| On Chengdu Plain, the "petroleum agriculture" production system, which is high in investment and pollution and low in effiency, has led to the excessive consumption of farmland system resources, seriously restricting the sustainable developement of regional agriculture. Therefore, it is imperative to construct the operating mode of farmland system which makes the efficient use and cyclic utilization of resources. The straw returning is a way that can improve the production conditons and increase the crop output, and an effective measure to realize the synergetic development of farmland economic system and farmland ecosystem. Thus, the technical study and extensive application of "straw returning system" have always been on the priority agenda of agricultural researchers, as well as the focus of governments and promoters of agricultural techniques.For this reasons, based on the fact that the Chengdu Plain is rich in straw resources and has enough room in the reutilization of straw resources, taking into consideration the fact that rice-wheat rotation system is extensively implemented on Chengdu Plain, this dissertation starts from three basic functions of farmland system, namely, cycle of maters, energy flow and value transfer, makes quantitative analysis of input/output and production effect of nutrient substances (N, P, K) in the farmland system in the models of straw returning on Chengdu Plain. At the same time, the dissertation elaborates on characteristics of energy flow and economic benefits in the system, compares and analyzes operating benefits of farmland system in different models of straw returning by establishing mathematical models, and then puts forward corresponding suggestions on straw returning on Chengdu Plain. The main research conclusions of the dissertation are as follows:(1) By consulting relevant materials and making data analysis, the dissertation discusses about the utilization structure and features of farmland system on Chengdu Plain at the present stage and introduces the major existing problems, i.e., quantity of farmland decreases year by year, farmland preservation faces serious challenges; farmland system is over-used, soil fertility is not high and acidification worsens; soil pollution deteriorates and soil environmental quality keeps falling down.(2) Nutrient effect analysis of farmland system shows that, compared with conventional fertilizing model (CF), the straw returning models effectively improve the nutrient internal circulation use rate in the rice-wheat rotation farmland system, which help fertilize the soil and increases production. From the perspective of nutrient sufficiency-insufficiency state and output-input ratio, the half-quantity model of straw returning (CFS1) is featured by insufficiency of potassium, and the 1.5-times-quantity model of straw returning (CFS3) leads to relatively low fertilizing effect because of the surplus nutrients. As a result, the full-quantity model of straw returning (CFS2) produces more effective and comprehensive nutrient circulation effect for the rice-wheat rotation farmland system.① Compared with CF model, CFS1, CFS2 and CFS3 models, by using nitrogen fertilizers (nitrogen in the returned straws) amounting to 33.15 kg/hm,66.29 kg/hm and 99.44 kg/hm2, help the nitrogen output in the rice-wheat rotation farmland system increase by 88.91 kg/hm2,55.15 kg/hm2 and 33.98 kg/hm2 with the surplus growing by -55.76 kg/hm2,11.15 kg/hm2 and 65.47 kg/hm2 and the output-input ratio rising by 18.24%,4.88% and -3.72%. We can conclude that, compared with CF model, the CFS1 and CFS2models effectively improve the nitrogen-use effect in the farmland system.② Compared with CF model, CFS1, CFS2 and CFS3 models, by using phosphorus fertilizers (phosphorus in the returned straws) amounting to8.35 kg/hm2,18.30 kg/hrnz and 25.05 kg/hm2, help the phosphorus output in the rice-wheat rotation farmland system increase by 8.53 kg/hm2,8.26 kg/hm and 12.15 kg/hm with the surplus growing by-0.18 kg/hm2,8.44 kg/hm2 and 12.90 kg/hm2 and the output-input ratio rising by 3.78%,1.83% and 3.24%. We can conclude that, compared with CF model, the straw returning models effectively improve the surplus of phosphorus and effect of using phosphorus in the rice-wheat rotation farmland system.③ Compared with CF model, CFS1, CFS2 and CFS3 models, by using potassium fertilizers (potassium in the returned straws) amounting to 111.77 kg/hm2,223.52 kg/hm2 and 335.29 kg/hm2, help the potassium output in the rice-wheat rotation farmland system increase by 103.80 kg/hm2,45.66 kg/hm2 and 78.88 kg/hm2 with the insufficiency quantity reducing by 7.95 kg/hm2,177.85 kg/hm2 (from insufficiency to sufficiency) and 256.40 kg/hm2 (from insufficiency to sufficiency) and the output-input ratio decreasing by 16.60%, 62.53% and 72.25%(the output-input ratio of potassium in the CF model is far higher than 100%). We can conclude that, compared with CF model, the straw returning models evidently improve the potassium insufficiency in the farmland system. Even more, the CFS2 and CFS3 models realize the surplus of potassium.④ In various models of straw returning, the internal circulation use rate, input quantity, surplus and circulation flux of nitrogen, phosphorus and potassium in the rice-wheat rotation farmland system all increase as the quantity of returned straws increases (potassium is insufficient in the CFS1 model), which is represented by CFS3> CFS2>CFS1. However, in general, the output-input ratio of nutrient substances reduces as the quantity of returned straws increases, which is represented by CFS1>CFS2>CFS3. Evidently, the increase in the quantity of returned straws effectively increases the surplus of nutrient substances in the farmland system while reducing the efficiency of converting and using nutrient substances.⑤ Straw returning considerably increases production of rice or wheat. Compared with CF model, the rice production in CFS1, CFS2 and CFS3 models increases by 27.13%,23.24% and 13.18% while the production of wheat increases by 8.41%,9.66% and 19.63%. It should be pointed out that, compared with CF model, the straw returning models evidently increase the output ratio of phosphorus in the farmland system. However, for output ratios of nitrogen and potassium, CFS2 and CFS3 models do not have the effect of increase. In different models of straw returning, the output ratios of nitrogen, phosphorus and potassium are all represented by CFS1>CFS2>CFS3. To be specific, the output ratio of nutrient substances reduces as the quantity of returned straws increases.(3) The emergy analysis of farmland system shows that, compared with CF model, the straw returning models increase the emergy output of farmland system and alleviate the pressure of farmland production on the environment while helping with sustainable and efficient development of the system. In particular, the CFS2 and CFS3 models have relatively strong performance in sustainability.① In different models of straw returning, both the total emergy input and output in the rice-wheat rotation farmland system have the trend of increasing as the quantity of returned straws increases, i.e. CFS3>CFS2>CFS1>CF. In the models of CF, CFS1, CFS2 and CFS3, the proportion of renewable organic emergy input of the farmland system in the total emergy input is 7.15%,21.57%,32.06% and 40.07% respectively. It shows that, as the quantity of returned straws increases, the proportion of renewable organic emergy in the total emergy input also increases gradually. The system production’s reliance on industrial fertilizers descends gradually and the self-maintenance ability of the system improves gradually.② Compared with CF model, the CFS1, CFS2 and CFS3 models, by adding the emergy of organic fertilizers (straws) by 4.04E+15 sej/hm2,8.08E+15 sej/hm2 and 1.21E+16 sej/hm2, help the emergy feedback rate and input density increase by 21.24%, 42.42%,63.55% and by 0.41E+12 sej/m2,0.82E+12 sej/m2 and 1.23E+12 sej/m2 respectively. The net emergy output ratio increases by 0.23,0.24 and 0.27 respectively. The emergy-labour productivity increases by 0.27E+14 sej/h,0.20E+14 sej/h and 0.19E+14 sej/h, the environmental load ratio decreases by 1.63,2.24 and 2.56, and the emergy sustainability index rises by 0.42,0.78 and 1.17. We can conclude that straw returning models, while increasing the net emergy output ratio and emergy-labour productivity in the rice-wheat rotation farmland system, reduce the system environmental load rate, effectively improve the sustainable development performance of farmland system and enhance potential of system development.③ In different models of straw returning, the emergy feedback ratio, emergy input density and net emergy output ratio increase as the quantity of returned straws increases in the rice-wheat rotation farmland system, which is represented by CFS3>CFS2>CFS1. Though the emergy-labour productivity in the farmland system presents the trend of CFS1 >CFS2>CFS3, as the quantity of returned straws increases and the system environmental load ratio gradually decreases (1.95,1.34 and 1.02 respectively), the sustainable development performance of the system transitions from unsustainability (CFSi) to sustainability (CFS2 and CFS3).(4) The economic benefit analysis of farmland system shows that, compared with CF model, the straw returning models effectively increase the economic benefits of farmland system. In particular, the CFS1 and CFS2 models have relatively high economic output-input ratio and net production benefits, thus having relatively strong competitiveness and economic benefit foundation for large-area application.① The economic output in the different models of straw returning is represented by CFS1>CFS2>CFS3>CF with the output standing at RMB 41,084Y/hm2, RMB 40,373 Y/hm2, RMB 39,345￥/hm2 and RMB 34,081￥/hm2 respectively. Compared with CF model, the output value of rice and wheat in the models of CFS1, CFS2 and CFS3 has increased by 20.55%,18.46% and 15.45% respectively.② In different models of straw returning, the net profits, labour productivity and economic output-input ratio in the rice-wheat rotation farmland system are represented by CFS1>CFS2>CFS3>CF. It shows that, compared with CF model, the production potential of farmland in the straw returning models has been released very well, thus bringing higher economic benefits; but in different models of returned straws, the economic benefits of rice-wheat rotation farmland system will descend as the quantity of returned straws increases.③ In the testing conditions, returns to scale of labour input and input of returned straws are both positive values. The production elasticity coefficient of labour input is 7.718, i.e., the input factor is in the stage of increasing returns; but the production elasticity coefficient of returned straws input is only 0.024, i.e., the input factor is in the stage of decreasing returns.(5) By consulting literatures and specialists, the dissertation initially establishes the comprehensive method of evaluating the operating benefits of rice-wheat rotation farmland system in the straw returning models. The evaluation shows the operating benefits of farmland system in the straw returning models are higher than those in CF model. In the CFS2 model, the operating benefits of farmland system are highest, followed by CFSi model and CFS3 model. In the CFS2 model, the nutrient substances of rice-wheat rotation farmland system, in the surplus state, have better effect, thus displaying considerable net production returns and economic output-input ratio, relatively small environmental load, and greater sustainable development performance of the system. As a result, as for the "straw returning system", the CFS2 model is most suitable for application in the rice-wheat rotation areas on Chengdu Plain in terms of the typical rice-wheat rotation farmland system on Chengdu Plain.The innovation of the research lies mainly in the following two aspects:(1) The innovation of the idea of the research. Farmland system is an open and composite ecosystem, whose operation involves the coupling of material flow, energy flow, and value flow of the system. Therefore, single technique or method or disciplinary approach can only solve one or some aspects of the problem found in the operation process of farmland system. The research makes farmland management theory, mateiral flow and energy flow theory as its theory foundations. Focusing on three basic functions of farmland system, namely, cycle of maters, energy flow and value transfer, this research analyzes the input/output situation of nutrients, yield effects, characteristics of energy flow and economic benefits in the rice-wheat rotation farmland system in the models of straw returning. On the basis of the the above-mentioned analysis, this research constructs the evaluation index system and evaluation method of the operating benefits of farmland system. This work not only offers a scientific basis to the efficient operation of regional farmland system and formulation of related policies, but also provides reference for research idea in future research.(2) The innovation of research methods and material. According to the contents of the research, in the choice of research methods, the research attaches importance to the comprehensive application of research methods used in natural science and social science;, which is not often applied in research of agrcultural economy management. Empirical research is one important feature of this research. This research conducts comparative analysis among the straw returning models through field investigation and experiment. This work offers reference of research methods and material for related research in the future. |
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