Carbon Turnover Kinetics with Depth in a French Loamy Soil

Abstract

Soil C dynamics below the plow layer have been little studied, in spite of proven large C stocks and suspected large C stabilization potential. The objective of the present study was to determine C‐turnover kinetics throughout the 1‐m profile of a cultivated loam soil of the Paris basin, France. The soil 13 C signature was determined to depths of 1.05 m in 32 replicated plots having received from 0 to 10 yr of maize after wheat. Above‐ and below‐ground maize‐residue biomass inputs were estimated throughout the 10‐yr period. After 10 yr, maize‐derived soil organic carbon (SOC) constituted about 10, 5, and 2% of the total SOC at 15‐, 50‐, and 100‐cm depths, respectively. About one‐third of recently deposited maize‐derived SOM present in the 1‐m soil profile was retrieved below the Ap horizon. The ratios of maize‐derived soil C to the cumulative maize above‐ and below‐ground inputs over the 10‐yr period averaged 17% across the soil profile. This ratio was lower in the Ap horizon (i.e., 13%) than in deeper soil horizons. Circumstantial evidences suggest that the distribution profile of recently deposited maize‐derived C was influenced by fine root activities, bioturbation, and dissolved organic carbon (DOC) transport, the latter being substantiated by a high correlation ( r 2 = 0.86) between SOC contents and amorphous Fe + Al contents. In conclusion, our study stresses the need to take into account the full 1‐m soil profile in C sequestration studies.