Spatial variability of symbiotic N2 fixation in grass-white clover pastures estimated by the 15N isotope dilution method and the natural 15N abundance method

Summary

Aiming at estimating the spatial variability in N2 fixation, and to evaluate the appropriateness of the 15N isotope dilution (ID) method and the natural 15N abundance (NA) method in reflecting spatial variability under the influence of cattle grazing, the symbiotic N2 fixation in grass-white clover mixture was studied. At the Foulum site, where the ID method was used, differences in the climatic conditions between the two years of investigations caused a considerable difference in plant growth rates and proportion of clover. Consequently, the total N2 fixation in ungrazed reference plots was significantly less in 1998 than in 1997, being 5.9 and 12.5 g N m-2, respectively. In both years there was a wide range in concentration of inorganic N in the soil with coefficients of variance of approximately 60 – 190% for ammonium and 70 – 340% for nitrate. Significant negative correlations between pNdfa, determined by the ID method, and the log-transformed values of inorganic N and total N in grass were found. The NA method was applied on 3 nearby commercial dairy farms. They also showed high coefficients of variation. The coefficient of variance for NO3--N ranged from 37% to 282% and for NH4+-N from 29% to 237%. Average estimates of pNdfa values, which in the NA method were calculated using apparent B values ranging from –2.10 to –2.59‰, were generally lower (0.7 – 0.87) for these farms than for the Foulum site (0.89 – 0.95) using the ID method. For the NA method the d15N values, i.e. deviation in 15N concentration from atmospheric N2, ranged from -7.0 to 5.7‰ for the grass N, which in several cases was lower than for clover N. Due to this high variability of the d15N values, probably caused by deposition and plant assimilation of 15N depleted urinary N in the pastures, the NA method was marginal for accurate determination of pNdfa. Consequently no significant correlation between the pNdfa determined by this method, and the log-transformed values of inorganic N in soil or total N in grass were found.