Non-Labile Plant C Controls Oxisol Aggregation.

Plant materials can act as natural soil conditioners by reducing the vulnerability of soil to physical degradation. We investigated (i) the effect of plant material addition from certain Poaceae and dicot species on soil organic C (SOC), total N, carbohydrate composition, total microbial and fungal biomass, and aggregation of an Oxisol; and (ii) the relationship among these properties and C mineralization patterns. An incubation experiment (25°C) was carried out for 180 d after addition of 11 plant materials (4 g C kg^‑1 soil) and a control (no plant material added). Mineralization of C during incubation was characterized by modeling organic C in two pools (labile and non-labile) using a first-order plus linear fitting. Compared to control and dicots, Poaceae plant materials showed higher mineralization rates for the non-labile C pool (k), higher soil pentose content (xylose + arabinose), lower galactose + manose to arabinose + xylose (GM/AX) ratio, and higher mean-weight diameter (MWD) of soil water-stable aggregates. A strong positive correlation was found between k and soil aggregate MWD (r = 0.95, P < 0.001). Principal component analysis showed that k, soil pentose content and soil aggregation were closely related. These results suggest that plant-derived pentoses have a mineralization rate that provides long-term effects of plant materials on Oxisol aggregation. This effect is independent of changes in remaining SOC. Our results provided evidence that C sequestration and aggregation are decoupled processes in Oxisols.