| The dynamic of soil organic carbon (SOC) has a close relationship with global climate change. Simulating and forecasting the dynamic of SOC by model has now become a popular method. Previous studies were primarily limited to medium and small mapping scale, while studies on the series mapping scale databases of large, medium and small scale were rarely reported. Furthermore, the influence of usage of different series mapping scale databases on the simulation precision of SOC model was not clear. In terms of modeling verification, it was usually carried out based on the results of long-term observation at field rather than regional scale, thus the simulation and forecast results had a large uncertainty.In this paper, taking the DNDC (DeNitrification and DeComposition) model which is comparatively mature at modeling biogeochemical process for example, selecting the Taihu Lake region whose soil type is almost paddy soil as the study area.The dynamic of SOC on more than 2.3 M.hm-2 paddy soil between 1982 and 2000 under six kinds of mapping scales i.e.1:14,000,000,1:4,000,000,1:1,000,000,1:500,000,1:200,000 and 1:50,000 was simulated and the results under different mapping scales were compared with the measured values of more than 1000 sampling points at year 2000.The model suitability was verified and assessed from the entire region, in order to provide a theoretical basis for further amendment of the model and assessment of simulation accuracy under different mapping scales. The main research conclusions were as follows.1) From the regional verification under different mapping scales, the relative errors between the simulated and the measured values of paddy soil SOC in Taihu Lake region in the year 2000 under mapping scale of 1:200,000,1:500,000 and 1:4,000,000 were all less than±5%, which indicated that the simulation results reached accurate level. The mapping scale of 1:200,000 which had the minimum relative error of 0.28% had the highest simulation accuracy. The relative errors between the simulated and the measured values under mapping scale of 1:50,000 and 1:1,000,000 were 6.4% and 5.1% respectively, which indicated that the simulation results were feasible. The relative error between the simulated and the measured values under mapping scale of 1:14,000,000 was 20.0%, which indicated that the simulation result was unreliable. Viewed from most studies at the present time, soil attribute data used in the simulation of DNDC model at national scale were predominantly from 1:14,000,000 soil map and the Second National Soil Survey, which would bring about comparatively big error of SOC simulation.2) The model suitability of paddy soil subgroup in Taihu Lake region under six mapping scales had great differences. The bleaching paddy soil had the highest simulation accuracy under 1:200,000 mapping scale. The hydromorphic paddy soil had the largest distribution area in Taihu Lake region and 1:4,000,000 was its ideal simulation scale. The percolated paddy soil whose area accounted for 16% of the whole paddy soil area had the highest simulation accuracy under 1:500,000 mapping scale. The gleyed paddy soil had maximum SOC content and its SOC simulation results, whose relatively errors were all above 10%, were the worst under each mapping scale compared with other subgroup. Relatively speaking, its simulation accuracy under the mapping scale of 1:500,000 was the best. The degleyed paddy soil whose area accounted for 18% of the whole paddy soil area and whose SOC content was outranked only by the gelyed paddy soil had the highest simulation accuracy under 1:500,000 mapping scale. The submergic paddy soil had the least distribution area in Taihu Lake region and its simulation result was the best under the 1:50000 mapping scale.3) Digital soil map of different mapping scale had different influence on the simulated estimation of SOC storage. In this study, with the decrease of mapping scale, the total SOC storage of surface paddy soil (0-30cm) in Taihu Lake region in the year 2000 simulated by the DNDC Model shown a trend of increase basically, especially the total SOC storage of 1:4,000,000 mapping scale was apparently higher than any other scale. Overall, the total SOC storage of the Taihu Lake region was mainly controlled by the hydromorphic paddy soil, the gleyed paddy soil, the degleyed paddy soil and the percolated paddy soil, for the SOC storage of these four paddy soil subtypes accounted for more than 93% of the total SOC storage under each mapping scale. The gleyed paddy soil and the hydromorphic paddy soil had the greatest influence on different mapping scale and their SOC storage under the 1:14,000,000 mapping scale was apparently higher than any other scale.4) Different mapping scale had significant impact on dSOC in Taihu Lake region. The area with increased SOC of surface paddy soil (0-30cm) from 1982 to 2000 of the 1:50,000 mapping scale with the most detailed soil attribute data was 1.47 M.hm-2, up to 63.4% of the whole paddy soil area, and 1.48 Tg C were sequestered in the soil over the past 19 years. However,0.78 Tg C,2.86 Tg C,2.33 Tg C,0.44 Tg C and 7.86 Tg C were lost respectively in the surface paddy soil (0-30cm) from 1982 to 2000 at the mapping scale of 1:200,000, 1:500,000,1:1,000,000,1:4,000,000 and 1:14,000,000. This indicated that mapping scale had a notable impact on SOC simulation result because different soil type had different SOC attribute and the ratio between total soil area and area of each soil type changed significantly with variation of soil mapping scale.5) Scenario analysis of paddy soil SOC in Taihu Lake region with different management practice and different climate factor indicated that more crop biomass return in the filed, no-tillage and basic conservation tillage practice such as straw return could increase SOC content effectively and reasonable midseason drainage and fertilizer application would benefit the accumulation of SOC. Climate factors had a quite complex influence on SOC, but the general trend was that the decomposition velocity of SOC speeded up along with the rise in temperature. The result showed that global warming in the future would be bound to bring about loss of SOC. This was in line with the result simulated by the RothC model.6) Different soil data unit had remarkable impact on SOC simulation accuracy of the DNDC model. The dSOC estimated by county-based database under 1:14,000,000 scale differed widely from that estimated by 1:50,000 soil map-based databases at the level of both entire region and county. The relative deviation of dSOC between most county-based database was more than 300%. The difference between dSOC estimated by county-based database and estimated by 1:50,000 soil map-based database was relatively minor. The dSOC and its total change in the past 19 years estimated by 1:50,000 soil map-based databases varied between the maximum and the minimum value simulated by county-based database under 1:50,000 scale. The result indicated that it was reasonable for the DNDC model taking a county of the smallest simulation unit and expressing regional dSOC by the scope of simulation value on one hand and detailed soil data unit was the guarantee of simulation accuracy of the biogeochemical process model on the other hand. Therefore, it would be necessary to use more detailed soil data for SOC simulation of national and regional scale in the future studies.
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