Variations in the natural abundance of 15N in ryegrass/white clover shoot material as influenced by cattle grazing

Biological N2 fixation in clover is an important source of N in low external-N input farming systems. Using the natural 15N-abundance method, variations in N2 fixation were investigated in grazed and mowed plots of a ryegrass/white clover field. Ryegrass 15N varied considerably, from 0.2 to 5.6 under mowed conditions and from –3.3 to 11.6 under grazed conditions. Variations in 15N white clover were lower than in ryegrass, especially in the mowed plots (SE = 0.05, n = 20). The variations in the percentage of nitrogen derived from the atmosphere (%Ndfa) in white clover were highest in the grazed plots where it ranged from 12 to 96% (mean = 64%) compared with the mowed plots where it ranged from 64 to 92% (mean = 79%). Thus, the N2 fixation per unit white clover DM in the grazed ley was lower and more variable than under mowing conditions.Urine from dairy cows equivalent to 0, 200, 400 and 800 kg N ha-1 was applied to a ryegrass/white clover plot 6, 4 or 2 weeks before harvest. Without urine application 15N of ryegrass was positive. By increasing urine application (15N = –1) two weeks before sampling, the 15N of ryegrass decreased strongly to about –7 (P < 0.001). However, this effect was only observed when urine was applied two weeks before sampling. When applying 800 kg N four and six weeks before sampling, 15N in ryegrass was not significantly different from the treatment without urine application. White clover 15N was unaffected by whatever changes occurred in 15N of the plant-available soil N pool (reflected in 15N of ryegrass). This indicates that within the time span of this experiment, N2 fixation per unit DM was not affected by urine. Therefore, newly deposited urine may not be the main contributing factor to the variation in %Ndfa found in the grazed fields. This experiment suggested that the natural abundance method can be applied for estimating %Ndfa without disturbance in natural animal-grazed systems.