Soil organic matter content and its influencing factors in the northeastern Sanjiang Plain

Soil organic matter (SOM) plays a crucial role in the global carbon cycle. Through statistical analysis and correlation analysis, this paper investigated the SOM content and its influencing factors for 105 groups of surface soil samples from the Hongwei-Daxing farms in the northeastern Sanjiang Plain. The results show that the SOM exhibited a content in the study area ranging from 1.72% to 13.80% (average: 4.78%) and significant enrichment in surface soil, with an enrichment coefficient of up to 10.29. The SOM content generally followed a normal pattern in its frequency distribution and an obvious regional pattern in its spatial distribution, with significantly higher SOM observed in the northeast than in the northwest of the study area. In typical soil profiles, both SOM and total nitrogen (TN) content showed a declining trend with the increase of soil depth, suggesting SOM aggregation in the surface soil layer. Meanwhile, the SOM content was positively correlated with some soil physiochemical indicators, such as moisture content, clay particle content, water-stable aggregates, iron oxide (Fe₂O₃), and TN, which represent favorable factors for SOM accumulation in the study area. In contrast, the SOM content exhibited negative correlations with the bulk density, pH value, and potassium oxide (K₂O). Moreover, the variations in soil carbon pools were quantitatively evaluated after the conversion of wetlands to croplands. The results indicate that the average SOM content followed the order of wetlands (8.87%) > dry lands (5.90%) > paddy fields (4.21%). Furthermore, the conversion resulted in a significant increase in soil pH and losses of SOM, TN, and total phosphorus (TP) to varying degrees. Among various soil types, boggy soil exhibited the highest average SOM content of 6.77%, representing a favorable factor for SOM accumulation in the study area. Among various parent materials, the Quaternary high-magnitude floodplain sediments presented the maximum average SOM content in the generated soil. This is attributed to their predominant composition: lacustrine mucky clays and loams, whose high content of clay particles favors SOM accumulation. These findings provide a scientific basis for enhancing the management and sustainable utilization of land resources, wetland protection, and the carbon sequestration function of the area.