Agricultural management practices play a major role in the process of SOC sequestration. However, the large background of stable carbon (C) already present in the soil and the long period of time usually required to observe changes in soil organic carbon (SOC) stocks have increased the necessity to identify soil C fractions with a fast response to changes in agricultural management practices. Consequently, we quantified the response of total SOC, permanganate oxidizable organic carbon (POxC), particulate organic carbon (POC) and the carbon concentration of water-stable macroaggregates, microaggregates within macroaggregates and the silt-plus clay-sized fraction (M-C, mM-C, s+cM-C, respectively) to changes in management. We chose a long-term tillage and N fertilization field experiment (18 years) located in NE Spain. In the first 5. cm depth under no-tillage (NT) compared with conventional tillage (CT), the POxC fraction and total SOC increased similarly (about 59%). However, other C pools studied (i.e., M-C, M-POxC, mM-C, POC and s+cM-C) had lower increases with values ranging from 17% to 31%. For the 5-20 and 20-40. cm soil depths, the POC was the most sensitive fraction to tillage with 46% and 54% decrease when NT was compared to CT, respectively. Likewise, the POC fraction presented the highest response to N fertilization in the three depths studied (i.e., 0-5, 5-20 and 20-40. cm). The mM-C and s+cM-C fractions presented the lowest sensitivity to changes in tillage and N fertilization management. Our results showed that the POC fraction had the greatest sensitivity to changes in agricultural management practices, proving its ability as an early indicator of optimized practices to sequester C in soil.
