Spatial distribution of rhizodeposit carbon of maize (Zea mays L.) in soil aggregates assessed by multiple pulse 13C labeling in the field

Rhizosphere effect is controlled by spatial distribution of rhizodeposits, which may be influenced by soil aggregation and soil moisture regime in relation to water uptake by roots. The objectives of this study were to measure soil organic carbon (SOC) concentration and its δ 13 C abundance by aggregate size in the rooted bulk soil and by distance in the root-free soil vertically and horizontally away from roots, and to measure DOC concentration and its δ 13 C abundance in pore water in the rooted bulk soil after a seasonal pulse labelings of 13 CO 2 to maize ( Zea mays L.). Pulse labeling was conducted in the field once a week for 11 weeks. Soil cells (50 mm in diameter and 100 mm long) mimicking root-free soils were imbedded vertically and horizontally 25–50 mm away from the main root of a maize crop. The rooted bulk soils were sampled to extract soil pore water at different suctions and to fractionate aggregates by wet sieving. The root-free soil cells were sliced by 1 mm intervals from the root end to 20 mm away. All the sampling was 12 days after the last labeling after the crop was harvested. The δ 13 C abundance before and after the continuous labeling was −24.20 ± 0.05 ‰ and −23.80 ± 0.05 ‰ in the rooted bulk soil. The labeling caused increases in δ 13 C abundance in all the aggregates in the rooted bulk soil and down to 14 mm away from the roots in both the root-free cells. The δ 13 C abundance was enriched in the >2 mm and 1–2 mm aggregates (−23.17 ± 0.12 ‰ and −23.26 ± 0.05 ‰) though the SOC concentration was not different among the >0.25 mm aggregates, indicating that rhizodeposits or their metabolites were protected and distributed widely in whole soil through soil aggregation. The δ 13 C abundance in pore water (−24.0 ± 0.01 ‰) was much lower than those soil aggregates and greatest from the >2 μm soil pores though the DOC concentration was greater from the <20 μm soil pores. The δ 13 C abundance was in general greater in the horizontal cell than in the vertical cell. The δ 13 C abundance decreased with the increasing distance to the roots in the vertical cell and peaked at the 5 and 6 mm distance to the roots in the horizontal cell (−23.66 ± 0.11 ‰ and −23.5 ± 0.10 ‰), possibly due to the drier condition unfavorable to microbial decomposition in the horizontal cell. The higher δ 13 C abundance in the horizontal cell than in the vertical cell was accompanied by a lower SOC concentration and a lower C: N ratio within 3 mm away from the roots, suggesting a stronger priming effect due to the longer residence time of rhizodeposits in the horizontal cell than in the vertical cell. Rhizodeposits or their metabolites were protected during soil aggregation and distributed to 14 mm beyond the rhizosphere in the natural soil-plant system. This extension is of significance in regulating the formation of soil structure and the priming of soil organic matter during the whole life cycle of plants, which needs further study.