| In order to systematic study the dynamic variation of soil water content, physical-chemical properties in 0-1000cm soil layer, the paper presents a study about grasslands with alfalfas growing for 3 years, 4 years, 6 years, 8 years, 12 years, 14 years, 18 years and 26 years, wheat fields, wasteland were control, we conducted research aiming to understand the relationship between alfalfa growth years and biological characteristics, productivity trends, the law of water requirements, as well as photosynthetic characteristics. The results were as follows:1. The relationship between alfalfa yield and botany showed that there were positive correlation between biological traits and alfalfa yield. Plant height was significantly positive correlation with internode length, per plant weight and yield, the same as the number of internode with branch, diameter and single stem weight. 3 yrs, 4 yrs, 6 yrs, 8 yrs had high yield potential which had prominent botany traits. The alfalfa grassland began to decline when alfalfa growing over eight years; when alfalfa grow to 18 yrs, 26 yrs, its populations fall into downfall seriously, botany traits were worst. The yield of two years in 2006 and 2007 showed that the sixth year was high-yield period, when alfalfa grew over six years, the yield decreased gradually.There were significant differences in root biomass with alfalfas growing for different lengths of time. The longer the life of alfalfa growth, the greater root biomass. 26 yrs alfalfa root biomass was significantly higher than that of the rest of years, there was no significant difference among 6 yrs, 8 yrs, 12 yrs, 14 yrs, 18 yrs, but they were significantly higher than that of 3 yrs and 4 yrs.3. The development patterns of alfalfa's height growth were similar with the way of"S"Logistic growth curves. Alfalfas growing for 3 yrs, 4 yrs, 6 yrs, 12 yrs and 14 yrs began to quick growth period on June 30, and over on the end of July, the fastest growth period continued for a month or so. Alfalfas growing over 14 yrs, the quick growth period was on June 18-19. Alfalfas growing for 6 yrs had maximal instant growth rate, the growth rate was 2.02cm/d, alfalfas growing for 3 yrs, 4 yrs and 8 yrs had higher growth rate, which beyond 1.55cm/d, alfalfas growing over 14 yrs (concluding 14 yrs), the growth rate of plant height was less than 0.85cm/d.4. Correlation analysis showed: the net photosynthetic rate was significantly positively correlated with PAR, Gs and Chl(r=0.8433**,0.8223**,0.8361**,P<0.01). The net photosynthetic rate was distinctly positively correlated with Tr (r=0.6613*,P<0.05), significantly negatively correlated with Ca(r=-0.8125**), there was no distinctly correlated with others environmental factors. Tr rate was significantly positively correlated with Chl, distinctly positively correlated with PAR, positively correlated with Ta and significantly negatively correlated with Ca (r=0.8361**,0.6486*,0.5361,-0. 8399**). The results of diurnal dynamics of Pn or Tr showed: PAR is a major factor influencing diurnal changes of net photosynthetic rate, followed by Chl, Gs,Ca,Tr,RH,Ta and VPD. Ca is a major factor influencing diurnal changes of Tr, the next is Chl, PAR, Ta,Gs,RH and VPD. In all growth years, 6 yrs alfalfa had highest net photosynthetic rate, which showed it had robust growth, strong photosynthetic characteristics and high yield potential. The net photo- synthetic rate of 26 yrs was higher than that of 18 yrs, it indicated that alfalfa's growth and development began to resume following with soil moisture and nutrient recovery, but recovery was slow. Therefore, according to the net photosynthetic rate of alfalfa, the study deemed that in the Loess Plateau, the best optimal use time for alfalfa to grow was six years. Research of Ci daily dynamics of alfalfa leaf showed: as the length of alfalfa growth prolongs, Ci increased gradually when alfalfa growth less than 8 years (including 8 yrs), Ci decreased gradually when alfalfa growth over eight years. It indicated that alfalfa growth spanned from the fourth to sixth year had higher Ca, Ca decreased rapidly when alfalfa growth over eight years, when alfalfa growth 26 years, Ca decreased in low level, it would affect alfalfa's photosynthesis. It indicated from another aspect that the best optimal use time for alfalfa to grow should not beyond eight years.5. The vertical variations of soil water in alfalfa grasslands indicated that 0-1000cm soil could be divide into four layers based on different variations of soil water:⑴0-40cm soil in which soil moisture varied drastically. The strong influence of external warmth and water conditions such as rainfall and temperature could result in sharp wet—dry variation of the layer that could exert strong influence on the water content in the topsoil of the layer, and the soil water content of the layer varied between 6.97 and 13.05%;⑵40-100cm soil in which soil moistures varied actively : in this layer, soil moisture varied less sharply than that in the layer on its top and the soil water content varied within 11.93-16.32% mainly due to the influence of evaporation, rainfall and alfalfa water consumption;⑶100-300cm soil in which soil moisture varied less actively, this layer was mainly affected by alfalfa consumption of soil water and as a result its soil water content appeared to have remarkably reduced variation and ranged within 9.76-13.41%;⑷300-1000cm soil in which soil moisture kept stable, it was mainly affected by alfalfa consumption of soil water and consequently its soil water content kept stable ranging within 11.19-14.81%.6. According to the soil wilting moistures and the soil water contents of the grasslands in its experiment, the study deemed that as long as the soil water content was below 11%, dry soil layer was thought to form with its lowest water content at the soil wilting moisture and was divided into three types, slightly dry soil layer(9%-11%), moderately dry soil layer(7%-9%) and highly dry soil layer(<7%). In the Loess Plateau, the limits and locations of dry soil layer differed among different land use patterns. In wheat land, no dry soil layer occurred; dry soil layer occurred in both wild land and alfalfa grasslands, it appeared slightly dry soil layer in wild land and in the grasslands with alfalfas growing for eight years and less than eight years, and in the soil deeper than 500cm below ground as moderately dry soil layer in the grasslands with alfalfas growing for more than eight years. Because of the short alfalfa growths in the grasslands with alfalfas growing for four years and six years suffered low consumption of soil water and thus slightly dry soil layer appeared in 250-300cm soil and 300-350cm soil (whose water contents range within 10.23-10.86 %); in the grassland with alfalfa growing for eight years , moderately dry soil layer appeared in 200-250cm soil (whose water content stood at 9.83%); in the grasslands with alfalfas growing for more than eight years, moderately dry soil layer appeared; moderately dry soil layer appeared in 200-250cm in the grassland with alfalfa growing for twelve years and in 250-300cm soil in the grasslands with alfalfas growing for eighteen years and twenty-six years, and the soil water contents of these two soil layers were lower than 9%. When alfalfa has grown for a certain length of time, its populations fall into downfall and the soil water content gets recovered to some extent, but the recovery is slow because of extreme soil water consumption and low rainfall, and average annual rate of recovery was 1.49% in the eight years. Alfalfas growing for eighteen years and twenty-six years fall into downfall and then their biomasses get sharply decreased; although their consumption of soil water gradually declines, this can only help recovering upper soil moisture and deeper soil moisture is still difficult to recover. At this annual rate of increase it will take thirteen years to recover the soil water content to the soil water content(12.5%)in 0-200cm soil in the grassland with alfalfa growing for eight years, and twenty-eight years to recover the soil water content to the soil water content(14.82%)in 0-200cm soil in the grassland with alfalfa growing for six years. Hence, the recovery of soil moisture is very slow and time-consumed. In 200-1000cm soil, the soil water contents in the grasslands with alfalfas growing for four years, six years and eight years were above 13.3% and those in the grasslands with alfalfas growing for twelve years and fourteen years were separately 11.56% and 11.67%, but those in the grasslands with alfalfas growing for eighteen years and twenty-six years amounted to only 10.10%; this illustrated that while the length of alfalfa growth prolonged, water consumption and soil drying were intensified in lower soil. Wheat land, wild land and the grasslands with alfalfas growing for four years, six years and eight years did not created unfavorable impacts on ecological environments of soil water; the grasslands with alfalfas growing for twelve years, eighteen years and twenty six years did exert relatively stronger unfavorable influences on ecological environments of soil moisture.7.According to the variations of soil water contents in 0-1000cm soil layer in alfalfa grasslands, alfalfa grasslands could be divided into three groups: the first group was the grasslands with alfalfas growing for 3 yrs, 4 yrs and 6 yrs, which had the most favorable water conditions, soil water contents were 14.84%,14.22%,13.85% respectively. The second group was the grasslands with alfalfas growing 8 yrs, which had more favorable water conditions, soil water contents was 12.98%. The third group was the grasslands with alfalfas growing for 12 yrs, 14 yrs, 18 yrs and 26 yrs, which had the poorest water conditions, soil water contents were 11.25%, 11.22%,10.27%,10.76% respectively.8. According to the index of yield dynamic variation, when alfalfa had grown for a certain length of time, its populations fall into downfall. Yield decreased gradually when alfalfa grew over six years, population in decline when alfalfa grew over eight years, fall into downfall seriously when alfalfa growth beyond 18 years. It can be seen from the defect of yield fluctuations areas (Fr) and the average yield (Ya), the yield was relatively stable in the early growth of alfalfa (1-6 years),while yield changed dreamticly in the latter part of the alfalfa growth period (growth over six years).There were significant differences in WUE with alfalfas growing for different lengths of time. The longer the years of alfalfa grew, the lower the WUE decreased. The grasslands with alfalfas growing for 4 yrs, 6 yrs and 8 yrs had higher WUE, which beyond 50kg.mm-1.hm-2, the WUE of the grasslands with alfalfas growing for 12 yrs was 20kg.mm-1.hm-2 , the grasslands with alfalfas growing for more than 18 yrs had lower WUE, which was only 15kg.mm-1.hm-2. The net yield of alfalfa was significantly positively correlated with soil water contents(r = 0.8695**).It indicated that soil water conditions was a major factor influencing alfalfa yield, to improve soil water contents could increased alfalfa's WUE.9. Alfalfa growth years had effected on physical-chemical properties. Soil porosity increased following with alfalfa growth years, soil bulk density declined gradually after alfalfas growing for 18 yrs. There were significantly positive correlativity between SOC and TN, AN and there was significantly positive correlativity between TN and AN too, correlative coefficients were 0.9844,0.9262,0.9968,respectively, improving SOC would lead to increase TN, AN, this provided scientific basis for crop-alfalfa rotation and alfalfa grassland management. Total P content in the soil showed deep aggregation. When alfalfa has grown for a certain length of time, root activity declined, root biomass began to decrease, its populations fall into downfall, the recovery of soil nitrogen began in upper soil and high consumption of deep-soil nitrogen in the early alfalfa growth make it difficult to recover soil nitrogen in deep soil. After alfalfa has grown for six years, fertilization should be practiced to maintain nitrogen balance in alfalfa grassland.10. Therefore, comprehensive analysis the dynamic variation of alfalfa productivity, soil water condition, physical-chemical properties in alfalfa grasslands and photosynthetic characteristics of alfalfa leaf, the study deemed that in the Loess Plateau, the growth year of alfalfa should not beyond eight years, the most suitable use time for alfalfa to grow was six years. After alfalfa had grown for six years, grass-crop rotation and reasonable fertilization should be practiced to recover soil moisture so as continue to raise land productivity.
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