NIR integration for the evaluation of compost quality with extruded wood fibers

Extruded wood fibers are an environmentally friendly alternative to peat in horticulture and are suitable to produce high-quality compost1. Extrusion increases the homogeneity and stability, which enables more consistent compost products2. An essential quality component of this compost is nitrogen stability, determined by the availability of ammonium (NH4+) and nitrate (NO3-). Traditionally, nitrogen stability is measured using the Zöttl test, but this is time-consuming and can give inconsistent results3,4. Therefore, near infrared spectroscopy (NIR) in combination with partial least squares (PLS) modeling is being investigated to provide a faster and more accurate assessment of compost wood fiber quality. Four sample batches of extruded wood fibers (Ehrensberger GmbH, Tenneck, AT) were prepared under different conditions regarding sample preparation, enrichment and incubation. NIR spectra were recorded, and NH4+ and NO3- were determined using steam distillation. The NIR data were analyzed using PLS methods to develop predictive models for the nitrogen contents and to check the correlations with the chemical analyses. Initial analyses showed significant correlations between the NIR spectra and the chemically determined NH4+ and NO3- values, confirming the potential of the NIR technique to replace the Zöttl test. The raw data models showed good agreement between predicted and actual values for NO3-, while there was potential for improvement for NH4+ at higher concentrations. Adjustments to the spectra pretreatment and modeling methodology further increased t. he prediction accuracies. The integration of NIR into compost wood fibre quality control offers advantages such as time and cost efficiency as well as the possibility of continuous monitoring. The results confirm the suitability of NIR in combination with PLS modeling as a reliable method for predicting nitrogen stability in compost substrates made from extruded wood fibers.