2. Materials and methods
2.1. Data collection
Data were collected from peer-reviewed papers published in both the Chinese and English literature before 2020. Papers were identified using the Web of Science (Clarivate Analytics) and the China Knowledge Resource Integrated Database (www.cnki.net). Search terms included “conservation tillage”, “no-till”, “ridge tillage”, “subsoiling” with selected papers restricted to those conducted in Northeast China. The following criteria were considered when selecting the properly paired experiments to avoid any publication bias: (1) except tillage measures (conservation tillage and conventional tillage), other experimental conditions must be the same; (2) the experiment duration under conservation tillage and conventional tillage must be the same; (3) data on crop yield and (or) SOC concentration/storage under conservation tillage and conventional tillage must be given; (4) the experiment must have been conducted under field conditions. Based on the above criteria, a total of 176 pairs of experimental data were available for analysis, among which 148 were crop yield data, and 65 were SOC data (including 37 pairs of data for both crop yield and SOC). Data were collected from about 70 articles, including 25 experiment sites located in Northeast China excluding the northeast part of Inner Mongolia Autonomous Region (Fig.1). The details of selected 25 sites are listed in Table S1. The experimental sites were mainly distributed in the main grain-producing areas such as Sanjiang Plain and Songnen Plain.