Sampling and measurement during the incubation experiment
During the 120-day incubation, soil respiration was recorded through measurement of CO2 evolution in the flask headspaces. Gas samples were collected at 3, 6, 9, 12, 18, 22, 27 and 30 days in the first month, and weekly in the following months. Three blank Erlenmeyer flasks for each sampling time and mineral treatment were analyzed for background CO2 concentrations. After each gas sampling, all remaining Erlenmeyer flasks were opened for 20 minutes to replace their CO2-enriched headspace with fresh air. Then soil moisture content in each flask was adjusted by weighing and adding a specified water volume to re-establish the original moisture content. The CO2 concentration in each gas sample was measured by titration using 0.1 N NaOH and corrected by subtracting the background CO2 concentration. The corrected CO2concentration was used to calculate daily emission.
At the end of the incubation, soil samples were taken for measuring C and N contents, X-ray diffractograms, PLFAs and amino sugars and to extract mineral-associated SOM for measuring C and N contents and chemical structures using solid-state 13C cross polarization/total sideband suppression (CP/TOSS) NMR spectroscopy. The mineral-associated SOM was extracted by hydrofluoric acid treatment and centrifugation. Briefly, 5-g air-dried soil samples were mixed with 40 mL 2% hydrofluoric acid in 100 mL plastic tubes, shaken for 2 hours at 25oC at 180 rpm and centrifuged at 1000 g for 10 minutes. Before and after centrifugation, visible floating materials was removed, and the supernatant was discarded. The treatments were repeated four more times. The final remaining soil residues were washed four times with deionized water to remove hydrofluoric acid, oven dried at 80oC until constant weight, and measured for C and N contents and chemical structures by using 13C NMR. Based on the C contents before and after the treatment of 2% hydrofluoric acid, C loss was estimated, being from 4.7% to 16.8% in all model soils except for the vermiculitic soil added with soya litter (37.6%) (Supplementary Table 2).