Carbon (C) allocation strategy plays a critical role in plant adaptability, with knock-on effects for community stability under environmental change. Based on optimal partitioning theory we asked two questions: (1) How is plant C allocation within tissues affected by long-term nutrient enrichment (N addition)? And (2) does N addition alter how plants allocate C under drought?

To address these questions, we conducted a greenhouse experiment using the widespread perennial C₃ grass, Leymus chinensis, under four treatments: “Watered”, “Dry”, “Watered+N” and “Dry+N”. ¹³CO₂ pulse labelling was used to trace C transport through the plant-soil system.

We found that, in combination, drought and N addition resulted in offsetting effects on C allocation. Greater aboveground biomass under N addition resulted in higher C loss via aboveground plant respiration even under drought, which plays a more important role in the adjustment of R/S ratio than does the trade-off between above and belowground compartments.

Compared to the concept of active phenotype adjustment for maximized growth rate in traditional optimal partitioning theory, our results imply that pre-drought allometry, which changes under long-term resource addition, also determine how plants respond to drought and their adaptability to changing environmental conditions.

The ¹³CO₂ pulse labeling experiments were immediately carried out after 35 days of drought treatment. Four pots from each of the treatments were randomly selected to measure natural background δ¹³C of each C pool from each block. The rest of the 36 pots in each block were equipped with a polymethyl methacrylate chamber (0.9 m × 0.6 m × 0.7 m, 95% light permeability) before labelling. To stabilize the transparent chamber and reduce leakage of ¹³CO₂, an iron groove with a seal ring was fixed to the soil in advance of pulse labeling. In addition to ice packs and four symmetrical electric fans, temperature stabilization and air circulation in the chambers was achieved by an external air circulation system composed of a pump and pressure-tight cool box. CO₂ concentration and interior air temperature were monitored by an infrared gas analyzer (LI-6400, LiCor Inc., Lincoln, NE, USA) and thermocouple thermometer. During pulse labelling, ¹³CO₂ (> 99.9% CO₂ with 99 atom% ¹³C, Cambridge Isotope Laboratories, Andover, MA, USA) was repeatedly added over a period of 90 min to maintain CO₂ concentrations between 330 - 480 ppm. All pulse labeling was conducted on two consecutive sunny days between 8:30 - 10:30 am.

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