Can the Biochemical Features and Histology of Wheat Residues Explain their Decomposition in Soil?

Abstract

The biochemical characteristics or quality of crop residues is an important factor governing soil residue decomposition. To improve C and N biotransformation models the process underlying this decomposition needs to be better understood and new quality criteria found to describe it. The aims of this explorative study were to (i) improve our understanding of residue decomposition from detailed studies of cell wall biochemical compositions and tissue architecture (ii) find new ways of exploring generic indicators of organic matter quality. To do this, the cell wall composition and tissue architecture of wheat leaves, internodes and roots, before and after their incorporation into soil were determined. Results showed that leaves which were poorly lignified decomposed faster in soil than internodes and roots. Cellulose was the most degraded polysaccharide irrespective of wheat residue. However, cellulose was much more degraded in the case of leaves as compared to internodes and roots. Leaves also presented a highly condensed lignin structure and the extent to which uncondensed leaf lignin was affected by soil decomposition suggests that the contribution of leaf lignin to C mineralization during incubation was very low. Roots which contained similar amounts of lignin than the internodes decomposed more slowly. Roots were enriched in phenolic acids, and more particularly p-coumaric acid (pCA) and presented a more condensed lignin structure than internodes. Phenolic acids are involved in the formation of lignin-polysaccharide complexes known to be recalcitrant to enzymatic attack. Microscopic investigations confirmed that the vessels were the most resistant tissues to decomposition in soil and this could be related either to their lignin content or to the quality of this lignin (condensed-like type lignin). Therefore, cell wall biochemical analyses have revealed that phenolic acids, which in their esterified form represent only 0.1-1% of plant dry matter, have cross link functions within the cell walls that could be of major interest in estimating soil residue degradability. Lignin quality (monomers, level of condensation) was another crucial criterion that could explain why residues with similar amounts of lignin decomposed at different rates in soil (roots vs. aerial parts). Visualization of residue cell walls before and after decomposition in soil underlined the interest of a microscopic approach coupled with image analysis. This study, corroborated by the extensive literature on forage digestibility, confirmed that the proportions of vascular tissue and sclerenchyma cells in plant material are determinant factors affecting plant degradability. In the future, classification of plant material based on these criteria could lead to the definition of new quality parameters for models of C and N biotransformation in soil.

Impact of fine litter chemistry on lignocellulolytic enzyme efficiency during decomposition of maize leaf and root in soil

“…Chemical characteristics of decomposing leaves and roots were expressed in relation to the initial dry matter (non-decomposed dry matter, ND-DM) by taking into account the loss of mass by decomposition, calculated from the cumulative amounts of mineralized C (Machinet et al 2009). These chemical analyses were performed on only two replicates at each date, as previous experiments demonstrated extremely low variation in similar results (Bertrand et al 2006;Machinet et al 2009Machinet et al , 2011a. This low number of replicates decreased the power of statistical analyses so that significant results should be viewed as conservative.…”

“…It contains 16.8 % clay, 76.3 % silt and 3.8 % sand, has an organic carbon content of 8.70 mg C g -1 and a pH (soil H 2 O) of 7.6 (Bertrand et al 2006). Soil was collected in autumn 2008 from 10 soil cores to a depth of 0-30 cm, then pooled and air-dried for 2 days to reduce moisture content to 120 mg water g -1 dry soil.…”

“…Plant residues consist of a soluble and an insoluble fraction (cell wall), which vary by species, tissue type and maturity. Cell walls consist mainly of insoluble polymers such as cellulose, hemicelluloses and lignin, which form a complex, chemical network that influences biological degradation (Chesson 1988(Chesson , 1997Bertrand et al 2006;Š najdr et al 2011). The recalcitrance of residues to decomposition is mainly related to cell wall polymer content, i.e., lignin (Melillo et al 1982;Berg and McClaugherty 2008).…”

Tissue density determines the water storage characteristics of crop residues

“…It is well established that the chemical composition of plant residues changes during decomposition in soil (Trinsoutrot et al 2000;Bertrand et al 2006); however, there are few studies addressing the water retention properties of plants during decomposition (Dresboll and Magid 2006). The water storage characteristics of crop residues are extremely important parameters for modeling the decomposition of crop residues, particularly when residues are left at the soil surface under notill conditions ).…”

Influence of non-cellulose structural carbohydrate composition on plant material decomposition in soil

“…Studies in the rumen environment have shown that arabinose is degraded more rapidly than xylose (Åman 1985;Nordkvist and Åman 1986;Albrecht et al 1987;Jung et al 1998). However, the rate of degradation of arabinoxylans in grasses has been found to decrease with increasing degree of branching, as indicated by an increase in the arabinose-to-xylose ratio (Albrecht et al 1987;Bertrand et al 2006). On the other hand, Vailhé et al (2000) found a positive correlation between the arabinose-to-xylose ratio and degradation rate.…”