High Heterogeneity of Root Carbon Allocation Affects Root Turnover Rate and Production of Bothriochloa ischaemum Under Drought Stress

The mechanism that causes the difference in carbon (C) turnover rate in root populations is unclear. The carbon utilization strategy is assumed to be the main causal factor responsible for differences in root turnover rate. In this study, we determined the correlations between root turnover rate, production, and proportions of C allocated to roots using 13 CO 2 as a labeling gas in a 13 C pulse labeling experiment. The proportions of δ 13 C were measured in various organs of the grass Bothriochloa ischaemum sampled 0, 6, 24, 48, 216, and 360 h after labeling in three treatments: control (CK), mild water stress (MS), and serious water stress (SS). We found that drought stress increased short-term C allocation to belowground. Fine roots have stronger C demand than coarse root under drought condition. The amount of 13 C gradually decreased in leaves and increased in soil with time after 13 C pulse labeling. Stem 13 C increased with the level of stress and peaked at 24 h, while both fine- and coarse-root 13 C peaked at 216 h. 13 C distributed to fine roots in MS was significantly higher than in the other treatments at 216 h. The fine-root turnover rate in SS treatment was positively correlated with root biomass but not the amount of 13 C. Larger C allocation to roots increased fine-root mass in MS, stimulated rapid fine-root turnover, and increased C input to both the rhizosphere and soil. The fine-root turnover in CK was significantly positively correlated with both 13 C amount and biomass, which indicated that increasing short-term C input accelerated turnover in the fine-root pool. The C allocation difference between the fine roots and coarse roots may be a key cause of the different turnover rate in the root population.