Effects of Water–Fertilizer Management on Soil Aggregate Stability and Organic Carbon Sequestration in Greenhouse Eggplant Fields of the Black Soil Region

Excess fertiliser and sub-optimal irrigation threaten soil health in greenhouse vegetable systems on black soils. This study explored how water–fertilizer regimes shape soil aggregate structure, stability, and soil organic carbon (SOC) sequestration in a meadow black soil eggplant system in Heilongjiang, China. Using a randomized block design with drip irrigation, three treatments were tested: conventional water and fertilizer (WF), conventional water with 20% fertilizer reduction (W80%F), and 20% water reduction with conventional fertilizer (80%WF). Results showed that 80%WF significantly increased macro-aggregate proportion, improved stability (mean weight diameter, MWD; geometric mean diameter, GMD), enhanced total organic carbon (TOC) content, and strengthened carbon sequestration, whereas W80%F weakened aggregate stability and reduced SOC in deeper layers. Water availability was the dominant factor for aggregate formation and SOC in surface and middle layers, while nutrients were more influential at depth. These findings demonstrate that moderate water reduction is more effective than fertilizer reduction in improving soil structure and carbon sink capacity, providing a scientific basis for precision water–fertilizer management and sustainable greenhouse agriculture in black soil regions.