| The most important physiological function of fine root is absorbing water and nutrients, to support tree growth. In root branching systems, first two orders were primarily absorptive roots, have much shorter life spans, but play a disproportionately important role in resource acquisition and ecosystem carbon and nutrient flux. The research in anatomical structure and nutrient (C and N) concentrations of first two orders has great significance for understanding the relationship between root structure and function, predicting root lifespan and estimating the contribution of first two orders in carbon and nitrogen cycles of forest ecosystem. In this study, seven important planting species in the Northeast China, Fraxinus mandshurica, Phellodendron amurense, Juglans mandshurica, Larix gmelinii, Pinus koraiensis, Pinus sylvestris var. mongolica and Picea koraiensis were used as experimental stands. We measured the anatomical structure, total C, total N, TNC and soluble N concentrations in the first two order roots, and analysed the correlations among the total C, total N, TNC and soluble N, and examined the effects of root anatomical structure on nutrient (C and N) concentrations, as well as the effects of root C and N concentrations on root life span. The results were showed as follows:(1) There were significant seasonal variations in anatomical structure of the first two order roots in seven tree species. In spring, roots have the smallest diameter and stele diameter, the lowest stele:root diameter ratio, and the thinnest cortex but highest cortex proportion and diarch roots percentage, whereas it is opposite in autumn. So roots may have stronger absorbing ability in spring than other seasons.(2) Among the first two order roots in seven tree species, TNC concentration was accounted for 26%～43% of total C concentration, soluble N concentration was accounted for 6%～28% of total N concentration. In seven tree species, fine root (1st-order,2nd-order) have the lowest total C but highest total N concentration in spring. The total C concentration appeared seasonal peak value in summer and total N concentration appear seasonal minimum value in autumn. In summer, root have the lowest TNC and soluble N concentrations. A significant negative correlation was found between total C and total N concentrations in the first two order roots in each tree species (P<0.01), but significant positive correlation between TNC and soluble N concentrations (P<0.01). TNC and soluble N concentrations significantly decreased with increasing total C concentration, but significantly increased with increasing total N concentration among the first two order roots. These results demonstrated that TNC, soluble N, total C, and total N concentrations were closely related in the first two order roots in these tree species.(3)There were significant correlations between anatomical structure and C and N concentrations in the first two order roots in seven tree species. In the first two order roots, TNC and soluble N concentrations significantly increased with increasing cortex thickness and cortex proportion (P<0.01), but decreased with increasing stele diameter and stele:root diameter ratio. The variety of total N concentration was same to soluble N concentration. However, total C concentration correlated positively with these four factors. The results suggest that the varieties of cortical thickness can cause the changes of N (total N and soluble N) and TNC concentrations, and total C concentration was affected together by cortical thickness and stele tissue in the first two order roots in these tree species.(4) In most tree species, mean life span of the first two orders was the shortest in spring. In spring, a significant positive correlation was found between TNC/soluble N ratio and mean life span (R2=0.715, P<0.01). The results indicated that in root C and N concentrations, TNC/soluble N ratio may play an key role in predicting the seasonal dynamics of fine roots. |
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