Carbon allocation to the rhizosphere is affected by drought and nitrogen addition

Abstract 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. We used a 13 CO 2 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 13 C) 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. We found that drought decreased the absolute amount of excess 13 C allocated to both soils and roots, likely due to less photosynthetic C fixation. In contrast, proportionally more excess 13 C 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 13 C 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. Synthesis . Mycorrhizal colonization related negatively to rhizodeposit decomposition rate but positively to both excess 13 C 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.