PATTERNS OF DENITRIFICATION LOSS FROM GRAZED GRASSLAND - EFFECTS OF N FERTILIZER INPUTS AT DIFFERENT SITES

Factors influencing the seasonal and daily variation in denitrification rates in grazed swards were examined at 5 experimental sites in England with wide ranging environmental/geographic conditions. There was a wide range of fertilizer inputs at each site. Rates of denitrification were estimated by a coring and field incubation technique using acetylene to inhibit the reduction of N2O to N2. Major features of the detailed results from two of the sites were: (i) the large ranges in rates of loss, (ii) the relatively low contributions to total annual loss during autumn and winter, (iii) the apparent association of high rates of loss with fertilizer additions made when the soil was wet or immediately preceding a rainfall event, and (iv) significant losses from soil at 10-30 cm in the profile. Multiple quadratic regression analysis of the effects of soil NO3--N, soil temperature and water was used to explain variability in rates of loss. When separate regressions were fitted within each site x year x season x fertilizer level subset, 51% of the variation in loss was explained on a poorly drained fine loam/silt but only 38% on a freely drained loam.