黄土高原不同干旱类型区苜蓿草地深层土壤干燥化效应

田间实地测了黄土高原不同干旱类型区不同生长年限苜蓿草地0～1000cm土层土壤湿度，分析和比较了各类苜蓿草地深层土壤干燥化效应特征。结果表明，在半湿润区、半干旱区和半干旱偏旱区，各类苜蓿草地土壤湿度平均值分别为10．84％、7．07％和5．45％，明显低于当地土壤稳定湿度值和荒草地土壤湿度值，土壤水分过耗量分别为540．2、641．1mm和455．0mm，平均土壤干燥化速度分别为61．2、101．9mm/a和99．0mm／a；3种类型区各类苜蓿草地年降水入渗深度分别为187．8、144cm和173cm，降水入渗深度以下深层土壤湿度保持稳定的干燥化状态；土壤干燥化强度随苜蓿草地生长年限延长而加剧，3年生苜蓿草地为中度干燥化强度，土壤干层厚度达到500～760cm，4年生以上苜蓿草地已达到严重干燥化和强烈干燥化强度，土壤干层厚度超过940～1000cm；通过粮草轮作使苜蓿草地土壤湿度恢复到当地土壤稳定湿度分别需要6、11a和18a以上。

Effects of deep soil desiccations on alfalfa grasslands in different rainfall areas of the Loess Plateau of China

Soil moistures in 0 --1000cm soil layers on different growth age alfalfa grasslands in different rainfall areas of the Loess Plateau were measured and soil desiccation effects on alfalfa grasslands were analyzed and compared. Average soil moistures in 0 -- 1000cm soil layers on alfalfa grasslands of semi-humid area, semi-arid area and semi-arid drought prone area were 10.84%, moisture on natural 7.07% and 5.45% respectively,which were obviously lower than local soil stable moisture and soil grassland. Soil water overuse amounts of alfalfa grasslands of the 3 areas were 540.2,641. lmm and 455.0mm, and average speeds of soil desiccation were 61.2mm, 101.9mm and 99.0mm per year separately. Average annual rainfall infiltration depths of alfalfa grasslands on the 3 different rainfall areas were 187.8, 144cm and 173cm separately, and soil moisture of deep soil layers below rainfall infiltration depth remained in a stable desiccated condition. As alfalfa growth aged, soil desiccation intensity strengthened. Soil desiccation intensities was medium desiccated on the 3 years alfalfa grassland, and were serious desiccated and strong desiccated on the 4 years or over alfalfa grasslands Thickness of desiccated soil layers were 500 -- 760cm on the 3 years alfalfa grasslands and were over 940 -- 1000cm 4 years or over alfalfa grasslands. Years required for soil moisture restoration of desiccated soil layers to local soil on the stable moisture with alfalfa-grain crop rotations on the 3 different rainfall areas were over 6 years, 11 years and 18 years separately.