Variations in soil organic carbon decompositions of different land use patterns on the tableland of Loess Plateau

Land use patterns are one of the critical factors affecting soil carbon sequestration or decomposition and greenhouse gas emissions. The accurate evaluation of its change is particularly crucial for the carbon cycle of the terrestrial ecosystem and global climate change. This paper examined the dynamic soil respiration by means of the indoor airtight culture method and static box-gas chromatography under the land uses of the farmland, natural grassland, shrub forestland, arbor and shrub mixed forestland, and arbor forestland on the tableland of Loess Plateau. The results showed that soil organic carbon mineralization and soil respiration rate were influenced by the land use type. (1) The content and rate of soil mineralizable carbon in grassland were significantly higher than that in forestland and farmland, and that in forestland significantly higher than that in the farmland (P < 0.05). The trend of organic carbon mineralization was rapid in the initial stage of culture (by the rate of 30.02~238.56 mg kg^-1 h^-1 in the first 0.5 h) and slowed down in the later stage (by the rate of 1.07~1.95 mg kg^-1 h^-1 in 1575 h). In the soil of grassland, the accumulation of mineralizable organic carbon in the 0-5-cm layer was 1.20~1.64 times that in the soil of forestland and 1.82 times that of farmland. Compared with the 0-5-cm soil layer, there were decreases in the 5-20-cm soil layer under all land uses, with a decline of up to 48% in farmland. (2) The soil carbon mineralization potential of different land use types ranged from 0.81 to 2.70 mg kg^-1, that of grassland was significantly higher than that of farmland (P < 0.05), and the soil organic carbon decomposition rate constant (k) under shrub forestland was significantly higher than that under other land use types (P < 0.05). The highest organic carbon mineralization capacity and the lowest sequestration were in the soil of farmland, while that in grassland had the lowest organic carbon mineralization capacity and the highest sequestration. (3) The cumulative amount of soil respiration was highest under the natural grassland, followed by the shrub forestland, and lowest under the farmland. The soil respiration rate was positively correlated with soil temperature, and its correlation with soil water content was positive in the wet season (May through October) and negative in the dry season (November through April in the following year). (4) The soil respiration rate showed obvious seasonal differences. After the conversion of farmland to forestland, it would be rising in the content of soil organic carbon and labile organic carbon, and be going down in the temperature sensitivity (Q₁₀). In summary, by converting farmland to forestland or grassland, the sequestration of soil organic carbon had greatly been enhanced, and the possibility of soil greenhouse gas emissions had reduced on the tableland of Loess Plateau.