Two pathways for reducing soil aggregate organic carbon mineralisation via minimum tillage under a long-term field experiment

Soil aggregates are the basic units of soil organic carbon (SOC) mineralisation. However, it is unclear through which pathways soil aggregates and their hydrolytic activities affect SOC mineralisation under minimum tillage. For this purpose, we conducted incubation experiments. Soil samples of conventional (CT_S) and minimal tillage [subsoiling (SS_S) and no tillage (NT_S)] with straw return to the field were collected from a long-term tillage experimental field. Soil aggregates with column sizes of 2-5 mm (large macro-aggregates; LMA), 0.25-2 mm (small macro-aggregates; SMA) and 0.053-0.25 mm (micro-aggregates; MA) were prepared, with bulk soil serving as a control. Results indicated that minimum tillage increased macro-aggregate (>0.25 mm) content (12.9 %-17.7 %) and soil carbon fraction content (2.7 %-46.9 %) but decreased soil hydrolase activity (5.0 %-45.8 %) and CO₂ cumulative emissions (8.1 %-28.0 %) compared with CT_S. Minimum tillage reduced the SOC mineralisation contribution of SMA compared with CT_S (66.6 %-70.3 %). Partial least squares path model analysis showed that there are two pathways by which the proportions of soil aggregate-size classes (ASC) and hydrolase activity affect SOC mineralisation in soil aggregates. First, minimum tillage increased the ASC of LMA, which negatively regulated the hydrolase activity of macro-aggregates, and then indirectly reduced SOC mineralisation by negatively regulating the content of soil carbon fractions. Second, minimum tillage decreased the ASC of MA, which positively regulated soil hydrolase activity and reduced SOC mineralisation. This study provides new insights into reducing GHG emissions from agricultural production from a soil aggregation perspective.