Environmental and Experimental Botany 44 2000 207 – 219
Uptake capacity of amino acids by ten grasses and forbs in relation to soil acidity and nitrogen availability
U. Falkengren-Grerup , K.F. Ma˚nsson, M.O. Olsson
Department of Ecology, Plant Ecology, Lund Uni6ersity, Ecology Building, S-
223 62
Lund, Sweden Received 12 October 1999; received in revised form 6 July 2000; accepted 6 July 2000
Abstract
Uptake capacity of organic nitrogen was studied in solution experiments on eight grasses and two forbs growing in acid soils with relatively high nitrogen mineralisation in southern Sweden. Uptake of a mixture of amino acids
alanine, glutamine, glycine, that varied between 1.6 and 6.3 mmol g
− 1
dw root h
− 1
, could not be explained by soil data from the species’ field distributions pH, total carbon and nitrogen, potential net mineralisation of ammonium
and nitrate. The ratio between organic and inorganic nitrogen methylamine uptake was B 0.05 for the forbs, higher for the grasses with a maximum of 1.42 for Deschampsia flexuosa. The ratio was negatively correlated with measures
related to soil acidity Ellenberg’s R-value, soil nitrate and total carbon but not, as hypothesised, with the total amount of mineralised nitrogen. The total demand on nitrogen by all components of the ecosystem would probably
have described the extent to which competition among and between plants and microbes induced nitrogen limitation. In a methodological study two grasses were exposed to pH 3.8, 4.5 and 6.0 and to 50, 100 and 250 mmol l
− 1
of three amino acids. Uptake was also compared between intact plants and excised roots. The treatment response varied
considerably between the species which stresses the importance of studying intact plants at field-relevant pH and concentrations. © 2000 Elsevier Science B.V. All rights reserved.
Keywords
:
Alanine; Deschampsia flexuosa; Elymus caninus; Excised roots; Glutamine; Glycine; Soil pH www.elsevier.comlocateenvexpbot
1. Introduction
There is a growing interest in the capacity of plants to take up organic nitrogen and of the
significance it has for the plants’ nitrogen nutri- tion Stribley and Read, 1980; Chapin et al., 1993;
Kielland, 1994, 1997; Raab et al., 1996, 1999. Early reports usually reflected the physiological
aspect, investigating concentration dependence of the amino acid uptake and the uptake systems
King, 1976; Soldal and Nissen, 1978 whereas the ecological significance of a range of species and
geographical areas has come into focus in several later papers. Species studied have been predomi-
nantly the non-mycorrhizal sedges Chapin et al., 1993; Kielland, 1994; Raab et al., 1996 and
woody,
ericoid and
ectomycorrhizial species
Corresponding author. Tel.: + 46-46-2224408; fax: + 46- 46-2224423.
E-mail address
:
ursula.falkengren-grerupplanteco.lu.se U. Falkengren-Grerup.
S0098-847200 - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 9 8 - 8 4 7 2 0 0 0 0 0 6 8 - X
Stribley and Read, 1980; Bajwa and Read, 1985; Abuzinadah and Read, 1989; Kielland, 1994;
Na¨sholm et al., 1998. Plants with ectomycorrhiza and ericoid mycorrhiza, growing in soils with low
mineralisation of inorganic nitrogen, have often been considered to be especially efficient in using
organic nitrogen Abuzinadah and Read, 1986. Organic nitrogen uptake has rarely been consid-
ered in arbuscular mycorrhizal species and then usually in cultivated plant species Schobert,
et al., 1988; Chapin et al., 1993; Jones and Dar- rah, 1994. The uptake supplied by the fungi itself
is usually low compared to the total demand by the plants George et al., 1995.
There is a large temporal and spatial variation in availability of amino acids in soils and a rapid
turnover into other organic or inorganic com- pounds. Soil solution concentrations in organic
soils in temperate and arctic regions are com- monly 5 – 15 mmol l
− 1
of amino acids such as alanine, aspartic acid, glutamic acid and glycine
and analysed amino acids may add up to 100 m
mol l
− 1
Abuarghub and Read, 1988; Chapin et al., 1993; Kielland, 1994; Raab et al., 1996,
1999. A high seasonal variation in total amino acids is exemplified in the study on Cyperaceae
species in four alpine meadows, being between 13 and 158 mmol l
− 1
Raab et al., 1999. Some results indicate that higher concentrations are
found in soil solutions in highly organic soils Raab et al., 1996, which the few results on
brown soils, as compared with podsols, also show when the amounts per g soil are similar Abuar-
ghub and Read, 1988.
Studies on organic nitrogen uptake by wild plants have been based largely on species from
infertile and highly organic soils where nitrogen mineralisation rates are low Chapin et al., 1993;
Kielland, 1994, 1997; Na¨sholm et al., 1998;Raab et al., 1999. In a study by Kielland 1994
on arctic soils glycine, aspartic acid and glutamic acid accounted for as much as 80 of the total
nitrogen uptake in Ledum palustre, while the same amino acids accounted for only 10 in
Eriophorum angustifolium. Organic nitrogen up- take has rarely been studied in more fertile soils
where mineralisation rates are relatively high. Ni- trogen is often the limiting nutrient even in these
soils, as stated for example for temperate decidu- ous forest soils of varying soil fertility Tamm,
1991. Laboratory experiments have frequently been
used to study amino acid uptake. Although a few studies have shown that pH has a significant
effect on uptake rates Soldal and Nissen, 1978 most experiments did not adjust pH to field-rele-
vant levels. The possibilities of transferring exper- imental results to field conditions are conditioned
by the experimental set-up, e.g. use of excised roots instead of intact plants, pH of solution and
concentrations of amino acids. Excision reduces water movement caused by shoot transpiration,
ceasing the passive movement of ions and disrupt- ing the active translocation to the shoot. Uptake
may further be inhibited by accumulation of ions in the roots and by a limitation of carbohydrates
needed for energy-dependent absorption of ions. The effects of excision on uptake seem to be
ion-specific. Uptake of, for example, ammonium and nitrate decreased to 25 of the uptake by
intact plants Bloom and Caldwell, 1988, whereas uptake of potassium was initially low but in-
creased during the experiment to levels close to those attained by intact plants Glass, 1978;
Bloom and Caldwell, 1988. It is, however, also argued that accurate laboratory methods give
similar uptake of potassium by excised roots and intact plants Huang et al., 1992. Most results
refer to studies on one species and there is only indirect evidence that species may respond in dif-
ferent ways to excision Huang et al., 1992. The various degrees to which species’ uptake is de-
pressed by root excision or influenced by concen- tration or pH of the tested nitrogen sources are
little known. A concentration-dependent uptake has been shown for single species but fewer stud-
ies compare species from similar habitats nor do they discuss the relevance of the chosen ex-
perimental set-up e.g. Kielland, 1994. Knowl- edge of uptake mechanisms of amino acids is still
limited and multiple sets of specific or general amino acid transport proteins seem to be possible
Fischer et al., 1998. Our study will therefore address both methodological and ecological as-
pects on the uptake of organic nitrogen in wild plants. As our study concerns uptake by seedlings
under laboratory conditions our results must be validated for soil conditions in the field and for
mature plants infected by arbuscular mycorrhiza. The following hypotheses are tested:
1. The capacity to take up amino acids varies between species and amino acids and depends
on concentration and pH in the solution. 2. Uptake is sink-source driven. Experiments
with excised roots, as compared to intact plants, underestimates uptake and to various
degrees in different species. 3. A species’ uptake capacity of amino acids is
negatively related to availability of inorganic nitrogen in the soil. This is reflected by varying
nitrogen mineralisation in soils of the species’ field distributions.
4. Uptake of amino acids relative to inorganic nitrogen is low for all studied species. The
importance of uptake of organic to inorganic nitrogen is highest in soils with low availability
of inorganic nitrogen.
2. Material and methods
