Spatial and temporal evolution of soil organic matter and its response to dynamic factors in the Southern part of Black Soil Region of Northeast China

Abstract

Soil organic matter content (SOMC) is decreasing in the Black Soil Region of Northeast China (BSRNC) due to the combined impacts of prolonged agricultural reclamation, climate change, and soil erosion. As an essential soil quality indicator, it is urgent to analyze the dynamic characteristics of Soil organic matter (SOM). This study aims to evaluate the spatial and temporal dynamics of SOMC and identify the factors driving these changes. Two sets of soil data collected in the 1980s and 2020 were compared in the southern part of BSRNC. Five machine learning models were used to estimate the spatial distribution of surface SOMC during these two periods, and the accuracy of the five models was evaluated. Simultaneously, the factors leading to SOM's spatial variability and temporal change were assessed. The results showed that Extreme Gradient Boosting (XGBoost) had the best performance with R² of 0.65 and 0.78 for the 1980s and 2020, respectively. Spatially, SOMC was lower and decreased more in the western saline agglomeration than in other parts of the study area. Soil properties (bulk density, silt, pH) and climate (temperature, precipitation) were key factors that affected the spatial variability in SOM. Temporally, SOMC decreased from 22.8 ± 4.5 g·kg⁻¹ in the 1980s to 20.3 ± 4.4 g·kg⁻¹ in 2020, and the average content reduced by 2.5 g·kg⁻¹ overall. This study revealed that the loss of SOMC increases with soil erosion. Land use also affects change in SOM. The most severe decrease in SOM occurred when forests were reclaimed as drylands (-6.2 g·kg⁻¹). In the past 40 years, increasing temperatures have been accompanied by a decrease in SOM, while increasing precipitation has had little positive effect on SOM. The coupled effect of land use change and soil erosion had the highest contribution rate to SOMC changes, at 8.66 %, followed by the independent effect of soil erosion at 6.40 %. To summarize, this study clarified spatial variability and temporal change in SOM and elucidated the mechanism of dynamic factors affecting SOM, which can guide the design of sustainable agricultural policies.