Does nitrogen concentration affect relative uptake rates of nitrate, ammonium, and glycine?

Abstract

Plant roots are exposed to a variety of nitrogen forms (e.g., nitrate, ammonium, amino acids) and take up these forms at different rates. Many studies have investigated whether plants prefer nitrate, ammonium, or amino acids; but studies may not be comparable because they used substrate concentrations between 100 and 2000 μmol L–1. This study tests the hypothesis that substrate concentrations from 10 to 1750 μmol L–1 affect plant preference for N forms. Nitrogen uptake by the herb Ocimum basilicum and the evergreen tree Eucalyptus regnans was examined by placing roots of intact seedlings in equimolar mixtures of nitrate, ammonium, and glycine in which one of the N forms was 15N‐labelled (and 13C‐labelled in the case of glycine). In both species, preference for N forms was affected by substrate concentration. At 10 μmol L–1 (O. basilicum) or 10 and 50 μmol L–1 (E. regnans), rates of N uptake did not differ among N forms. At substrate concentrations of 50 μmol L–1 and greater O. basilicum took up ammonium the fastest, glycine the slowest, and nitrate at an intermediate rate. At substrate concentrations from 100 to 1750 μmol L–1, E. regnans took up ammonium the fastest with glycine and nitrate taken up at slower rates. The absence of significant differences at lower concentrations was a true biological effect rather than a function of larger relative errors. This study demonstrates that substrate concentration has a large effect on plant preference for N forms, and sounds a warning for studies of N nutrition that do not consider the concentration‐dependence of plant preference for N forms.

Inorganic and organic nitrogen uptake by nine dominant subtropical tree species

“…These dominant trees showed similar uptake rates of nitrate to those trees in subtropical rainforests, but 3 times uptake of glycine and ammonium (Schmidt & Stewart 1997). Our estimated uptake rates of all three N forms were similar to those of temperate and sub-alpine trees (Warren 2009a(Warren , 2009b. Our results demonstrated that investigated dominant subtropical tree species preferentially take up ammonium, contributing about 80% of total N uptake by these dominant trees.…”

“…Such high-latitude and high-altitude regions are often dominated by tundra (Nordin et al 2004), boreal (Finzi & Berthrong 2005, Leduc & Rothstein 2010, alpine (Xu et al 2004) and temperate (Bardgett et al 2003, Warren 2006, Harrison et al 2007, Warren & Adams 2007, Warren 2009a, 2009b forests. Many studies of inorganic and organic N uptake have been done in these ecosystems.…”

Amino acid cycling in plankton and soil microbes studied with radioisotopes: measured amino acids in soil do not reflect bioavailability

“…If these same kinetics apply, then the uptake should be proportional to the concentration of labeled substrate. This has been found by Warren (2009) for an evergreen tree (Eucalyptus regnans) and the herb basil (Ocimum basilicum); the author cautioned researchers to pay more attention to the concentrations used in experiments. Dual labeling of carbon and nitrogen also show that intact amino acid molecules enter tree roots (Näsholm et al 1998;McFarland et al 2002).…”

“…For example, hundreds of experiments have measured the rate of movement of amino acids into roots by adding labeled amino acids at concentrations of 100-1000 lM. The effect of the concentrations on uptake rates is almost never considered but Warren (2009) demonstrates that substrate concentration greatly affects plant preference for N forms.…”

Climatic conditions, soil fertility and atmospheric nitrogen deposition largely determine the structure and functioning of microbial communities in biocrust-dominated Mediterranean drylands

“…In lowproductivity semiarid Mediterranean sites, recent studies have shown how increased N deposition can reduce both the accumulation of organic matter in soils (Ochoa-Hueso et al 2013a) and productivity of certain local plant species (Ochoa-Hueso et al 2013b), which may be partially attributed to the negative effects of excessive ammonium (Ochoa-Hueso et al 2013b). An increase in N deposition may also promote alterations in the availability of different organic and inorganic N forms (Schimel and Bennett 2004), which may affect competitive interactions among species due to their different preferences for different N forms (Nordin et al 2001;Warren 2009). …”