Response of soil aggregation and associated organic carbon to organic amendment and its controls: A global meta-analysis

Organic amendments are considered to be an effective way to improve soil structure and increase soil organic carbon storage. Determining the effects of organic amendments on soil aggregate carbon pools and their potential drivers on a global scale are crucial for predicting changes in soil carbon pools. We performed a meta -analysis of 4,293 pairwise data from 249 studies in global farmland ecosystems to determine the extent to which climate factors, soil properties, and agronomic practices modulate the response of aggregation and associated organic carbon (OC) to organic amendment. The results showed that organic amendment increased the pro-portion of > 2 mm and 0.25-2 mm aggregates by 32 % and 12 %, aggregate stability (MWD and GMD) by 17 % and 22 %, and OC content in all aggregates by 16-23 % compared with that of the mineral only fertilization treatment. Specifically, organic amendments had more positive effects on > 2 mm aggregate and associated OC in areas with an average annual rainfall of < 600 mm, an average annual temperature of 8-15 C, as well as surface (0-20 cm) soil with loamy texture and initial soil OC content (<10 g kg-1), total nitrogen content (<0.5 g kg(- 1)) and pH (<6). Variance partitioning analysis showed that changes in soil aggregation and associated OC caused by organic amendment application were mainly explained by agronomic practices (9-35 %) and soil properties (10-21 %). The results of random forest model showed that the amendment C/N and initial soil properties (initial soil OC and total nitrogen content) were the predominant controls on the effects of organic amendment. Overall, our results suggest that organic amendment application is an effective strategy for enhancing soil aggregation and associated OC, but changes in response are governed by the characteristics of the amendments themselves and the inherent properties of the initial soil conditions.