Topsoil organic carbon storage of China and its loss by cultivation

Topsoil is very sensitive to human disturbance under the changing climate. Estimates of topsoil soil organic carbon (SOC) pool may be crucial for understanding soil C dynamics under human land uses and soil potential of mitigating the increasing atmospheric CO2 by soil C sequestration. China is a country with long history of cultivation. In this paper, we present an estimate of topsoil SOC pool and cultivation-induced pool reduction of China soils based upon the data of all the soil types identified in the 2nd national soil survey conducted during 1979-1982. The area of cultivated soils of China amounted to 138x10(6) ha while the uncultivated soils occupied 740x10(6) ha in 1980. Topsoil SOC density ranged from 0.77 to 1489 t Cha(-1) in uncultivated soils and 3.52 to 591 t Cha(-1) in cultivated soils with the average being 50 +/- 47 t Cha(-1) and 35 +/- 32 t Cha(-1), respectively. Geographically, the maximum mean topsoil SOC density was found in northeastern China, being of 70 +/- 104 t Cha(-1) for uncultivated soils and of 57 +/- 54 t Cha(-1) for cultivated soils, respectively. The lowest topsoil SOC density for uncultivated soils was found in East China, being of 38 +/- 33 t Cha(-1) and that for cultivated soils in North China, being of 30 +/- 30 t Cha(-1). There is still uncertainty in estimating the total topsoil SOC of uncultivated soils because a large portion of them was not surveyed during the 2nd Soil Survey. However, an estimate of total SOC for cultivated soils amounted to 5.1 Pg. On average, cultivation of China's soils had induced a decrease of SOC density of 15 t Cha(-1) giving rise to an overall pool reduction at 2 Pg. This is significantly smaller than the total SOC pool decline of 7 Pg due to cultivation of natural soils in China reported by Wu et al. (Glob. Change Biol. 2003, 9: 305-315), who made a pool estimation of whole soil profile assuming 1 m depth for all soils. As the mean topsoil SOC density of China was lower than the world average value given by Batjes (J. Soil Sci. 1996, 47: 151-163), China may be considered as a country with low SOC density and may have great potential for C sequestration under well defined management. However, the dynamics of topsoil C storage in China agricultural soils since 1980's and the effects of modern agricultural developments on C dynamics need further study for elucidating the role of China agriculture in global climatic change.