Nutrient Concentrations In Fine Roots

Ecology, Jan, 2000 by Wendy S. Gordon, Robert B. Jackson

WENDY S. GORDON [1]

ROBERT B. JACKSON [2]

Section of Integrative Biology, University of Texas at Austin, Austin, Texas 78712 USA

Abstract. Fine roots are an important source and sink for nutrients in terrestrial biogeochemistry. We examined the following hypotheses for fine root nutrients by analyzing data from 56 published studies: (1) that there is a general, inverse relationship of fine root nutrient concentrations with root diameter, and (2) that retranslocation of nutrients out of fine roots is minimal. We analyzed nutrient concentrations of roots [less than or equal to]5 mm in diameter as a function of root diameter and root status (live, dead, and undifferentiated), including a comparison for coniferous and broad-leaved trees. For fine roots [less than]2 mm in diameter, average C:N and C:N:P ratios were 43:1 and 522:12:1, significantly narrower than for 2--5 mm roots (79:1 and 920:12:1). Live roots [less than]2 mm in diameter contained significantly more N, P, and Mg and less C than did roots 2-5 mm in diameter, but no significant differences were observed for K or Ca. Mean N and P concentrations were 11.0 and 0.9 g/kg, respect ively, for live roots [less than]2 mm diameter, compared to 6.5 and 0.6 g/kg in roots 2-5 mm in diameter. Mean N concentrations in live and dead fine roots were identical and may imply little retranslocation of root N with senescence, but conflicting evidence from Ca:N ratios highlights the need for further research. These results have practical implications for various ecological methods and for the representation of roots in biogeochemical models.

Key words: C:N:P ratios; calcium; carbon; coniferous vs. broadleaf trees; fine roots; magnesium; nitrogen; nutrient concentrations and retranslocation; phosphorus; potassium.

INTRODUCTION

Fine roots are an important source and sink for nutrients in terrestrial ecosystems. Plants depend on fine roots ([less than]2 mm in diameter) for water and mineral uptake. Across a range of ecosystems, net primary production can be greater belowground than above (e.g., Caldwell 1987), and nutrient concentrations in fine roots may be higher than those in foliage (e.g., Meier et al. 1985) and their life-spans considerably shorter (Vogt et al. 1983). Nutrient release from decomposing roots is a pathway of significant nutrient flux in terrestrial ecosystems (Joslin and Henderson 1987, Fahey et al. 1988). In forests, for example, the amount of carbon and nutrients returned to the soil from fine root turnover may equal or exceed that from leaf litter (Joslin and Henderson 1987, Raich and Nadelhoffer 1989). Minimal retranslocation of nutrients from roots upon senescence may also contribute to the importance of fine roots in nutrient cycling (Aerts 1990, Nambiar and Fife 1991).

Given the relatively short life-spans of fine roots, understanding the relationship between fine root diameter and nutrient contents, and the extent to which nutrients are retranslocated prior to turnover, is important for estimating nutrient cycling in terrestrial ecosystems. In this paper we compile data from the literature to test the generality of an observed inverse relationship between nutrient concentrations and root diameter. We also apply our findings to the important, though unresolved, issue of nutrient retranslocation from fine roots. We consider six essential elements--carbon, nitrogen, phosphorous, potassium, calcium, and magnesium--selecting these elements because of their importance to a range of plant physiological activities and because sufficient data exist for their analysis. We use the term nutrient to encompass all six elements, even though carbon is not commonly considered a plant nutrient.

METHODS

To study patterns of nutrient concentrations in fine roots, we examined published accounts for roots [less than or equal] mm in diameter, building on the database of Jackson et al. (1996, 1997). The synthesized studies included data from a range of ecosystems and biomes, including grass, shrub, and tree functional types from temperate, tropical, boreal, and tundra systems. The preponderance of data came from experiments with temperate and coniferous trees. Study sites were a mixture of natural and manipulated ecosystems, including old growth, secondary growth, old fields, and plantations. Data from fertilized systems were excluded, as were results from pot or greenhouse experiments and studies with seedlings. Criteria for inclusion were that each study provide information on root status (i.e., live, dead, or undifferentiated) and that if a range of root sizes were reported (e.g., 1-3 mm, 3-5 mm) the reported range did not exceed 2.5 mm. For information on the 56 studies used for the database and the spreadsh eet used in the calculations see the Appendix.

To promote comparability across studies, we adopted additional conventions for analyzing the data. The maximum soil depth sampled in all studies was 1 m, but the vast majority of experiments looked at roots from a single layer [less than or equal] 30 cm deep; in the few cases where multiple depths were sampled we averaged across depths for a particular study. If data from multiple sites were reported, the data were pooled unless the sites differed in a meaningful way such as soil type, species composition, or climatic variables. Values from stands of different ages were also averaged. To prevent a single study from disproportionately influencing results, no study contributed more than four values to a given analysis.