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.

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.

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.

Study of Dynamics of Floodwater Nitrogen and Regulation of Its Runoff Loss in Paddy Field-Based Two-Cropping Rice with Urea and Controlled Release Nitrogen Fertilizer Application

The article deals with the effects of urea and controlled release nitrogen fertilizer （CRNF） on dynamics of pH, electronic conductivity （EC）, total nitrogen （TN）, NH4^＋-N and NO3 -N in floodwater, and the regulation of runoff TN loss from paddy field-based two-cropping rice in Dongting Lake, China, and probes the best fertilization management for controlling N loss. Studies were conducted through modeling alluvial sandy loamy paddy soil （ASP） and purple calcareous clayey paddy soil （PCP） using lysimeter, following the sequence of the soil profiles identified by investigating soil profile. After application of urea in paddy field-based two-cropping rice, TN and NHa＋-N concentrations in floodwater reached peak on the 1st and the 3rd day, respectively, and then decreased rapidly over time; all the floodwater NO3--N concentrations were very low; the pH of floodwater gradually rose in case of early rice within 15 d （late rice within 3 d） after application of urea, and EC remained consistent with the dynamics of NH4^＋-N. The applied CRNF, especially 70% CRNF, led to significantly lower floodwater TN and NH4^＋ concentrations, pH, and EC values compared with urea within 15 d after application. The monitoring result for N loss due to natural rainfall runoff indicated that the amount of TN lost in runoff from paddy field- based two-cropping rice with urea application in Dongting Lake area was 7.47 kg ha^-1, which accounted for 2.49% of urea- N applied, and that with CRNF and 70% CRNF application decreased 24.5 and 27.2% compared with urea application, respectively. The two runoff events, which occurred within 20 d after application, contributed significantly to TN loss from paddy field. TN loss due to the two runoffs in urea, CRNF, and 70% CRNF treatments accounted for 72, 70, and 58% of the total TN loss due to runoff over the whole rice growth season, respectively. And the TN loss in these two CRNF treatments due to the first run-off event at the 10th day after application to early rice decreased 44.9 and 44.2% compared with urea, respectively. In conclusion, the 15-d period after application of urea was the critical time during which N loss occurred due to high floodwater N concentrations. But CRNF decreased N concentrations greatly in floodwater and runoff water during this period. As a result, it obviously reduced TN loss in runoff over the whole rice growth season.

^(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.

Systematic Studies of Nitrogen Loss from Paddy Soils Through Leaching in the Dongting Lake Area of China

A two-year lysimeter study was conducted to study the effects of different fertilizers and soils on nitrogen leaching loss in a double rice cropping system by considering three major types of paddy soils from the Dongting Lake area. The results showed that N concentration in the leachate did not differ significantly among the treatments of urea, controlled release N fertilizer and pig manure and that all these fertilizers produced higher total nitrogen （TN） concentrations in the leachate compared to the case where no fertilizer was applied. The TN leaching loss following urea treatment accounted for 2.28%, 0.66%, and 1.50% of the amount of N applied in the alluvial sandy loamy paddy soil （ASL）, purple calcareous clayey paddy soil （PCC）, and reddish-yellow loamy paddy soil （RYL）, respectively. Higher TN loss was found to be correlated with the increased leachate volume in ASL compared with RYL, and the lowest TN loss was observed in the PCC, in which the lowest leachate volume and TN concentration were observed. Organic N and NH＋-N were the major forms of N depleted through leachate, accounting for 56.8% and 39.7% of TN losses, respectively. Accordingly, soil-specific fertilization regimens are recommended; in particular, the maximum amount of fertilizer should be optimized for sandy soils with a high infiltration rate. To avoid a high N leaching loss from rice fields, organic N fertilizers such as urea or coated urea should primarily be used for surface topdressing or shallow-layer application and not for deep-layer application.