Department of Animal, Plant and Soil Sciences, Centre for AgriBioscience, La Trobe University, Melbourne Campus, Bundoora, VIC, Australia.
Ann Bot. 2019 Nov 27;124(6):1033-1042. doi: 10.1093/aob/mcz082.
The rhizosphere priming effect (RPE) has been explained from the perspective of microbial responses to root exudates and nutrient availability. This study introduced a chemical process that could also contribute to RPE: root exudates (organic acid ligands) could liberate mineral-protected carbon (C) in soil for microbial degradation.
Wheat (Triticum aestivum L.) near-isogenic lines varying in citrate efflux were grown for 6 weeks in a C4 soil supplied with either low (10 μg g-1) or high P (40 μg g-1). Total below-ground CO2 was trapped and partitioned for determination of soil organic C decomposition and RPE using a stable isotopic tracing technique. Mineral dissolution was examined by incubating soil with citric ligand at a series of concentrations.
High P increased RPE (81 %), shoot (32 %) and root biomass (57 %), root-derived CO2-C (20 %), microbial biomass C (28 %) and N (100%), soil respiration (20 %) and concentrations of water-extractable P (30 %), Fe (43 %) and Al (190 %), but decreased inorganic N in the rhizosphere. Compared with Egret-Burke, wheat line Egret-Burke TaMATE1B with citrate efflux had lower inorganic N, microbial biomass C (16 %) and N (30 %) in the rhizosphere but greater RPE (18 %), shoot biomass (12 %) and root-derived CO2-C (low P 36 %, high P 13 %). Egret-Burke TaMATE1B also had higher concentrations of water-extractable P, Fe and Al in the rhizosphere, indicating the release of mineral-protected C. In addition, citrate ligand facilitated Fe and Al release from soil, with their concentrations rising with increasing ligand concentration and incubation time.
While high P supply increased microbial growth and RPE possibly due to higher total root exudation, citrate efflux from the root might have facilitated the liberation of mineral-bound C, leading to the higher RPE under Egret-Burke TaMATE1B. Mineral dissolution may be an important process that regulates RPE and should be considered in future RPE research.
根际激发效应（RPE）已从微生物对根分泌物和养分供应的响应角度得到解释。本研究引入了一个可能有助于 RPE 的化学过程：根分泌物（有机酸配体）可以将土壤中受矿物质保护的碳（C）释放出来，以供微生物降解。
在供应低磷（10 μg g-1）或高磷（40 μg g-1）的 C4 土壤中，种植了根质体柠檬酸外排差异的小麦（Triticum aestivum L.）近等基因系，为期 6 周。通过稳定同位素示踪技术，收集并分离总地下 CO2，以确定土壤有机碳分解和 RPE。通过在一系列浓度下用柠檬酸配体孵育土壤，研究了矿物质的溶解情况。
高磷增加了 RPE（81%）、地上部分（32%）和根生物量（57%）、根衍生 CO2-C（20%）、微生物生物量 C（28%）和 N（100%）、土壤呼吸（20%）和水可提取磷（30%）、Fe（43%）和 Al（190%）的浓度，但降低了根际中的无机氮。与 Egret-Burke 相比，柠檬酸外排的小麦 Egret-Burke TaMATE1B 具有较低的根际无机氮、微生物生物量 C（16%）和 N（30%），但具有较高的 RPE（18%）、地上生物量（12%）和根衍生 CO2-C（低磷 36%，高磷 13%）。Egret-Burke TaMATE1B 还具有较高的水可提取磷、Fe 和 Al 浓度，表明受矿物质保护的 C 得到释放。此外，柠檬酸配体促进了土壤中 Fe 和 Al 的释放，其浓度随配体浓度和孵育时间的增加而升高。
虽然高磷供应可能由于总根分泌物的增加而增加了微生物的生长和 RPE，但根质体柠檬酸的外排可能促进了受矿物质保护的 C 的释放，导致 Egret-Burke TaMATE1B 下更高的 RPE。矿物质的溶解可能是调节 RPE 的一个重要过程，在未来的 RPE 研究中应予以考虑。
