Micropore surface area of alkali-soluble plant macromolecules (humic acids) drives their decomposition rates in soil

Previous studies suggested that micropore surface area (MSA) of alkali-soluble bio-macromolecules of aerial plant residues of maize constitutes an important factor that explains their humification in soil, that is, preservation against biological degradation. On the other hand, root plant residue contributes to the soil humus balance, as well.Following the experimental design used in a previous paper published in this journal, this study shows that the biochemical recalcitrance of the alkali-soluble acid-insoluble fraction of the root plant material, contributed to the root maize humification of both Wild-type maize plants and its corresponding mutant brown midrib (bm3), this latter characterized by reduced lignin content.Humic acids (HAs) existed in root (root-HAs) were less degraded in soil than corresponding HAs existed in shoot (shoot-HAs): shoot-HAs bm3 (48%) > shoot-HAs Wild-type (37%) > root-HAs Wild-type (33%) > root-HAs bm3 (22%) (degradability shown in parenthesis).These differences were related to the MSA of HAs, that is, root-HAs having a higher MSA than shoot-HAs: shoot-HAs bm3 (41.43 ± 1.2 m2 g−1) < shoot-HAs Wild-type (43.43 ± 1.7 m2 g−1) < root-HAs Wild-type (51.7 ± 3.6 m2 g−1) < root-HAs bm3 (54.08 ± 3.9 m2 g−1).Taking into account both the previous data obtained for maize shoots and the results of this study, it was possible to find a very good correlation between degradability of HAs and HA-MSAs (r = −0.88, P < 0.08, n = 4), confirming that MSA was able to explain bio-macromolecules recalcitrance in soil.