Variation in soil carbon stocks with depth along a toposequence in a sub-humid climate in North Africa (Skikda, Algeria)

•Deep soil C accounts for >50% of total C stocks in the piedmont and alluvial plain.•Deep soil C is associated with soil fractions <20 μm, mainly in the lowland soils.•Topsoil C is associated with soil fractions >50 μm, mainly in the mountain soils.•Deep soil C stocks are not inert and are potentially mineralizable.•Top and deep soil C stocks are potentially mineralizable at the same rate.

Deep soil carbon (>30 cm) is the primary pool of soil organic carbon (SOC). However, deep SOC is often not included in carbon inventories or management strategies, especially in North-Africa. The objective of the study was to determine the distribution and the main factors governing SOC storage along a representative toposequence in North-Eastern Algeria. The soil organic matter (SOM) was characterized by SOC content, carbon stocks, particle size distribution and mineralization potential.The amount and the dynamics of the deep SOC depended on the topographical position. In the mountain soils, most of the SOC stock (100 t C ha−1) was in the topsoil (0-30 cm) and associated with the >50 μm fractions (plant materials) whereas in the piedmont and alluvial plain soils, most of the SOC stock (168 t C ha−1) was accumulated in subsoil (30-100 cm) and associated with the <20 μm fractions (mineral fractions). The SOC in the subsoil contributed to SOC potential of mineralization and should not be underestimated, especially in the alluvial cultivated plains. This carbon should be considered as an important component in agro-ecological agriculture but it is also a potential source of CO2.