Temporal-spatial variability of soil organic carbon stocks in a rehabilitating ecosystem

In global change research, changes of soil organic carbon (SOC) reservoirs in tropical and subtropical regions are still unknown. The temporal-spatial variability of SOC stocks was determined in a basin of over 579 km(2) in subtropical China from 1981 to 2002. ArcCIS8.1 software was utilized for spatial analysis of semivariance, ordinary kriging (OK), and probability kriging (PK). Grid and hierarchical approaches were employed for the sampling scenario in 2002 with 106 Global Position System (GPS) established spots sampled. Bulk topsoil samples (0-30 cm) were collected at three random sites on each spot. The SOC content for 1981 came from the SOC map of the Second National Soil Survey. Geostatistical results of the nugget to sill ratio (0.215-0.640) in the rehabilitating ecosystem indicated a moderate spatial dependence for SOC on this large scale. The range of SOC changed from 2.04 km in 1981 to 7.15 km in 2002. The mean topsoil SOC increased by 4.6% from 10.63 g kg(-1) (1981) to 11.12 g kg(-1) (2002). However, during this 21-year period 25.2% of the total basin area experienced a decrease in SOC. Also, the probability kriging results showed that the geometric mean probabilities of SOC less than or equal to 6.0 g kg(-1), less than or equal to 11.0 g kg(-1) and > 15.0 g kg(-1) were 0.188, 0.534 and 0.378, respectively in 2002, comparing to 0.234, 0.416 and 0.234 in that order in 1981, respectively. The SOC storage in the topsoil increased by 17.0% during this time with the main increase occurring in forests and cultivated land, which amounted to 82.5% and 17.0% of the total increase, respectively.