Modelling organic matter mineralization and exploring options for organic matter management in arable farming in Northern China

The primary objectives of this thesis were to (i) to identify key factors in soil organic matter (SOM) dynamics in arable land of northern China, (ii) to predict long-term SOM dynamics under various scenarios, and (iii) to give suggestions for the most efficient use of the available organic resources. Modelling was chosen as a tool. Experimental data were collected from relevant sources, and used to test some C mineralization models, to develop and test a new model, to study various factors affecting mineralization, to compare efficiencies of various organic materials in SOM accumulation and to predict long-term SOM dynamics in northern China.

It was found that the average relative mineralization rate has a linear relationship with time in double logarithmic scales under constant environmental conditions. Based on this, several functions were derived for the description of the dynamics of SOM built from added substrate. This new model proved valid under diverse conditions for all types of substrates encountered in practice.

The model showed that differences in mineralization rates between different substrates fade away over time irrespective whether they are caused by different substrate properties or by different environmental conditions. Substrates mineralizing more quickly than others in the beginning may become more stable in the long run, and a positive effect of a raised temperature on C mineralization may disappear over time, or even become negative.

The model predicts that under the conditions of northern China, the efficiency of organic substrates in SOM accumulation decreases in the order: roots>straw>farmyard manure>green manure. Roots were identified as the key input of SOM in this region. SOM accumulated from different materials showed differences in quality. Roots and stubble can maintain SOM content at 10 g kg ^-1at the current level of grain yield. SOM content will rise with return of e.g. straw or farmyard manure (FYM). With increase in SOM content, the quality of SOM will improve, and the release of nutrients from SOM will rise. Increase in crop yield by e.g. use of chemical fertilizers, will bring about improvement of both content and quality of SOM in this region. For a sustainable soil fertility and grain production at high levels, the application of either straw or FYM to the soil should be increased. Long-term efforts are needed.