Meta analysis of the effects of global organic material returning on soil organic carbon sequestration in Mollisols

Mollisols (from Latin mollis, "soft") are the types of highly fertile soil with a thick layer of humus, high organic matter content, and strong nutrient capacity. The natural granary has played an indispensable role in the crucial agricultural and livestock production activities in many countries. A perfect tradeoff can be required for high crop yields with less nutrient loss in the soil under intensified farming practices. Among them, the organic matter greatly contributes to the high fertility, particularly at the medium to high latitudes in the cold and cool regions. Excessive depletion of organic matter can directly impact grain production in the sustainable development. An important approach to restoring organic carbon can be returning the organic materials to the soil via organic matter incorporation. Two common approaches are employed in conventional cultivation: straw incorporation and organic fertilizer application. However, there is an ongoing debate regarding which one is more beneficial for sequestering organic carbon. It is of significant importance to investigate the conditions under which straw incorporation or organic fertilizer application is more advantageous for carbon sequestration. Additionally, the organic matter incorporation is also dependent on various factors, such as fertilization duration and carbon input. The organic materials vary significantly in the decomposition with the increasing fertilization duration, leading to different effects on the soil organic carbon retention. Carbon input is one of the most important limiting factors in soil organic carbon sequestration and the carbon-to-nitrogen ratio. Previous studies have focused mostly on controlled experiments to investigate the effects of different organic matter incorporation on soil organic carbon sequestration. Less attention was paid to the different fertilization durations and carbon inputs under organic matter incorporation. In this study, a meta-analysis approach was utilized to examine the impact of straw incorporation and organic fertilizer application on soil carbon sequestration under various fertilization durations and carbon inputs in Mollisols. A systematic search of peer-reviewed articles was conducted to apply the following criteria for reliability: 1) The experiments included both laboratory and field trials, consisting of control groups without fertilizer application, and experimental groups with either straw incorporation or organic fertilizer application; 2) The trials were performed on Mollisols to record the latitude, longitude, and climatic conditions of the experimental sites; 3) Relevant indicators of soil property with their means, standard deviations, and the number of replicates was directly extracted from the articles' text, tables, or figures. A total of 41 articles with 2012 observations were obtained after the screening process. The results indicated that the organic manure return was more effective in increasing the soil organic carbon content, compared with the straw return. There was a significant increase of 42.99% in soil organic carbon, 39.97% in total soil nitrogen, and 74.01% in total soil phosphorus in Mollisols. By contrast, the straw return resulted in a significant increase of 14.96% in soil organic carbon, 15.03% in total soil nitrogen, and 20.03% in total soil phosphorus. Moreover, the organic manure return was more effective than the straw return with the increase of treatment years, in terms of the soil organic carbon content. There were different effects of organic manure return to the field on the soil carbon sequestration under different carbon input conditions. The organic carbon sequestration of organic manure returning to the field can be significantly higher than that of straw under medium carbon input conditions. Some suggestions were proposed that the organic manure return under long-term medium carbon input was more beneficial to the increase of organic carbon sequestration. Overall, the findings can provide valuable insights for the scientific selection of organic materials in the field return in Mollisols.