Response of farmland soil aggregate-associated organic carbon to straw return: a meta-analysis

Straw return is a practical method for effectively improving soil organic matter (SOM) and is favored owing to its simplicity and cost-effectiveness. In this study, we screened 35 relevant experimental field studies to investigate the effects of straw return on farmland soil aggregate organic carbon content. We used meta-analysis and random-forest models to evaluate the response effects of farmland soil aggregate carbon content to straw return, as well as the contribution of influential factors of farmland soil aggregate carbon content, such as geographical region, return mode, return amount, and years of straw return. This could provide a theoretical basis for enhancing soil quality and carbon sequestration, as well as for developing a reasonable aggregate carbon pool management strategy. Compared to that of the control treatment (no straw return), the carbon content of soil aggregates in China increased considerably when straw was returned to the farmlands. The positive effects of straw return on farmland soil aggregate carbon content varied remarkably across different regions in China. The highest positive effect of straw return on farmland soil aggregate carbon content was observed in Southwest China, ranging from 26.5% to 43.4%, whereas north China had the lowest positive effect of straw return on farmland soil aggregate carbon content, ranging from 2.0% to 10.8%. The beneficial impacts of straw return on farmland soil aggregate carbon content were moderate in Northeast, Northwest, East, and Central China. In addition, among the four straw return methods, including high stubble retention, mulching, rotary tillage and deep ploughing, rotary tillage had the largest influence on the rise of farmland soil large aggregate carbon content and small aggregate carbon content, ranging from 13.5% to 18.7%. High stubble retention had the highest improvement effect on farmland soil microaggregate carbon content, with a value of 19.5%. With respect to the years of straw return, the positive effects of straw return on farmland soil aggregate carbon content improved along with increasing amounts and the years of straw return. Furthermore, the beneficial effect of straw return on farmland soil aggregate carbon was most significant eight years after straw return. Additionally, the results of random-forest model showed that geographical region and straw return mode are important factors affecting farmland soil aggregate carbon with various particle sizes under the conditions of straw return, and their contribution rate was in the range of 16.2%–28.1% and 8.0%–22.4%, respectively. We also discovered that the crucial physical process by which straw return improves the organic carbon content of farmland soil on a broad scale is an increase in the physical carbon sequestration of soil aggregates. As aforementioned, it is suggested that the straw should be returned to the farmland soil within a 15 cm depth using the rotary tillage method after crushing, and simultaneously an appropriate amount of chemical fertilizer (e.g., nitrogen and phosphorus fertilizer) should be applied to maximize the potential of soil aggregates for carbon sequestration.