Their most applied indicators and their trends after AD have been summarised. Ultraviolet–visible and fluorescence spectroscopy have been identified as the most relevant techniques for aqueous phase study whereas mid-infrared and ¹☼ cross-polarisation magic angle spinning nuclear magnetic resonance were the most appropriate for the solid phase. This review aimed to fill up this gap through the identification and evaluation of: (i) the most frequently applied techniques, their principles, deployment methods and limitations, (ii) the quantitative spectroscopic indices to define OM stability, (iii) the common trends of these parameters due to AD effect on the OM and (iv) the relevance of each technique based on the frequency of statistically significant results reported. This unclear scenario brings to the forefront the need for a meta-analytical study providing benchmarking values and trends. In these studies, a lack of agreement on analytical techniques, indicators and reference values is evident. Spectroscopic methods are useful and easy to deploy alternatives to assess several aspects in anaerobic digestion (AD) studies such as process performance, waste classification and both OM composition and transformation. However, organic matter (OM) stability in digestates is usually either poorly assessed or done through laborious methods. Soil carbon protection following land use diversification was evident for at least 80 years.ĭigestate landspreading is a key aspect of the circular economy. High carbon concentrations in native forest micro-sites exhibiting low pH occurred predominantly as aliphatic and carbohydrate fractions, even at depths down to 0.5 m. Fractional areas for NMR spectra ranged between 0.16 and 0.34 for aliphatic, 0.33 and 0.50 for carbohydrate, 0.12 and 0.40 for aromatic, 0.04 and 0.11 for phenolic, and 0.01 and 0.17 for carbonyl carbon. Principal component analysis and projection to latent structures regression of the spectra showed changes in carbohydrate and phenolic fractions reflected the current land use, with the introduction of pasture and plantation forest decreasing the aliphatic and increasing the aromatic fractions of the soils. Mid infrared (IR), near infrared (NIR), and 13C nuclear magnetic resonance (NMR) spectra were recorded on solid soil samples from three neighbouring sites with similar history but different current land use - namely, native podocarp forest, pasture land cleared from native forest, and exotic softwood plantation converted from improved pasture.
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