Under the condition of complete nutrient solution,the effect of three kinds of nitrogen forms (ni-trate nitrogen,ammonium nitrogen and nitrate nitrogen,ammonium nitrogen was 1∶1 and three nitrogen levels (0,105,210 mg/L)on growth nodulation and nitrogen- fixation of alfalfa were studied at flowering stage by sand culture method.The results showed that different nitrogen treatments on plant height,biomass,nodule number,nodule weight,nitrogenase activity and total nitrogen content of alfalfa were significantly higher than those of CK.Under the same nitrogen level,the plant height was the highest and higher under the effect of NO3 --N and NH 4 +-N mixed culture and under NO3 --N next,and NH 4 +-N culture.The performance of bio-mass,nodule number,nodule weight,nitrogenase activity and total nitrogen content were the best under NO3 --N and NH 4 +-N mixed culture,it was the better and lowest under NH 4 +-N and NO3 --N culture respectively. The performance of stem leaf ratio was the best under NO3 --N training,it was better under NH 4 +-N training than that of NO3 --N and NH 4 +-N mixed culture.There was significant difference in nitrogen forms between treatments (P <0.05).Along with the increase of nitrogen level,the plant height,biomass,nodule number,nod-ule weight,nitrogenase activity and total nitrogen content of alfalfa were increased,the leaf stem ratio was de-creased.When the concentration of NO3 --N+NH 4 +-N was 210 mg/L,the plant height,biomass,nodule num-ber,nitrogenase activity and total nitrogen content of alfalfa reached the maximum value,the leaf stem ratio had a minimum value.The nodule weight reached the maximum value when the concentration of NO3 --N+NH 4 +-N was 105 mg/L.In conclusion,alfalfa grows best in mixed nitrogen culture.%在完全营养液条件下,采用砂培法,研究了3种氮素形态配比 NO3--N,NH 4+-N 和 NO3--N∶NH 4+-N 为1∶1和0、105、210 mg/L 3个氮素水平对紫花苜蓿盛花期生长及结瘤固氮的影响。结果表明：不同形态氮素处理下紫花苜蓿的株高、生物量、根瘤数、根瘤重、固氮酶活性和全氮含量均显著高于CK。同一氮素水平下,株高表现为 NO3--N 和 NH 4+-N 混合培养下效果最好,NO3--N 培养下次之, NH 4+-N 培养下最低；生物量、根瘤数、根瘤重、固氮酶活性和全氮含量 NO3--N 和 NH 4+-N 混合培养下效果最好,NH 4+-N 培养下次之,NO3--N 培养下最低；茎叶比则是 NO3--N 培养下最大,NH 4+-N培养下次之,NO3--N 和 NH 4+-N 混合培养下的最低,各氮素形态处理间差异显著(P <0.05)。随着氮素水平的增加,各形态配比下紫花苜蓿的株高、生物量、根瘤数、根瘤重、固氮酶活性和全氮含量呈现增加的变化趋势,茎叶比呈现减小的变化趋势。NO3--N+NH 4+-N 的浓度为210 mg/L 时,紫花苜蓿的株高、生物量、根瘤数、固氮酶活性和全氮含量取得最大值,茎叶比取得最小值,根瘤重则在 NO3--N+NH 4+-N 的浓度为105 mg/L 时取得最大值,紫花苜蓿在混合态氮培养下生长最好。
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