Effects of Multiple Reference Plants, Season, and Irrigation on Biological Nitrogen Fixation by Pasture Legumes using the Isotope Dilution Method

The popular and widely used 15 nitrogen (N)-isotope dilution method for estimating biological N fixation (BNF) of pasture and tree legumes relies largely on the ability to overcome the principal source of error due to the problem of selecting appropriate reference plants. A field experiment was cond...

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Bibliographic Details
Published in:Communications in soil science and plant analysis 2007-07, Vol.38 (13-14), p.1841-1860
Main Author: Goh, K. M.
Format: Article
Language:English
Subjects:
Agronomy. Soil science and plant productions
Biological and medical sciences
Biological nitrogen fixation
forage legumes
Fundamental and applied biological sciences. Psychology
irrigation
N-isotope dilution method
reference plants
season
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Summary:The popular and widely used 15 nitrogen (N)-isotope dilution method for estimating biological N fixation (BNF) of pasture and tree legumes relies largely on the ability to overcome the principal source of error due to the problem of selecting appropriate reference plants. A field experiment was conducted to evaluate the suitability of 12 non-N 2 -fixing plants (i.e., nonlegumes) as reference plants for estimating the BNF of three pasture legumes (white clover, Trifolium repens L.; lucerne, Medicago sativa; and red clover, Trifolium pratense L.) in standard ryegrass-white clover (RWC) and multispecies pastures (MSP) under dry-land and irrigation systems, over four seasons in Canterbury, New Zealand. The 15 N-isotope dilution method involving field 15 N-microplots was used to estimate BNF. Non-N 2 -fixing plants were used either singly or in combination as reference plants to estimate the BNF of the three legumes. Results obtained showed that, on the whole, 15 N-enrichment values of legumes and nonlegumes varied significantly according to plant species, season, and irrigation. Grasses and herb species showed higher 15 N-enrichment than those of legumes. Highest 15 N-enrichment values of all plants occurred during late summer under dry-land and irrigation conditions. Based on single or combined non-N 2 -fixing plants as reference plants, the proportion of N derived from the atmosphere (% Ndfa) values were high (50 to 90%) and differed between most reference plants in the MSP pastures, especially chicory (Cichorium intybus), probably because it is different in phenology, rooting depth, and N-uptake patterns compared to those of legumes. The percent Ndfa values of all plants studied also varied according to plant species, season, and irrigation in the MSP pastures. Estimated daily amounts of BNF varied according to pasture type, time of plant harvest, and irrigation, similar to those shown by percent Ndfa results as expected. Irrigation increased daily BNF more than 10-fold, probably due to increased dry-matter yield of pasture under irrigation compared to dry-land conditions. Seasonal and irrigation effects were more important in affecting estimates of legume BNF than those due to the appropriate matching of N 2 -fixing and non-N 2 -fixing reference plants.
ISSN:0010-3624
1532-2416
DOI:10.1080/00103620701435605