Investigating the chemical characteristics of soil organic matter fractions suitable for modeling

Current models of soil organic matter (SOM) turnover tend to invoke pools that are defined by their contrasting first-order reactivity constants but which cannot be directly measured. New models may be based around fractions defined by procedures that can be used to isolate them experimentally. The drawback of such fractions is that they may display properties that are not sufficiently distinct or which vary in time or space. In this study the properties of two fractions from soils of contrasting geographical origin and under different nutrient management were examined using C-13 nuclear magnetic resonance (NMR) spectroscopy. The fractions were free SOM (FR-SOM, discrete organic particles located between stable aggregates), and intra-aggregate SOM (IA-SOM, discrete organic particles within stable aggregates). The composition of both fractions was highly consistent across soil types and environments, but the fractions differed significantly in the proportion of C present in five of the seven functional C groups identified by NMR (P < 0.05). The results confirmed that IA-SOM contains a greater proportion of microbial products and more resistant C as compared with the FR-SOM. Nutrient management affected fraction composition in four functional groups most abundant in plant material or farmyard manure (P < 0.05). The effects were considerably less pronounced when the analysis was restricted to plots receiving inorganic (or zero) N. Overall the results supported the view that free and intra-aggregate organic matter occupy contrasting positions in the decomposition sequence, and are likely to display reactivities sufficiently distinct to operate as discrete pools in new SOM models.