Temporal dynamics and vertical distribution of newly-derived carbon from a C₃/C₄ conversion in an Ultisol after 30-yr fertilization

Long-term fertilization has a considerable effect on the dynamics of soil organic carbon (SOC). However, quantifying the contribution of fertilization practices to SOC is still a challenge. In this study, we selected a 30-yr fertilization experiment planted with a shift from C₃ to C₄ crops. Our objectives were (i) to determine temporal dynamics of SOC and newly-derived C (f_new) with the cultivation time and their vertical distribution along with soil profile; (ii) to assess the effect of the fertilization on SOC and f_new. Three treatments were involved: no fertilization (Control), NPK application (NPK), and continuously planted with C₃ crops adjacent to the experiment as a reference (NAT). The SOC was physically separated into cPOM, fPOM, iPOM, s + c_m and s + c_f fractions, and their δ¹³C values were determined. Our results show the f_new in the bulk soil and C fractions presented an exponential increase over the cultivation time in the NPK treatment but this was only observed for the bulk soil and the s + c_f fraction in the Control treatment. The f_new had a priority to be stored within microaggregates, and then enriched in the silt and clay sized fraction over time. The SOC and f_new in the bulk soil and C fractions decreased with soil depth so that the difference between the Control and NPK treatments was observed only in the 0–20 cm depth. Our results demonstrate that the long-term fertilization increases the new C proportion in the bulk soil and C fractions but this contribution was limited to the plough layer.