Contribution of maize root derived C to soil organic carbon throughout an agricultural soil profile assessed by compound specific 13C analysis

Cutin and suberin structural units might be stabilized in subsoils and contribute to the aliphatic structures observed in stabilized soil organic matter (SOM). We studied their dynamics in subsoils by measuring the concentrations and 13C contents of cutin and suberin markers in soil profiles under wheat (C3) and after 9 years of maize cropping (C4 plant). Alkandioic acids were considered as markers for roots, mid-chain hydroxy acids were only present in shoots and ω-hydroxy acids were identified in both roots and shoots. The diacid concentrations greatly increased below the ploughed layer after 9 years of maize cropping, possibly due to a higher root density of maize compared to wheat or to a faster turnover of fine roots and increased exudation of maize compared to wheat. From 0-75 cm, 9 years of maize cropping did not affect the distribution of shoot biomarkers but increased their concentrations. By contrast, below 75 cm, the shoot marker concentrations drastically decreased from the wheat control to the 9 year maize soil. The difference of δ13C observed for shoot markers was always lower than that observed for ω-hydroxy acids, and below 15 cm, it was close to that observed for SOC. The difference in δ13C of diacids was much more variable along the profile. Since the concentrations of the different markers were not at equilibrium, it was not possible to estimate their turnover. This study suggests several caveats for the use of molecular markers of roots and shoots to study the dynamics of SOM in deep soils: the higher heterogeneity compared to the ploughed layer, the presence of long history record of past vegetation that may hinder the short time scale changes tracked with the 13C isotope technique, and the difficulty in evaluating root inputs in the soil systems.