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

Biological N-2 fixation in clover is an important source of N in low extern al-N input farming systems. Using the natural N-15-abundance method, variat ions in N-2 fixation were investigated in grazed and mowed plots of a ryegr ass/white clover field. Ryegrass delta(15)N varied considerably, from 0.2 t o 5.6 parts per thousand under mowed conditions and from -3.3 to 11.6 parts per thousand under grazed conditions. Variations in delta(15)N white clove r were lower than in ryegrass, especially in the mowed plots (SE = 0.05 par ts per thousand, n = 20). The variations in the percentage of nitrogen deri ved 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 p lots where it ranged from 64 to 92% (mean = 79%). Thus, the N-2 fixation pe r unit white clover DM in the grazed ley was lower and more variable than u nder mowing conditions. Urine from dairy cows equivalent to 0, 200, 400 and 800 kg N ha(-1) was app lied to a ryegrass/white clover plot 6, 4 or 2 weeks before harvest. Withou t urine application delta(15)N of ryegrass was positive. By increasing urin e application (delta(15)N = -l parts per thousand) two weeks before samplin g, the delta(15)N of ryegrass decreased strongly to about -7 parts per thou sand(P < 0.001). However, this effect was only observed when urine was appl ied two weeks before sampling. When applying 800 kg N four and six weeks be fore sampling, delta(15)N in ryegrass was not significantly different from the treatment without urine application. White clover delta(15)N was unaffe cted by whatever changes occurred in delta(15)N of the plant-available soil N pool (reflected in delta(15)N of ryegrass). This indicates that within t he time span of this experiment, N-2 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 experime nt suggested that the natural abundance method can be applied for estimatin g %Ndfa without disturbance in natural animal-grazed systems.