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

BackgroundA high use-efficiency of fertilizer N remains essential to sustain high crop productivity with low environmental impact. However, little is known on the long-term lability of mineral fertilizer N. AimsTo quantify crop uptake and leaching of N-15-labelled mineral fertilizer that has been retained in an agricultural soil for 25-30 years in crops with variable growing season. MethodsA field plot received N-15-labelled mineral fertilizers over a period of 5 years and was then kept under arable cropping for 12 years. After relocation to 16 lysimeters, the topsoil grew set-aside grassland for the next 13 years. Then crop uptakes and leaching losses of N-15 remaining in soil was tested over a 2-year period by either converting set-aside grass to production grassland, or by replacing it with spring barley (+/- autumn cover crop) or vegetation-free fallow. All treatments received unlabelled mineral N fertilizers. ResultsCrop uptake and leaching of N-15 were generally highest in the first test year after termination of the set-aside. The leaching of residual N-15 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 fixed relationship between leaching losses of N-15 and total N. ConclusionsAfter residing in soil for 25-30 years, the lability of labelled mineral N fertilizer residues appeared slightly higher than the lability of bulk soil N. Autumn vegetation was crucial for reducing leaching losses.