Plant availability and leaching of 15N-labelled mineral fertilizer residues retained in an agricultural soil for 25 years: a lysimeter study

Abstract Purpose: The crop uptake and leaching of labelled fertilizer N applied 25-30 years ago was measured in different agricultural cropping systems to improve the understanding and quantification of the long-term fate of fertilizer N. Methods: 15N-labelled mineral fertilizers were added to a field plot with different crops over 5 years. After 12 years arable cropping the top-soil was relocated to lysimeters with set-aside grassland for the next 13 years. The lability of the residual 15N in soil was then tested over a period of two years by either converting to fertilized and cut grass, or by replacing the set-aside grassland with spring barley (+/- autumn cover crop) or vegetation-free fallow, all receiving unlabelled mineral N fertilizers. The crop uptake and leaching of 15N was determined over the 2-year period.Results: The 15N residing in soil before set-aside decreased by 24% during the 13 years under set-aside grassland. Crop uptake and leaching of 15N were generally highest in the first test year after termination of the set-aside. The leaching of residual 15N in soil declined in the order: vegetation-free soil (4.7%), spring barley (1.9%), spring barley + cover crop (0.7%), and production grassland (0.2%). Corresponding losses for the second leaching period were 2.7%, 0.9%, 0.4%, and 0.06%. There was a constant relationship between the leaching loss of 15N and total N. Conclusions: After residing in soil for 25-30 years, the lability of labelled mineral N fertilizer residues were slightly higher than the lability of bulk soil N. Autumn vegetation was crucial for reducing leaching losses.