DENITRIFICATION LOSSES FROM A NATURAL GRASSLAND IN THE BASQUE COUNTRY UNDER ORGANIC AND INORGANIC FERTILIZATION

Denitrification losses from a poorly drained clayey loamy soil under natural pasture were measured over a two-year period using the acetylene inhibition technique. Plots received two different applications of fertilizer as calcium ammonium nitrate or cow slurry (a total of 145-290 kg N ha(-1) in 1991 and 120-240 kg in 1992). In the first year, N losses in the mineral treatments were about 4 times greater than losses in the slurry treatments. In the second year losses in the slurry treatments increased in such a way that losses in the higher slurry application became similar to those far the two mineral treatments. Soil nitrate was the factor producing differences between treatments. In this way, N mineralization in periods between fertilizations coinciding with high soil water contents was responsible in the second year for the increase in N losses in the slurry treatments. Denitrification rates greater than 0.1 kg N ha(-1) day(-1) occurred at soil water contents > 33 % (air filled porosity < 26 %) and soil nitrate contents > 1 mg N kg(-1) dry soil. Spring and autumn were the seasons of highest risk of denitrification because of N fertilizations coinciding with periods of soil saturation with water. Winter losses were low, but this is a period when there is a risk of denitrification in wetter seasons, particularly for a slurry application management.