Keith Paustian

Soil denitrification in three cropping systems characterized by differences in nitrogen and carbon supply

The objective of our study was to determine the amount and temporal dynamics of N losses via denitrification and to assess the importance of different rate-regulating factors in three cropping systems: (i) barley fertilized with 120 kg N ha-1 yr-1 as Ca(NO3)2, (ii) grass ley (meadow fescue) receiving 200 kg N ha-1 yr-1 and (iii) an unfertilized nitrogen-fixing lucerne ley. The C2H2-inhibition technique was used on intact soil cores samples on 50 occasions in 1982 in barley and grass and on 68 occasions in 1983 in barley, grass and lucerne.Although there was great spatial variability in denitrification rates, differences in rates between cropping systems over the season were almost always significant. Lucerne denitrification rates were higher than those of the barley (p < 0.01) and grass ley (p < 0.001), while grass ley denitrification was higher than in barley for 1983 samples taken within rows only (p < 0.01). Rates measured within plant rows were significantly higher than rates between rows in 1983. Within-row rates were about 15 times higher than between-row rates in lucerne ley (p < 0.001), 8.6 times in grass ley (p < 0.001) and 1.7 times in barley (p < 0.05). In 1982, only the grass ley showed a statistically significant difference (1.4, p < 0.05) in-between versus within-row denitrification rates. The upper 10-cm of the topsoil generally showed higher rates than the soil 10–20 cm below the surface.Seasonal N losses were estimated to be 0.3 and 0.6 g N m-2 yr-1 in barley (1982 and 1983), 0.4 and 1.4 g N m-2 yr-1 in grass ley (1982 and 1983) and 1.7 g N m-2 yr-1 in lucerne ley (1983). About 3 and 5% of the N fertilizer applied to the barley was denitrified, while the values for the grass ley were 2 and 7% for 1982 and 1983. The denitrification losses in the lucerne ley corresponded to 4% of the atmospheric nitrogen fixed.Although the annual denitrification losses were small in relation to the nitrogen supply either as commercial fertilizer or via biological nitrogen fixation, denitrification played a major role as a nitrogen sink for the cropping systems investigated. Except for barley during 1982, about half or two thirds of the nitrogen lost was due to denitrification.