| This paper takes artificial alfalfa grassland in The arid area of southern Ningxia as object of study. During 2004~2005 year, the soil moisture status of different year old alfalfas grasslands are mensurated in soil layer with 0~700 cm and 0~1000 cm . And during 2003 to 2005, with the 10-year-old alfalfa grassland as previous stubble and take spring wheat, millet and potato as rotation grains, a three-year alfalfa-grain rotation orientation experiment has been carried out in HaiYuan, south Ningxia. The differences of 27 kinds of rotation patterns have been analyzed and compared in crops water and nutrition use characteristic, crops total yield, water use efficiency(WUE), precipitation productive efficiency, nitrogen use efficiency(NUE), and soil nutrition and enzyme activety in the crop rotation end. Research conclusion has important theory and practice significance for grassland management and alfalfa-grain rotation in arid area of southern Ningxia .The main results of the study are as followings:1. Deep soil profile moisture of different growth age alfalfa fields has been measured during successive 2 years (2004 and 2005 years) in southern Ningxia dry area. Results indicate: With the increasing of alfalfa's growth age, soil desiccation degrees intensify in whole. Average contain moisture rates in 0~700cm soil layer of 4,7,12 growth age alfalfa fields, which has measured in 2004, is in turn 5.30%,5.22% and 5.01%; And those in 0~800cm depth of 3,6,10 growth age alfalfa fields, which has measured in 2005, is in turn 6.26%,5.60% and 5.27%. After 3 growth year, alfalfa taproot has reached under 300cm depth. So soil moisture of 300~800cm depth be consumed in intensity and 0~300cm soil moisture increase than before. According to moisture distribution and dynamic characteristics analysis on deeper soil profile of different growth age alfalfa fields, the soil profile be divided into four layers: Soil layer that moisture easy to change by precipitation and evaporation(0~100cm), Soil layer of precipitation continue to seep (100 to 300cm),Dry soil layer that water exhausted by alfalfa root system (300cm to 800cm) and layer regulated by water under deep soil that stored up(800~below 1000cm). Before 6 growth age, alfalfa's soil moisture consumption depth and thickness are increasing by each year quickly. And in 6 growth age, both exceed 1000cm depth, and contain water in soil severely decrease, which 0~1000cm soil layer average moisture is only 5.73%. Alfalfa growth decline during its 7~12 growth age, soil moisture's consumption depth and thickness are decreasing slightly, which contain moisture of 300~700cm dry soil layer only maintain 4% level, and those of 800~1000cm depth layer soil moisture has increase .2. Alfalfa-grain rotation has the feasibility in restores soil moisture of the degenerated alfalfa's dry layer soil. In two consecutive years, the study measured the soil moisture content of farmland on which alfalfa had grown for different periods and the soil water content of farmland on which alfalfa had grown for ten years and then rotated with other crops in different years and then compared with the soil water content of those in different croplands. The study indicated that while alfalfa growth was prolonged the depth and thickness of the soil dry layer first increased and then declined. The depth of soil dry layer was 720 cm in the farmland with alfalfa growing for three years, at most 1000 cm in the farmland with alfalfa growing for six years , and 920 cm in the farmland with alfalfa growing for ten years and soil dry layer spanned from zero to nearly 700 cm soil in the farmlands with alfalfa growing for 3~12 years. In alfalfa farmlands, the average moisture content declined in 0~800 cm soil as the growth of alfalfa was prolonged. In 2004, the average soil moisture content was measured to be 5.30%, 5.22% and 5.01% in 0~700 cm soil in the farmlands with alfalfa growing for 4, 7 and 12 years respectively. In 2005, the soil moisture was measured to be 6.26%,5.60% and 5.27% in 0-800 cm soil in farmlands with alfalfa growing for 3, 6 and 10 years respectively. But the soil moisture tended to rebound in 800~1000 cm after alfalfa grew for a certain period. A soil depth of 300 cm is the maximum depth to which the rainwater could infiltrate. Thus the soil dry layer, tended to stay permanently once it formed in soil below 300 cm deep. The soil moisture remained only 4.0% in 300~700 cm soil in the farmlands with alfalfa growing for 7~12 years. The thickness and moisture content of the soil dry layer differed greatly between alfalfa farmland and cropland, but alfalfa-crop rotation made the soil moisture of the soil dry layer recover substantially worse than those of the croplands and this effect was intensified in different soil layers as the rotation was prolonged. Furthermore, the 18-year alfalfa-crop rotation made the farmland with alfalfa growing for 18 years recover its soil moisture content to a level similar to that of cropland.3. It is studied that soil physical and chemical properties of different year old alfalfas and alfalfa-millet crop rotation in Arid Region of Southern Ningxia during from 2002 to 2004 year. The results indicated that with the increasing of alfalfa growth year, soil ecological environment be improved. Alfalfa was growing during from 3 to 22 years old, soil bulk density reduced 0.213 g/cm3, soil porosity added 8.03%, and soil hold water saturation ratio increased 14.17%, so soil hold capacity was strengthened. During from 6 to 22 years old of alfalfa, soil organic matte and nitrogen content in 0~60cm depth increased 1.60 g/kg and 11.02 mg/kg respectively. And pH value felt for 0.09, which supply better soil growing environment for rotation crops. The more ahead that alfalfa rotation year is, the better effect of soil moisture restoration is and the higher Water Use Effect of rotation crop is. When millets be harvested in farmlands after 6,10 and 22 years old alfalfa, which soil water restoration content in 0~200cm depth were 63.06 mm,55.22 mm and -42.55 mm, respectively. Millet yields that alfalfa-grain crop rotation , after 6,10 and 22 years old alfalfa ,were 1725.95 kg/hm2,1485.80 kg/hm2 and 1560.75 kg/hm2, and their WUE were 12.0116kg/mm.hm2,8.4325kg/mm.hm2 and 5.8161kg/mm.hm2, respectively. The alfalfa optimum growth year in grass-grain rotation is 5~6 year old.4. Experiment has been carried out that three kinds of crops including spring wheat, potato and millet are rotated on the 10-year-old degenerate alfalfa grassland in the drought areas of southern Ningxia on the loess plateau during 2003~2005. Results indicate, in alfalfa-crop rotation farmland, 0~60cm soil layer is apt to be influenced by precipitation and ground evaporation, the rotation crop mainly consumes water in 0-120cm soil layer; Within the range of 120 cm - 200cm soil layer, with the increasing of rotation year, the soil moisture show that trend is resuming constantly.5. The rotation experiments on the 10-year-old degenerated alfalfa grassland, which carried out for 3 years and included 27 kinds of alfalfa-crop rotation patterns, indicate that is mainly the first year rotation that crops restore the alfalfa soil moisture. The rotation grains'type and combination method are different, so causes that the soil moisture consumption's intensity and the depth are different, and causes the soil moisture content to have the big difference. Regardless of under what kind of precipitation year, potato has much higher yields, water use efficiency and precipitation production efficiency than other two rotation crops, which make full of the limited precipitation resources and adapt climatic conditions of south Ningxia, so is first choice crop of alfalfa-grain rotation. After spring wheat harvest, the fallow period was advantageous in the rainy season precipitation infiltrates in soil, maintained farmland moisture content balanced. It suggest that, after alfalfa ploughing, the crop rotation first year should plant potato, and when soil moisture consuming too much, should rotate spring wheat to save water in growing period and convergence rainwaterl down the depth soil layer during the fallow period, and promotes the farmland water balance.6. After the alfalfa grassland be plowed and rotated by the crops, the soil organic matter content drops unceasingly, especially the high-yield crops plant in succession causes to drop much quickly; The nitrogen is greatly influenced by the crop output and soil moisture. The crop rotation may enhance the phosphorus validity. The soil moisture also affects the crops absorption for the phosphorus element. The potato demand the phosphorus element is much. After the crop rotation, the pH value begin to increase and then decrease, ultimately is lower than those of the original alfalfa grassland, therefore,the alfalfa-crop rotation may reduce the salting of soil degree.7. There are significant differences at above indexes of the third year rotation spring wheat of 9 kinds of alfalfa-grain rotation patterns. Organic matter,Tot. N,Alk.-hydr.N,Avai.K average contents and pH values of 0~60cm soil layers all decreased, which breadths is 0.05~2.24 g/kg,0.019~0.325 g/kg,0.118~12.280mg/kg,8.87~166.88 mg/kg and 0.03~0.31, respectively. It has cumulation effect that No.1 and No.2 year rotation grains absorb soil water and nutrients,which decrease by rotation year adds. It is dominating that rotation crops consume soil nutrient at the first rotation year and consumption quantity was different with crop species. When the first rotation crop is spring wheat, consumption of this rotation pattern's soil N and P is the least of all, it is second when it is millet, and it is the most when it is potato. As millet favor to Avai.K, soil Avai.K was use greatly when former stubble grains was millet, it is more little than millet when former stubble grains was potato, and it is the least when spring wheat. Soil organic matter and Avai.P decrease greatly when those three grains rotate with free arrange in groups. In all rotation patterns, potato and millet use greatly soil water than spring wheat, so it leads to soil keep high pH value. In the alfalfa-grain rotation patterns, grain succession pattern induce to soil urease activity decline, which breath is much more than others. Potato succession pattern lead to soil Alkaline Phosphatase activity fall, soil invertase activity of crop succession pattern is more high than others.8."10 years old alfalfa-Potato- Potato-Spring wheat"is optimal rotation pattern that soil restoring-water effects is better than other patterns and take full advantage of limit rainwater resources. Its 0~200 cm depth soil moisture, crops'total yields, WUE,Precipitation Productive Efficiency and NUE are higher than others, which are in turn 254.58mm,5214.5 kg/hm2,9.3786 kg/hm2·mm-1,6.8711kg/hm2·mm-1and 19.612kg/kg.hm-2. and has significantly differences(P< 0.05 or P< 0.01)between those of other patterns respectively.
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