Influence of biochemical quality on C and N mineralisation from a broad variety of plant materials in soil

We studied C and N mineralisation patterns from a large number of plant materials (76 samples, covering 37 species and several plant parts), and quantified how these patterns related to biological origin and selected indicators of chemical composition. We determined C and N contents of whole plant material, in water soluble material and in fractions (neutral detergent soluble material, cellulose, hemicellulose and lignin) obtained by stepwise chemical digestion (modified van Soest method). Plant materials were incubated in a sandy soil under standardised conditions (15 °C, optimal water content, no N limitation) for 217 days, and CO2 evolution and soil mineral N contents were monitored regularly. The chemical composition of the plant materials was very diverse, as indicated by total N ranging from 2 to 59 mg N g-1, (i.e. C/N-ratios between 7 and 227). Few materials were lignified (median lignin = 4% of total C). A large proportion of plant N was found in the neutral detergent soluble (NDS) fraction (average 84%) but less of the plant C (average 46%). Over the entire incubation period, holocellulose C content was the single factor that best explained the variability of C mineralisation (r = -0.73 to -0.82). Overall, lignin C explained only a small proportion of the variability in C mineralisation (r = -0.44 to -0.51), but the higher the lignin content, the narrower the range of cumulative C mineralisation. Initial net N mineralisation rate was most closely correlated (r = 0.76) to water soluble N content of the plant materials, but from Day 22, net N mineralisation was most closely correlated to total plant N and NDS-N contents (r varied between 0.90 and 0.94). The NDS-N content could thus be used to roughly categorise the net N mineralisation patterns into (i) sustained net N immobilisation for several months; (ii) initial net N immobilisation, followed by some re-mineralisation; and (iii) initially rapid and substantial net N mineralisation. Contrary to other studies, we did not find plant residue C/N or lignin/N-ratio to be closely correlated to decomposition and N mineralisation.