Six-Year Nitrogen-Water Interaction Shifts the Frequency Distribution and Size Inequality of the First-Order Roots of Fraxinus mandschurica in a Mixed Mature Pinus koraiensis Forest.

Cunguo Wang,Wei Guo,Zhenzhen Geng,Mai-He Li,Si Shen,Daming Jin,Tian-Hong Zhao,Zhao Chen,Jiandong Li,Ying Cao

doi:10.3389/fpls.2017.01691

Abstract

The variation in fine root traits in terms of size inequality at the individual root level can be identified as a strategy for adapting to the drastic changes in soil water and nutrient availabilities. The Gini and Lorenz asymmetry coefficients have been applied to describe the overall degree of size inequality, which, however, are neglected when conventional statistical means are calculated. Here, we used the Gini coefficient, Lorenz asymmetry coefficient and statistical mean in an investigation of Fraxinus mandschurica roots in a mixed mature Pinus koraiensis forest on Changbai Mountain, China. We analyzed 967 individual roots to determine the responses of length, diameter and area of the first-order roots and of branching intensity to 6 years of nitrogen addition (N), rainfall reduction (W) and their combination (NW). We found that first-order roots had a significantly greater average length and area but had smaller Gini coefficients in NW plots compared to in control plots (CK). Furthermore, the relationship between first-order root length and branching intensity was negative in CK, N, and W plots but positive in NW plots. The Lorenz asymmetry coefficient was >1 for the first-order root diameter in NW and W plots as well as for branching intensity in N plots. The bimodal frequency distribution of the first-order root length in NW plots differed clearly from the unimodal one in CK, N, and W plots. These results demonstrate that not only the mean but also the variation and the distribution mode of the first-order roots of F. mandschurica respond to soil nitrogen and water availability. The changes in size inequality of the first-order root traits suggest that Gini and Lorenz asymmetry coefficients can serve as informative parameters in ecological investigations of roots to improve our ability to predict how trees will respond to a changing climate at the individual root level.

Highlights

The first-order roots of the fine root system play a key role in nutrient and water absorption because they are located in the root hair zone (He et al, 2005), which is the metabolic hotspot for exudations that mobilize less mobile nutrients (Pregitzer, 2003) and is the point of association with symbiotic mycorrhizal fungi (Guo et al, 2008)
The results reported here suggest that the nitrogen–water interaction has a striking effect on the frequency distribution of first-order root length and area (Figures 1A, C), characterized by a lower branching intensity (Table 1) and a lower Gini coefficient of first-order root length in nitrogen with reduced rainfall (NW) treatment plots (Table 2)
Our work is clearly different from many previous studies that only characterized differences in fine root traits at the population level among species, with little information on how these traits vary at the individual level within the fine root system

Summary

Introduction

The first-order roots (the distal roots) of the fine root system play a key role in nutrient and water absorption because they are located in the root hair zone (He et al, 2005), which is the metabolic hotspot for exudations that mobilize less mobile nutrients (Pregitzer, 2003) and is the point of association with symbiotic mycorrhizal fungi (Guo et al, 2008). The results of fine root traits and their general response trends are typically depicted using means or medians and the associated statistics, such as standard errors or standard deviations (Lim et al, 2015; Liu et al, 2015). These metrics eliminate the variation at the individual root level based on the assumption that the variation is experimental error only (Trewavas, 2003). It may be important to pay attention to the ecological implications of individual root variation occurring within the fine root system, an important mechanism for adapting to the drastic variation in water and nutrient supplies occurring with global environment change (Forde, 2009; Russell et al, 2014; Zadworny et al, 2016)

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