2.3 Soil sampling
Soil cores were collected from 0-20 cm layer after maize harvest in October 2018 and 2019, three replicates in each plot were drilled and then homogenized to form a constitute sample for each layer. Soil samples were sieved through an 8-mm stainless steel to remove visible materials, transported to laboratory and stored at 4 °C in a cooler. The samples were divided into three sets of subsamples. One set was air-dried at room temperature to determine the aggregate size distribution using an aggregate analyzer; one set was used to determined soil water content and microbial biomass carbon (MBC), water soluble organic carbon (WSOC), and extractable organic carbon (EOC), as well as basal respiration (BR); and the other set was air-dried and ground to pass a 0.15 mm sieve for determination of SOC and KMnO4-oxidized organic carbon (KMnO4-C).