Low efficacy of different crop residue management on C and N stocks after five decades

Increasing the soil organic carbon (SOC) content is gaining growing attention nowadays due to its double function of restoring soil fertility and mitigating climate change. This study aims to investigate the effect of different residue management including residue removal, residue incorporation, and residue incorporation + 1 t ha(-1) poultry manure, on SOC, soil inorganic carbon (SIC), total soil carbon (TSC), total Kjeldahl nitrogen (TN) and C:N ratio on a long-term experiment located in North-East Italy. After 55 years of residue retention, SOC content increased by similar to 12 % in the tilled topsoil and 6 % in the subsoil. Among the 0-60 cm soil profile, this corresponded to a SOC storage of similar to 6 t ha(-1) that was achieved in response to similar to 127 t ha(-1) residue-derived C input. Therefore, assuming that C sequestration was linear during the experimentation, an average annual conversion rate from residue-derived C to SOC can be estimated as 4.7 % which is comparable to what is usually reported in the literature. The addition of poultry manure only marginally affected the SOC stock while increasing the 0-30 cm TN stock of 0.5 t TN ha(-1) , demonstrating how it acts more as a mineral source of N rather than affecting the soil organic matter (OM) dynamics. Any significant effect of treatments was instead found on SIC, TSC, and C:N. Crop residue incorporation increased the SOC stock, but its low conversion efficiency might suggest a different use (e.g., bioenergy production). Despite not improving the OM dynamics, poultry manure can be used as an alternative to mineral fertilizers, reducing fossil fuel consumption and giving new insight into the circular economy.