POTENTIAL FOR DENITRIFICATION AT DEPTH BELOW LONG-TERM GRASS SWARDS

The denitrification potential (PDN) of soil and chalk samples taken to depth (down to > 6 m) below a number of long-term grassland and arable managements was determined under controlled laboratory conditions. Cut and grazed grassland (either grass-clover with no fertilizer N or grass with 420 kg N ha(-1)) and a continuous arable management were examined. PDN rates were always greater in the top soil samples and declined progressively with depth. However, even at depths of 6 m, significant PDN was demonstrated under all treatments. This was enhanced by the addition of available C. There were apparent differences between the previous management treatments particularly in the upper soil samples, but these were also found at greater depths in some cases. PDN was always less in the samples from the grass-clover swards than in comparable samples from fertilized swards. Adding extra labile C had little or no affect on PDN in the grass-clover samples. Calculation of PDN on an area basis indicated that even in the absence of added C, and even in those treatments with low PDN rates, there was substantial potential for denitrification. Adding C increased the overall potential by, on average, nearly 2-fold for the grassland samples and by over 4-fold for samples from the long-term arable system. Even if only a small proportion of this potential was achieved in practice, this has major significance for decreasing the amount of NO3- reaching aquifers.