Leaf nitrogen concentration and chlorophyll meter readings as predictors of tall fescue nitrogen nutrition status

Determining the nitrogen nutrition index (NNI) of a crop requires measuring the amount and nitrogen concentration of standing biomass. This limits its use at farm conditions where simplicity and agility are required. In this study, two proxies of NNI were assessed: the nitrogen concentration of leaves located in the sward upper 5–7 cm layer (NUSL, g N kg−1 dry matter), and its greenness, as measured by a SPAD 502 handheld chlorophyll meter (SPADUSL, SPAD units). Seven field experiments carried out at Balcarce (Argentina), on two soil types, were conducted from 2008 to 2010, during autumn, late winter/early spring, spring and summer regrowths of tall fescue swards (Lolium arundinaceum (Schreb.) Darbysh.). Different nitrogen application rates were imposed to generate contrasting conditions of nitrogen availability. NNI, NUSL and SPADUSL were simultaneously measured four to seven times during sward regrowths. NUSL was closely associated with NNI. However, the regression parameters changed with elapsed time since sward initial cut. Once developmental effects were accounted for by fitting lineal regressions separately to thermal time intervals related to leaf appearance rate, equation parameters did not differ among years, seasons and sites: NNI = 0.024 (NUSL − 3.27), R2 = 0.89, N = 78; NNI = 0.026 (NUSL − 2.37), R2 = 0.81, N = 108; and NNI = 0.030 (NUSL − 3.26), R2 = 0.84, N = 102 for swards accumulating, respectively, <260, 260–440, and >440 growing degree days (GDD, base temperature 4 °C) since the initial cut. Obtained equation parameters compared well to those reported for other C3 grasses, suggesting that the NNI–NUSL relationship is approximately constant among members of this functional subgroup. Likewise, SPADUSL was highly associated with NUSL and, therefore, with NNI. Developmental effects were also evident, but only two equations arised: NNI = −0.81LN (−0.82LN (SPADUSL/68.42)), R2 = 0.69, N = 72; and NNI = 0.035 (SPADUSL–18.50), R2 = 0.75, N = 169, for swards accumulating, respectively, <260 or >260 GDD since initial cut. The effect of the higher intrinsic variability of SPADUSL on its NNI predictive precision can be compensated increasing its sampling intensity, except at early stages of sward development when higher leaf N concentrations derive in the saturation of the chlorophyll meter. Still, SPADUSL emerges as an adequate method for performing an instantaneous, approximated, ‘in the field’ estimation of swards N status, thus allowing a prompt correction of N deficiencies through fertilization.

► Two proxies of NNI were evaluated for the assessment of tall fescue N status in seven field experiments covering different seasons, years and sites.
► N content in the uppermost sward leaves (NUSL) was closely associated with tall fescue NNI.
► Once developmental effects were accounted for, the NNI–NUSL relationship parameters did not differ among years, seasons and sites.
► SPAD readings in the uppermost sward leaves (SPADUSL) also associated strongly with tall fescue NNI.
► SPADUSL emerges as an adequate tool for performing an instantaneous, approximated sward N status assessment at the field level.