Dynamics of Total and Humic Carbon in a Long-Term Field Experiment Determined by 13C Natural Abundance.

Soil organic carbon (C) preservation in agroecosystems is crucial point to maintain soil fertility and productivity, and to reduce losses of CO2 in the atmosphere. Agricultural management practices can differently affect the level of soil organic C (SOC). In the present paper or chapter the results of a long-term field experiment (30 years) were investigated to evaluate the effect of mineral fertilization and organic amendments on soil organic C content and on the humic acids (HA) that represent the most important and stable reservoir of soil organic C. The effect of the plant species was also evaluated by comparing wheat and corn monocultures. The amount of corn-derived C in soil and HA at the end of the experiment was calculated by 13C natural abundance measurements. After 30 years of cultivation, the SOC significantly decreased in both unfertilized (Control) cropping systems, especially with continuous corn. Mineral fertilization (Min) and organic amendments (Org) always caused an increase in SOC, especially with Org treatment on continuous corn. The C always increased in HA, except in the unfertilized plots of corn monoculture. Again the highest increase was observed with Org treatment. The amount of corn-derived C in total organic C (TOC) increased in the following order: unfertilized < Min < Org treatments, ranging from 19 to 29%. The turnover time of the older C3-derived C increased in the same order ranging between 55 and 86 years in the Control and Org treatment, respectively. In the HA the proportion of corn-derived C was similar in the Control and Org treatment (26.4%), lower in the Min treatment (23.7%). Nevertheless if we consider the total amount of corn-derived C in soil and the proportion recovered in the HA, the highest was measured in the unfertilized control. In general a proportion ranging from 35% (Org) to 40% (control) of the total corn-derived C in soil was recovered as humic C, confirming the important role of this pool as a C reservoir in soil