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Comparison between isotope dilution and acetylene reduction methods to estimate N 2 fixation rate of white clover in grass/clover swards
Author(s) -
LAIDLAW A. S.,
CHRISTIE P.,
WITHERS J. A.
Publication year - 1990
Publication title -
grass and forage science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.716
H-Index - 56
eISSN - 1365-2494
pISSN - 0142-5242
DOI - 10.1111/j.1365-2494.1990.tb01953.x
Subject(s) - stolon , trifolium repens , dilution , zoology , acetylene , isotope dilution , fertilizer , chemistry , botany , horticulture , agronomy , biology , chromatography , mass spectrometry , physics , organic chemistry , thermodynamics
In two field experiments acetylene (C 2 H 2 ) reduction by white clover in mixed swards was compared to N 2 ‐fixed measured by 15 N dilution. In both experiments, samples for C 2 H 2 reduction were 7.5 cm diameter turves taken from plots within which microplots of 24 cm diameter were delimited and to which 15 N was applied as ammonium sulphate ( 15 NH 4 ) 2 SO 4 ). C 2 H 2 reduction was assayed every 6–7 d. The rate of C 2 H 2 reduction per unit length of stolon was applied to the estimated stolon length within the appropriate microplot at the time of assay, and the amount of C 2 H 2 that would have been reduced within the microplot was estimated by integration. In experiment 1, turves taken from grass/clover swards to which 0, 1·5, 3·0, 4·5 or 6·0 g N m −2 had been applied were incubated in sealed chambers (10% C 2 H 2 , 90% air). The mean ratio of C2H2 reduced to N2 fixed during 5 weeks was 0.74:1. Application of N fertilizer lowered the proportion of assimilated N derived from N 2 fixation from 95% in unfertilized swards to 83% in those receiving 6 g N m −2 (60 kg N ha −1 ). In experiment 2, clover roots and stolons from plots that previously had been grazed were dissected from turves and incubated in a stream of C 2 H 2 and air (i.e. the open system). The maximum rate of ethylene (C 2 H 4 ) produced during the first 12 min was taken as a measure of true nitrogenase activity. The relationship between C 2 H 2 reduced and N 2 fixed was significant (r=0.80**). The mole ratio was 0.55:1 for the 6 weeks duration of the experiment, the low ratio possibly being due to disturbance of the nodules adversely affecting acetylene reduction. Mole ratios from both experiments were well short of the theoretical 4·3:1. Using the open system does not, therefore, overcome the shortcomings of the acetylene reduction technique for measuring N 2 fixation of white clover in mixed swards.