Responses of soil C stock and soil C loss to land restoration in Ili River Valley, China

In the past decades, massive destroying of natural vegetation and extension of agricultural lands strongly affected soil carbon (C) cycling. Returning cropland to forestry or grass land is one of restoration measures sponsored by the Chinese government. Land use change exerts a great influence on soil microclimate and litter quality, and is therefore an important determinant of soil C dynamics. However, knowledge on the effects of land restoration on soil C in northwest China was limited. To address this problem, a study was undertaken in the arid Ili River Valley to estimate soil C dynamics for representative land uses: grassland and forest plantations (transformed from croplands), and cropland. The results showed that there were significant differences in soil C stock among the vegetation types. Soil C stock increased in poplar plantation eight years after conversion from cropland, whereas it significantly declined after conversion to spruce plantation. The 15-year-old poplar plantation had the highest soil C stock (59.60 Mg C ha−1), including C sequestration in forest floor and in mineral soil (0-30 cm). High litter mass in the old poplar forest facilitated the accumulation of soil C. There were significant differences in soil respiration rate and cumulative soil C loss among the land use types, with the highest in clover grassland. Furthermore, the grassland had a higher Q10 value, which implies that an increase in soil temperature would lead to more soil C loss under global warming. Our results suggested that conversion of croplands to fast-growing woody crops might be beneficial to soil C sequestration, which was helpful for land management and mitigation of climate change.