The impact on soil aggregates status of two tillage practices (moldboard plowing, TT, and chisel plowing, RT) applied to a farm previously cultivated under no-tillage (NT) was studied. The experiment was carried out on a Leptic Typic Xerorthent soil at the "Las Navas" dryland experimental farm located in Jerez de la Frontera (Cadiz, SW Spain). Several organic C pools such as total organic carbon (TOC), water soluble carbon (WSC), permanganate oxidizable carbon (POxC), and microbial biomass carbon (MBC) were analyzed, together with two enzymatic activities related to soil organic matter (SOM) oxidization metabolism: dehydrogenase activity (DH) and beta-glucosidase activity (beta-Glu). 13C cross polarization magic angle spinning nuclear magnetic resonance ( 13C CPMAS NMR) spectroscopy was used to characterize the composition of the SOM and its degradation status. Two years after the implementation of the new management practices, analysis of the aggregate distribution of the topsoil (0-10 cm) showed that, even after a short term, TT enhanced aggregate disruption. The apparent reduction in soil quality of TT plots was evident from the lower contents of total organic carbon (TOC), permanganate oxidizable carbon (POxC) and microbial biomass carbon (MBC) if compared with RT and NT. Moreover, TT soil showed also a lower beta-glucosidase activity. As confirmed by 13C CPMAS NMR, the SOM of the TT fractions revealed higher alkyl C to O-alkyl ratios than their RT and NT counterparts. Also signals commonly referred to lignin structures were absent in the spectra of the TT fractions, but were still present in those of the larger fractions of the NT treatment. After a short term evaluation, RT samples did not show the same declining trend as observed for the TT treatment. For this reason, chisel plowing seems to offer a viable occasional management option when required during long-term NT.
