Significantly more carbon (C) is stored in deep soil than in shallow horizons, yet how the decomposition of deep soil organic C (SOC) will respond to rising temperature remains unexplored on large scales, leading to considerable uncertainties to predictions of the magnitude and direction of C‐cycle feedbacks to climate change. Herein, short‐term temperature sensitivity of SOC decomposition (expressed as Q₁₀) from six depths within the top 1 m soil from 90 upland forest sites (540 soil samples) across China is reported. Results show that Q₁₀ significantly increases with soil depth, suggesting that deep SOC is more vulnerable to loss with rising temperature in comparison to shallow SOC. Climate is the primary regulator of shallow soil Q₁₀ but its relative influence declines with depth; in contrast, soil C quality has a minor influence on Q₁₀ in shallow soil but increases its influence with depth. When considering the depth‐dependent Q₁₀ variations, results further show that using the thermal response of shallow soil layer for the whole soil profile, as is usually done in model predictions, would significantly underestimate soil C‐climate feedbacks. The results highlight that Earth system models need to consider multilayer soil C dynamics and their controls to improve prediction accuracy.

Plant respiration can acclimate to changing environmental conditions and vary between species as well as biome types, although belowground respiration responses to ongoing climate warming are not well understood. Understanding the thermal acclimation capacity of root respiration (Rroot) in relation to increasing temperatures is therefore critical in elucidating a key uncertainty in plant function in response to warming. However, the degree of temperature acclimation of Rroot in rainforest trees and how root chemical and morphological traits are related to acclimation is unknown. Here we investigated the extent to which respiration of fine roots (≤2 mm) of four tropical and four warm-temperate rainforest tree seedlings differed in response to warmer growth temperatures (control and +6 °C), including temperature sensitivity (Q10) and the degree of acclimation of Rroot. Regardless of biome type, we found no consistent pattern in the short-term temperature responses of Rroot to elevated growth temperature: a significant reduction in the temperature response of Rroot to +6 °C treatment was only observed for a tropical species, Cryptocarya mackinnoniana, whereas the other seven species had either some stimulation or no alteration. Across species, Rroot was positively correlated with root tissue nitrogen concentration (mg g−1), while Q10 was positively correlated with root tissue density (g cm−3). Warming increased root tissue density by 20.8% but did not alter root nitrogen across species. We conclude that thermal acclimation capacity of Rroot to warming is species-specific and suggest that root tissue density is a useful predictor of Rroot and its thermal responses in rainforest tree seedlings.