Spatial Distribution And Morphological Distribution Of Soil Organic Carbon And Its Relationship With Land Use And Environmental Conditions In Estuary Fluvent

At present,approximately 1,500 Pg of carbon is stored in the soil in the form of organic carbon.The soil organic carbon pool is 2.5 times the size of the terrestrial plant carbon pool and two times the size of the atmospheric carbon pool.Therefore,the soil has a tremendous potential for carbon sequestration.Varied scales of the carbon storage and carbon sequestration potential can indirectly reflect the agricultural production and environmental quality of a region.Investigation of carbon storage and carbon sequestration is important for scientifically using and protecting limited soil resources,slowing down the soil greenhouse gas emissions,increasing soil carbon sequestration and improving soil quality.However,there are many uncertainties about the stock capacity and inventory change of the soil carbon pool.In particular,the influence mechanism of different land uses exerted by artificial and natural factors on regional soil carbon sources and sinks is still unclear.To fully understand and accurately evaluate the impact of different land uses on the spatial distribution of the regional 5oil carbon pool and its evolutionary patterns,this study explored the compositional proportion and spatial variation pattern of soil organic carbon under different land uses(cropland,vegetable field,woodland and wetland).Chongning Island,which has a unique location,was selected as the study region,and we adopted the method of combining field investigations and location experiments to analyze the spatial distribution of the topsoil organic carbon and the organic carbon storage of deep soil sections.Based on this method and using the soil organic carbon components grouping technique that integrates physical grouping,chemical grouping and biological stability-based grouping methods,the soil organic carbon components,contents and properties of the organic carbon of aggregates were quantitatively investigated,whereas the mineralization patterns of the organic carbon and their influencing factors were qualitatively examined.The following conclusions were drawn:(1)The range of soil organic carbon content in the topsoil layer(0-20 cm)of Chongming Island varied from 2.74-32.5 g/kg with an average of 10.53 g/kg,which was a relatively low level with a high potential for carbon sequestration.The coefficient of variation of organic carbon content was 36.85%,indicating a moderate degree of variation.In terms of soil organic carbon classification,Level 4 soil dominated,accounting for 59%of the soil,followed by Level 3 soil,which accounted for 29%of the soil;compared with observations from 2013,the overall soil organic carbon content was significantly improved.Land-use type had a significant impact on topsoil organic carbon content;cropland had the highest average soil organic carbon content and had significantly higher amounts than vegetable fields,woodlands or wetlands,whereas vegetable fields had significantly higher amounts than woodlands but not significantly different amounts from wetlands.Comparedwith 2013,the soil organic carbon content of cropland,vegetable fields and woodlands increased by 31.55%,14.29%and 8.14%,respectively.The comparison of different soil types showed that the average organic carbon content of paddy soil was the highest and significantly higher than the content of the coastal saline soil,whereas the content of fluvo-aquic soil was not significantly different from either of the other two soil types.Compared with the 2003 data,the soil organic carbon contents of paddy soil,fluvo-aquic soil and coastal saline soil increased by 25.17%,15.61%and 19.32%,respectively.(2)The range of the soil organic carbon content in the deep soil layer(0-100 cm deep)of Chongming Island varied from 1.84-7.04 g/kg,with an average of 3.94 g/kg,which was a relatively low level with a high potential for carbon sequestration.The coefficient of variation of the organic carbon content was 31.73%,indicating a moderate degree of variation.The ranges of soil organic carbon content and carbon storage at different soil depths(0-100 cm)on Chongming Island varied from 8.73-1.72 g/kg and 2.31-0.47 × 106 kg/km2,respectively,exhibiting significant differences between the various soil layers.The soil organic carbon content significantly decreased with soil depth,and the carbon storage of topsoil(0-20 cm)accounted for 43.92%of the total carbon storage.The spatial distribution chart of organic carbon contents at different soil depths was generated using inverse distance weighted interpolation,which showed that the trend of soil organic carbon distribution was similar in each soil layer;soil carbon contents were high in the south and west but low in the north and east.The comparison of different land-use types showed that soil carbon density at depths from 0-100 cm showed the following pattern:vegetable field>cropland>wetland>woodland.Carbon storage showed the following pattern:cropland>wetland>woodland>vegetable field.(3)Analysis of the active ingredients of soil organic carbon under different land uses showed that the contents of labile oxidizable carbon(LOC)of cropland and vegetable fields were significantly higher than the LOC contents of woodlands and wetlands in the 0-40 cm soil layer.In the 40-80 cm soil layer,the LOC content of wetlands was the highest,and in the 80-100 cm soil layer,the LOC content showed no significant difference among land-use types.In soil layers of the same depth,the dissolved organic carbon(DOC)content of cropland was higher than for each of the other three land-use types.In the 0-40 cm soil layer,the microbial biomass carbon(MBC)content was significantly higher than for each of the other three land-use types,whereas in soil layers deeper than 40 cm,the MBC of woodlands was significantly lower than for each of the other three land-use types.Analyses of the seasonal dynamics of the active ingredients of organic carbon indicated that the LOC content of woodlands or wetlands showed an initial decrease followed by an increase,reaching its maximum and minimum values in the winter and summer,respectively.The LOC content of cropland showed an initial increase followed by a decrease,reaching its maximum value in the spring.The LOC content of vegetable fields or cropland showed the opposite trend.The DOC content of cropland or woodlands showed an initial increase followed by a decrease,reaching its maximum and minimum values in the autumn and spring,respectively.The DOC content of wetlands and vegetable fields showed an initial decrease followed by an increase;the MBC content of cropland or woodlands essentially showed an initial decrease and then an increase,while the MBC content of wetlands or vegetable fields showed the opposite trend.Analyses of the soil section distributions and seasonal change patterns of organic carbon and its active ingredients indicated that temperature and carbon source inputs such as crop residues were major factors impacting the change in the content of active ingredients of organic carbon;tillage management measures such as fertilizer application and straw returning were the main factors affecting changes in the content of cropland soil organic carbon.(4)Tillage management measures had a significant impact on soil carbon sequestration.Compared with tilled topsoil,the contents of macroaggregates(>2 mm),mid-sized aggregates(0.25-2 mm)and microaggregates(0.106-0.25 mm)increased by 11.25%,9.45%and 5.99%,respectively,in the untilled 0-20 cm topsoil layer.Under the same tillage conditions,the content of macroaggregates or mid-sized aggregates under the "untilled+straw returning+fertilizer application" treatment increased by 6.76%and 12.64%,respectively,compared with the "untilled+fertilizer application" treatment,whereas the content of microaggregates decreased.Due to conservation tillage measures such as no-till farming and straw returning,soil structure damage was reduced,increasing the stability of aggregates and contributing to the formation of macroaggregates.The formation of aggregates was accompanied by the accumulation of and an increase in organic carbon,while under the physical protection of coarse aggregates,new carbon,such as crop residues,enhanced the soil's potential for carbon sequestration.Soil organic carbon was mainly solidified with mid-sized aggregates(0.25-2 mm)because they were the most abundant and had higher microbial biomass amounts and enzymatic activities.Conservation tillage measures such as no-till farming and straw returning under the condition of mid-sized aggregates facilitated the formation and stabilization of the aggregates.Tillage measures such as tillering could have helped bring crop residues from the topsoil into the deeper soil layers and increased the content of organic carbon in these layers,indicating that the increase in soil carbon was mainly derived from the input of crop carbon because tillering could increase the fixation of organic carbon in deep soil sections.(5)The effect of different water contents,temperatures and straw returning amounts on the mineralization of soil organic carbon was analyzed through laboratory simulations.The results showed that the soil organic carbon mineralization curves of different land-use types all exhibited the trend of an initial increase followed by a decrease.Under aerobic conditions,the average daily organic carbon mineralization peaked between days 4 and 6,whereas under the flood conditions,it peaked between days 6 to 8.Under different temperatures(5?,15?,25? and 35?),the average daily soil organic carbon mineralization rate increased with increasing temperature,with a shortened time of increase and a longer time of stabilization achieved in the late stage.The result of an exponential model fit of the soil organic carbon mineralization rate and temperature showed that from highest to lowest,soil temperature sensitivity can be ordered as follows:cropland>vegetable fields>woodlands>wetlands.Compared to CK soils without straw returning,the average CO2 emissions and CO2 maximum emissions increased with the increasing amount of straw returning when the amount of straw returning was 0.5%,1.0%or 2.0%.In the CO2 rapid-release phase,the average CO2 emissions of the four levels of straw returning were 39.44,52.11,56.67 and 70.67 ml · kg-1·d-1,respectively,whereas in the CO2 slow-release phase,they were 15.63,19.01,20.62 and 21.67 ml·kg-1·d-1,respectively.