| The Loess Plateau is one of the most suitable areas for apple planting in China. The apple planting area has exceeded 1.2 million hectares, almost 55% of the country’s total apple area. The total output of the apple was more than 15 million t on the Loess Plateau, more than the half of tatal apple yield of China. In recent years, the apple planting area is still growing faster on the loess plateau. Most of the apple orchards planted at a high density at the early stage of construction, canopy density was high, the ventilation and light condition was poor, seriously affected apple’s products and quality. There is no irrigation condition in most of the apple orchards. Annual precipitation can not meet the demand of apple trees consumption. The water deficit is main factor that restrict the orchards product, it causes the occurrence of deep dried soil layer and thickening gradually. Soil reservoir regulation capacity decline or even disappear, seriously affect the sustainable and stable development of the apple orchards on the Loess Plateau.Based on widely investigation and field observations of yield, deep soil water and soil nutrients of apple oechards, the Win EPIC model developed by USA was used to simulate apple yields and deep soil moisture changing trend of dryland apple orchaards in different rainfall locations of the loess plateau during 1965-2009. The study compared the change rule of simulated annual yield and the occurrence rule of deep soil desiccation of apple orchards in different rainfall areas. In order to investigate long-term effects of deep soil desiccation on yields of dryland apple orchards, and to determine reasonable planting density, fertilization level and suitable use age of apple orchards, the paper also simulated the dynamic changes of water productivity and deep soil desiccation of apple orchards under six different planting density treatments and six fertilization treatments during 1965-2009 at Luochuan and Changwu. The results provided the scientific basis for sustainable development of the rain-fed apple orchaeds on loess plateau. The main achievements showed as following:(1) The model validation results of apple yield and deep soil moisture of dryland apple orchards at Luochuan, Changwu, Baishui, Yan’an and Jingning showed as following: the simulated results of apple yield and dynamic change of soil moisture for dryland apple orchards were relatively precise. The Win EPIC could be used the rain-fed apple orchards on the loess plateau.(2) In Luochuan, Baishui, Yan’an and Jingning, the simulated yield of the 1-45 years old apple orchards averaged 26.05, 23.89, 22.29 and 20.51t/hm2, respectively. In 1-20-year-old apple orchards, average annual water consumptions were higher than annual rainfalls, which resulted in soil drying in deep soil layers. Soil desiccation rates in Luochuan, Baishui, Yan’an and Jingning were 59.6, 56.9, 63.9, 64.9 mm per year, respectively. The 0-15 m soil moisture profiles sharply varied, with annual declining the humidity. Dry soil layers occurred in 13-year-old orchards of Luochuan, 11-year-old orchards of Baishui, 7-year-old orchards of Yan’an, 6-year-old orchards of Jingning and thickness of desiccated soil layer increased annually. In 20-year-old orchards of the four sites, dry soil layers already exceeded 11 m thick, and soil moistures after that year remained steady status in 3-15 m soil layers.(3) During the 45 year growth period, simulated annual apple yield of D1~D6 density(714~1429 plants/hm2) apple orchards increased in the early growth stage and then decreased with fluctuation at Luochuan and Changwu. The higher planting density of apple orchards had, the higher annual yield could obtain in early growth stage. But it was decreased with the tree-age for excessive consumption of soil water, and the falling time occurred earlier under the higher planting density. In later growth stage, difference of apple yield was not significantly among different planting density apple orchards. Averages of simulated annual water consumption of Luochuan and Changwu apple orchards among six density treatments were very close to each other, and its value was 627 mm and 605 mm, respectively. But annual water consumption in prophase was significantly higher than anaphase, and it were higher than annual rainfall of the corresponding period. The higher plating density of apple orchards had, the higher water consumption could have during early growth phase, and the water consumption were higher for lower planting density treatments during medium growth phase. In later growth stage, simulated annual water consumption was not significantly different among six density apple orchards. And their values were similar to local annual rainfall. Water stress days of 1~45 year-old apple orchards showed a growth trend which was opposed to the change trend of annual rainfall. With the planting density increased, water stress occurred earlier and water stress days got larger, monthly available soil water amounts in 0-15 m soil profile decreased rapidly. The higher planting destiny of treatment had, the faster increasing rate of desiccated soil layer depth could have and the earlier time for desiccated soil layer occurred as well. To summarize, taking yield, water consumption, water stress days, soil water contents and soil moisture of apple orchards into consideration, we suggested the suitable planting densities of apple orchards was 833~1000 plants/hm2 at Luochuan and 714~833 plants/hm2 at Changwu. The reasonable years for soil water sustainable use was 20~25years.(4) During the 45 year growth period, simulated annual apple yield of F0~F5 fertilization treatments apple orchards increased in the early growth stage and then decreased with fluctuation at Luochuan and Changwu. The average yield was 24.42, 27.27, 28.69, 29.63, 30.49 and 29.43 t/ha respectively at Luochuan, it was 23.62, 26.52, 28.48, 28.85, 28.61 and 28.58 t/ha respectively at Changwu. The yield of Luochuan apple orchards was higher than Changwu, and the annual decreasing rate was lower than Changwu. Soil moisture distribution declined acutely in 0-15 m soil profile on six treatments orchards at Luochuan and Changwu, and desiccated soil layers thickened gradually in pace with tree growth old. The higher fertilization treatment of apple orchards had, the faster increasing rate of desiccated soil layer depth could have and the earlier time for desiccated soil layer occurred as well. Averages of simulated annual water consumption of apple orchards among six fertilization treatments was 623.2, 625.6, 627.8, 628.0, 628.5 and 629.7 mm respectively at Luochuan. And it was 593.7, 598.2, 600.9, 603.5, 605.7 and 608.9 mm respectively at Changwu. The annual water consumption in prophase was significantly higher than anaphase, and it were higher than annual rainfall of the corresponding period. As fertilization increased, the averages of simulated annual water consumption and water overuse of apple orchards was increased, as well as the water use efficiency. Take the yield, sustainable utilization of soil water and economic benefits into consideration, the suitable fertilization level of Luochuan and Changwu dryland was N 360-480 kg/hm2, P 180-240 kg/hm2 and N 240-360 kg/hm2 and P 120-180 kg/hm2, respectively. |
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