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.