Organic matter quality in ecological studies: theory meets experiment

Despite its importance for the understanding of element cycles in ecosystems, organic matter (OM) quality has remained an elusive property that is difficult to measure. In this study, two new approaches, both of which taking into account the complete biochemical composition of the organic material during the decomposition process, have been combined to solve this problem. First, following the continuous-quality theory where quality is defined as a measure of substrate availability to the decomposers, initial litter OM qualities of a range of plant species from two experiments on litter decomposition were estimated and resulted in highly accurate fits of observed mass loss during decomposition. Applying the same theory, qualities of the litters at all stages of decomposition were then calculated. By comparison, the initial qualities of the same litters were estimated from conventional chemical fractions and resulted in much lower accurate fits. Second, near-infrared reflectance spectroscopy (NIRS), a highly precise physical method of characterising biochemical composition of OM, was used to obtain a unique spectral signature of each sample. Calibrations were performed between spectral data and calculated qualities on the first half of the sample set and the calibration equations were applied to the second half of the sample set. Results show that theoretical litter OM quality can be calibrated and predicted precisely using NIRS. OM quality, defined according to a theoretical concept of substrate availability to decomposers, then contains and summarises all the relevant biochemical information. We demonstrate how the combination of NIRS and theory allows us to accurately measure OM quality. Measurement of OM quality provides an access to a fundamental property of organic matter and opens up new possibilities for studying element cycles in ecosystems.