Nitrogen use efficiency (NUE) is defi

Nitrogen use efficiency (NUE) is defined in this study as a measureofkggrainNuptakeperkgNapplied(Ladhaetal.,2005).NUE was estimated as the slope of a linear regression of the average crop grain N uptake in the field replicates against N application, as demonstrated in Fig. 2b. At a few locations, the response in N uptake was not linear at high N application rates. If the R2 value of a line fitted to the data points below 150kgNha−1 was higher thanalinefittedtoalldatapoints,thentheslopeofthelinebelow 150kgNha−1 wasusedinthedataanalysisinstead.IftheR2 values remained below 0.97 the estimate of NUE was considered too low andremovedfromthedataset.Inthisway,36NUEestimateswere removed for the winter wheat and 32 for the spring barley. Slopes from regressions with an R2 value below 0.97 were excluded from the analysis. There were a few locations with very high organic matter contents. In order to prevent these observations from influencing analyzes, observations with SOC values greater than 4% were excluded from the analysis (n=16 and 11 for winter wheat and spring barley, respectively). Furthermore, some values of ORGN appeared unrealistically large in the wheat data set and therefore observations from locations with ORGN values greater than 4000kgNha−1 were also excluded from the analysis (n=18).