C and N mineralization of undisrupted and disrupted soil from different structural zones of conventional tillage and no-tillage systems in northern France

Abstract

Differences in soil structure created by tillage systems are often believed to have large impacts on C and N mineralization, in turn influencing total soil C and N stocks, CO 2 emissions and soil mineral N supply. The objectives of our work were therefore (i) to study C and N mineralization in undisrupted fresh soils from long-term conventional till (CT) and no-till (NT) systems in northern France and (ii) to evaluate at which scale soil structure plays a significant role in protecting organic matter against C and N mineralization. The in situ heterogeneity of soil structure was taken into account during sampling. Two megastructure zones induced by tillage and compaction were identified in the ploughed layer of CT: zones with loose structure (CT Loose) and clods with dense structure (CT Dense). The soil samples in NT were taken from layers that differed in both structure and organic matter content (NT 0–5 and NT 5–20). Soil from the two zones of different megastructure in CT showed similar levels of protection and similar C and N mineralization. Undisrupted soil from NT 0–5 showed greater absolute and specific C and N mineralization than CT Loose, CT Dense and NT 5–20. Limited soil structure destruction (sieving through 2 mm) had no effect on C and N mineralization. Increased disturbance (sieving down to 250 μm) only induced a significant increase of both C and N mineralization in the 5–20 cm layer of NT. Further disruption of soil structures (sieving through 50 μm) resulted in greater C and N mineralization for all treatments except C mineralization in the upper layer of NT. Protection in the four structural zones in CT and NT was, in general, greatest in the NT deeper layer and least in the NT upper layer. Our results therefore suggest that physical protection in the 5–20 cm soil layer can partly account for larger C and N stocks in NT, but that the large C and N concentrations in the 0–5 cm soil layer are determined by mechanisms other than physical OM protection.