Abstract:

This research explored the variation and characteristics of soil carbon components across four major types of natural grassland （meadow steppe， warm steppe， steppe desert and desert steppe） in Ningxia typical temperate natural grassland. The carbon components were investigated soil organic carbon and active organic carbon （readily oxidized organic carbon， microbial biomass carbon and dissolved organic carbon） at 49 fixed sites， using both survey and direct sampling methods. It was found that soil organic carbon contents to 40 cm soil depth were， respectively， 34.23， 12.84， 5.76 and 3.82 g·kg^-1， and the soil organic carbon storage was， respectively， 13.43， 5.75， 2.58 and 2.29 kg·m^-2 in the four natural grassland categories. For the four grassland categories， the readily oxidized organic carbon content ranged from 0.75-7.43 g·kg^-1， microbial biomass carbon content ranged from 102.52-554.77 mg·kg^-1， and dissolved organic carbon content ranged from 69.66-89.61 mg·kg^-1. The overall ranking was meadow steppe>warm steppe>steppe desert>desert steppe. The levels of readily oxidized organic carbon storage were 2.56， 1.44， 0.62 and 0.48 kg·m^-2， respectively； soil microbial biomass carbon storage was 218.31， 170.50， 81.99 and 68.26 g·m^-2， respectively， with meadow steppe significantly higher than the other grassland types （P<0.05）. The soil dissolved organic carbon storage was 34.36， 35.21， 37.22 and 43.14 g·m^-2， respectively， and the value for desert steppe was significantly larger than the other grassland types （P<0.05）. The range of values in the four grassland types for readily oxidized organic carbon was18.42%-29.72%， with the warm steppe being the highest. The range of values for microbial biomass carbon was 1.54%-3.83%， and meadow steppe grassland was the lowest. The range of values for dissolved organic carbon was 0.23%-2.01%， with ranking desert steppe>steppe desert>warm steppe>meadow steppe， and with significant differences （P<0.05） between the four grassland types. Soil organic carbon storage showed a significant （P<0.05） or extremely significant （P<0.01） positive correlation with soil readily oxidized organic carbon storage， microbial biomass carbon storage， total nitrogen， total phosphorus and total potassium， and showed significant （P<0.05） or extremely significant （P<0.01） negative correlation with soil dissolved organic carbon storage， soil bulk density and pH values. In summary， soil organic carbon stability was lowest in desert grassland， while the activity and bioavailability of the soil organic carbon pool in warm grassland was the highest. The importance of the soil organic carbon storage of natural grassland in Ningxia should not be underestimated.

Key words: Ningxia, natural grassland, soil organic carbon, soil active organic carbon