Fate of residual 15N-labeled fertilizer in dryland farming systems on soils of contrasting fertility

Up to 50% of nitrogen (N) fertilizer can remain in soil after crop harvest in dryland farming. Understanding the fate of this residual fertilizer N in soil is important for evaluating its overall use efficiency and environmental effect. Nitrogen-15 (N-15)-labeled urea (165kgNha(-1)) was applied to winter wheat (Triticum aestivum L.) growing in three different fertilized soils (no fertilizer, No-F; inorganic nitrogen, phosphorus and potassium fertilization, NPK; and manure plus inorganic NPK fertilization, MNPK) from a long-term trial (19 years) on the south of the Loess Plateau, China. The fate of residual fertilizer N in soils over summer fallow and the second winter wheat growing season was examined. The amount of the residual fertilizer N was highest in the No-F soil (116kgha(-1)), and next was NPK soil (60kgha(-1)), then the MNPK soil (43kgha(-1)) after the first winter wheat harvest. The residual fertilizer N in the No-F soil was mainly in mineral form (43% of the residual N-15), and for the NPK and MNPK soils, it was mainly in organic form. The loss rate of residual N-15 in No-F soil over summer fallow was as high as 48%, and significantly (P<0.05) higher than that in the NPK soil (22%) and MNPK soil (19%). The residual N-15 use efficiency (RNUE) by the second winter wheat was 13% in the No-F soil, 6% in the NPK soil and 8% in the MNPK soil. These were equivalent to 9.0, 2.0 and 2.2% of applied N-15. The total N-15 recovery (N-15 uptake by crops and residual in 0-100cm soil layer) in the MNPK and NPK soils (84.5% and 86.6%, respectively) were both significantly higher than that in the No-F soil (59%) after two growing seasons. The N-15 uptake by wheat in two growing seasons was higher in the MNPK soil than in NPK soil. Therefore, we conclude that a high proportion of the residual N-15 was lost during the summer fallow under different land management in dryland farming, and that long-term combined application of manure with inorganic fertilizer could increase the fertilizer N uptake and decrease N loss.