （10Me 18：0 and 10Me 16：0） were the lowest in the PK treatment and the highest in the NPK treatment, which means that soil nitrogen status affected the diversity of actinomycetes, whereas nitrogen cycling was related to the actinomycets. Under CK treatment, the ratio of Gram-positive to Gram-negative bacteria was lower compared with that in fertilizer addition treatments, indicating that fertilizer application stimulated Gram-positive bacterial population in paddy soil. The fatty acid 18：2to6,9, which is considered to be predominantly of fungal origin, was at low level in all the treatments. The ratio of cyl9：0 to 18：1 to7, which has been proposed as an indicator of stress conditions, decreased in PK treatment. Changes of soil microbial community under different fertilizer treatments of paddy soil were detected in this study; however, the causes that lead to changes in the microbial community still needs further study.

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.

Adsorption and desorption of Cu(Ⅱ) and Pb(Ⅱ) in paddy soils cultivated for various years in the subtropical China

The adsorption and desorption of Cu（Ⅱ） and Pb（Ⅱ） on upland red soil,and paddy soils which were originated from the upland soil and cultivated for 8,15,35 and 85 years,were investigated using the batch method.The study showed that the organic matter content and cation exchange capacity （CEC） of the soils are important factors controlling the adsorption and desorption of Cu（Ⅱ） and Pb（Ⅱ）.The 15-Year paddy soil had the highest adsorption capacity for Pb（Ⅱ）,followed by the 35-Year paddy soil.Both the 35-Year paddy soil and 15-Year paddy soil adsorbed more Cu（Ⅱ） than the upland soil and other paddy soils.The 15-Year paddy soils exhibited the highest desorption percentage for both Cu（Ⅱ） and Pb（Ⅱ）.These results are consistent with the trend for the CEC of the soils tested.The high soil CEC contributes not only to the adsorption of Cu（Ⅱ） and Pb（Ⅱ） but also to the electrostatic adsorption of the two heavy metals by the soils.Lower desorption percentages for Cu（Ⅱ） （36.7% to 42.2%） and Pb（Ⅱ） （50.4% to 57.9%） were observed for the 85-Year paddy soil.The highest content of organic matter in the soil was responsible for the low desorption percentages for the two metals because the formation of the complexes between the organic matter and the metals could increase the stability of the heavy metals in the soils.

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.

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.

Nitrous oxide emission and reduction in a laboratory-incubated paddy soil response to pretreatment of water regime

A laboratory incubation experiment was conducted to investigate nitrous oxide(N 2O) emission and reduction in a paddy soil(Stagnic Anthrosol) response to the pretreatment of water regime. The paddy soil was maintained under either air dried(sample D) or submerged(sample F) conditions for 110 d before the soil was adjusted into soil moisture of 20%, 40%, 60%, 80% and 100% water holding capacity(WHC) respectively, and then incubated with or without 10%(v/v) acetylene for 138 h at 25℃. At lower soil water content (≤60% WHC), N 2O emission from the sample F was 2 29 times higher than that from the sample D( P <0 01). While, N 2O emission from the sample F was only 29 and 14 percent of that from the sample D at the soil moisture of 80% and 100% WHC, respectively( P <0 01). The maximal N 2O emissions observed at soil moisture of 80% WHC were about 24 and 186 times higher than the minima obtained at the soil moisture of 20% WHC for the sample F and D, respectively. But at the soil moisture of 80% and 100% WHC, N 2O emission from the sample F with acetylene(F+ACE) was comparable to that of the sample D with acetylene (D+ACE). The results showed that the F sample produced N 2O ability in denitrification was similar to the sample D, however, the sample F was in the better reduction of N 2O to N 2 than the sample D even after the soil moisture was adjusted into the same level of 80% or 100% WHC. Therefore, the pretreatment of water regime influenced the strength and product composition of denitrification and N 2O emission from the paddy soil.

Effects of free iron oxyhydrates and soil organic matter on copper sorption-desorption behavior by size fractions of aggregates from two paddy soils

Effects of free iron oxyhydrates （Fed） and soil organic matter （SOM） on copper （Cu^2＋） sorption-desorption behavior by size fractions of aggregates from two typical paddy soils （Ferric-Accumulic Stagnic Anthrosol （Soil H） and Gleyic Stagnic Anthrosol （Soil W）） were investigated with and without treatments of dithionite-citrate-bicarbonate and of H2O2. The size fractions of aggregates were obtained from the undisturbed bulk topsoil using a low energy ultrasonic dispersion procedure. Experiments of equilibrium sorption and subsequent desorption were conducted at soil water ratio of 1：20, 25℃. For Soil H, Cu^2＋ sorption capacity of the DCB-treated size fractions was decreased by 5.9% for fine sand fraction, by 40.4% for coarse sand fraction, in comparison to 2.9% for the bnlk sample. However, Cu^2＋ sorption capacities of the H2O2-treated fractions were decreased by over 80% for the coarse sand fraction and by 15% for the clay-sized fraction in comparison to 88% for bulk soil. For Soil W, Cu^2＋ sorption capacity of the DCB-treated size fraction was decreased by 30% for the coarse sand fraction and by over 75% for silt sand fraction in comparison to 44.5% for the bulk sample. Cu^2＋ sorption capacities of the H2O2-treated fractions were decreased by only 2.0% for the coarse sand fraction and by 15% for the fine sand fraction in comparison to by 3.4% for bulk soil. However, Cu^2＋ desorption rates were increased much in H2O2-treated samples by over 80% except the clay-sized fraction （only 9.5%） for Soil H. While removal of SOM with H2O2 tendend to increase the desorption rate, DCB- and H2O2-treatments caused decrease in Cu^2＋ retention capacity of size fractions, Particularly, there hardly remained Cu^2＋ retention capacity by size fractions from Soil H after H2O2 treatment except for clay-sized fraction. These findings supported again the dominance of the coarse sand fraction in sorption of metals and the preference of absorbed metals bound to SOM in differently stabilized status among the size fractions. Thus, enrichment and turnover of SOM in paddy soils may have great effects on metal retention and chemical mobility in paddy soils.

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.

Effect of microbial mediated iron plaque reduction on arsenic mobility in paddy soil

The potential of microbial mediated iron plaque reduction, and associated arsenic （As） mobility were examined by iron reducing bacteria enriched from As contaminated paddy soil. To our knowledge, this is the first time to report the impact of microbial iron plaque reduction on As mobility. Iron reduction occurred during the inoculation of iron reducing enrichment culture in the treatments with iron plaque and ferrihydrite as the electron acceptors, respectively. The Fe（II） concentration with the treatment of anthraquinone-2, 6-disulfonic acid （AQDS） and iron reducing bacteria increased much faster than the control. Arsenic released from iron plaque with the iron reduction, and a significant correlation between Fe（II） and total As in culture was observed. However, compared with control, the increasing rate of As was inhibited by iron reducing bacteria especially in the presence of AQDS. In addition, the concentrations of As（III） and As（V） in abiotic treatments were higher than those in the biotic treatments at day 30. These results indicated that both microbial and chemical reductions of iron plaque caused As release from iron plaque to aqueous phase, however, microbial iron reduction induced the formation of more crystalline iron minerals, leading to As sequestration. In addition, the presence of AQDS in solution can accelerate the iron reduction, the As release from iron plaque and subsequently the As retention in the crystalline iron mineral. Thus, our results suggested that it is possible to remediate As contaminated soils by utilizing iron reducing bacteria and AQDS.

Differences in Organic C Mineralization Between Aerobic and Submerged Conditions in Paddy Soils of Southern Jiangsu Province,China

Moisture regime plays a crucial role in the mineralization of soil organic carbon （SOC）. In this paper, the dynamics of SOC mineralization in typical paddy soils of Changshu, Jiangsu Province, China, was investigated by incubation test in laboratory. The differences in SOC mineralization under aerobic and submerged conditions of paddy soils were also studied. Results showed that the daily mineralization of SOC under different moisture regimes was significantly different in the whole incubation period, at the beginning of the incubation, it decreased quickly under aerobic condition, but increased rapidly under submerged condition, and both remained constant after 10 d of incubation. The differences in SOC mineralization were found to be mainly at the beginning period of the incubation and decreased along with the incubation time. Thus, the difference was not significantly different at the later incubation period. The respiration intensity, daily and cumulative mineralization of SOC under aerobic condition was 2.26-19.11, 0.96-2.41, and 0.96-2.41 times than those .under submerged condition, respectively. Statistic analyses showed that the higher the contents of microbial biomass carbon and nitrogen, the more significant difference in respiration intensity between aerobic and submerged conditions, but the higher the contents of microbial biomass nitrogen and dissolved organic carbon, the more significant difference in daily mineralization of SOC between the two conditions. The decrease in soil microbial activity under submerged condition was the main reason leading to the decrease in respiration intensity, but the decrease in SOC mineralization was also correlated with the changes in dissolved organic carbon over the whole incubation period.

Effects of Root Penetration Restriction on Growth and Mn Nutrition of Different Winter Wheat Genotypes in Paddy Soils

Effects of root penetration restriction on the growth and Mn nutrition of different wheat genotypes were studied in paddy soils using a method of nylon net bags (400 mesh) buried in the soil. The results showed that the spatial distribution of Mn in paddy soils played an important role in the growth and Mn nutrition of wheat crops. In the treatment where wheat roots were restricted in the plough layer by nylon net bags, the symptoms of Mn deficiency in wheat occurred much earlier and more seriously than usual. Of the two tested wheat genotypes, 80-8 was tolerant to Mn deficiency while 3295 was sensitive to Mn deficiency, respectively. The restriction of root penetration intensified symptoms of Mn deficiency of the Mn-deficient sensitive genotype (3295). The experiment demonstrated that well-developed roots with a strong ability to penetrate into the Mn-rich deep soil layer might explain the better tolerance of Mn deficiency in the tolerant genotypes.

Influence of Exogenous Lanthanum on Fertility Parameters of Red Soil and Paddy Soil

The effects of exogenous La on the fertility parameters such as cation exchange capacity ( CEC), exchangeable basic cations, and exchangeable acidity in red soil and paddy soil were studied with soil column simulation. The results show that with increasing amount of the added La, the proportion of exchangeable La in soils increases and there is more exchangeable La in red soil than in paddy soil. When the concentration of La is more than 600 mg(.)kg(-1) the proportion of exchangeable La almost remains constant. When the concentration of La is less than 1200 mg(.)kg(-1) there is no significant effect on CEC in red soil. But when the concentration of La is more than 1200 mg(.)kg(-1,) it has significant effect on CEC in paddy soil. The application of La resulted in increasing exchangeable aluminum, Ca and Mg in soil solution, and decreasing exchangeable Ca and Mg retained in soils. But when the concentration of La is less than 150 mg(.)kg(-1), it has no significant influence on CEC, exchangeable Ca and Mg, and exchangeable acidity in red soil and paddy soil.

Aromatic compound degradation by iron reducing bacteria isolated from irrigated tropical paddy soils

Forty-six candidate phenol/benzoate degrading-iron reducing bacteria were isolated from long term irrigated tropical paddy soils by enrichment procedures.Pure cultures and some prepared mixed cultures were examined for ferric oxide reduction and phenol/benzoate degradation.All the isolates were iron reducers,but only 56.5%could couple iron reduction to phenol and/or benzoate degradation,as evidenced by depletion of phenol and benzoate after one week incubation.Analysis of degradative capability using Biolog...

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.

Effect of Intensive Inorganic Fertilizer Application on Microbial Properties in a Paddy Soil of Subtropical China

A field experiment with rice-rice rotation was conducted since 2002 in southeast China for investigating the response of soil microbial properties to intensive nitrogen fertilizer application. The tested soil was a subtropical paddy soil derived from Quaternary red clay. Differences between treatments existed in different application rates of urea when the experiment was designed. Urea was applied in five rates, i.e., 0, 0.5, 1, 1.5, and 2 U, equivalent to 0, 0.5, 1, 1.5, and 2 times the local average amount of urea application （900 kg urea ha-~ yr-~, equivalent to 414 kg N ha-1 yr-~）. In 2007, soil total nitrogen, available nitrogen, and soil organic carbon contents were increased by 10.2-27.9, 8.0-16.0, and 10.2-30.6%, respectively, in treatments with urea application rates of 0.5 to 2 U compared to control （0 U）. Microbial biomass carbon and nitrogen were also increased by 3.1-30.8 and 1.3-13.9%, respectively, in treatments with urea application. Basal respiration in treatments with urea input were 9.4-29.1% higher than that in control. However, changes of bacterial functional diversity had different trends. Urea fertilization enhanced bacterial functional diversity until treatment of 1 U, but re-decreased it from treatment of 1.5 U. Principal components analysis indicated that there were intimate relationships among soil organic matter, nitrogen nutrient, microbial biomass, and respiration. Nevertheless, microbial diversity was related to soil moisture contents after urea application. We conclude here that the application of N fertilizer improved soil microbial biomass and respiratory activity. But, microbial diversity was reduced when excessive urea was applied in the tested paddy soil.

Adsorption characteristic of bensulfuron-methyl at variable added Pb^(2+) concentrations on paddy soils

The combined pollution of heavy metal Pb^2＋ and bensulfuron-methyl （BSM）, originating from chemical herbicides, in agroecological environments has become commonplace in southern China. The adsorption of BSM on three paddy soils in the presence of Pb^2＋ was examined using high-performance liquid chromatograph （HPLC）. Results indicated that adsorption of BSM could accurately be described by a Freundlich isotherm equation with correlation constant （R） 〉 0.98, irrespective of the presence of spiked Pb^2＋. Of the various factors influencing BSM sorption, soil pH appeared to be the most influential. The constant Kf of Freundlich isotherm equation tended to increase with increasing Pb^2＋ concentration in soil which indicated that the spiked of Pb^2＋ in paddy soils would promote the sorption of BSM. AGo of BSM in three paddy soils was less than 40 kJ/mol in all treatments, indicating the adsorption of BSM is mainly physical in nature. The elution of soil dissolved organic matter （DOM） enhanced the adsorption of BSM in paddy soils. The mechanisms involved in the promotion effects of the spiked Pb^2＋ on BSM adsorption might be the modified surface characteristics of paddy soil solids due to the soil acidification and the increase of soil organic matter concentration because of DOM binding.