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紫苜蓿(Medicago sativa)是一种经济和生态价值较高的优良牧草,但其耗水量大,在西北半干旱地区仅靠天然降水难以满足紫苜蓿的正常生长发育。宁夏盐池北部地处毛乌素沙地南缘,地下水埋深较浅,地下水有可能成为紫苜蓿的潜在水源,弥补天然降水的不足。本试验在地势平坦的缓坡丘陵梁地和丘间低地,选择8年生旱地紫苜蓿试验地作为研究对象,采用稳定同位素技术,研究了不同海拔的4个坡位(海拔自低到高分别为:坡1、坡2、坡3和坡4)紫苜蓿的水分来源及其生长生理表现。结果表明:坡位对0–300 cm土壤剖面含水量有显著影响,海拔最低的坡1土壤含水量最高。土壤水和植物茎秆水δ18O-δD坐标点大部分位于中国西北地区地方大气降水线(LMWL)的右侧,说明植物利用的水源氢氧同位素组成受到蒸发的影响而发生了富集作用。0–450 cm土壤剖面水δ18O值随着海拔高度的增加而增大。同一坡位土壤水δ18O值随着土壤深度的增加逐渐下降。深层土壤水δ18O值与地下水δ18O相近,说明地下水通过土壤毛细管上升而补充其上层土壤水分。0–40 cm土壤水δ18O值随季节波动较大,270 cm以下土壤水δ18O值较为稳定。4、7、8月份坡1紫苜蓿茎秆水δ18O值显著低于其他3个坡位(p<0.001)。在4、6、7三个月,坡位1紫苜蓿对深层土壤水(270 cm以下)的利用率最高。而在8月份,坡1、坡3、坡4紫苜蓿主要利用150–270cm、270–450 cm土层土壤水以及地下水,坡2对表层(0–20 cm)土壤水利用率最高。坡1紫苜蓿的产量、整株Δ13C值及气孔导度显著高于其他3个坡位。本研究表明:在平均年降水量只有280 mm的西北半干旱地区种植旱地紫苜蓿要尽量选择地势较低的滩地,使其能够利用到埋深较浅地下水,以满足植物生长发育的需要并取得较好的生态和经济效益。
Medicago sativa is an excellent forage with high economic and ecological value, but its water consumption is large. It is difficult to meet the normal growth and development of the alfalfa only by natural precipitation in the semi-arid area of the northwest. The northern part of Yanchi in Ningxia is located in the southern margin of the Mu Us Desert. The groundwater depth is shallow and groundwater may become a potential source of water for alfalfa to make up for the lack of natural precipitation. In this study, we selected 8-year-old dry land purple alfalfa as the research object in the gentle slope hilly and hilly land with flat terrain. Stable isotope techniques were used to study the 4 slope positions at different altitudes (elevation from low to high were: Slope 1, Slope 2, Slope 3 and Slope 4). The Water Sources and Growth Physiological Characteristics of Alfalfa. The results showed that: the slope position had a significant effect on the water content in 0-300 cm soil profile, and the lowest elevation slope 1 had the highest soil moisture content. Most of δ18O-δD coordinate points of soil water and plant stem water are located on the right side of the local atmospheric precipitation line (LMWL) in Northwest China, indicating that the hydrogen and oxygen isotope compositions of water sources used by plants are enriched due to evaporation. The δ18O value of 0-450 cm soil profile water increases with the increase of altitude. The δ18O values of soil water in the same slope gradually decreased with the increase of soil depth. The δ18O value of the deep soil water is similar to that of the groundwater δ18O, which indicates that the groundwater recharges the upper soil moisture through the soil capillary. The δ18O values of 0-40 cm soil water fluctuated with seasons, and the δ18O values of soil water below 270 cm were stable. In April, July and August, the δ18O value of the stems of purple alfalfa was significantly lower than that of the other three slopes (p <0.001). At 4, 6, 7 and 7 months, the highest utilization rate of purple soil alfalfa was in the top soil layer (270 cm depth). In August, alfalfa of slope 1, slope 3 and slope 4 mainly utilized soil water of 150-270 cm and 270-450 cm soil layer and ground water, while slope 2 had the highest water utilization rate of surface soil (0-20 cm). Slope 1 purple alfalfa yield, whole plant Δ13C value and stomatal conductance were significantly higher than the other three slope positions. This study shows that the planted purple alfalfa in the semi-arid area of northwestern China with an average annual rainfall of only 280 mm should try its best to choose the lower land and make it possible to utilize the shallow groundwater to meet the needs of plant growth and development Better ecological and economic benefits.