Agricultural management practices and dynamics of C in a long-term field experiment followed by isotopic and thermal analysis

Long-term field experiments are crucial for studing the effects of different management practices on organic C storage capacity of agricultural soils. One of the most important pools of soil organic C (SOC) is that of the humic substances and in particular of humic acids (HA-http//ihss.gatech.edu/ihss2) that are complex macromolecules modified from plant compounds or newly synthesized during decomposition, that accumulate in soil. Because of their resistance to microbial degradation, they represent a crucial component of SOC where C tends to be stored for long periods and an important reservoir of nutrients. In the long-term field experiment at the Cadriano farm (Bologna University) different fertilization treatments have been compared for over thirty years on a continuous-wheat (used as a reference field) and a continuous-maize cropping. These two plant species have different C isotope composition because the C3 and C4 photosynthetic pathways differently fractionate against the heavier isotope of CO2-C, therefore the isotopic approach was used to quantify the C3 and C4-derived C in soil. In particular three fertilization treatments were compared: no fertilization (control) mineral fertilization and organic amendment, and the content of old C3 and new C4-derived C in soil and in the humic acids (HA) was determined. Moreover a structural characterization of HA was carried out with thermogravimetric (TG) and differential thermal analysis (DTA). After 36 years of continuous maize cropping the old C3-derived C similarly decreased in spite of the treatments. The new C4-derived C constantly increased with all the treatments, significant higher amounts compared the control were measured with the mineral, and even more with the organic treatment. In the HA the amount of C3-derived C decreased over the studied period in the control and in the mineral treatment, whereas it increased with the organic treatment. The amount of C4-derived C in the HA linearly increased over the studied period in all the treatments with the higher values measured for the mineral and organic one. TG-DTA data showed that the 36 years organic amendment caused in the HA an increase in aromatic compounds compared to the control and the mineral treatment.