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Tree roots are highly heterogeneous in form and function. Previous studies revealed that fine root respiration was related to root morphology, tissue nitrogen (N) concentration and temperature, and varied with both soil depth and season. The underlying mechanisms governing the relationship between root respiration and root morphology, chemistry and anatomy along the root branch order have not been addressed. Here, we examined these relationships of the first- to fifth-order roots for near surface roots (0-10 cm) of 22-year-old larch (Larix gmelinii L.) and ash (Fraxinus mandshurica L.) plantations. Root respiration rate at 18 °C was measured by gas phase O2 electrodes across the first five branching order roots (the distal roots numbered as first order) at three times of the year. Root parameters of root diameter, specific root length (SRL), tissue N concentration, total non-structural carbohydrates (starch and soluble sugar) concentration (TNC), cortical thickness and stele diameter were also measured concurrently. With increasing root order, root diameter, TNC and the ratio of root TNC to tissue N concentration increased, while the SRL, tissue N concentration and cortical proportion decreased. Root respiration rate also monotonically decreased with increasing root order in both species. Cortical tissue (including exodermis, cortical parenchyma and endodermis) was present in the first three order roots, and cross sections of the cortex for the first-order root accounted for 68% (larch) and 86% (ash) of the total cross section of the root. Root respiration was closely related to root traits such as diameter, SRL, tissue N concentration, root TNC : tissue N ratio and stele-to-root diameter proportion among the first five orders, which explained up to 81-94% of variation in the rate of root respiration for larch and up to 83-93% for ash. These results suggest that the systematic variations of root respiration rate within tree fine root system are possibly due to the changes of tissue N concentration and anatomical structure along root branch orders in both tree species, which provide deeper understanding in the mechanism of how root traits affect root respiration in woody plants.

Root respiration is closely related to root morphology, yet it is unclear precisely how to distinguish respiration-related root physiological functions within the branching fine root system. Root respiration and tissue N concentration were examined for different N fertilization treatments, sampling dates, branch orders and temperatures of larch (Larix gmelinii L.) and ash (Fraxinus mandshurica L.) using the excised roots method. The results showed that N fertilization enhanced both root respiration and tissue N concentration for all five branch orders. The greatest increases in average root respiration for N fertilization treatment were 13.30% in larch and 18.25% in ash at 6°C. However, N fertilization did not change the seasonal dynamics of root respiration. Both root respiration and root tissue N concentration decreased with increase in root branch order. First-order (finest) roots exhibited the highest respiration rates and tissue N concentrations out of the five root branch orders examined. There was a highly significant linear relationship between fine root N concentration and root respiration rate. Root N concentration explained >60% of the variation in respiration rate at any given combination of root order and temperature. Root respiration showed a classical exponential relationship with temperature, with the Q(10) for root respiration in roots of different branching orders ranging from 1.62 to 2.20. The variation in root respiration by order illustrates that first-order roots are more metabolically active, suggesting that roots at different branch order positions have different physiological functions. The highly significant relationship between root respiration at different branch orders and root tissue N concentration suggests that root tissue N concentration may be used as a surrogate for root respiration, simplifying future research into the C dynamics of rooting systems.