1. Department of Soil Management and Soil Care Department of Applied Analytical and Physical Chemistry INFLUENCE OF NITRIFICATION INHIBITORS ON SOIL N DYNAMICS AFTER INCORPORATION OF VEGETABLE CROP RESIDUES: A FIELD STUDY Georges Hofman Barbara Chaves Stefaan De Neve Pascal Boeckx Oswald Van Cleemput

2. Outline  Introduction  Materials and methods  Results  Discussion  Conclusion

3. Introduction The Netherlands The Walloon Region Problems with NO 3 - -N leaching = Vegetable region

4. Introduction CropCrop residues  Large amounts crop residues (up to 60 t fresh material ha -1 )  High N content (2 - 4% N on dry matter)  Large and fast N release from crop residue (up to 150 kg N ha -1 )  High risk of NO 3 - leaching

5. Objective Can nitrification inhibitors (DCD and DMPP) reduce nitrification and NO 3 - leaching after incorporation of vegetable crop residues?

6. Materials and methods Block 3 Block 2 Block 1 Randomised block design with 3 replicates Silt loam Cauliflower residues ~ 452 kg N ha -1 sandy loam DMPP ~ 1,9 kg a.c. ha -1 DCD ~ 19 kg a.c. ha -1 Cauliflower SoilCauliflower + DCD Cauliflower + DMPP

7. Materials and methods 6 sampling dates with an auger to a depth of 90 cm in 3 layers: 0-30 cm, 30-60 cm, 60-90 cm Mineral N extraction with (1N) KCl continuous flow autoanalyser

8. NH 4 + -N and NO 3 - -N

9. Mineral N

10. Discussion  0-30 cm Mineral N in DCD > mineral N in cauliflower only  N release from DCD itself (67% N = 13 kg N ha -1 )  priming effect induced by DCD (Gioacchini et al., 2002; Chaves et al., 2006 )  30-60 cm and 60-90 cm Mineral N in DMPP < mineral N in cauliflower only and DCD  Only DMPP reduced NO 3 - leaching

11. Conclusions DMPP and DCD  Increased NH 4 - -N in top 30 cm  Reduced NO 3 - -N in top 30 cm DMPP  Reduced NO 3 - -N in 30-90 cm layer  DMPP can reduce NO 3 - leaching DCD  Increased mineral N in 0-30 cm layer (N release from DCD and priming effect)  No reduction in NO 3 - leaching

12. Thank you for your attention!!