Organ carbon reallocation and deep nitrogen acquisition enable rubber tree adaptation to harvest stress: Revealed by nonlinear isotopic optimization

The carbon cycle paradox in tropical rubber plantations—Sustained latex carbon export without triggering the expected photosynthetic compensatory enhancement—challenges our understanding of managed forest carbon balance. Using year–round δ 13 C and δ 15 N measurements across three plant organs (leaf, stem, root) and various soil depths in Xishuangbanna rubber plantations, we developed a nonlinear programming framework that simultaneously optimizes isotope source contributions and fractionation values to quantitatively resolve seasonal dynamics of plant and soil carbon–nitrogen sources. Results show: (1) when NLP optimized, time–varying fractionation values were used, nonlinear optimization achieved 2–4 fold higher precision than Bayesian methods, effectively addressing fractionation variability; (2) rubber tree leaf carbon relied primarily on inter–organ reallocation (91%, stem 55.9%, root 35.1%) rather than photosynthetic fixation (9%), challenging the traditional photosynthesis–centric theory; (3) nitrogen acquisition strategies exhibited seasonal vertical shifts, with deep soil contributions reaching 50% in pre–rainy season, reflecting depth–dependent nitrogen processes; (4) carbon–nitrogen responses showed temporal scale differentiation, with 7–day lag for carbon vs. 1–day rapid response for nitrogen systems, and relative humidity > 50% was strongly associated with a switch in nitrogen source acquisition strategies. The study unveils dual adaptive mechanisms in rubber trees under harvest stress–organ carbon reallocation and deep nitrogen acquisition–and provides a new analytical paradigm for isotopic ecology; these insights may inform sustainable management of tropical plantations. • Time–varying fractionation NLP is 2–4 × more precise than MixSIAR. • Rubber tree leaves obtain 91% carbon from internal reallocation. • Deep soil nitrogen increases to ∼50% contribution during pre-rainy season. • Plant carbon lags precipitation by ∼7 days; N responds within ∼1 day to humidity. • Humidity > 50% is associated with shifts in N–source use.