Variability of soil organic carbon in plough layers and its impact factors in a coastal reclamation area on south coast of Hangzhou Bay

Soil organic carbon (SOC) has been identified as a key element in the global carbon balance. The studies on spatial distribution and variation of SOC are very helpful for soil monitoring and soil mapping. There are few reports on spatial variability of SOC concentration under different crops in plough layers in coastal reclamation areas. This study is intended to determine spatial distribution of SOC storage and its impact factors in the coastal reclamation area on south coast of Hangzhou Bay.A total of 2 353 soil profiles in plough layer were collected from 2007 to 2009 in coastal reclamation area of Hangzhou Bay. SOC concentrations of soil samples were determined according to the dichromate oxidation method with a factor of 0.58. Geo-statistical methods were used for interpolation and spatial grids of crop fields, and random forest method was employed to estimate explanation capability and eliminate inter-correlations among impact factors.The average contents of SOC in plough layer is 9.78 g/kg, varied from 1.04 g/kg to 38.22 g/kg, indicating a moderate degree of variability (variable coefficient=65.24%). For three different cropping patterns, including Vegetables, Rice-Rice and Orchards, the soil points were 1 858, 327 and 168, respectively. Specifically, Rice-Rice has the highest SOC content (18.13 g/kg), followed by Vegetables (8.44 g/kg) and Orchards (8.30 g/kg). Under different soil types, significant differences were observed among paddy soil, coastal saline and fluvio-aquic soil (P< 0.05), SOC content in coastal saline soil was close to those in fluvio-aquic soil, and the highest SOC concentration was observed under paddy soil (21.42 g/kg). Besides, for different reclaimed ages, SOC content ranged from 7.14 g/kg to 10.96 g/kg. Except the reclaimed age of 1724—1918, no significant difference was found among other reclaimed ages (P> 0.05). Additionally, under each reclaimed age, the significant differences were observed between Rice-Rice and Orchards (P<0.05), Vegetables and Rice-Rice (P<0.05), and significant differences were also detected among different soil types (P< 0.05). Moreover, there were highly significant inter-correlations between soil types and reclaimed ages (r=0.64, P<0.001), and the highest individual explanation capability of cropping patterns on SOC variability was 22.84% after eliminating the inter-correlations, followed by soil types (8.68%) and reclaimed ages (4.12%) in this region. Finally, geo-statistical interpolation analysis revealed that high SOC concentration was mainly located at southeast and northwest of study area, whereas low SOC content was observed in central north. This high and low distribution of SOC concentration in this study region is consistent with the cropping patterns of Rice-Rice and Vegetables, respectively.In conclusion, a spatial distribution of SOC concentration was achieved in the study region, and SOC content is much higher in the rice planted region and/or paddy soil than those of vegetables, orchards, coastal saline soil and fluvio-aquic soil. The cropping pattern is a more important factor on variation of SOC content than soil type and reclaimed ages in this coastal region, and this could be helpful to adopt appropriate cropping strategies to improve the soil carbon storage in the coastal reclaimed region.