Nitrogen concentration in New Zealand vegetation foliage derived from laboratory and field spectrometry

Multiple linear regression models of leaf nitrogen concentrations were developed from measured leaf values of selected native New Zealand plant species and spectral absorption obtained from laboratory NIR spectrometry( NIRS 6500). Two sets of equations were produced from samples that were from either dried and ground or fresh and intact foliage. Canopy nitrogen was predicted from these laboratory-derived equations and applied to GER 2600 spectroradiometer data collected above small plant canopies (r(2) = 0.65 to 0.77). Reflectance data from the NIRS 6500 and GER 2600 were not directly comparable, therefore calibration of predicted canopy nitrogen values was necessary to place the predicted nitrogen in the observed range of values. Highest accuracy occurred where wavelengths selected in the laboratory analysis were not 'attributable' to nitrogen bonds rather to complex carbohydrate compounds(e.g. 2160 nm). In addition, as laboratory equations were derived from small sample sizes, canopy nitrogen was predicted with greatest accuracy when only a single 'attributable' wavelength was utilised.