求翻译：：通过设置田间小区试验，采用15 N自然丰度法测定不同施氮水平下接种根瘤菌的固氮效能及对紫花苜蓿生长和种子产量的影响。是什么意思？

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fields: By setting up small district pilot, using 15 N natural abundance determination under different Shih nitrogen level of nitrogen-fixing Rhizobium strains vaccination efficacy and the alfalfa growth and seed production.

: Through establishes the field plot experiment, uses 15 N natural abundance law determination to execute under the nitrogen level to vaccinate differently the root nodule bacteria nitrogen fixation potency and to the alfalfa growth and the seed output influence.

: Community testing by setting the field, using the natural abundance of 15 n determination under different nitrogen application rates of nitrogen-fixing Rhizobium inoculation effectiveness and its impact on growth and seed yield of alfalfa.