| Soil erosion will cause lateral transport of soil organic carbon on land slopes,produce erosion and accumulation,and partly enter lakes,reservoirs and even the ocean through river runoff,which significantly changes the process of soil organic carbon release and decomposition,greatly affecting the strength and direction of carbon sources and sinks in the global terrestrial ecosystem.It is also one of the main reasons leading to the“missing carbon sink”in the current global carbon balance estimation.However,the current simulation and assessment of the mechanisms,processes,and effects of soil organic carbon lateral transport at the catchment scale are relatively lacking,which greatly limits the accuracy of the scientific community's research on terrestrial carbon sinks.The abundant rainfall has created abundant surface runoff in the rivers in the province,most of which have the characteristics of"short and strong",and the main rivers have relatively independent and complete water system units,which mostly originate in the province and eventually flow into the East China Sea.This unique catchment characteristic provides a good condition and basis for estimating the process of soil organic carbon lateral transport at the catchment scale and analyzing the influence intensity of factors such as dominant climate and human activities on the carbon cycle.In this study,the eight major river catchments in Fujian Province are taken as the research objects.County-level soil erosion data from the second(1995-1996)and fourth(2010-2012)national soil erosion survey data sets were used to estimate soil organic carbon(SOC)erosion and sedimentation intensity in the eight major watersheds of Fujian Province.And the effects of precipitation and vegetation productivity changes on SOC erosion were discussed.Further in the Minjiang River catchment,the soil erosion model(ANSWERS-2000)was used to simulate the model to quantify the SOC lateral transport and redistribution in the Minjiang River catchment.The factors such as precipitation,land use types,vegetation productivity,and population number were discussed for SOC lateral transport and the impact of redistribution.The main research results are as follows:(1)The SOC erosion and sedimentation intensities of the eight major river basins in Fujian Province were quantitatively estimated,and the main causes of SOC erosion changes were analyzed.During the second and fourth national soil erosion surveys,the average SOC erosion rates in the eight watersheds of Fujian Province were 7.53±2.81 g C m-2yr-1and 26.75±9.4 g C m-2yr-1,respectively.The average precipitation were1350±216 mm yr-1,1928±374 mm yr-1,and the average torrential rain precipitation were248±97 mm yr-1and 494±188 mm yr-1,the average NDVI values were 0.709 and 0.652,respectively.From the second to fourth national soil erosion surveys,the average precipitation in the eight catchments increased,especially the increase in torrential rain precipitation and heavy rain precipitation,and the vegetation coverage decreased significantly,the SOC erosion rate in the eight river catchments increased by 2.6 times,which is significantly higher than the national and global averages during the same period.The increase in precipitation from second to fourth national soil erosion surveys,especially the increase in the intensity of heavy rains,may be a key influencing factor that significantly increases the SOC erosion rate.At the same time,the reduction of vegetation productivity is also an important factor that aggravates the erosion of SOC.(2)The distribution characteristics of eroded SOC on land,rivers and oceans in the Minjiang River catchment of Fujian Province from 2008 to 2016 were quantitatively estimated.And the main reasons for changes in SOC erosion and redistribution were analyzed.From 2008 to 2016,73±3.9%of the entire Minjiang River catchment experienced soil erosion and SOC level migration to varying degrees.The annual average SOC erosion in the Minjiang River catchment is 0.078–1.229 Mt C yr-1,and the estimated SOC erosion rate is 0.049 t C ha-1yr-1.Among them,the erosion rate in 2010was 0.221 t C ha-1yr-1,which was much larger than the other 8 years,and was 6.6 times the average SOC erosion rate in the other 8 years.61.78±17.08%of the SOC induced by soil erosion in the Minjiang River catchment was deposited in the watershed,6.46±1.90%was deposited in the river course,and the remaining 31.76±18.90%were exported to the ocean.The SOC input to the ocean in 2008–2016 was 0.146 Mt C yr-1,while the SOC input to the ocean in 2010 reached 1.008 Mt C,which is 26 times the other 8-year average.Areas below 500 meters accounted for 51.40%of the total area of the Minjiang River catchment,and SOC erosion in the above areas accounted for 76.05%of the total SOC erosion in the Minjiang River catchment.The average SOC erosion rates of evergreen broad-leaved forest,mixed forest,grassland,and farmland in the Minjiang River catchment are 0.036 t C ha-1yr-1,0.027 t C ha-1yr-1,0.0378 t C ha-1yr-1and 0.081 t C ha-1yr-1.SOC erosion in farmland and low altitude areas where human activities are frequent is relatively serious.(3)The distribution characteristics of eroded SOC on land,rivers and oceans in the Minjiang River catchment of Fujian Province from 1958 to 2007 were quantitatively estimated.And the main reasons for changes in SOC erosion and redistribution were analyzed.From 1958 to 2007,74.6±6.2%of the entire Minjiang River catchment experienced soil erosion and SOC level migration to varying degrees.In general,the SOC erosion in the Minjiang River catchment is on a downward trend.The average annual SOC erosion is 0.263–2.553 Mt C yr-1,and the SOC erosion rate is 0.145 t C ha-1yr-1.In the 30 years after the reform and opening up,the amount of SOC erosion in the Minjiang River catchment decreased by 32%compared with the 20 years before the reform and opening up,which was lower than the global average.From 1958 to 2007,58.37%of the SOC output from soil erosion in the Minjiang River catchment was deposited in the watershed,10.32%was deposited in the river channel,and 31.32%was imported into the ocean.Among them,34.26%of the SOC induced by soil erosion in1958-1977 was imported into the ocean,and 28.44%of the SOC that was eroded in1978-2007 was imported into the ocean.Correlation analysis shows that SOC erosion and precipitation in the Minjiang River catchment have a moderate exponential correlation between 1958 and 2007,and SOC level migration increases with the increase in precipitation;SOC erosion has a weak positive correlation with rainfall in rainfall.From1982 to 2007,the average annual NDVI fluctuations in the Minjiang River catchment changed,but the overall trend was gradually increasing.From 1958 to 2007,the population showed a significant upward trend,with an average annual growth rate of22%.Correlation analysis shows that the SOC erosion of Minjiang River catchment from1958 to 2007 has a weak negative correlation.On the long-term scale,the improvement of vegetation productivity is the main factor influencing the reduction of SOC erosion.The research results indicate that the SOC erosion rate and precipitation in the main catchments of Fujian Province are closely related,especially the heavy rainfall has significantly increased the SOC rate.On a long-term scale,in the case of steady fluctuations in precipitation,significantly increasing the vegetation coverage can reduce the SOC of horizontal loss due to soil erosion.Low-altitude areas,especially farmland ecosystems,cause serious SOC erosion due to frequent interference from human activities.Therefore,scientific afforestation should be vigorously promoted to improve the vegetation coverage,optimize the structure of vegetation and land use types,and reduce the destruction of vegetation by human disturbance.This is an effective measure to reduce the loss of SOC levels caused by mountain and hilly soil erosion in coastal areas under current global climate change,and it is also an effective measure to protect soil carbon pools. |
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