Decreased soil aggregation and reduced soil organic carbon activity in conventional vegetable fields converted from paddy fields

Soil structure and organic carbon are key factors that reflect soil quality in sustainable agricultural production. Many studies have concentrated on soil aeration under intensive anthropogenic agricultural practices, but the relationship between soil aggregate stability and soil organic carbon (SOC) functional groups is poorly understood, particularly in degraded greenhouse vegetable fields converted from paddy fields. To solve this problem, soil samples were collected from local vegetable fields and adjacent paddy fields. Conventional open-air vegetable fields and greenhouse vegetable fields, both with a 6-8-year cultivation history following conversion from paddy fields, were considered. SOC functional groups were characterised by C-13 cross-polarisation magic-angle-spinning nuclear magnetic resonance (C-13 CPMAS NMR). The results showed that the mean weight diameter (MWD) values of water-stable soil aggregates in the open-air vegetable fields and greenhouse vegetable fields decreased by 64.6% and 66.7%, respectively, compared with those in the paddy fields. Similar results were obtained for the soil aggregate distribution, and soil aggregates in the greenhouse vegetable fields and open-air vegetable fields were mainly in the 0.25-2 mm size classes. Among all SOC functional groups, O-alkyl C exhibited the greatest decrease after the conversion of paddy fields to vegetable fields, no matter whether greenhouse vegetable fields or open-air vegetable fields. The degradation of soil structure decreased the physical protection of SOC functional groups by soil aggregates, and therefore, decreased the total SOC content and labile C functional group fractions in bulk soil. At the soil aggregate scale, the decrease of O-alkyl C mainly occurred in microaggregates (0.053-0.25 mm), while aromatic C decreased significantly in macroaggregates (>0.25 mm) in the greenhouse vegetable fields converted from paddy fields. Correlation analysis showed that O-alkyl C was significantly positively related to MWD and the proportion of large soil macroaggregate (>2 mm). This result suggested that soil aggregates become fragile in long-term vegetable cultivation after conversion from paddy fields, and a large loss of O-alkyl C might be the main reason for this phenomenon.