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  Contribution of roots to soil organic carbon: From growth to decomposition experiment

  Carbon (C) derived from roots, rhizodeposition of living roots and dead root decomposition, plays a critical role in soil organic C (SOC) sequestration. Recent studies suggest that root inputs exert a disproportionate influence on SOC formation, and therefore, it is necessary to test separately the effects during root growth (i.e. rhizodeposition) and dead root decomposition (i.e. belowground litter) undernaturalconditions. A field experiment was carried out in grasslands with three typical plants: Stipa bungeana (St.B), Artemisia sacrorum (Ar.S), and Thymus mongolicus (Th.M) to differentiate the effects of root growth vs decomposition on C inputs by using in-growth soil cores and litter bag methods. For root growth experiment, the SOC content increased (2.5–3.8 g·kg−1) with root biomass, especially in soil under Ar.S. C input increased mineral-associated organic C (MAOC%) from 57% to 65%. The SOC δ13C was consistent with roots δ13C, indicating the roots were the primary source of SOC. The root decomposition experiment showed that the increase in SOC (0.74–2.6 g·kg−1) was highest at 90 days. Root decomposition rate was fast before 45 days, and SOC increased with the MAOC% during this period. After 45 days, particulate organic C % (POC%) was raised and was higher than in the control soil. The increase in SOC under root growth (2.5–3.8 g·kg−1) after one year was greater as compared to the rate under root decomposition (-0.4–0 g·kg−1). It suggested that C released during root growth was more effectively retained in the soil than that caused by dead root decomposition. The rise in MAOC during root growth and decomposition mainly explains the increase in SOC. Our results provided direct field based evidence illustrating the specific contribution of root growth and decomposition to SOC.
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  Carbon allocation to the rhizosphere is affected by drought and nitrogen addition

  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 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. 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. 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.