NITROGEN-FIXATION AND TRANSFER IN VETCH AND VETCH-OATS MIXTURES

N-15-labelled organic matter and ammonium sulphate fertilizer (20 and 60 kg N ha-1) were applied as N-15 sources to estimate N2 fixed in field-grown pure vetch (Vicia sativa L.) and vetch grown in two mixtures with oats (Avena sativa). The decline in N-15 enrichment in the soil during the seven sequential harvests and deduced from the atom % N-15 excess values in the oats reference crop was significantly slower in the organic matter labelled plots than with the N-15-labelled ammonium sulphate plots. Total N uptake patterns of oats and vetch differed markedly for the last two or three harvests. Also, estimates of N2 fixed (Ndfa) in vetch differed with the N-15 source. The mean % Ndfa (or total Ndfa) at 106 days after emergence was 76% (95 kg N ha-1) with the ploughed-in organic matter, 45% (61 kg N ha-1) or 41% (41 kg N ha-1) when 20 or 60 kg N ha-1, respectively, of N-15-labelled ammonium sulphate was applied. Estimates of N transferred from the legume to the cereal were similarly influence by the N-15 source. Little (maximum 5 kg N ha-1) or no N transfer from vetch to oats was detected, using the organic matter labelled plots. In contrast, several significant values (up to 29 kg N ha-1) of N transferred were estimated using the inorganic N-15-labelled plots. With the greater effects of fast N-15: N-14 ratio decline in soil on reference crop errors, we estimate that the values of N2 fixed and of N transferred under the more stable N-15: N-14 conditions, i.e. with N-15-labelled organic incorporated into soil, should be the most accurate. Thus, vetch derived on the average, over 70% of its N from fixation, while N transfer from vetch to oats was not significant.