Decomposition dynamics of plant materials in relation to nitrogen availability and biochemistry determined by NMR and wet-chemical analysis

Improved understanding of the interactive relationships of plant material decomposition kinetics to biochemical characteristics and nitrogen availability is required for terrestrial C accounting and sustainable land management. In this study, 15 typical and/or native Australian plant materials were finely ground and incubated with a sandy soil at 25°C and 55% water holding capacity without nitrogen (-N) or with nitrogen (+N) addition (77 mg N kg soil as KNO ). The C mineralisation dynamics were monitored for 356 days and the initial biochemical characteristics of the plant materials were determined by NMR and wet-chemical analyses. Under the -N treatment, C mineralisation rates of the plant materials were positively correlated with their initial N contents during the first several weeks, and then negatively correlated with lignin and polyphenols contents during the late stages of incubation. Thus the ratios of lignin/N, polyphenols/N and (lignin+polyphenols)/N had more consistent correlation with the cumulative amounts of C mineralised throughout the incubation than did any single component. In terms of the C types determined by NMR analysis, the C mineralisation rates were initially related positively to carbonyl C contents, and then negatively to aryl and O-aryl C contents from day 3 onwards. Addition of NO -N accelerated C mineralisation during the early stages, but resulted in lower cumulative C mineralisation at the end of the incubation for most plant materials. Under the +N treatment, the decomposition rates were correlated with the contents of lignin and the sum of cellulose+acid detergent-extractable non-phenolic compounds, or with aryl, O-aryl and N-alkyl+methoxyl C contents. Regardless of the N treatment, the ratios of aryl/carbonyl, O-aryl/carbonyl and (aryl+O-aryl)/carbonyl C had the closest and most consistent correlations with the cumulative C mineralisation among all biochemical indices examined. A double exponential model with defined mineralisation rate constants for the active and slow pools was used to describe the C mineralisation dynamics. The biological meanings of the kinetically estimated active and slow pool sizes are interpreted and their relationships to the initial chemical/biochemical composition of the plant materials are explored