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Critical Reading;Select an Answer
Plants, and all other living things, require nitrogenfor growth; it is an essential component of nucleicacids and proteins. Although air is mostly nitrogen,this gaseous form is inaccessible to plants and mustbe fixed into ammonium to render it biologicallyrelevant. Soil bacteria called rhizobia fix nitrogen, but to do this they must first take upresidence inside the roots of legumes like pea, alfalfa, clover, and soybean.
Soon after a legume begins to grow, rhizobia invade its root hairs and multiply, causing theplant to form specialized organs—nodules—that contain the proliferating bacteria. Thissymbiotic arrangement benefits both parties: legumes can thrive without nitrogen fertilizersonly if they have functional nitrogen-fixing nodules, while the bacteria receive the energyneeded to multiply and fix nitrogen from the plant. When the plant dies, the fixed nitrogen isreleased into the soil so other plants can use it. This process has significant implications foragriculture, as nitrogen is the most common nutrient deficient in the earth's soil and, thus,the one most commonly supplied by chemical fertilizers.
Rhizobia are a diverse group taxonomically, genetically, and metabolically. They can be foundin distant genera. Their symbiotic trait appears to have arisen independently multiple times byhorizontal transfer of genes. However, it is not thought that this horizontal gene transferis sufficient to confer symbiosis, or to explain the biodiversity of rhizobia. There must beselective pressures preventing or permitting the expression of the acquired symbiosis trait andadaptive mechanisms to deal with these pressures. But neither the pressures nor the measurestaken to circumvent them are known.
What can reasonably be inferred about the fixed nitrogen released when a plant dies?
(A) It is biologically insignificant because it reverts to its gaseous form.
(B) It is not sufficient to maintain adequate nitrogen levels in soil.
(C) It is less useful for other plants than the nitrogen found in chemical fertilizers.
(D) It is not a consideration in farmers’ decisions regarding agricultural practices.
重点单词讲解：
bacteria [bæk'tiəriə] n. (复数)细菌
gene [dʒi:n] n. 基因
deficiency [di'fiʃənsi] n. 缺乏，不足，缺点
adaptive [ə'dæptiv] adj. 适合的，适应的，能适应的
circumvent [.sə:kəm'vent] vt. 围住，绕行，用计谋应付
transfer [træns'fə:] n. 迁移，移动，换车
arrangement [ə'reindʒmənt] n. 安排，商议，整理，布置，商定，[音]改编，改编曲
bacteria [bæk'tiəriə] n. (复数)细菌
nutrient ['nju:triənt] adj. 营养的，滋养的
deficient [di'fiʃənt] adj. 不足的，不充份的，有缺陷的
gaseous ['gæsiəs] adj. 气体的，气态的
答案：B
解析：
Choice B is the best answer. The author states thatsoil commonly is nitrogen-deficient and thatnitrogen is often supplied by fertilizers, even thoughthe fixed nitrogen released when legume plants withnitrogen-fixing nodules die can be used by other plants. This suggests that the fixed nitrogenthose legume plants release is not enough to supply all of the nitrogen that is needed in soil;fertilizers are needed to address the remaining nitrogen deficiency.