Soil N enrichment mediates carbon allocation in a dominant grass during drought

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.