Abstract: A study of the soil composition of Carbon、Nitrogen and Phosphorus in a plot of successive years of planting alfalfa crops shows that the content of compounds of soil C and N reaches the lowest leavel in 5^th year. Hereafter ,the soil content of the two elements begins to rise. A 15-year-old alfalfa crop entered its declining stage, and the soil in which the crop is growing contains far less C and N contents than the soil cultivating a 23-year-old alfalfa crop, in which the nutrient contents of C and N have restored to a certain extent.However, the phosphorus content in the same plot increases and reduces in a contrary way. The content of Ca₂-P and Fe-P decrease with the plant growth years until it is 23-year-old when contents of these elements begin to increase; while the increase and reduction of Ca₈-P、Al-P and O-P, contents moving a contrary way-the soil growing a 5-year-old alfalfa crop is impregnate with moderate liable organic phosphorus; while in the soil growing a 10-year,a 15-year,and a 23-year-old alfalfa crop,the change of fractionation of organic P is in the following order: moderate reliable organic phosphorous>moderate liable organic phosphorous>high reliable organic phosphorous>liable organic phosphorous.

Key words: Organic matter, Organic nitrogen, Inorganic phosphor, Organic phosphor

摘要： 对黄土高原沟壑区不同种植年限苜蓿(Medicago sativa)地土壤碳、氮、磷组分的分析结果表明:连续种植5年,土壤碳、氮各组分含量最低,而年限延长含量则有所提高;15年生苜蓿生长已进入衰退期,各组分含量远低于土壤肥力得到一定程度恢复的23年生苜蓿地.P与C、N组分的变化不同,Ca₂-P和Fe-P含量随年限延长逐渐降低,到23年时又有升高趋势,而Ca₈-P、Al-P、O-P变化趋势则与之相反,5年生苜蓿地以中活性有机磷为主,10年、15年、23年生苜蓿土壤有机磷组分的变化表现为:中稳性有机磷(MROP)>中活性有机磷(MLOP)>高稳性有机磷(HROP)>活性有机磷(LOP).

关键词: 有机碳, 有机氮, 无机磷, 有机磷