Photoacoustic fourier transform mid-infrared spectroscopy estimates plant-available phosphorus in bio-based fertilizers and amended soils

Abstract

Phosphorus (P) is a key nutrient for crop production, and there is growing interest in sourcing it from organic materials such as manure and digestate. Rapid estimation of phosphorus availability in organic amendments and their in-soil application is critical for supporting management decisions and maximizing phosphorus use efficiency. In this study, we investigated the use of photoacoustic Fourier transform mid-infrared spectroscopy (FTIR-PAS) to predict plant-available phosphorus (PAP) in bio-based fertilizers (BBF) and BBF-amended soils (Soil-A and Soil-B). Spectra were recorded using an FTIR-PAS instrument (Nicolet 6700, ThermoScientific, USA) for 60 BBF samples (e.g., cow, chicken, and pig manure, and substrate) and 60 samples of BBF mixed with two soil types: Soil-A (Luvisols with sandy-loam texture) and Soil-B (Humic-podzol with loamy-sand texture). Measurements were taken on the first and seventh days of incubation. Partial least squares regression (PLSR) models were developed using a training set of 48 samples and validated with a testing set of 12 samples for three cases: BBF, BBF-amended Soil-A, and BBF-amended Soil-B, at both incubation time points. FTIR-PAS demonstrated higher accuracy in predicting PAP for BBF ($R^2=0.84$; RMSE = 1.51 g kg⁻¹) compared to BBF-amended soils. Prediction accuracy was higher for Soil-A ($R^2=0.72$–0.81; RMSE = 26.6–30.32 mg kg⁻¹) than for Soil-B ($R^2=0.19$–0.22; RMSE = 46.3–50.6 mg kg⁻¹). The accuracy of FTIR-PAS decreased slightly (a 3% reduction in $R^2$) with increasing incubation time up to seven days. In conclusion, FTIR-PAS shows promise for estimating PAP in BBF and BBF-amended soils, but soil texture and type require special consideration. Further research is needed to improve prediction accuracy for soils, as the current models exhibit limitations in this regard.