Qiao Xu1, Xiaojuan Wang1, Caixian Tang1. 1. Department of Animal, Plant and Soil Sciences, Centre for AgriBioscience, La Trobe University, Melbourne Campus, Bundoora, VIC, Australia.
Abstract
BACKGROUNDS AND AIMS: 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. METHODS: 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. KEY RESULTS: 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. CONCLUSIONS: 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.
BACKGROUNDS AND AIMS: The rhizosphere priming effect (RPE) has been expn>lained from the perspn>ective of microbin>an class="Chemical">al 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. METHODS: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. KEY RESULTS: 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 TaMATE1Balso 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. CONCLUSIONS: 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.
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