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Challenging the paradigm of nitrogen cycling: no evidence of in situ resource partitioning by coexisting plant species in grasslands of contrasting fertility.

Wilkinson A, Hill PW, Vaieretti MV, Farrar JF, Jones DL, Bardgett RD - Ecol Evol (2014)

Bottom Line: As a result, the extent to which hypothetical niches are realized in nature remains unclear.On the other hand, in low-productivity grasslands, where the availability of dissolved inorganic N is low, and soil availability of dissolved organic N is greater, we predicted that plants would preferentially use N from amino acids and peptides, prior to microbial mineralization.We found that, contrary to our hypothesis, the majority of plant species across both grasslands took up most N in the form of NH4 (+), suggesting that inorganic N is their predominant N source.

View Article: PubMed Central - PubMed

Affiliation: Lancaster Environment Centre, Lancaster University Bailrigg, Lancaster, LA1 4YQ, U.K ; Faculty of Life Sciences, Michael Smith Building, The University of Manchester Oxford Road, Manchester, M13 9PT, U.K.

ABSTRACT
In monoculture, certain plant species are able to preferentially utilize different nitrogen (N) forms, both inorganic and organic, including amino acids and peptides, thus forming fundamental niches based on the chemical form of N. Results from field studies, however, are inconsistent: Some showing that coexisting plant species predominantly utilize inorganic N, while others reveal distinct interspecies preferences for different N forms. As a result, the extent to which hypothetical niches are realized in nature remains unclear. Here, we used in situ stable isotope tracer techniques to test the idea, in temperate grassland, that niche partitioning of N based on chemical form is related to plant productivity and the relative availability of organic and inorganic N. We also tested in situ whether grassland plants vary in their ability to compete for, and utilize peptides, which have recently been shown to act as an N source for plants in strongly N-limited ecosystems. We hypothesized that plants would preferentially use NO3 (-)-N and NH4 (+)-N over dissolved organic N in high-productivity grassland where inorganic N availability is high. On the other hand, in low-productivity grasslands, where the availability of dissolved inorganic N is low, and soil availability of dissolved organic N is greater, we predicted that plants would preferentially use N from amino acids and peptides, prior to microbial mineralization. Turves from two well-characterized grasslands of contrasting productivity and soil N availability were injected, in situ, with mixtures of (15)N-labeled inorganic N (NO3 (-) and NH4 (+)) and (13)C(15)N labeled amino acid (l-alanine) and peptide (l-tri-alanine). In order to measure rapid assimilation of these N forms by soil microbes and plants, the uptake of these substrates was traced within 2.5 hours into the shoots of the most abundant plant species, as well as roots and the soil microbial biomass. We found that, contrary to our hypothesis, the majority of plant species across both grasslands took up most N in the form of NH4 (+), suggesting that inorganic N is their predominant N source. However, we did find that organic N was a source of N which could be utilized by plant species at both sites, and in the low-productivity grassland, plants were able to capture some tri-alanine-N directly. Although our findings did not support the hypothesis that differences in the availability of inorganic and organic N facilitate resource partitioning in grassland, they do support the emerging view that peptides represent a significant, but until now neglected, component of the terrestrial N cycle.

No MeSH data available.


Related in: MedlinePlus

The total percentage of added 15N recovered from within the microbial biomass (black bars), root material (pale gray bars), and shoot material (dark gray bars), following the 15N-labeled application (A) NO3−, (B) NH4+, (C) alanine, and (D) tri-alanine, and 13C following the 13C-labeled application (E) alanine and (F) tri-alanine.
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fig04: The total percentage of added 15N recovered from within the microbial biomass (black bars), root material (pale gray bars), and shoot material (dark gray bars), following the 15N-labeled application (A) NO3−, (B) NH4+, (C) alanine, and (D) tri-alanine, and 13C following the 13C-labeled application (E) alanine and (F) tri-alanine.

Mentions: Generally, there were very few differences between the amount of 15N and 13C recovered in plant and microbial biomass (Fig.4), although there were some exceptions. In the high-productivity grassland, three times more 15N was recovered in the microbial biomass compared to plant biomass following organic N addition (Fig.4C; alanine: F(1,3) = 23.15, P = 0.017, and Fig.4D; tri-alanine: F(1,4) = 38.30, P = 0.003). In the low-productivity grassland, slightly more 15N was recovered in plant biomass compared to microbial biomass following labeled NH4+ addition (Fig.4B; F(1,4) = 10.22, P = 0.033), and seventeen times more 13C was recovered in plant biomass (Fig.4F; F(1,4) = 51.85, P = 0.002) following labeled tri-alanine addition. Nonetheless, in most cases, equal amounts of 13C and 15N from added substrates were recovered in plant material and microbial biomass in both grasslands. However, plant material always had a much higher concentration of 15N and 13C (nmol excess g−1) than microbial biomass in all substrate addition treatments (Fig. S3).


Challenging the paradigm of nitrogen cycling: no evidence of in situ resource partitioning by coexisting plant species in grasslands of contrasting fertility.

Wilkinson A, Hill PW, Vaieretti MV, Farrar JF, Jones DL, Bardgett RD - Ecol Evol (2014)

The total percentage of added 15N recovered from within the microbial biomass (black bars), root material (pale gray bars), and shoot material (dark gray bars), following the 15N-labeled application (A) NO3−, (B) NH4+, (C) alanine, and (D) tri-alanine, and 13C following the 13C-labeled application (E) alanine and (F) tri-alanine.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4314261&req=5

fig04: The total percentage of added 15N recovered from within the microbial biomass (black bars), root material (pale gray bars), and shoot material (dark gray bars), following the 15N-labeled application (A) NO3−, (B) NH4+, (C) alanine, and (D) tri-alanine, and 13C following the 13C-labeled application (E) alanine and (F) tri-alanine.
Mentions: Generally, there were very few differences between the amount of 15N and 13C recovered in plant and microbial biomass (Fig.4), although there were some exceptions. In the high-productivity grassland, three times more 15N was recovered in the microbial biomass compared to plant biomass following organic N addition (Fig.4C; alanine: F(1,3) = 23.15, P = 0.017, and Fig.4D; tri-alanine: F(1,4) = 38.30, P = 0.003). In the low-productivity grassland, slightly more 15N was recovered in plant biomass compared to microbial biomass following labeled NH4+ addition (Fig.4B; F(1,4) = 10.22, P = 0.033), and seventeen times more 13C was recovered in plant biomass (Fig.4F; F(1,4) = 51.85, P = 0.002) following labeled tri-alanine addition. Nonetheless, in most cases, equal amounts of 13C and 15N from added substrates were recovered in plant material and microbial biomass in both grasslands. However, plant material always had a much higher concentration of 15N and 13C (nmol excess g−1) than microbial biomass in all substrate addition treatments (Fig. S3).

Bottom Line: As a result, the extent to which hypothetical niches are realized in nature remains unclear.On the other hand, in low-productivity grasslands, where the availability of dissolved inorganic N is low, and soil availability of dissolved organic N is greater, we predicted that plants would preferentially use N from amino acids and peptides, prior to microbial mineralization.We found that, contrary to our hypothesis, the majority of plant species across both grasslands took up most N in the form of NH4 (+), suggesting that inorganic N is their predominant N source.

View Article: PubMed Central - PubMed

Affiliation: Lancaster Environment Centre, Lancaster University Bailrigg, Lancaster, LA1 4YQ, U.K ; Faculty of Life Sciences, Michael Smith Building, The University of Manchester Oxford Road, Manchester, M13 9PT, U.K.

ABSTRACT
In monoculture, certain plant species are able to preferentially utilize different nitrogen (N) forms, both inorganic and organic, including amino acids and peptides, thus forming fundamental niches based on the chemical form of N. Results from field studies, however, are inconsistent: Some showing that coexisting plant species predominantly utilize inorganic N, while others reveal distinct interspecies preferences for different N forms. As a result, the extent to which hypothetical niches are realized in nature remains unclear. Here, we used in situ stable isotope tracer techniques to test the idea, in temperate grassland, that niche partitioning of N based on chemical form is related to plant productivity and the relative availability of organic and inorganic N. We also tested in situ whether grassland plants vary in their ability to compete for, and utilize peptides, which have recently been shown to act as an N source for plants in strongly N-limited ecosystems. We hypothesized that plants would preferentially use NO3 (-)-N and NH4 (+)-N over dissolved organic N in high-productivity grassland where inorganic N availability is high. On the other hand, in low-productivity grasslands, where the availability of dissolved inorganic N is low, and soil availability of dissolved organic N is greater, we predicted that plants would preferentially use N from amino acids and peptides, prior to microbial mineralization. Turves from two well-characterized grasslands of contrasting productivity and soil N availability were injected, in situ, with mixtures of (15)N-labeled inorganic N (NO3 (-) and NH4 (+)) and (13)C(15)N labeled amino acid (l-alanine) and peptide (l-tri-alanine). In order to measure rapid assimilation of these N forms by soil microbes and plants, the uptake of these substrates was traced within 2.5 hours into the shoots of the most abundant plant species, as well as roots and the soil microbial biomass. We found that, contrary to our hypothesis, the majority of plant species across both grasslands took up most N in the form of NH4 (+), suggesting that inorganic N is their predominant N source. However, we did find that organic N was a source of N which could be utilized by plant species at both sites, and in the low-productivity grassland, plants were able to capture some tri-alanine-N directly. Although our findings did not support the hypothesis that differences in the availability of inorganic and organic N facilitate resource partitioning in grassland, they do support the emerging view that peptides represent a significant, but until now neglected, component of the terrestrial N cycle.

No MeSH data available.


Related in: MedlinePlus