The objectives of this study are to predict the long-term soil organic matter (SOM) dynamics in arable land in northern China, and to suggest measures for the maintenance and improvement of SOM content. With a newly developed model calibrated to relevant data collected from this, region, the course of SOM content and SOM quality was calculated over a time span of 50 years. Two levels of annual grain yields were considered: 'mean yields' of 7500 kg/ha and 'high yields' of 15 000 kg/ha, and three scenarios for organic inputs into the soil: (1) roots and stubble; (2), as (1) plus one-third of straw produced in situ; (3) as (2) plus farmyard manure (FYM) made with another one-third of the straw produced in situ. It was predicted that with mean yields SOM contents can be maintained at 10, 13 and 15 g/kg under the three scenarios, and with high yields at 15, 19 and 21 g/kg. The present SOM contents are often below 10 g/kg. Depending on whether present SOM contents are below or above the steady-state levels, they will increase or decrease respectively. The fraction of annual mineralization of SOM will increase with the magnitudes of annual organic inputs, indicating improvement in SOM quality. Hence, in most arable fields in northern China, both quantity and quality of SOM will rise upon increases in annual crop production, or when a large portion of straw is returned to the soil, directly or indirectly via FYM.