Nitrogen addition changed the relationships of fine root respiration and biomass with key physiological traits in ozone-stressed poplars

Increasing ozone (O3) and nitrogen (N) addition may have contradictory effects on plant photosynthesis and growth. However, it remains unclear whether these effects on aboveground parts further change the root resource management strategy and the relationships of fine root respiration and biomass with other physiological traits. In this study, an open-top chamber experiment was conducted to investigate the effects of O3 alone and in combination with nitrogen (N) addition on root production and fine root respiration of poplar clone 107 (Populus × euramericana cv. ‘74/76’). Saplings were grown with (100 kg ha−1 year−1) or without (+0 kg ha−1 year−1) N addition under two O3 regimes (non-filtered ambient air or non-filtered ambient air + 60 ppb of O3). After about two to three months of treatment, elevated O3 significantly decreased fine root biomass and starch content but increased fine root respiration, which occurred in tandem with inhibited leaf light-saturated photosynthetic rate (Asat). Nitrogen addition did not change fine root respiration or biomass, neither did it alter the effect of elevated O3 on the fine root traits. However, N addition weakened the relationships of fine root respiration and biomass with Asat, fine root starch and N concentrations. No significant relationships of fine root biomass and respiration with soil mineralized N were observed under elevated O3 or N addition. These results imply that changed relationships of plant fine root traits under global changes should be considered into earth system process models to project more accurately future carbon cycle.