Institute of Plant Science and Genetics in Agriculture, The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, Rehovot, 7610001, Israel.
Department of Natural Resources, Beef Cattle Section, Newe Ya'ar Research Center, Agricultural Research Organization, Ramat Yishay, 30095, Israel.
New Phytol. 2019 Mar;221(4):1866-1877. doi: 10.1111/nph.15526. Epub 2018 Nov 10.
Plants, especially perennials, growing in drylands and seasonally dry ecosystems are uniquely adapted to dry conditions. Legume shrubs and trees, capable of symbiotic dinitrogen (N ) fixation, often dominate in drylands. However, the strategies that allow symbiotic fixation in these ecosystems, and their influence on the nitrogen cycle, are largely unresolved. We evaluated the climatic, biogeochemical and ontogenetic factors influencing nitrogen fixation in an abundant Mediterranean legume shrub, Calicotome villosa. We measured nodulation, fixation rate, nitrogen allocation and soil biogeochemistry in three field sites over a full year. A controlled experiment evaluated differences in plant regulation of fixation as a function of soil nutrient availability and seedling and adult developmental stages. We found a strong seasonal pattern, shifting between high fixation rates during the rainy season at flowering and seed-set times to almost none in the rainless season. Under controlled conditions, plants downregulated fixation in response to soil nitrogen availability, but this response was stronger in seedlings than in adult shrubs. Finally, we did not find elevated soil nitrogen under N -fixing shrubs. We conclude that seasonal nitrogen fixation, regulation of fixation, and nitrogen conservation are key adaptations influencing the dominance of dryland legumes in the community, with broader consequences on the ecosystem nitrogen cycle.
生长在旱地和季节性干燥生态系统中的植物，尤其是多年生植物，具有独特的适应干旱条件的能力。能够进行共生固氮的豆科灌木和乔木，通常在旱地中占主导地位。然而，这些生态系统中允许共生固定的策略及其对氮循环的影响在很大程度上仍未得到解决。我们评估了影响丰富的地中海豆科灌木 Calicotome villosa 固氮的气候、生物地球化学和个体发育因素。我们在一个完整的年度内在三个野外地点测量了结瘤、固氮率、氮分配和土壤生物地球化学。一项对照实验评估了植物对固定作用的调节随土壤养分供应以及幼苗和成年发育阶段的差异。我们发现了一个强烈的季节性模式，在雨季开花和种子形成期间，固氮率很高，而在无雨季节几乎没有固氮。在受控条件下，植物会根据土壤氮供应而下调固氮，但这种反应在幼苗中比在成年灌木中更强。最后，我们在固氮灌木下没有发现土壤氮升高。我们得出的结论是，季节性氮固定、固定作用的调节和氮的保存是影响旱地豆科植物在群落中占主导地位的关键适应，对生态系统氮循环有更广泛的影响。
