Soil is an important carbon pool in terrestrial eco-systems, even its tiny change may promote the increase in atmospheric CO₂ concentration and thus affect the bio-geo chemical cycling of the earth system. The carbon storage in arid and semi-arid grassland soil composes 15% of global carbon pool. So carbon cycling and related factors in this region is important to predicting the variation in arid region carbon pool on the background of global change.

Carbon isotope ¹³C can be used to indicate the source and cycling of soil carbon. Research found that along the depth of soil profile, the carbon content decreases but the isotope value increases. The possible reason: 1) Suess effect-burning of fossil fuel makes the atmospheric ¹³C depleted carbon accumulated on the soil surface; 2) the isotope ratio in plant roots is 1-2 unit higher resulting in that the isotope ratio in the lower zone is higher than top zone. 3) The carbon fractionation by microbes during the process of soil organic carbon decomposition, resulting in that ¹²C leaving the system through respiration while ¹³C was retained and accumulated in the soil, forming the gradual increase in the ¹³C signal along the depth gradient in the soil profile.

Dr WANG Chao, Prof BAI E and others, using the platform of the 2000 km sample zone of North China Grassland, analyzed 27 soil profiles for their variation in carbon isotope and carbon content. The results show that the isotope value increases along the depth on all soil profiles. The negative correlation is significant and well indicates the cycling rate of soil carbon.

They also analyzed the effect of temperature, precipitation and soil properties on the cycling rate of soil carbon. The work provides new idea for the research in the soil carbon cycling on the regional and global scales and provides new parameters for the modeling of bio-geo chemistry.

The work was published entitled as Depth Profiles of Soil Carbon Isotopes along a Semi-arid Grasslandtransect in Northern China in the journal of Plant and Soil. The work was supported by 973 Youth Project and NNSFC.