Soil carbon transformation in long-term field experiments with different fertilization treatments

Abstract

Soil carbon transformation was observed in long-term stationary field experiments (longer than 20 years) at two sites with different soil-climatic conditions (Luvisol, Chernozem). The following crops were rotated within the trial: row crops (potatoes or maize)-winter wheat-spring barley. All three crops were grown each year. Four different fertilization treatments were used: (a) no fertilizer (control); (b) sewage sludge (9.383 t dry matter/ha/3 years); (c) farmyard manure (15.818 t dry matter/ha/3 years); (d) mineral NPK fertilization (330 kg N, 90 kg P, 300 kg K/ha/3 years). At the Luvisol site, the control treatment showed a tendency to decrease organic carbon (C<sub>org</sub>) in topsoil. At organic fertilization treatments the content of C<sub>org</sub> increased: sewage sludge – +15.0% (Luvisol) and +21.8% (Chernozem), farmyard manure – +19.0% (Luvisol) and +15.9% (Chernozem). At the NPK fertilization, the increase was +4.8% (Luvisol) and +4.7% (Chernozem). The increased C<sub>org</sub> content was also associated with an increase of microbial biomass carbon (C<sub>mic</sub>) and extractable organic carbon (0.01 mol/L CaCl<sub>2</sub> and hot water extraction). The ratio of C<sub>mic</sub> in C<sub>org</sub> was within the range 0.93–1.37%.