Effects of Forest Swamp Succession and Soil Depth on Distribution Pattern of Soil Nematode Communities

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The forest swamp ecosystem, as a special wetland ecosystem, is a key link in the material cycle and an important carbon sink in the carbon cycle. The global carbon cycle is of great significance, but the impact of forest swamp succession and soil depth on soil active organic matter and nematode community structure and diversity is unclear. This study used the “space instead of time” method to investigate the succession process of forest swamps from grasslands (WC) and shrubs (WG) to forests (WS) in national nature reserves. The results showed that during the forest succession process, the dominant nematode communities in the WC and WG stages were dominated by the genera Apis and Labroidei, while the dominant genera increased in the WS stage. The total abundance of nematodes increased, and the number of groups was ordered WG > WC > WS. The diversity in soil nematode communities according to Shannon–Wiener (H′), Pielou (J), and Trophic diversity (TD) was WS > WG > WC, which is related to vegetation, soil physical and chemical properties, and microbial community structure. The maturity index (MI) was WG > WS > WC. The soil food web was dominated by bacterial channels and had characteristics in forest metabolic activity and regulation ability. At different soil depths, there were significant differences in the community, with species such as the spiny cushioned blade genus being key. The number and group size of nematodes varied from 0–10 cm > 10–20 cm > 20–30 cm. The relative abundance of feeding nematodes changed with depth, while diversity indices such as H′, J, and TD decreased with depth. Ecological function indices such as MI and PPI showed depth variation patterns, while basic indices (BI) and channel indices (CI) showed significant differences. In terms of soil variables, during the forest succession stage, soil organic carbon (SOC), soluble organic nitrogen (DON), easily oxidizable organic carbon (ROC), microbial biomass carbon (MBC), and microbial biomass nitrogen (MBN) showed a gradually increasing trend with WC-WG-WS, while total nitrogen (TN), soluble organic carbon (DOC), soil temperature (ST), and soil moisture (SM) showed opposite changes. There were significant differences in soil ST, SM, and DON values with succession (p < 0.05). At different soil depths, except for DON and ROC, which increased first and then decrease with depth, the values of other physical and chemical factors and active carbon and nitrogen components at depths of 0–10 cm were higher than those at other depths and decreased with depth. An analysis of variance showed significant differences in MBC and MBN values at different soil depths (p < 0.05), which is of great significance for a deeper understanding of the mechanism of soil nematode community construction and its relationship with the environment.