Abstract: Soil aggregates are crucial for evaluating soil quality and fertility, as they significantly contribute to the storage of soil organic carbon and support the functional stability of forest ecosystems. Nitrogen and phosphorus deposition can influence the carbon cycle within forest ecosystems, thereby altering nutrient inputs that affect soil microbial communities and enzymatic activities. In this study, soil samples were gathered from the 0–20 cm depth in both Broad-leaved Korean pine (Pinus koraiensis) forests and KPP, which were representative in northeast China. After wet screening, soil aggregates of 4 different fractions were obtained. Soil aggregates of > 2, 2–0.25, 0.25–0.053 and < 0.053 mm were included. Corresponding forest litter was added to each fraction, and three levels of nitrogen and phosphorus (NP) addition involving low (L), medium (M), and high (H) along with a control treatment (CK), were applied. A 360–d laboratory incubation experiment was conducted under controlled conditions. The following parameters were measured: soil total organic carbon (TOC), microbial carbon (MBC), dissolved organic carbon (DOC), readily oxidized carbon (ROC), particulate organic carbon (POC), mineral bound organic carbon (MOC), light group organic carbon (LOC), mean weight diameter (MWD), geometric mean diameter (GMD), to assess alterations in soil carbon storage and the stability of soil aggregates. High NP addition markedly decreased soil aggregate stability and inhibited soil mineralization. However, it enhanced soil organic carbon accumulation, particularly DOC and LOC, in most aggregate fractions, while potentially suppressing MBC in small aggregate fractions. Carbon stabilization mechanisms differed between forest types: natural forests relied more on large aggregates (> 0.25 mm), whereas plantation depended more on microaggregates (< 0.25 mm). Consequently, soils dominated by large aggregates are better suited for M and L NP additions, whereas soils rich in microaggregates should avoid H NP inputs. Overall, the results indicate that medium and low levels of NP enhance aggregate stability and promote mineralization rates, whereas H NP application compromises structural stability and suppresses mineralization. These results enhance our understanding of how nitrogen and phosphorus deposition influence the organic carbon cycle in temperate forest ecosystems.

Key words: Pinus koraiensis,  Plantation, Soil aggregates, Nitrogen and phosphorus deposition, Carbon pool