Effects of Farmland Scale on Soil Organic Matter Change in Black Soil Areas of China in the Past 40 Years

ABSTRACT Differences in farmland scale led to variations in agricultural practices and management, which in turn influence the direction and rate of changes in soil organic matter (SOM). This study collected 574 topsoil samples (0–20 cm) from the Youyi Farm in a typical black soil region of Northeast China. Cloud‐free Landsat images from 1984 to 2023 were obtained via Google Earth Engine and bare soil images were synthesized in 10‐year intervals. The study area was classified using the K‐means clustering algorithm to construct a two‐cluster probabilistic hybrid model, enhancing the accuracy of SOM predictions. Finally, SOM spatial distribution data were obtained for each 10‐year period to evaluate the impact of different farmland scales on SOM variation. The results showed that: (1) using a probabilistic hybrid model effectively improved the prediction performance of SOM, with R 2 reaching 0.71, RMSE at 0.76% and RPD at 1.96. (2) Over the past 40 years, SOM content at Youyi Farm has shown an overall downward trend, with the average SOM content decreasing from 3.57% ± 0.65% to 3.51% ± 0.58%. Negative changes in SOM were observed in 67.15% of the farmland. (3) SOM decreased most slowly when field sizes ranged from 180 to 210 ha in the study area, as both excessively large and excessively small farmland scales accelerated SOM decline. Future conservation of black soil and intensive agricultural land use should consider rational planning of farmland scale.