No-till: A key tool for sequestering C and N in microaggregates on a Mediterranean Vertisol

•Microaggregate-associated C and N stocks were higher in no tillage compared with conventional tillage.•Microaggregates comprised 13% of the total aggregates.•Macroaggregates on average accounted for 48% of the total aggregates.•Crop rotation and N fertilization did not contribute to the pool of C and N sequestration.

Soil aggregate stability is crucial for agroecosystem sustainability and for mitigating the effects of greenhouse gases. Within soil aggregates, increases in the microaggregate-associated C and N contents are of paramount importance. The aim of this study was to determine the effect of agricultural practices on macro- and microaggregate-associated C and N through a 27-year continuous field experiment in a dryland Mediterranean Vertisol. The treatments included a tillage system (no-till and conventional), 2-year crop rotations [wheat (Triticum aestivum L.)-faba bean (Vicia faba L.) and wheat-sunflower (Helianthus annuus L.)], and the application of N fertilizer (0 and 100 kg ha−1) to wheat. Soil samples were collected from depths of 0 to 15, 15 to 30, 30 to 60, and 60 to 90 cm. Higher proportions of microaggregates and macroaggregates were observed under no-till than under conventional tillage. Crop rotations did not affect the microaggregate concentration, and the microaggregate-associated C and N stocks (0-90 cm) were higher in the no-till treatment compared with conventional tillage (6.9 vs. 4.4 Mg ha−1 and 957 vs. 672 kg ha−1, respectively). The microaggregate-associated C stocks obtained from the no-till treatment were significantly higher at depths of 0-15, 15-30, and 60-90 cm. At certain depths, the no-till treatment resulted in greater microaggregate N stocks than conventional tillage, regardless of crop rotation. Conventional tillage yielded significantly lower macroaggregate-associated C and N stocks (0-90 cm) than no-till. This finding was also observed at certain depths in both crop rotations. The wheat-faba bean treatments yielded greater macroaggregate C stocks than the wheat-sunflower treatments under both tillage systems at certain depths. We determined that no-till is a key practice for sequestering C and N in microaggregates and that crop rotation and N fertilization do not contribute to the C and N sequestration. As a result, no-till appears to be the only truly effective practice for the long-term encapsulation of C and N in microaggregates.