1. Photosynthetic carbon (C) allocated below-ground can be shared with mycorrhizal fungi in exchange for nutrients, but also added into soil as rhizodeposits that potentially increases plant nutrient supply by supporting microbial nutrient mineralization from organic matter. How water and nitrogen (N) availability affects plant C allocation to the rhizosphere, including both arbuscular mycorrhizal fungi (AMF) symbionts and rhizodeposits, remains largely unknown.
2. We used a ¹³CO₂ pulse labelling experiment to assess the effects of drought and N addition on below-ground allocation of C to soils and roots (quantified as excess ¹³C) and tested their relationships with AMF colonization in an Australian grassland. We also examined relationships between AMF and previously reported root respiration and decomposition of rhizodeposits in this study.
3. We found that drought decreased the absolute amount of excess ¹³C allocated to both soils and roots, likely due to less photosynthetic C fixation. In contrast, proportionally more excess ¹³C was allocated to soils but less to root biomass with drought, suggesting that relatively more C was allocated to rhizodeposits and to AMF hyphal growth and extension. However, N addition reversed drought effects on below-ground C allocation by retaining proportionally more excess ¹³C in roots and less in soils, congruent with higher soil N and phosphorus availability, root biomass and number of root tips compared to drought without N addition. This suggests that the alleviation of nutrient limitation promoted plants to expend relatively more C on root growth and root trait adjustment, but less C on rhizodeposition and mycorrhizal symbiosis.
4. Synthesis. Mycorrhizal colonization related negatively to rhizodeposit decomposition rate but positively to both excess ¹³C in root biomass and root respiration, suggesting a possible trade-off in C allocation between mycorrhizal symbiosis and rhizodeposition. We conclude that below-ground C allocation in this grassland can be mediated by mycorrhizal colonization and is strongly affected by water and nutrient availability.

中文翻译：

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根际碳分配受干旱和氮添加的影响

1. 分配在地下的光合碳 (C) 可以与菌根真菌共享以换取养分，但也可以作为根沉积物添加到土壤中，通过支持有机质中的微生物养分矿化来潜在地增加植物养分供应。水和氮 (N) 可用性如何影响植物 C 分配到根际，包括丛枝菌根真菌 (AMF) 共生体和根际沉积物，在很大程度上仍然未知。
2. 我们使用¹³ CO ₂脉冲标记实验来评估干旱和 N 添加对 C 到土壤和根部的地下分配（量化为过量¹³ C）的影响，并测试了它们与澳大利亚草原上 AMF 定植的关系。在本研究中，我们还研究了 AMF 与先前报道的根呼吸和根际沉积物分解之间的关系。
3. 我们发现干旱减少了分配给土壤和根部的过量¹³ C的绝对量，这可能是由于光合 C 固定减少了。相比之下，在干旱时，更多的过量¹³ C 分配给土壤，但较少分配给根生物量，这表明分配给根际沉积物和 AMF 菌丝生长和延伸的 C 相对较多。然而，氮的添加通过按比例保留更多的过量^{13 来}逆转干旱对地下碳分配的影响C 在根中而在土壤中较少，与没有添加 N 的干旱相比，与更高的土壤 N 和磷可用性、根生物量和根尖数量一致。这表明养分限制的缓解促使植物在根系生长和根系性状调整上消耗相对较多的碳，而在根系沉积和菌根共生上消耗较少的碳。
4. 合成。菌根定植与根沉积物分解率呈负相关，但与根生物量和根呼吸中超过¹³ C呈正相关，表明菌根共生和根沉积物之间的 C 分配可能存在权衡。我们得出的结论是，该草地的地下碳分配可以由菌根定植介导，并受水和养分可用性的强烈影响。