Predictive Capacity of Visible-Near Infrared Spectroscopy for Quality Parameter Assessment of Compost

Abstract

Screening tests are basic procedures commonly used to assess compost quality. Important parameters for quality assessment are the germination capacity and the suppression of plant pathogens which have to be measured by time-consuming laboratory methods. The objective was to test whether visible (vis) and near infrared (NIR) spectroscopy (vis-NIR) is useful to analyse parameters important for compost quality. Ninety seven compost samples from Switzerland were analysed by conventional methods and by vis-NIR. The content of organic (Corg) and inorganic C (Cinorg), total N (Ntot), mineralisable N after 56 days (Nmin_d56), total P (Ptot), K, Ca and salt, the C/N ratio, pH and microbiological characteristics [hydrolysis of fluorescein diacetate (FDA-hydrolysis) as indicator of total enzyme activity and cellulase activity] were determined. Furthermore, plant tolerance and the suppression of pathogens were tested using germination tests with salad, cress, ryegrass and bean or a Rhizoctonia solani bioassay, respectively. The samples were scanned in the range of 400–2500 nm (visible light and NIR) using a Foss NIRSystems spectrometer 6500. A modified partial least squares regression method and the whole spectrum were used to develop cross-validation equations for all constituents. For this, the first to third derivative was calculated. The prediction accuracy was evaluated as excellent for Corg and good for N, and the C/N ratio based on the RSC values (ratio of standard deviation of laboratory results to standard error of cross-validation) and the coefficients of determination ( r2). Approximate quantitative predictions were possible for the contents of Ptot, K, Ca and salt, whereas for the constituents Cinorg, Nmin_d56, FDA-hydrolysis and the germination tests with cress and salad only between high and low values could be discriminated. Unsuccessful predictions as indicated by RSC values lower than 1.5 and r2 values below 0.50 were obtained for pH, cellulase activity, germination tests with ryegrass and bean and the disease suppression test using R. solani. Overall the results of the present study indicate that vis-NIR spectroscopy has the potential to be used for quality assessment of composts and to replace time-consuming methods such as germination tests using salad and cress. However, the use for monitoring purposes requires further research to clarify whether other complex quality parameters such as disease suppression indicators may also be predicted successfully.