Instantaneous accounting for leaf water in X-ray fluorescence spectra of corn grown in manure- and fertilizer-amended soils

Abstract

Manure-amended soils have shown a high degree of temporal and geographic variability in labile phosphorus (P). Real time information and practical ways for determining foliar composition are needed to obtain direct evidence of plant uptake, and the supplying capacity of the soil. Field and laboratory experiments were conducted to determine leaf water content effects on spectral characteristics of corn (Zea mays) uppermost leaves when grown in mineral P fertilizers- (0–250mgPkg−1) or manure-amended soils (0, 15, and 30kgPha−1). In situ X-ray fluorescence scans of fresh green leaves yielded foliar P concentrations that paralleled those obtained using oven-dried harvested leaf samples. Leaf structure and size of seedlings can cause significant variations in leaf-to-spectrometer contact, inter-element interference, and leaf water content and its attenuating effects on the yield of weak fluorescence radiation of low Z-elements (<19). Intensities of scattered radiation associated with the X-ray tube anode were significantly correlated with leaf water content (θw), which was used to (i) assess crop water status and (ii) normalize fluorescence intensities of P to a common basis. The θw−P reduced concentration relationship was best-described by a sigmoidal function y=0.12+0.94/(1+exp(-(θw-0.37)/-0.17)) with r2=0.938 and RMSE=0.02. Therefore, we proposed its use to obtain P concentration on a dry weight basis and unbiased estimates of crop P status. The in situ fluorescence sensing system presents a new paradigm in nutrient management to insure the sustainability of agroecosystems and development of field-specific nutrient management practices.