DENITRIFICATION LOSSES OF NITROGEN-FERTILIZER APPLIED TO WINTER-WHEATFOLLOWING LEY AND ARABLE ROTATIONS AS ESTIMATED BY ACETYLENE INHIBITION AND N-15 BALANCE

We studied the fate of 222 kg N ha-1 applied in spring as (KNO3)-N-15 to winter wheat test crops which followed either continuous arable cro pping (Arable) or a rotation in which a 3-year grass/clover ley preced ed the wheat (Ley). Denitrification losses (measured by an acetylene-i nhibition method) of over 1 kg N ha-1 d-1 were measured for short peri ods following heavy rain in mid-May. However the generally dry and coo l weather resulted in accumulated losses by denitrification between fe rtilizer application and anthesis equivalent to only 5.3% and 3.6% (+/ -2%) of the applied N for the arable and ley treatments respectively. The smaller loss from the lev was despite this treatment containing mo re inorganic N and available carbon. N-15 balance indicated that, at a nthesis, 1.5% and 11.5% (+/-7%) of the labelled N was lost from the ar able and ley treatments respectively. Given the precision of the N-15 and the acetylene-inhibition methods, the results are not significantl y different. However, the larger difference between methods for losses from the ley treatment may be an underestimate because N-15 balance d oes not measure losses of unlabelled N. These were probably very small on the arable treatment but could have increased total N loss by 25% to c. 32 kg ha-1 on the ley treatment compared with the 8 kg ha-1 meas ured as denitrificd. Such a large difference is unlikely to be an erro r but was probably due to ammonia volatilization from this crop which was severely infected by mildew. The results were thus a poor test of the acetylene-inhibition method, but revealed another loss process whi ch could be significant in some situations.