| As one of soil conservation tillage methods, no tillage (NT), which is an efficient way to improve agricultural labor efficiency and to conserve water and soil quality, has been widely applied in farmlands worldwide. However, there is an ongoing debate of the positive versus negative effects of NT on soil properties and nutrient losses in previous studies. Considering both economic and environmental benefits, therefore, further efforts aimed at the effect of continuous NT on soil properties and water environment are required to provide scientific foundation in developing practical field management measures under NT through the combined studies at field and watershed scales.Based on the former research results and the theories of soil, ecology, and environment sciences, effects of NT on field soil properties and watershed hydrology and water quality were fully addressed using field investigation, measurement and analysis, and model simulation. A number of NT pratical paddy fields and a typical agricultural watershed (i.e., ChangLe watershed) in Shaoxing area in southeastern China were taken as the study objectives. The variations of soil physical, chemical, biological properties in response to continuous NT were discussed considering different soil textures (i.e., clay loam and clay). Based on the measured variables and parameters from the NT paddy field analyses, the Soil and Water Assessment Tool (SWAT) model was then established to predict the responses of water quantity and quality to continuous NT for farmlands in the ChangLe watershed. Finally, according to the results derived from field and watershed scale studies, the suitable duration years for continuous NT was discussed to protect soil and water quality, which is beneficial for NT technology improvement and water pollution control in southeastern China.The main research results and conclusions of the dissertation are included:(1) Without straw mulching, although soil compactions at0-20cm layers were increased with increasing NT applied years in both the clay loam and clay paddy soils, its increased intensity was higher in the clay soil than that in the clay loam soil, resulting in shallower plough layer occurring in the NT clay soil. The clay soil also presented greater soil bulk densities than the clay loam soil after same years of continuous NT.(2) Both soil organic matter and available nitrogen contents were increased with NT applied years in the0-10cm layer of clay loam soil, while they were significantly decreased in the0-10cm layer of clay soil. They both presented decreasing trends with NT applied years in the10-20cm layer of both clay loam and clay soil. No obvious variations were observed in soil total phosphorus, available phosphorus, total potassium and available potassium in both clay and clay loam soils in response to continues NT.(3) For NT applied paddy fields, the diversity and total biomass of soil microbial phospholipid fatty acid (PLFA) were significantly increased compared to the conventional tillage (CT). The dominant soil PLFAs maintained long-term stability. Similar community structure of soil microbes was observed among paddy fields with continuous NT of2,4,6and8years, while it was significantly different from that in the CT soils. The characteristic PLFAs for bacteria under continuous NT were significantly higher than that under CT. The ratio of total saturated PLFAs:total monounsaturated PLFAs and ratio of cyclopropyl:precursor PLFAs were both increased after NT, suggesting that soil microbes of paddy field suffered from physiological stress under CT.(4) After1-6years of continuous NT, the clay loam paddy soil presented more positive consequences than the clay ones in terms of soil physical and chemical properties without straw mulching. Considering tillage cost and soil quality protection, overall ploughing is suggested to be performed once for clay paddy field after every3-4years of continuous NT in southern China.(5) Soil and Water Assessment Tool (SWAT) simulation results indicated that conservational tillage resulted in an increase of baseflow while decreases in surface runoff and total water yield at watershed scale. Meanwhile, NT could significantly decrease sands losses from watershed fields to the rivers.(6) If paddy fields with conventional tillage were transfer with no-tillage, watershed organic nitrogen, organic phosphorus, sediment P and nitrate loss loads via surface runoff would be decreased by34.02%,28.73%,4.03%and4.05%, respectively. For nitrate, however, its loss loads in lateral flow and groundwater would be increased by0.37%and1.47%, respectively. Overall, TN and TP loss loads would be decreased3.13%and16.60%, respectively, under continuous NT. The minimum TN and TP loss loads would be occurred after4and3years’continuous NT for farmlands.(7) The results of field scale studies indicated that overall ploughing is suggested to be performed once after every3-4years of continuous NT in southern China, considering tillage cost and soil protection. The results of watershed scale studies also indicated that the minimum TN and TP loss loads were observed in3-4years of continuous NT, and then increased with increasing NT applied years. Combined the results from field scale and watershed scale studies, continuous NT applied years should be not exceeded4years, which will be beneficial to achieve continuable utilize of no tillage paddy field and to control watershed water pollution.The innovative points for this study are as follows:(1) The effects of NT on soil properties were addressed concerned the role of soil texture. The result that the clay loam paddy soil is more suitable to adopt continuous NT pattern than the clay ones provided a theoretical basis for further promoting the improvement and application of NT technology. It offers new knowledge to explain the ongoing debate of the positive versus negative effect of NT on paddy soil properties.(2) Using the SWAT model, this study fully addressed agricultural watershed hydrology and water quality cycling in response to NT. Modeled results provided quantitative information on changes in sands, nitrogen and phosphorus losses under continues NT. It will be beneficial to establish scientific measures and strategies for control watershed nonpoint source pollution.(3) Combined the results from field scale and watershed scale studies, for first time, overall ploughing is suggested to be performed once after about every4years of continuous NT in paddy field. It offers an important technical support to improve and extend NT technology. |
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