黄土高原雨养条件下紫花苜蓿生物固氮调控氮磷重吸收机制

Nutrient resorption in a plant is beneficial to the enhancement of plant adaption to infertile soil or environment where nutrient availability is restricted by adverse environmental factors, i.e. drought. There is infertile soil in the Loess Plateau of China and the precipitation is not always enough for plant growth. Notably the rainfall is uneven among seasons or years, which makes it more difficult for the plant to absorb nutrients from soils. These stresses restrict the development of agriculture and livestock breeding and the ecosystem conservation in this region. So elucidating the characteristics of nutrient resorption and its regulation mechanisms is of great importance to promote the agricultural productivity and to restore the ecological function of the environment. Previous researches have revealed that the resorption is species-specific and influenced by soil nutrient availability and plant nutrient status. However, there is no consistent conclusion so far and no clear explanation. As known, frequent removal of the shoots may result in nutrient (mainly N) loss of the system, and leguminous species, i.e. lucerne, has some strong ability to biologically fix N from the atmosphere (BNF). So nutrient resorption and its regulation in these perennial legumes may be different from other species. Here, we proposed some hypotheses below: 1) The N and P resorptions in leguminous species with BNF, i.e. lucerne, are regulated by BNF. 2) The N and P resorptions are enhanced under drought. 3) In a mature (lucerne) grassland, the function that N addition reduces N resorption is weakened. 4) For perennial legumes, N and P resorptions are age- and cut-specific. In this study, some field and controlled pot experiments in the glasshouse will be conducted. We shall investigate the characteristics of N and P resorptions of lucerne in the rainfed Loess Plateau of China, testing the above hypotheses and revealing how BNF regulates N and P resorptions in lucerne.

营养重吸收能增强植物适应养分贫乏或因逆境（如干旱）导致的养分获取困难环境的能力。黄土高原土壤贫瘠，降水不足且季节性和年际变异大，严重限制了农牧业发展和生态保育。弄清植物营养重吸收规律及其调控机制对提升黄土高原雨养农区生产力、促进生态恢复具有实践意义。重吸收有物种特异性，并受土壤养分和体内营养的影响，但缺乏一致结论，也没有明确的机制性解释。多年生牧草常因地上部分移出而导致系统元素（主要为氮）大量减少，且豆科植物（如紫花苜蓿）常具有较强的生物固氮（BNF）能力，因此，多年生豆科牧草可能具区别于其他植物的重吸收特性和机制。试提假设：1）具BNF的豆科植物（如紫花苜蓿）的氮磷重吸收受其BNF的调控。2）干旱下，氮磷重吸收加强。3）在成熟草地中，氮添加对氮重吸收的弱化作用减弱。4）氮磷重吸收具年龄和茬次特异性。拟研究黄土高原雨养条件下紫花苜蓿氮磷重吸收特性，验证假设，以揭示BNF调控氮磷重吸收的机制

叶片养分重吸收是植物适应逆境的重要机制。黄土高原土壤贫瘠，降水季节性和年际变异大，严重限制了农牧业发展和生态保育。弄清叶片养分重吸收规律及其调控机制有助于通过适宜管理措施增强牧草适应能力，对提升黄土高原雨养农区生产力、促进生态恢复具有实践意义。研究了不同品种、生长阶段下控水、施肥对紫花苜蓿叶片氮、磷重吸收特性的影响及其调控。主要发现：1）叶片氮、磷重吸收效率（NRE、PRE）和重吸收度（NRP、PRP）具种内品种间差异性，但并不显著，与其他植物的种间差异显著。叶片氮、磷重吸收因建植年龄、生长年龄和茬次而显著不同。随年龄增大，NRE、PRE先增大后减小，重吸收度变化则不一致；第一茬叶片氮、磷重吸收显著强于第二茬。一般地，NRE、PRE分别与成熟叶氮、磷浓度显著正相关，与衰老叶氮、磷浓度显著负相关。土壤速效养分影响叶片氮、磷重吸收，但并不显著。2）施氮肥对NRE、PRE无显著影响，施磷肥显著提高NRE，对PRE的影响受施氮肥水平的调节；施氮、磷肥提高NRP，但PRP仅受施氮肥影响。一次性施磷肥对氮重吸收无显著影响，而其对磷重吸收的影响因苜蓿年龄而变化。一般地，重吸收效率与成熟叶N:P负相关、与衰老叶N:P正相关，重吸收度与成熟叶氮、磷浓度负相关。3）随供水增加，磷重吸收增强；控水改变施肥对氮、磷重吸收的影响；同施磷钾肥导致PRE降低。NRE、PRE受土壤养分相对限制性调控。叶片酸性磷酸酶活性与PRP显著正相关，与PRE显著负相关，尤其是衰老叶酸性磷酸酶。4）苜蓿生物固氮率具品种和年龄特异性，受控水和施肥影响。生物固氮影响叶片氮、磷重吸收特性，但年龄、控水、施肥等对二者关系有显著影响。本研究揭示了黄土高原雨养农区苜蓿叶片氮、磷重吸收的品种、生长阶段特异性，及其在控水、施肥等下的变化规律，阐明了其与叶片养分浓度、土壤养分供应间的关系，并揭示了苜蓿生物固氮与叶片养分重吸收间的关联。将有助于深入理解豆科植物的适应性，为合理实施管理、提高草地生产可持续性提供理论依据。

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