| Paddy rice-winter wheat rotation is one of the largest agricultural planted systems. In China, the rice-wheat rotation occupies 10.5 million hectares in the middle-lower reaches of Yangtze River, China. In recent years, the rice and wheat production has been greatly increased, with the improving of cultivation techniques, breeding of high-yielding variety, and applying of inorganic fertilizer in China. By contrast, more soil nutrients were taken away, while more residues were produced. The incorporation of crop straw is an effective mode to use the crop residue. Some farmers in many regions return the crop redidue in every other season now. However, the study about the effect of crop straw return in every other season or the mutual effect when return in every season was rare. For this, a 3-year-6-season experiment with four treatments was conducted in rice-wheat rotation in the middle-lower Yangtze area of China. Four treatments as:both wheat and rice straw return (WR), rice straw without wheat straw return (R), wheat straw without rice straw return (W), no straw return (CK). In which, we investigated the effects on soil fertility, greenhouse gas emissions, crop growth, yield and the quality in return and succeeding seasons, and analyzed the agricultural productivity and the greenhouse effect according to the numerical evaluation. The main results were shown as follows:1. Dynamic process of soil fertility between seasonsDifferent residue return treatments were conducted to research the dynamic process of fertility factors in paddy-upland rotation and the effect under continuous straw return. The results showed that:there were obvious "wave" variations in soil pH, AP, and AK for paddy-upland rotation. The pH increased in rice but decreased in wheat season, while AP and AK went with the opposite way. The pHs increased with both crop residue returned, but no mutual effect between them. AP contents did not show regularly variation among four treatments. Rice straw return showed more important role for AK content than wheat straw return. SOM and total nitrogen contents improved with rice straw or wheat straw returned. The mutual effect between rice straw return and wheat straw return going stronger.2. Crop yieldDue to wheat yield was lower than CK, rice residue return decreased the total yield of wheat and rice in the 3 years. Crops yields were higher in two seasons where wheat straw was incorporated, and increased significantly in rice yield. Furthermore, the mutual effect between the two residues was not significant. Additionally, the path analysis indicated that panicle number was the key factor to wheat yield; spikelets per panicle and panicle number were the key factors to rice yield.3. Nutrient uptakeRice residue return decreased nitrogen uptake of wheat, and increased in rice. And the effect of rice straw on the following rice went stronger with experiment extension. Wheat residue return showed no significant effect of wheat residue on the nitrogen uptake of rice, while increased in the following wheat season. The mutual effect between the two residues on rice showed no significant difference. And the mutual effect on wheat was also not significant in initiated but reached the significant level in the third year.The phosphorus content in rice was higher than that in wheat, which may be related to the availability of soil phosphorus in paddy or upland soil. The phosphorus uptake of rice or wheat plant was less, and did not show obvious regularly variation among four treatments. This may be caused by the weak effect on soil phosphorus of crop straw and the low content in wheat or rice (0.2-0.3%).Residue return increased the potassium content of crops. The potassium uptake results indicated that, rice residue return decreased potassium uptake of wheat in return season but increased in most of the following rice seasons. And the potassium uptakes were higher in two crops where wheat straw was incorporated. Additionally, the mutual effect between the two residues on potassium uptake was not significant. These indicate that the population biomass is the main reason for decreasing of potassium uptake of wheat under rice residue return.4. Crop qualityThe difference between appearance quality and milling quality of rice was not significant among four treatments. Treatment W significantly increased the amylopectin content, peak viscosity, cool viscosity, and breakdown viscosity and decreased the gelatinization temperature and setback viscosity in relation to CK. The protein content of treatment W was significantly decreased according to reducing the globulin, gliadin, and glutenin contents. Compared with W, treatment R had the opposite effect on rice qualities, in spite of the difference was not significant between treatment R and CK. In addition, the mutual effect between the two residues was not significant. Hence, treatments WR and W were favorable for improving cooking and eating qualities of rice.There was no significant difference in the appearance quality of wheat grain when crops residues returned for 3 years. Treatment R decreased the crude protein and gluten content, but increased the starch content of flour. Compared with R, treatment W had the opposite effect on wheat qualities. The mutual effect between the two residues was not significant. And treatment WR almost showed a return-effect on crude protein and starch contents, as R. But the trends of gluten content were same as W.5. Greenhouse gas emissionThe total seasonal N2O emission from paddy field was less, and the difference among the treatments was not significant. The CH4 flux rate of the four treatments were mainly concentrated during the first 30-35 days after transplanting in the rice growing season, peaked approximately one to two weeks after transplanting. Treatment W significantly increased CHU emissions in comparison to treatment CK in rice growing season. Due to the mutual effect of the two residues was not significant, treatment WR had the same increasing effect as W. And treatment R had a decreasing trend for the CH4 emission of paddy fields.The total seasonal CH4 emission of wheat field was less, and the difference among the treatments was not significant. Treatment WR and W showed a small carbon source effect when treatment R and CK behaved the carbon sink effect in wheat field. The N2O emission results indicated that there were two obvious peaks approximately after applying the urea at jointing and booting stages. There was no obvious N2O emission during sowing to turning green. Treatment W could decrease the N2O emission from wheat field. Furthermore, treatment WR and R increased the N2O emission of wheat fields. The difference of the N2O emission among four treatments was not significant.6. Comprehensive evaluation of residue return on agricultural productivity and greenhouse effectThe average of evaluation scores of soil fertility in wheat and rice season were as follows:WR>R>W>CK. The annual yield productivity results indicated that wheat straw return was conducive to increase the yield productivity; rice straw return was not conducive to the increase the yield productivity, which may be caused by the decreasing of wheat yield in R. According to the global warming potential (GWP), the results showed that treatments WR and W had an increasing effect on greenhouse effect; treatment R showed a decreasing effect in 20a scale, and a small increasing effect in 500a scale. The difference in anniversary greenhouse effect among the treatments was caused by the difference of CH4 emissions from paddy fields.These results suggest that crop residue return is conducive to maintain the soil fertility, and residue return in every season (WR) is a optimal way to increase soil fertility. Wheat straw return in every rice season (W) is easy to achieve the high-yield goal from the perspective of annual yield productivity. And rice residue return in every wheat season is conducive to reduce the greenhouse gas emission of rice-wheat farmland from the perspective of greenhouse effect. |
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