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

Abstract

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 CO 2 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 ¼ 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 ¼ )0.73 to )0.82).Overall, lignin C explained only a small proportion of the variability in C mineralisation (r ¼ )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 ¼ 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.

How relevant is recalcitrance for the stabilization of organic matter in soils?

“…Using this approach, mass loss, elemental composition, specific compounds, or isotopic signature have been monitored at specific sampling dates over periods of up to 3 years. Most of these studies show that N-rich and water-soluble compounds as well as celluloses are rapidly degraded (Williams and Gray, 1974;Nelson et al, 1994;Kerem et al, 1999;Marschner and Noble, 2000;Webster et al, 2000;Jensen et al, 2005), while lignin and other polyphenols tend to accumulate during the initial stages of litter decomposition (Williams and Gray, 1974;Azam et al, 1985;Kalbitz et al, 2006). Since plant compounds are not uniformly labeled with 13 C, this parameter has also been used as a tool to identify compositional changes during litter decomposition (Schweizer et al, 1999).…”

“…Early studies of litter decomposition showed that lignin content was inversely related to mass loss (Williams and Gray, 1974). However, several recent studies have shown, that a selective preservation of lignin appears to be only relevant during the early stages of litter decomposition and that later-on lignin degradation occurs at the same or even higher rate as the overall litter decomposition (Gleixner et al, 1999;Kerem et al, 1999;Jensen et al, 2005;Prescott, 2005;Kalbitz et al, 2006;Sollins et al, 2006). Recent studies using 13 C-CPMAS-NMR and pyrolysis techniques have confirmed that lignin is altered relatively quickly and does not appear to be stabilized in the long-term in any soil fraction (Baldock and Nelson, 2000;Gleixner et al, 2002;Kiem and Kögel-Knabner, 2003).…”

Organic No-Till with Roller Crimpers: Agro-ecosystem Services and Applications in Organic Mediterranean Vegetable Productions

“…soil temperature and moisture) and intrinsic factors related to plant materials characteristics (i.e. lignin and cellulose content, total and soluble N content, C/N ratio; Jensen et al, 2005). In particular, the C/N ratio of the plant material, even if considered only an approximate guide to the likely net mineralisation, is often able to provide a valuable prediction of N mineralisation and can be effectively utilised in the current practice (Canali et al, 2011).…”

How Important is the Quality of Organic Amendments in Relation to Mineral N Availability in Soils?

“…However, some researchers have found that the pool of SON was of equal size to soluble inorganic N [76], and it became important to evaluate the SON content and its contribution during N mineralization. Some researchers reported that the initial net N mineralization rate from organic amendments was most closely correlated (r = 0.76) with the water-soluble N content, for a broad variety of plant materials in soil [52]. Water-soluble C and N pools have been hypothesized to play a central role in the transport and supply of C and N to microbial populations in soils and thus, the regulation of subsequent microbially mediated N transformations [24].…”