Decomposition and driving factor of organic materials in the black soil belt of Northeast China

Organic materials can often be high-efficiently incorporated into soils to enhance the fertility of black soil in Northeast China. The organic material and the temperature are the key influencing factors on the decomposition of organic materials. However, the low spring temperatures and slow decomposition rate of organic materials have limited to the large-scale production in modern agriculture. Only a few studies have been focused on the dynamic decomposition of organic materials in the black soil region. The influencing factors have been rarely defined to optimize the agricultural practices of organic materials incorporation into field for the soil fertility in this cold region. Therefore, this study aims to investigate the decomposition in the different kinds of organic materials, and the driving factors. A field experiment was performed on the decomposition network of organic materials. Six kinds of organic materials were taken from the five varied sites along the Northeast black soil belt. Then the structural equation modeling was constructed to quantify the contribution of the potential influencing factors. Finally, the main driving factor was identified. The study found that: (1) There were the significant differences in the organic materials with the highest carbon to nitrogen ratio (C/N) in three crop straws, whereas, the lowest C/N in the organic fertilizers. While the decomposition index showed the opposite trend as to C/N. (2) The cumulative rates of decomposition and carbon release in the six organic materials showed the outstanding seasonal and regional patterns after 720 days of decomposition. Namely, these rates were much higher in summer and autumn than that in spring and winter, indicating the higher cumulative temperature. Meanwhile, there was the increase with the high accumulated temperature at the test site. The cumulative rates of decomposition and carbon release increased by 15.8%-41.0% and 47.8%-114.0%, respectively, from Heihe City with the lowest cumulative temperature to Changtu County with the highest cumulative temperature. The cumulative rate of decomposition of different organic materials was ranked in the descending order of the crop straw > fungal bran > livestock manure; The cumulative rate of carbon release was in the descending order of the livestock manure> fungal bran> crop straw. (3) The decomposition of organic materials was accompanied with carbon release and nitrogen enrichment. All treatments seemed to achieve the optimal decomposition. Nevertheless, the C/N of maize straw and rice straw in Hailun and Heihe was remained above 25 after 480 days, due to the lower cumulative temperatures. The degradation equation of decomposition residual rate was used to fit the decomposition. And the rate constant of decomposition of crop straw (0.23-0.25) was significantly higher than that of fungal bran (0.15) and livestock manure (0.08~0.13) (P<0.05). The structural equation model was established to clarify the relationship among soil properties, climatic factors, decomposition time and properties of organic materials. The decomposition features showed that the total degree of each factor were 80% and 75%, respectively, for the cumulative rates of decomposition and carbon release. And the properties of organic materials were contributed more than the rest factors. The degrees of explanation to cumulative rate of decomposition and carbon release were 44% and 35%, respectively, indicating the most important driving factor on the decomposition of organic materials. The finding can provide a theoretical basis and technical guidance for the efficient utilization of organic material resources, in order to optimize the agricultural management practices in the black soil region of Northeast China.