Fate of 15N-fertilizers in the soil-plant system of a forage rotation under conservation and plough tillage

For agriculture sustainability and biosphere conservation in the 21st century, to improve N use efficiency and to reduce the fertilization rates is fundamental. The N dynamics in the soil-plant system can be altered after the adoption of conservation,practices, making necessary to evaluate the fertilizer efficiency and, if needed, to adapt N fertilization. Accordingly, during two years, we assess the effect of conventional plough tillage (PT) and conservation minimum tillage (MT) on the N dynamics in a 14-year-old ryegrassmaize forage rotation. We used a N-15-tracing approach to evaluate the recovery efficiency of N (REN) taking into account both the N immobilized in soil (0-5, 5-15, 15-30, 30-45 and 45-60 cm) and that exported at harvest. Adjacent PT (n = 9) and MT (n = 9) plots were randomly assigned in triplicate to three treatments to which (NH4NO3)-N-15-N-15 (10 atom% N-15) was applied in one of the three first fertilizations (N-15(october), N-15(March)- and N-15(May)-fertilizer), the others being done with unlabelled N. Plant N concentration (% N) was affected (p < 0.001; n = 18) by the crop [80% of variance explained: ryegrass-1 (2.6 +/- 0.9%) > ryegrass-2 (1.9 +/- 0.4%) > maize-2 (1.4 +/- 0.1%) > maize-1 (1.1 +/- 0.2%)] and the crop-tillage interaction (22% of variance explained). Jointly considering all data, more N-15-fertilizer was recovered in the MT (25 +/- 4%) than in the PT soil profile (19 +/- 6%) at the end of the experiment whereas the N exported with the crops was unaffected by the tillage system and varied from 5-6% (N-15(october)-fertilizer) to 45-49% (N-15(March)-fertilizer) and 52-53% (N-15(may)-fertilizer; despite only three instead of four subsequent crops were studied).The N-15 unaccounted for in the case of N-15(October)-fertilizer (72 +/- 5%) was more than twice that in N-15(March)-(34 +/- 7%) and N-15(May)-fertilizer (25 +/- 14%). Considering soil, site and weather conditions, denitrification and nitrate leaching during the ryegrass-1 crop were the most likely processes explaining the high losses of the N-15(October)-fertilizer. Results suggested a higher initial immobilization of the applied 15N in the soil organic matter (SOM) of MT, that reduces N-15 availability to the first crop, followed by an increase of the residual availability of the fertilizer N-15 to the subsequent 2-3 crops. (C) 2016 Elsevier B.V. All rights reserved.