Denitrification losses of nitrogen fertilizer applied to winter wheat following ley and arable rotations as estimated by acetylene inhibition and 15 N balance

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Abstract
SUMMARY: We studied the fate of 222 kg N ha −1 applied in spring as K 15 NO 3 to winter wheat test crops which followed either continuous arable cropping (Arable) or a rotation in which a 3‐year grass/clover ley preceded the wheat (Ley). Denitrification losses (measured by an acetylene‐inhibition method) of over 1 kg N ha −1 d −1 were measured for short periods following heavy rain in mid‐May. However the generally dry and cool weather resulted in accumulated losses by denitrification between fertilizer 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 ley was despite this treatment containing more inorganic N and available carbon. 15 N balance indicated that, at anthesis, 1.5% and 11.5% (± 7%) of the labelled N was lost from the arable and ley treatments respectively. Given the precision of the 15N and the acetylene‐inhibition methods, the results are not significantly different. However, the larger difference between methods for losses from the ley treatment may be an underestimate because 15 N balance does 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 measured as denitrified. Such a large difference is unlikely to be an error 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 which could be significant in some situations.

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