Purpose

Beneficial soil microbes, such as rhizobia, engage in facultative symbioses in the roots of leguminous host plants to exchange nitrogen for products of photosynthesis, and these symbioses can be altered by biotic and abiotic factors. Here, we investigated how soil nitrate supply and aboveground insect herbivory interact to influence biological nitrogen fixation in Medicago sativa (alfalfa or lucerne).

Methods

Using field and greenhouse experiments, we quantified above- and belowground allocation of rhizobially fixed nitrogen using isotopic nitrogen ratios in plants with different combinations of herbivory and nitrate supplementation. We caged Empoasca fabae (potato leafhopper) on fixing and non-fixing cultivars of M. sativa and supplemented soils with varied nitrate concentrations.

Results

We detected strong changes in legume above- and belowground allocation of fixed nitrogen in response to both herbivory and nitrate supply. Moderate nitrate soils, irrespective of herbivory, induced little to no fixed nitrogen allocation across both field and greenhouse experiments. In the field only, non-supplemented soil increased aboveground allocation of fixed nitrogen following herbivore damage but resulted in no changes belowground. In contrast, non-supplemented and high nitrate soils in the greenhouse increased above- and belowground fixed nitrogen allocation relative to moderate nitrate soils.

Conclusion

Our results demonstrate herbivory drives distinct plant allocation strategies across soil nitrate levels, advancing our understanding of how rhizobia influence legumes both above- and belowground. Herbivory-induced changes in rhizobia-legume symbioses are likely widespread across both agricultural and natural ecosystems.

中文翻译：

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草食性增强豆科植物-根瘤菌共生功能，增加生物固定氮的地上分配，但仅在没有额外硝酸盐的土壤中

目的

有益的土壤微生物，如根瘤菌，在豆科寄主植物的根部进行兼性共生，以交换氮以换取光合作用的产物，这些共生可以通过生物和非生物因素改变。在这里，我们研究了土壤硝酸盐供应和地上昆虫食草动物如何相互作用以影响苜蓿（苜蓿或苜蓿）的生物固氮。

方法

使用田间和温室实验，我们使用具有不同食草和硝酸盐补充组合的植物中的同位素氮比率量化了根瘤菌固定氮的地上和地下分配。我们将Empoasca fabae（马铃薯叶蝉）关在M. sativa 的固定和非固定品种上，并补充了不同硝酸盐浓度的土壤。

结果

我们检测到豆科植物地上和地下固定氮分配的强烈变化，以响应食草和硝酸盐供应。中度硝酸盐土壤，不考虑食草动物，在田间和温室实验中几乎没有或没有固定的氮分配。仅在田间，未补充的土壤在食草动物受损后增加了地上固定氮的分配，但未导致地下发生变化。相比之下，温室中未补充和高硝酸盐土壤相对于中等硝酸盐土壤增加了地上和地下的固定氮分配。

结论

我们的结果表明，食草动物在土壤硝酸盐水平上推动了不同的植物分配策略，促进了我们对根瘤菌如何影响地上和地下豆类的理解。食草引起的根瘤菌-豆科植物共生变化可能在农业和自然生态系统中广泛存在。