In agricultural ecosystems the maintenance of Soil Organic Carbon (SOC) depends on soil tillage, on the amount of crop residues after harvest, rhyzodeposition and on the addition of organic amendments. Mineralization and leaching of dissolved organic C are, on the contrary, the mechanisms mainly responsible for the C loss from the system. The balance between inputs and outputs can result in an accumulation or viceversa in a decline of SOC content depending on the agricultural managements. To better highlight the SOC dynamics, it's also important to follow the fate and turnover of organic C contained in several components of SOC, in particular that of the humic substances which are considered important sink for organic C in soils. The effects of the agricultural management on SOC content and on the humic substances, can be appreciated only after long period of time, in long term field experiments. However, in order to study the turnover of SOC and to quantify the amount of new C which is being incorporated, a change in vegetation from C₃ to C₄ species is required. In this situation the C₄-derived C entering in a C₃ soil is quantified by the ¹³C natural abundance measurement technique. In the present study we present data from a 30 year field experiment where a monoculture of maize (Zea mays L. spp. mays) started in 1973 on a C₃ soil. The change of SOC, total N, and humic fraction C was monitored in this field and in the reference site cultivated with wheat (Triticum durum L.). The maize-derived C entering the SOC and the humic substances during this period of time was quantified with the ¹³C natural abundance technique on the basis of the isotopic shift of the C₄ field with respect to the C₃ reference field. In spite of a decline of SOC content that was accompanied by a decline of total soil N, the amount of maize-derived C progressively increased accounting for about 10% and 19% of total SOC after 15 and 30 years of maize monoculture, respectively. The same trend was observed also for the humic fraction, in spite of a significant decrease of C, the portion of maize-derived C significantly increased after 15 and 30 years of maize accounting for 12% and 26%, respectively. The percentage of maize-derived C that was stored in the humic substances progressively increased throughout the experiment from 27% after 15years, to 36% after 30 years, confirming the important role of the humic fraction in the storage of SOC.