| As a special ecosystem with both carbon source and carbon sink functions,agricultural ecosystems are affected by global climate change,and it is important to understand their carbon sequestration capacity for scientific management of agricultural production.As an important agricultural production area in Hubei Province,the Jianghan Plain has a wide distribution of farmland with abundant crop species and sufficient yields.Estimating carbon storage based on the cropping system can deeply explore the production potential of local crops and thus contribute to the enhancement of carbon sequestration capacity of the Jianghan Plain farmland ecosystem.Inthis paper,the spatial distribution of land use,spatial distribution of replanting index,spatial distribution products of major crops and distribution data of oilseed rape extracted by remote sensing index threshold method in consecutive years from 2001 to 2020 in Jianghan Plain are used as the basis for synthesizing the cropping system of farmland ecosystem in Jianghan Plain.Area matrix shift,mass center migration,spatial autocorrelation analysis and landscape pattern index methods were used to describe the spatial and temporal variation characteristics of farmland ecosystem cropping system in Jianghan Plain for the past 20 years.The carbon storage module of the InVEST model was used to quantitatively assess the carbon storage of farmland ecosystems in the Jianghan Plain from 2001 to 2020 by combining the carbon pool data of farmland with different cropping systems,to analyze the characteristics of spatial and temporal changes in carbon storage of farmland ecosystems in the Jianghan Plain in the past 20 years,and to further understand the effects of changes in cropping systems on carbon storage.The following conclusions were obtained:1.From 2001 to 2020,the farmland area of each cropping system in the farmland ecosystem of the Jianghan Plain was ranked from largest to smallest in the order of first-year rice > first-year dry crop > second-year dry crop > dry-rice rotation > fallow > second-year rice > third-year dry crop > dry-rice-rice rotation > third-year rice.During the study period,the overall area of farmland containing rice cultivation showed a decreasing trend,and the area of pure dry-crop farmland showed an increasing trend.The largest conversion area of each cropping system in the last 20 years was the conversion from one-year rice to one-year dry crop,with a conversion area of 401,000 hm2.More and more rice fields in Jianghan Plain are converted to dry crop crops,changing the previous situation of blindly planting large areas due to good rice planting environment and enhancing the diversity and science of agricultural production in Jianghan Plain.2.From 2001 to 2020,the center of mass of fallow,one-year-old rice,two-year-old rice and dry-rice crop rotation farmland shifted to the southeast,and was concentrated in Qianjiang City and adjacent districts and counties.The center of mass of one-year dry crop and two-year dry crop farmland moved to the west because of spreading to the west and south,among which the center of mass of one-year dry crop farmland was concentrated in the south of Zhongxiang City and the center of mass of two-year dry crop farmland was concentrated in the north of Qianjiang City.The area of the fields under the one-year,three-maturity cropping system was small,and there was no corresponding cropping system in some years,and the distribution of the plasmas was very scattered during the study period.3.the spatial distribution of farmland cropping systems in the Jianghan Plain from 2001 to 2020 showed significant positive correlation and a clustering pattern,and the overall aggregation of farmland cropping systems in the Jianghan Plain from 2001 to 2009 increased year by year,and the farmland cropping systems from 2016 to 2020 had a trend of spatial dispersion.Based on the local autocorrelation LISA clustering map,the local spatial autocorrelation of the spatial distribution of farmland cropping systems in Jianghan Plain mainly showed high-high aggregation and low-high aggregation,but the overall decrease in the proportion of distributed area during the study period.The high-low aggregation type has a stable share of less than 1% in 2001-2011,while it increases significantly in 2020,accounting for 15.31%.4.Calculating indices for each cropping system of farmland ecosystem in Jianghan Plain from 2001-2020 in terms of landscape aggregation,landscape fragmentation and landscape diversity,it can be found that the farmland of one-year rice,one-year dry crop,dry-rice rotation,one-year two-year dry crop and dry-rice-rice rotation remained highly intact and aggregated all year round.The extremely high separation index(SPLIT)indicates a high degree of fragmentation of one-year triple cropping farmland(one-year triple rice,dry-rice-rice,and oneyear triple dry crop),although the area is small.2014-2018 farmland cropping systems move toward unevenness and increase toward evenness by 2020.5.2001-2020,the total carbon storage value of farmland ecosystem in Jianghan Plain decreases from 117.21 Tg to 110.2 Tg,and the carbon storage is higher in the central part of Jianghan Plain and both sides of Yangtze and Han River basins.2001-2005,2005-2010,2010-2015 and 2015-2020 carbon storage changes are mainly unchanged,2001-2020 The central part of Jianghan Plain is dominated by the increase of carbon storage in 2020,and the decrease of carbon storage in 2010-2020.6.Combining the changes of cropping system and carbon storage in the farmland ecosystem of Jianghan Plain from 2001 to 2020,it is found that the farmland with the largest contribution to carbon storage by cropping system is the annual monoculture rice farmland with an annual average carbon storage of 47.59 Tg and the strongest carbon sequestration capacity in 2001,which accounts for 58.56% of the total carbon storage in the farmland ecosystem.The increase of carbon storage in the process of switching out of each farmland cropping system was most obvious from dryland crop cultivation to rice cultivation.It indicates that in the process of change of each cropping system in the farmland ecosystem of Jianghan Plain,despite the decrease of rice cropping area,its carbon storage contribution still occupies an important position in the whole farmland ecosystem. |
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