Soil aggregate and organic nitrogen distributions as influenced by vermicompost application in vegetable greenhouse

Vermicompost application can improve soil physical structure and increase soil nitrogen (N) sequestration, yet its specific impact on soil aggregates in relation to changes in organic N fractions remains underexplored, especially in protected vegetable fields. We compared the effects of vermicompost substitution (commercial organic fertilizer, COF; reduced COF + vermicompost, RCOF + VC; vermicompost, VC) on soil dry aggregate size distribution, aggregate stability, particulate organic N (PON) and mineral-associated organic N (MON) distributions within aggregates, as well as their interrelationships in a protected continuous tomato cropping system. Compared with COF, RCOF + VC was not beneficial for soil macro-aggregation and aggregate stability in 20-40 cm, leading to diminished physical protection and loss of organic N fractions. In comparison, VC had no significant influence on soil aggregate structure, while was effective in N retention by preventing organic N degradation, especially in 0-20 cm. In all treatments, most PON and MON (averaging 86.05%) were distributed in macro-aggregates, which played more important role in regulating the quality of soil agglomeration structure than micro-aggregates. Although differing in quantity, PON and MON within macro-aggregates functioned equally in macro-aggregation, whereas for aggregate stability, MON played a more pivotal role. Vermicompost (30000 kg hm^- 2) can completely replace commercial organic fertilizer in terms of maintaining aggregate structure and occluded organic N fractions.