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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. Abbreviations: CT: conventional tillage; M-C: dichromate oxidizable organic carbon of the macroaggregates; mM-C: dichromate oxidizable organic carbon of the microaggregates within macroaggregates; M-POxC: permanganate oxidizable organic carbon of the macroaggregates; NT: no-tillage; POC: particulate organic carbon; POxC: permanganate oxidizable organic carbon; s+cM-C: dichromate oxidizable organic carbon of the silt-plus clay-sized soil particles of the macroaggregates.

There is a strong need to identify the combination of tillage and N fertilization practices that reduce the amount of nitrous oxide (N 2 O) emissions while maintaining crop productivity in dryland Mediterranean areas. We measured the fluxes of N 2 O in two field experiments with 3 and 15 years since their establishment. In the long-term experiment, two types of tillage (NT, notillage, and CT, conventional intensive tillage) and three mineral N fertilization rates (0, 60 and 120 kg N ha Our results showed that in rainfed Mediterranean agroecosystems, the use of NT and pig slurry are effective means of yield-scaled N 2 O emissions reduction.3

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