Elevated CO2 (eCO2) and high temperatures are known to affect plant nitrogen (N) metabolism. Though the combined effects of eCO2 and chronic warming on plant N relations have been studied in some detail, a comprehensive statistical review on this topic is lacking. This meta-analysis examined the effects of eCO2 plus warming on shoot and root %N, tissue protein concentration (root, shoot and grain) and N-uptake rate. In the analyses, the eCO2 treatment was categorized into two classes (<300 or ≥300 ppm above ambient or control), the temperature treatment was categorized into three classes (<1.5, 1.5–5 and >5 °C above ambient or control), plant species were categorized based on growth form and functional group and CO2 treatment technique was also investigated. Elevated CO2 alone or in combination with warming reduced shoot %N (more so at ≥300 vs. <300 ppm above ambient CO2), while root %N was significantly reduced only by eCO2; warming alone often increased shoot %N, but mostly did not affect root %N. Decreased shoot %N with eCO2 alone or eCO2 plus warming was greater for woody and non-woody dicots than for grasses, and for legumes than non-legumes. Though root N-uptake rate was unaffected by eCO2, eCO2 plus warming decreased N-uptake rate, while warming alone increased it. Similar to %N, protein concentration decreased with eCO2 in shoots and grain (but not roots), increased with warming in grain and decreased with eCO2 and warming in grain. In summary, any benefits of warming to plant N status and root N-uptake rate will generally be offset by negative effects of eCO2. Hence, concomitant increases in CO2 and temperature are likely to negate or decrease the nutritional quality of plant tissue consumed as food by decreasing shoot %N and shoot and/or grain protein concentration, caused, at least in part, by decreased root N-uptake rate.

中文翻译：

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二氧化碳升高和慢性变暖对植物 %N、蛋白质含量和 N 吸收率的综合影响的荟萃分析

已知升高的 CO2 (eCO2) 和高温会影响植物氮 (N) 代谢。尽管已经对 eCO2 和长期变暖对植物 N 关系的综合影响进行了一些详细的研究，但缺乏对该主题的全面统计回顾。这项荟萃分析检查了 eCO2 加上变暖对枝条和根 %N、组织蛋白浓度（根、枝条和籽粒）和 N 吸收率的影响。在分析中，eCO2 处理分为两类（高于环境或对照<300 或≥300 ppm），温度处理分为三类（高于环境<1.5、1.5-5 和>5°C或对照），根据生长形式和功能组对植物物种进行分类，并研究了CO2处理技术。单独升高的 CO2 或与变暖相结合会降低枝条 %N（在高于环境 CO2 ≥300 对 <300 ppm 时更是如此），而根部 %N 仅通过 eCO2 显着降低；单独变暖通常会增加枝条 %N，但大多不影响根 %N。单独使用 eCO2 或 eCO2 加温后，木质和非木质双子叶植物的枝条 %N 降低幅度大于草，豆科植物比非豆科植物更大。虽然根系 N 吸收率不受 eCO2 的影响，但 eCO2 加上变暖会降低 N 吸收率，而单独变暖会增加它。与 %N 相似，芽和谷粒（但不是根）中的蛋白质浓度随着 eCO2 降低，随着谷粒变暖而增加，随着 eCO2 和谷粒变暖而降低。总之，变暖对植物氮状态和根系氮吸收率的任何好处通常会被 eCO2 的负面影响所抵消。因此，