The dynamics of nitrate fertilizer in soil in the autumn and the effect of farmyard manuring

The effect of long-term farmyard manure application on denitrification and leaching of nitrate during autumn was studied in uncropped plots. Denitrification was measured directly using the acetylene inhibition technique. Bromide, being a good tracer for nitrate movement in soil, was applied together with 15N-labelled nitrate fertilizer to test a method for distinguishing leaching from denitrification. This study was conducted on plots of a long-term experiment at Rothamsted which have received farmyard manure (FYM) or an inorganic fertilizer treatment (PK) annually since 1852. Nitrate (as K15NO3) and bromide (as KBr) were applied to both the FYM and PK plots at about 25 kg N or Br ha-1. Soil and weeds were sampled 28 and 64 days after fertilizer application. Despite considerable denitrification, the initial recovery of 15N in plant and soil considerably exceeded that of Br. This was due to the greater immobilisation of 15N in plants and soil. However, in the final sampling period, measured losses of 15N and Br were similar within each treatment, indicating leaching was the dominant loss process. After 64 days, the losses of applied 15N were 45% and 46% for the PK and FYM plots, respectively. Measured denitrification from the FYM plots was considerable (29 kg N ha-1) but was small (< 5 kg N ha-1) from the PK plot. (author). 17 refs, 2 figs, 6 tabs