EFFECT OF ONE-YEAR ROTATIONAL SET-ASIDE ON IMMEDIATE AND ENSUING NITROGEN LEACHING LOSS

This paper reports results from a 3-year field experiment which examin ed Nitrogen (N) leaching loss from land under various set-aside manage ments. Four treatments were examined: three ploughed plots which were sown with wheat, ryegrass or maintained fallow; the fourth treatment w as unploughed and natural weed growth (volunteers) permitted. The 1-ye ar set-aside was followed by two winter wheat test crops. Ceramic suct ion cups were installed at a depth of 90 cm and used to collect draina ge water. N leaching loss was calculated by multiplying drainage volum e, calculated from meteorological data, by its inorganic N concentrati on. Set-aside management significantly affected N leaching loss over t he three years. During the set-aside year, the peak nitrate concentrat ion from the unploughed treatment growing volunteer weeds was signific antly lower than that from ploughed plots. Of the latter, by the sprin g, crop (i.e, wheat and ryegrass) assimilation of N significantly redu ced N concentration compared to the fallow. The four set-aside treatme nts had a carry-over effect to the following year (first wheat test cr op) resulting in significant differences in N losses. Leaching followi ng the ryegrass treatment was very small and we believe that the grass residues minimised rates of net-N mineralization. The influence of se t-aside management continued to the second wheat test crop where N los s was greater under the all wheat rotation because take-all had reduce d yield and therefore crop N uptake.