土壤有机质含量是影响土壤肥力水平的重要指标，也是开展耕地质量建设、科学施肥工作的基础。利用2005—2014年测土配方施肥项目数据与全国第二次土壤普查数据进行对比，分析了近30年来中国农田耕层土壤有机质的变化趋势。结果显示，30年来我国农田耕层土壤有机质含量呈整体上升趋势，目前，全国耕层土壤有机质平均含量为24.65 g kg^-1，较全国第二次土壤普查时期提高4.85 g kg^-1，提高24.49%。其中，30 g kg^-1~40 g kg^-1等级比例增加3.64个百分点，20 g kg^-1~30 g kg^-1等级比例增加5.68个百分点，10 g kg^-1~20 g kg^-1等级比例增加5.36个百分点。需要引起注意的是，大于40 g kg^-1等级比例减少了1.38个百分点。

【Objective】Soil organic matter (SOM) is an important indicator of soil fertility, and also the basis for carrying on build-up of cropland quality and scientific fertilization. Based on the data of the project of soil test based fertilization and the Second National Soil Survey in 2005-2014, analysis was performed for variation of soil organic matter in top soils of the croplands of China. The objectives of the present study are (1) to report current status of SOM in the croplands of China; (2) to explore changes in SOM content in the croplands of China over the last three decades in nationwide; and (3) to discuss causes of the changes. 【Method】 In this paper, the data of SOM in croplands were cited from the Soil Survey Data of China and Soil Fertility of China, covering a total of 131 627 000 hm² of croplands and the data of the national soil test based formulated fertilization project from the “Basic Nutrient Dataset of the Soils under the Project of Soil Test-Based Formulated Fertilization” covering a total of 8 467 083 data points. As a total of 2 948 agricultural counties (district or city) and farms participated in the project, covering a total of 121 716 000 hm²of croplands calculated according to the data of cropland area by the end of 2008 published by the National Statistics Bureau of China. The distribution of soil sampling sites of the project of soil test-based formulated fertilization was consistent with the data of cropland distribution at the end of 2008, covering 66 700 m²~133 400 m²(on average) in plain areas, 20 010 m²~53 360 m² in hilly areas, and 6 670 m²~13 340 m² of vegetable land All soil samples were collected from topsoil (mainly 0~20 cm for crop field and 0~30 cm for vegetable field) in autumn after crops were harvested. Contents of soil organic matter were measured with the oil heating and potassium dichromate-volumetric method. A total of 8 467 083 soil samples were collected nationwide. The data of the soil-test-based formulated fertilization were mathematical means of the soil samples for each province. The massive data bases acquired from the project of soil test-based formulated fertilization and the second national soil survey were analyzed for variation of soil organic matter content in cropland of the country over the last three decades. Based on the data of soil organic matter contents in the croplands of the Second National Soil Survey, the croplands could be sorted into six grades, i.e.>40 g kg^-1, 30 g kg^-1~40 g kg^-1, 20 g kg^-1~30 g kg^-1,10 g kg^-1~20 g kg^-1, 6 g kg^-1~10 g kg^-1 and ≤6 g kg^-1【Result】Results show that the average content of soil organic matter in plough layer of the country is found to be 24.65 g kg^-1, with the highest in Heilongjiang province being 40.43 g kg^-1and the lowest in the Ningxia Hui Autonomous Region being 13.61 g kg^-1, and 4.85g kg^-1 or 24.49% higher than the average of the second national soil survey. The content of soil organic matter in the croplands of the grade of 30 g kg^-1~40 g kg^-1 increased by 3.64 percentage points, in those of the grade of 20 g kg^-1~30 g kg^-1 by 5.68 percentage points, and in those of the grade of 10 g kg^-1~20 g kg^-1 by 5.36 percentage points, but in those of the grade of > 40 g kg^-1 decreased by 1.38 percentage points. In most provinces or regions, soil organic matter increased or remained almost unchanged in content over the past three decades, but in Beijing and Qinghai, it was found to have been decreased slightly. The increases in content of soil organic matter in the plough layers of the country could be attributed to the farming practices of straw incorporation, zero or minimum tillage, and application of organic and green manures. Also, leaving crop stubs in the field is an important practice increasing soil organic matter content.【Conclusion】The average content of soil organic matter in the plough layers of different croplands in China is found to be 24.65 g kg^-1. Most of the croplands have been varying in the range from 10 to 30 g kg^-1, and exhibiting a rising trend in soil organic matter content over the past three decades, although in some regions croplands originally high in soil organic matter content have decreased somewhat. So the content of soil organic matter in the plough layers will keep on increasing with increasing organic matter input into the croplands in future.