2.1. Site description and soil sampling
Soils were sampled from a long-term experimental field located in the Shenyang station of Chinese Academy of Sciences, Liaoning province, Northeastern China (41°32′ N, 122°23′ E). The climate is typical continental monsoon, with a mean annual temperature of 7.9°C and mean annual precipitation of 604 mm (1981-2018,https://power.larc.nasa.gov). The long-term old-field (OF) and bare-fallow (BF) treatments were established in 2003 and were paddy fields before treatments. The OF plot received fresh organic input for 15 years (2003-2018) with freely growing weeds but no anthropogenic interference. The BF plot was kept free of organic input for 15 years (2003-2018) by frequent hand weeding. Soils from these OF and BF treatments were sampled in 2018. Another BF plot had been kept free of organic input for 23 years until soil sampling for this study (Zhang et al., 2021). Soils were collected from the plow layer (0-20 cm depth) in the long-term experimental field. Within each plot, soils were sub-sampled at five randomly selected sites and then homogenized into one sample. Prior to the incubation experiment, soil samples were air-dried, sieved to 2 mm, and homogenized thoroughly. Visible roots, plant detritus, and stones were removed carefully by handpicking.