Conservation tillage and N fertilization affect soil aggregate distribution, carbon storage and enzymatic activities

Conservation agriculture is globally recommended for increasing carbon (C) stock in soil and reducing greenhouse gases emissions by modifying soil physical, chemical and biological processes. In the present study, soil aggregate fractions, soil organic C (SOC) and soil enzymatic activities in bulk soil and microaggregates within macroaggregates (mM) were measured in a long-term field experiment comparing conventional tillage (CT) and minimum tillage (MT) and N fertilizer to the rotation bread wheat (Triticum aestivum L.) – soybean (Glicine max L. Merr.). Under MT, a higher proportion of free microaggregates and a lower proportion of silt and clay was recorded compared to conventional tillage, suggesting a greater potential to form macroaggregates, despite their proportion was not modified by tillage. However, when macroaggregates were further fractionated, MT resulted in higher proportion of microaggregates. In bulk soil and in mM, all soil enzymatic activities were higher under MT than CT. Conversely, SOC in bulk soil was not modified by tillage, but was higher under MT in mM. These results demonstrate the crucial role of mM for C sequestration under reduced tillage. Thus, this fraction is proposed as an effective diagnostic tool to assess variations in carbon storage induced by agricultural practices.