Nutrient heterogeneity in fine roots of six subtropical natural tree species

Abstract

Fine roots, commonly defined as roots < 2 mm in diameter, are often treated as a homogeneous mass compartment. However, this approach ignores the distinct branching structure of fine root systems, roots of different branch orders play different roles in belowground carbon and nutrient cycling. To date, it is remarkable how little we know about the chemical properties in fine root and the correlation between fine root and leaf in chemical properties, especially in subtropical evergreen broadleaved forest. So, in this study, intact fine root segments and leaves of six dominant tree species (Cinnamomum micranthum; Tsoongiodendron odorum Chun; Cinnamomum chekiangense; Castanopsis fabri; Altingia gracilipes; and Castanopsis carlesii) were sampled in an evergreen broadleaved forest located at the Wanmulin Nature Reserve, Jian'ou, Fujian province. We focused on: (1) the relationship between branch order and root nutrient concentrations; (2) the correlation between the chemical and morphological traits of fine root and between the chemical traits of root and leaf; (3) the effect of root order and tree species on root chemical traits. For Cinnamomum micranthum, C content increased as root order increased, while there was no distinct trend for the other five species. There was a negative relationship between root branch order and N and P concentration, and a positive relationship between root branch order and C/ N ratios for the six tree species, while the N/ P ratios had no distinct pattern with branch order. The one-way ANOVA demonstrated that tree species had significant effect on C,N,P concentration, C/ N ratio, and N/ P ratio (P < 0.01). Though root order had significant or marked effect on N,P concentration and C/ N ratio (P<0. 01, P<0. 05), it had no significant effect on C concentration and N/ P ratio (P>0. 05) for the six tree species. The two-way ANOVA showed that tree species ×root order interaction had significant or marked effect on C,N,P concentration and C/ N ratio (P<0. 01, P<0. 05), while it had no effect on N/ P ratio (P>0. 05). The analysis of correlation revealed that variation in C/ N ratio was determined by N concentration in the lower root order and by C concentration in the highest root order, and the variation in N/ P ratio was determined by N concentration. Among tree species, C concentration was coupled with N concentration in the higher root order and the N concentration coupled with P concentration in the lower root order. Specific root length was correlated with N and P concentrations as well as C/ N ratio (P <0. 01), but had no significant correlation with C concentration and N/ P ratio for the six tree species. The comparison among first root order, bulk fine root and leaf for the six tree species indicated that, while there was no significant correlation, N,P concentration and N/ P ratio of the first root order were more similar to those of leaf than those of bulk fine root. It concludes that there was large heterogeneity in nutrient traits among different root orders for these subtropical evergreen broadleaved species, especially N and P concentrations, and the root tip to some extent can serve as an analogue to leaf with regard to nutrient traits.