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.

Substitution of chemical fertilizer by Chinese milk vetch improves the sustainability of yield and accumulation of soil organic carbon in a double-rice cropping system

The double-rice cropping system is a very important intensive cropping system for food security in China. There have been few studies of the sustainability of yield and accumulation of soil organic carbon (SOC) in the double-rice cropping system following a partial substitution of chemical fertilizer by Chinese milk vetch (Mv). We conducted a 10-year (2008–2017) field experiment in Nan County, South-Central China, to examine the double-rice productivity and SOC accumulation in a paddy soil in response to different fertilization levels and Mv application (22.5 Mg ha^–1). Fertilizer and Mv were applied both individually and in combination (sole chemical fertilizers, Mv plus 100, 80, 60, 40, and 0% of the recommended dose of chemical fertilizers, labeled as F100, MF100, MF80, MF60, MF40, and MF0, respectively). It was found that the grain yields of double-rice crop in treatments receiving Mv were reduced when the dose of chemical fertilizer was reduced, while the change in SOC stock displayed a double peak curve. The MF100 produced the highest double-rice yield and SOC stock, with the value higher by 13.5 and 26.8% than that in the F100. However, the grain yields increased in the MF80 (by 8.4% compared to the F100), while the SOC stock only increased by 8.4%. Analogous to the change of grain yield, the sustainable yield index (SYI) of double rice were improved significantly in the MF100 and MF80 compared to the F100, while there was a slight increase in the MF60 and MF40. After a certain amount of Mv input (22.5 Mg ha^–1), the carbon sequestration rate was affected by the nutrient input due to the stimulation of microbial biomass. Compared with the MF0, the MF100 and MF40 resulted in a dramatically higher carbon sequestration rate (with the value higher by 71.6 and 70.1%), whereas the MF80 induced a lower carbon sequestration rate with the value lower by 70.1% compared to the MF0. Based on the above results we suggested that Mv could partially replace chemical fertilizers (e.g., 40–60%) to improve or maintain the productivity and sustainability of the double-rice cropping system in South-Central China.

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.

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.

Effect of Long-Term Rice Straw Return on Soil Glomalin, Carbon and Nitrogen

A long-term experiment was conducted to investigate how long-term fertilization and rice straw incorporation into soil affect soil glomalin, C and N. The combined application of chemical fertilizer and straw resulted in a significant increase in both soil easily extractable glomalin (EEG) and total glomalin (TG) concentrations, as compared with application of only chemical fertilizer or no fertilizer application. The EEG and TG concentrations of the NPKS (nitrogen, phosphorus, and potassium fertilizer application + rice straw return) plot were 4.68% and 5.67% higher than those of the CK (unfertilized control) plot, and 9.87% and 6.23% higher than those of the NPK (nitrogen, phosphorus, and potassium fertilizer applied annually) plot, respectively. Application of only chemical fertilizer did not cause a statistically significant change of soil glomalin compared with no fertilizer application. The changes of soil organic C (SOC) and total N (TN) contents demonstrated a similar trend to soil glomalin in these plots. The SOC and TN contents of NPKS plot were 15.01% and 9.18% higher than those of the CK plot, and 8.85% and 14.76% higher than those of the NPK plot, respectively. Rice straw return also enhanced the contents of microbial biomass C (MBC) and microbial biomass N (MBN) in the NPKS plot by 7.76% for MBC and 31.42% for MBN compared with the CK plot, and 12.66% for MBC and 15.07% for MBN compared with the NPK plots, respectively. Application of only chemical fertilizer, however, increased MBN concentration, but decreased MBC concentration in soil.

Effect of Sprinkler and Border Irrigation on Topsoil Structure in Winter Wheat Field

A two-year experiment was carried out on the effect of sprinkler irrigation on the topsoil structure in a winter wheat field. A border-irrigated field was used as the control group. The total soil porosity, pore size distribution, pore shape distribution, soil cracks and soil compaction were measured. The sprinkler irrigation brought significant changes to the total soil porosity, capillary porosity, air-filled porosity and pore shape of topsoil layers in comparison with the border irrigation. The total porosity and air-filled porosity of the topsoil in the sprinkler irrigation were higher than those in the border irrigation. The changes in the air-filled and elongated pores were the main reasons for the changes in total porosity. The porosities of round and irregular pores in topsoil under sprinkler irrigation were lower than those under border irrigation. Sprinkler irrigation produced smaller soil cracks than border irrigation did, so sprinkler irrigation may restrain the development of macropore flow in comparison with border irrigation. The topsoil was looser under sprinkler irrigation than under border irrigation. According to the conditions of topsoil structure, it is preferable for crops to grow under sprinkler irrigation than under border irrigation.

Analysis of Heavy Metal Sources for Vegetable Soils from Shandong Province, China

Heavy metal pollution in agricultural soils has serious negative influence on human health. Concentrations of Cd, Hg, As, Pb, Cr, Cu, Zn, and Ni in top soils （0-20 cm） of greenhouses and farmlands from four main vegetable production areas Shouguang, Laiyang, Jinxiang, and Zhangqiu in Shandong Province, one of the most rapidly developing regions in China, were measured in this study. Shouguang is mainly occupied by greenhouse vegetables and the other three areas are mainly open field culture. Total of 149 soil samples were collected. The average concentrations of the eight heavy metals of the tested 149 soil samples were all below the threshold values according to ＂Farmland environmental quality evaluation standards for edible agricultural products （HJ332-2006）＂ of China. However, most of the studied heavy metals were present at higher concentrations than those of the natural background levels in local agricultural soils. Among the total 149 soil samples, 22 samples were contaminated by Cd, Ni, Cu, or Hg. Comparisons showed that the main pollution element in greenhouse vegetable soils was Cd, while that of open field vegetable soils was Cu. The results of principal components analysis （PCA） suggested that concentrations of Cr, As, and Ni were mainly controlled by parent rocks; Hg and Pb were affected by anthropogenic activities such as vehicle and industrial fumes and waste water irrigation. Meanwhile, concentrations of Cd, Cu, and Zn were affected mainly by the use of agrochemicals. Most of the heavy metals were positively correlated with each other in concentration. Appropriate measures should be taken to effectively control heavy metal levels in vegetable soils and thus protect human health.

Effect of N and K Fertilizers on Yield and Quality of Greenhouse Vegetable Crops

The application of large amounts of fertilizers, a conventional practice in northern China for the production of vegetable crops, generally leads to substantial accumulation of soil nutrients within a relatively short period of time. A fixed field experiment was designed to study the effects of nitrogen (N) and potassium (K) fertilizers applied to optimize the yield and quality of typical vegetable crops. Application of N and K fertilizers significantly increased the yields of kidney bean. The largest yields were obtained in the first and second years after application of 1500 kg N and 300 kg K2O ha-1. In the third year, however, there was a general decline in yields. Maximum yields occurred when intermediate rates of N and K (750 kg N and 300 kg K2O ha-1) were applied. However, no significant differences were observed in the concentrations of vitamin C (VC) in kidney bean among different years and various rates of fertilizer treatments. Yields of tomato grown in rotation after kidney bean showed significant responses to the application of N and K in the first year. In the second year, the yields of tomato were much lower. This suggested that the application of N fertilizer did not have any effect upon tomato yield, whereas application of K fertilizer did increase the yield. Application of K fertilizer was often associated with increased sugar concentrations.

Scenario Analysis of Tillage, Residue and Fertilization Management Effects on Soil Organic Carbon Dynamics

Based on data from 10-year field experiments on residue/fertilizer management in the dryland farming region of northern China, Century model was used to simulate the site-specific ecosystem dynamics through adjustment of the model’s parameters, and the applicability of the model to propose soil organic carbon (SOC) management temporally and spatially, in cases such as of tillage/residue/fertilization management options, was identified via scenario analysis. Results between simulations and actual measurements were in close agreement when appropriate applications of stover, manure and inorganic fertilizer were combined. Simulations of extreme C/N ratios with added organic materials tended to underestimate the measured effects. Scenarios of changed tillage methods, residue practices and fertilization options showed potential to maintain and enhance SOC in the long run, while increasing inorganic N slowed down the SOC turnover rate but did not create a net C sink without any organic C input. The Century model simulation showed a good relationship between annual C inputs to the soil and the rate of C sequestration in the top 20 cm layer and provided quantitative estimations of changes in parameters crucial for sustainable land use and management. Conservation tillage practices for sustainable land use should be integrated with residue management and appreciable organic and inorganic fertilizer application, adapted according to the local residue resource, soil fertility and production conditions. At least 50% residue return into the soil was needed annually for maintenance of SOC balance, and manure amendment was important for enhancement of SOC in small crop-livestock systems in which crop residue land application was limited.

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.

Physiological Basis of Photosynthetic Function and Senescence of Rice Leaves as Regulated by Controlled-Release Nitrogen Fertilizer

The physiological mechanism of photosynthetic function and senescence of rice leaves was studied by using early rice variety Baliangyou 100 and late rice variety Weiyou 46, treated with controlled-release nitrogen fertilizer （CRNF）, urea and no nitrogen fertilizer. CRNF showed obvious effects on delaying the senescence and prolonging photosynthetic function duration of rice leaves. Compared with urea, CRNF could significantly increase the chlorophyll content of functional leaves in both early and late rice varieties, and this difference between the treatments became larger as rice growth progressed; CRNF increased the activities of active oxygen scavenging enzymes super oxide dismutase （SOD） and peroxidase （POD）, and decreased the accumulation amount of malondialdehyde （MDA） in functional leaves during leaf aging; Photosynthetic rate of functional leaves in CRNF treatment was significantly higher than that in urea treatment. The result also indicated that CRNF could effectively regulate the contents of indole-3-acetic acid （IAA） and abscisic acid （ABA） in functional leaves; IAA content was higher and ABA content was lower in CRNF treatment than those in urea treatment. Therefore, application of CRNF could increase the rice yield significantly due to these physiological changes in the functional leaves.

Preparation and Testing of Cementing and Coating Nano-Subnanocomposites of Slow/Controlled-Release Fertilizer

To select the cementing and coating materials of slow/controlled-release fertilizer, the natural kaoline and abandoned foam plastics were used to prepare nano-subnanocomposites through the methods of organic material intercalation, semiemulsification, and cut at high velocity techniques. Besides, two kinds of nano-subnanocomposites were tested in this study using several instruments, including the scanning electron microscope （SEM）, the X-ray diffraction （XRD）, the infrared ray spectrum （IR）, and the laser granularity. The main results were as follows： 1） The organic material was intercalated in the layers of kaoline clays, and the natural kaoline exfoliated into nanometer-sized layers. The organic agent and clays formed nanocomposites through hydrogen bond combination. 2） The SEM pictures of polystyrenestarch nano-subnanocomposites showed that many pores and rugas were present on the surface of film at sizes ranging from 10 to 20 nm. These nano-subnanocomposites were used as the cementing and coating materials of slow/controlledrelease fertilizer.

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.

The impact of nitrogen amendment and crop growth on dissolved organic carbon in soil solution

Dissolved organic carbon(DOC) is an important component of the terrestrial carbon cycle.However,the sources and controlling factors of DOC in soils remain uncertain.In this study,the effects of nitrogen(N) amendment and crop growth on DOC in soil solution were examined at a maize-wheat rotated field located in the central Sichuan Basin in southwestern China.Nitrogen treatments in this study included 150 kg N ha-1 season-1,200 kg N ha-1 season-1 and the control without any fertilizer application.During the whole experimental period,we observed significant decreases(p<0.05) in DOC concentrations in the sampled soil solutions associated with increase in N inputs at the bare soil plots,but no change in DOC at the plots with crop growth.The estimated average contributions of plantderived DOC were 16%,24% and 32% of total DOC in the summer maize season and 21%,32% and 38% in the winter wheat season along with the gradient of N fertilizer application rates.The results implied thatthe crop growth could play a key role in the soil DOC production,and the N input enhanced DOC production by increasing crop growth.The relationship between the DOC concentrations and the crop root biomass was statistically significant for both the maize and winter wheat seasons.Our observations indicated that crop growth exerted greater influence on the seasonal variability of DOC concentration in soil solutions at the experimental site,which overwhelmed the effect of soil native organic matter decomposition on DOC concentrations in soil solutions.

Effect of Organic Manure,Passivator and Their Complex on the Bioavailability of Soil Cd

A field experiment of organic manure, passivator and their complex was conducted to study the soil Cd bioavailability in the Cd polluted paddy soil. The results showed that the rice yield increased with the applications of organic manure, passivator and their complex, especially, the rice yields of applying 3 000 and 6 000 kg/hm^2 of organic manure increased significantly by 18.6% （ P 〈0.05） and 20.9% （P 〈0.05） because of the increase of rice economic coefficient. There had no significant change of the soil pH values and the soil available Cd contents, but the Cd distribution ratios in rice were lowered by applying organic manure, and the Cd contents of rice applying 3 000 and 6 005 kg/hm^2 of organic manure dropped by 14.3% （ P 〉 0.05） and 21.4% （ P 〉 0.05） compared with chemical fertilizer treatment, respectively. But applying with passivator and passivator fertilized with 3 000 and 6 000 kg/hm^2 of organic manure, the soil pH values increased and the soil available Cd contents decreased significantly, so that to restrain the rice＇s Cd absorption and accumulation. The rice Cd contents lowered by 28.6% （ P 〈 0.05）, 28.6% （ P 〈 0.05） and 42.9% （ P 〈 0.05）, the stem C,d contents lowered by 8.9% （ P 〉 0.05）, 29.7% （ P 〈 0.05） and 43.6% （ P 〈 0.05）, and the leaf Cd contents decreased by 18.8% （ P 〈 0.05）, 25.0% （ P 〈 0.05） and 25.0% （ P 〈 0.05）, respectively. It enhanced the inactivate effect of passivator significantly when fertilized with organic manure.

Function of controlled release fertilizer in maintaining high agronomic use efficiency and minimizing environmental pollution in rice field of southern China

Development and use of controlled release fertilizer (CRF) in southern China, potential advantages of CRF in increasing rice yield and nutrient use efficiency were introduced, as well as its role of minimizing rice field’s environmental contamination was discussed. Meanwhile, some suggestions were proposed.

Integrated application of February Orchid(Orychophragmus violaceus) as green manure with chemical fertilizer for improving grain yield and reducing nitrogen losses in spring maize system in northern China

The development of more efficient management systems is crucial to achieving high grain yields with high nitrogen use efficiency（NUE）. February Orchid-spring maize rotation system is a newly established planting system with the benefits of ground cover and potential wind erosion in northern China. A field experiment was conducted to evaluate the effects of integrated application of February Orchid as green manure with reduction of chemical fertilizers（INTEGRATED） on spring maize yield, N uptake, ammonium volatilization, and soil residual mineral N in northern China. Compared to farmers＇ traditional fertilization（CON）, integrated application of February Orchid as green manure with 30% reduction of nitrogen fertilizers（INTEGRATED） increased maize grain yield and biomass by 9.9 and 10.2%, respectively. The 0–100 cm soil residual Nmin at harvest was decreased by 58.5% and thus nitrogen use efficiency was increased significantly by 26.7%. The nitrogen balance calculation further demonstrated that the INTEGRATED approach performed better than CON with lower apparent nitrogen loss（decreased by 48.9%） which evidenced by the ammonium volatilization of top-dressing fertilizer was decreased by 31.1%, the N_（min） movement to the deeper soil layers was reduced, and the apparent nitrogen leaching loss nearly equal to 0 under the INTEGRATED treatment. Therefore, in northern China, integrated application of green manure and chemical fertilizers is an efficient management approach for improving maize yields and NUE simultaneously.

Effects of Corn Stover Incorporated in Dry Farmland on Soil Fertility

Seven years' field experiments on application of corn stover and/or cattle manure combined with chemical fertilizers were carried out in Shouyang Dryland Farming Experimental Station. Results showed that the increased available N in the plough layer was mainly influenced by the application of cattle manure; the available P was mainly influenced by the application of chemical fertilizer; the available K was mainly influenced by the incorporation of corn stover. The organic matter contents in the soils treated with corn stover or cattle manure were kept in balance under the experimental conditions. Corn yield and water use efficiency were influenced significantly not only by fertilizer N but also by incorporated corn stover. The results suggested that the highest N uptake, yield and water use efficiency could be obtained at rates of 105 kg fertilizer N, 6000 kg corn stover, and 1500 kg cattle manure per hectare. The experiments supplied information on nutrient recycling and use of corn stover as sources of fodder and organic fertilizer for balancing application of organic and inorganic fertilizer, improving soil fertility and increasing crop yield with incorporation of corn stover in soil.

Effect of rice-rice-rape rotation on physicochemical property and bacterial community of rhizosphere soil

Through collecting rhizosphere soil sample from a 30-year long-term fixed location test site that use“rice-ricerape”crop rotation(RRR)and“rice-rice-fallow”continuous cropping systems(RRF),this paper investigated effects of long-term crop rotation on physicochemical property and bacterial community of rhizosphere soil.Results showed that total nitrogen(TN),total phosphorus(TP)and available potassium(AK)contents in rhizosphere soil under long-term RRR were decreased by 28.09%,15.69%and 6.25%respectively.Alkali-hydrolyzable nitrogen(AN)and available phosphorus(AP)contents were 10.59%and 13.25%higher than those of soil in RRF respectively.Three soil samples collected during different periods also showed that RRR resulted in a lower rhizosphere soil pH than RRF.Clone library analysis revealed that significant difference in rhizosphere soil bacterial community was observed between RRR and RRF continuous cropping.Abundance ofα-Proteobacteria,β-Proteobacteria andγ-Proteobacteria were higher in rhizosphere soil of RRR compared to RRF.pH of rhizosphere soil was significantly correlated with Acidobacteria level,while total organic carbon(TOC)content was significantly correlated with Proteobacteria level.Long-term RRR enhanced conversion of N and P in rhizosphere soil,increased bio-availability to crop,and promoted diversity of soil bacterial community.Bacterial diversity in RRR could be ecological significance in maintaining soil fertility and functionality.

^(15)N Isotope Techniques for Estimating Effects of Urea-N Fertilizer Application Rate on Yields and Nutrient Contents of Pakchoi Cabbage and Asparagus Lettuce and Nitrogen Utilization Efficiency

A pot experiment combined with15N isotope techniques was conducted to evaluate effects of the varying rates of urea-N fertilizer application on yields,quality,and nitrogen use efficiency (NUE) of pakchoi cabbage (Brassica chinensis L.) and asparagus lettuce (Lactuca saiva L.).15N-labbled urea (5.3515N atom %) was added to pots with 6.5 kg soil of 0.14,0.18,0.21,0.25,and 0.29 g N/kg soil,and applied in two splits:60 percent as basal dressing in the mixture and 40 percent as topdressing.The fresh yields of two vegetable species increased with the increasing input of urea-N,but there was a significant quadratic relationship between the dose of urea-N fertilizer application and the fresh yields.When the dosage of urea-N fertilizer reached a certain value,nitrate readily accumulated in the two kinds of plants due to the decrease in NR activity; furthermore,there was a linear negative correlation between nitrate content and NR activity.With the increasing input of urea-N,ascorbic acid and soluble sugar initially increased,declined after a while,and crude fiber rapidly decreased too.Total absorbed N (TAN),N derived from fertilizer (Ndff),and N derived from soil (Ndfs) increased,and the ratio of Ndff and TAN also increased,but the ratio of Ndfs and TAN as well as NUE of urea-N fertilizer decreased with the increasing input of urea-N.These results suggested that the increasing application of labeled N fertilizer led to the increase in unlabeled N (namely,Ndfs) presumably due to "added nitrogen interaction" (ANI),the decease in NUE of urea-N fertilizer may be due to excess fertilization beyond the levels of plant requirements and the ANI,and the decrease in the two vegetable yields with the increasing addition of urea-N possibly because the excess accumulation of nitrate reached a toxic level.