Determination of chemical and biological properties of composts using near infrared spectroscopy

Successful use of compost to maintain plant health and soil fertility requires consistent monitoring of compost quality. For this purpose, near infrared (NIR) spectroscopy might be a useful alternative to standard procedures which are often time-consuming and laborious. Ninety-eight yard-waste compost samples were analysed by conventional methods and NIR spectroscopy. Reference analysis included the determination of age, organic C (C-org) and total N (N-t) contents, C/N ratio, microbial biomass (C.,c), the ratio of C-mic to organic C (C-mic/C-org), basal respiration, metabolic quotient (qCO(2)), hydrolysis of fluorescein diacetate (FDA-HR), specific enzyme activity, i.e. FDA-HR related to C-mic, and suppression of pathogens. All samples were scanned in the visible light and near infrared regions (400-2500 nm). Cross-validation equations were developed using the whole spectrum (first and second derivative) and a modified partial least-square regression method. NIR predicted basal respiration and age successfully [ratio of standard deviation and standard error of cross-validation (RPD) was 4.3 or 2.9, respectively]. All other properties, i.e. C-org and N-t contents, C/N ratio, C-mic, C-mic/C-org, qCO(2), FDA-HR, specific enzyme activity and suppression of pathogens at an inoculation level of 5 parts per thousand related to rating or fresh weight, respectively, were predicted with moderate success (1.4 <= RPD <= 2.0). However, the coefficients of determination for specific enzyme activity and suppression of pathogens related to fresh weight were rather low (r(2) = 0.49 and 0.47, respectively). The results presented indicate that NIR spectroscopy is able to determine important compost quality parameters. However, further research is needed concerning the basis of and limitations for the determination of specific enzyme activity and suppressiveness by NIR spectroscopy.