Plant growth is often limited by low soil phosphorus (P) availability, soil nitrogen (N) availability may affect plants’ responses to P supply. We studied the growth and physiological responses of alfalfa to soil P supply at different N levels. Alfalfa (Medicago sativa L.) plants were grown in an alkaline soil supplied with different levels of P (0, 5, and 20 mg kg−1) as monopotassium phosphate, and N (50 and 100 mg kg−1) as ammonium nitrate. Plant biomass and P concentrations always showed positive responses to P addition but not to N addition, nodulation was inhibited by lower P supply and higher N supply. Roots released more phosphatase and carboxylates, mainly tartrate, into the rhizosheath at lower soil P supply and higher N supply. Roots always acidified the rhizosheath, but rhizosheath pH did not vary considerably among treatments. This study demonstrates the release of tartrate as a major carboxylate as affected by soil P supply and N supply, and highlights the importance of investigating plant adaptive strategies for P acquisition from soil with different N availability for proper application of P and N fertilizers.

中文翻译：

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缺磷条件下苜蓿（Medicago sativa L.）释放的主要羧酸盐酒石酸盐和土壤氮供应的影响

植物生长通常受到低土壤磷 (P) 有效性的限制，土壤氮 (N) 有效性可能会影响植物对 P 供应的反应。我们研究了苜蓿在不同氮水平下对土壤磷供应的生长和生理反应。紫花苜蓿 (Medicago sativa L.) 植物生长在碱性土壤中，供应不同水平的 P（0、5 和 20 mg kg-1）作为磷酸二氢钾，N（50 和 100 mg kg-1）作为硝酸铵. 植物生物量和磷浓度对磷的添加总是表现出积极的反应，而不是对氮的添加，较低的磷供应和较高的氮供应抑制了结瘤。在较低的土壤 P 供应和较高的 N 供应下，根向根鞘释放更多的磷酸酶和羧酸盐，主要是酒石酸盐。根部总是酸化根鞘，但根鞘的 pH 值在处理之间没有显着差异。