NITROGEN TRANSFORMATIONS AFTER THE SPREADING OF PIG SLURRY ON BARE SOIL AND RYEGRASS USING N-15-LABELED 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 transf ormations of a pig slurry. The ammonium fraction of the slurry was lab elled with ((NH3)-N-15)(2)SO4. The slurry was spread manually on micro plots in mid-June, at the rate of 3 l/m(2), on a cut ryegrass sward, a nd compared with bare soil. Absorption of N-15-labelled NH4 by the gra ss occurred very rapidly, attaining 41% after 13 days and showing no f urther change at 27 days. The gaseous losses, mainly through volatiliz ation of ammonia, were considerable. N-15 recovery in soil and plant m aterial 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) vol atilization, 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-labelle d 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 s oil, even though a significant portion of the plant is involved in int ernal recycling. (C) 1997 Elsevier Science B.V.