N mineralization rates and environmental factors under different land use types

The study area is situated in the CLP in northwestern China. In the past, part of the natural grassland was converted to arable land in order to increase food production. In recent decades, with the acceleration of urbanization and industrialization, rural labor forces have moved to cities in search of better employment opportunities, leading to the abandonment of a large amount of arable land. Meanwhile in 1983, a greening project was implemented in the study area to cope with soil erosion and ecosystem degradation, leading to the conversion of some natural grasslands to afforested lands.
In July 2020, three sites in each of afforested grassland and natural grassland areas and one site on abandoned land were selected for sampling, with elevations ranging from 1800 m to 2100 m. At each site, three sample plots (10 m × 10 m for each) were randomly established at each elevation, 1800-1900 m, 1900-2000 m and 2000-2100 m, using three or five quadrats (0.50 m × 0.50 m) in each plot. The soil samples (five samples from0-0.15 m, and 0.15-0.30 m layers of soil at each elevation) were initially stored in the refrigerator at 4°C. Samples were then removed and placed in a 28°C incubator for one week of pre-culture. After the pre-culture, a portion of each sample was determined for ammonium nitrogen (NA1) and nitrate nitrogen content (NN1) using a SmartChem 200 element analyzer (AMSS Rome, Italy). The remaining pre-cultured soil samples, equivalent to 20 g of dry soil, were placed in a 250 ml conical flask and 20 ml of distilled water was added to submerge the soil. All conical flasks were sealed with rubber stoppers and placed in a constant temperature incubator at 40°C for one week. At the end of the incubation, ammonium nitrogen (NA2) and nitrate nitrogen content (NA2) were determined. The average soil net ammoniumfication rate (RA), net nitrification rate (RN) and net Nnitrogen mineralization rate (RM) were calculated (García-Angulo et al., 2020) as follows:
RA (mg kg-1 soil d-1) = (N_A2 - N_A1)/7
RN (mg kg-1 soil d-1) = (N_N2 - N_N1)/7
RM (mg kg-1 soil d-1) = ((N_A2 + N_N2 ) - (N_A1+ N_N1 ))/7
Soil samples for microbial metagenomics analysis (three samples for each elevation) were refrigerated at about -80 ºC and sent to Majorbio Bio-Pharm Technology Co. Ltd. (Shanghai, China) for analysis. The KEGG annotation was conducted using Diamond (http://www.diamondsearch.org/index.php, version 0.8.35) against the Kyoto Encyclopedia of Genes and Genomes database (http://www.genome.jp/keeg/) with an e-value cutoff of 1e-5.
There are the data of N mineralization rates, vegetation biomass, soil properties, and functional gene abundance under three land use types.