Carbon and nitrogen stocks in relation to organic matter fractions, aggregation and pore size distribution in no-tillage and conventional tillage in northern France

The magnitude of and mechanisms for long-term differences in soil organic matter stocks under no-tillage and conventional tillage are still relatively poorly known. We quantified differences in total C and N stocks after 32 years of no-tillage (NT) and conventional tillage (CT) in plots with a long-term cultivation history before differentiation and the same annual C and N returns to the soil. The role of physical protection of organic matter (OM) in these stock differences was further investigated by examining the changes at different levels of structural complexity, i.e. organic matter fractions, aggregation and pore-size distribution. Four structural zones were sampled: loose and dense soil zones under CT and the 0-5 cm (rich in OM) and 5-20 cm (massive structure) soil layers under NT. The C and N stocks, calculated for an equivalent mass of dry soil, were only 10-15% larger under NT than under CT. Mineral-associated N and particulate organic matter accounted for about 50% of the difference in N stocks. However, 66% of the total difference in C stocks was due to differences in the particulate organic matter (58%) and free residues (8%) fractions. The additional C and N under NT were almost exclusively situated in aggregates larger than 250 μm in diameter. Our results suggest that physical protection of OM under NT contributes significantly to the differences in C and N stocks between NT and CT by (i) enhanced macroaggregate formation in the 0-5 cm layer due to greater microbial activity and OM content and (ii) a better protection of soil organic matter in the 5-20 cm layer due to the presence of small pores and lack of soil disruption by tillage or climate.