Nitrogen transformations after the spreading of pig slurry on bare soil and ryegrass using N-15-labelled ammonium

A short field experiment (27 days) was carried out in summer 1995, to study the effect of an actively growing grass sward on nitrogen transformations of a pig slurry. The ammonium fraction of the slurry was labelled with ((NH3)-N-15)(2)SO4. The slurry was spread manually on microplots in mid-June, at the rate of 3 l/m(2), on a cut ryegrass sward, and compared with bare soil. Absorption of N-15-labelled NH4 by the grass occurred very rapidly, attaining 41% after 13 days and showing no further change at 27 days. The gaseous losses, mainly through volatilization of ammonia, were considerable. N-15 recovery in soil and plant material on day 27 was 42.5% (+/-1.2) on the bare soil, versus 57.4% (+/-3.1) on the ryegrass. The grass sward significantly reduced: (i) volatilization, as shown by the difference of 14.9% in N-15 recovery, on the 6th day; (ii) immobilization, which was 25% (+/-2.2) on day 27 on bare soil and 16.4% (+/-2.9) in the presence of ryegrass. N-15-labelled inorganic nitrogen was completely depleted beneath the ryegrass, 27 days after application, whereas ammonium was depleted and the nitrate was equal to 16.4% (+/-1.6) of the applied NH4 on the bare soil. II is clearly apparent that the ammonia from the slurry is more efficiently used when applied to an actively growing sward, rather than to bare soil, even though a significant portion of the plant is involved in internal recycling. (C) 1997 Elsevier Science B.V.