Soil organic matter spatial distribution change over the past 20 years and its causes in Northeast

Soil organic matter (SOM) content is one of main factors influencing the CO2 exchange between cropland and atmosphere. Therefore, SOM spatial distribution change on cultivated land can have a profound effect on food production and climate change. In this paper, remote sensing and GIS technology were used to obtain the spatial distribution of cultivated land of Northeast China in 2003 through ETM+ interpretation. Based on 750 soil samples collected in 2005, the SOM spatial distribution in 2005 was determined through Kriging interpolation. The SOM spatial distribution change of cultivated land over the past 20 years was analyzed by comparing it with the second soil census in 1985. The results show that the total SOM of cultivated land has declined over the past 20 years in northeast China. The percentage of crop areas with SOM of 3-4%, 4-8% and >8% dropped from 24.09%, 34.07% and 3.23% in 1985 to 19.79%, 29.60% and 0% in 2005. From the spatial distribution, areas with SOM increase cover 14.2% of the total cultivated land, 48.0% remain SOM stable, and areas with SOM decrease account for 37.8%. Crop areas with SOM decline were mainly located in the middle and south of Jilin Province and Liaoning Province. Crop areas with SOM unchanging and increasing were situated mostly in the north of Heilongjiang Province. The range of SOM with the largest area decrease is different for different soil types. Areas with 3-4% SOM for meadow soil and chernozem have the largest reduction. The main causes of SOM decrease include mainly human activities and soil erosion.