Changes And Influencing Factors Of Soil Inorganic Carbon In Farmland In The Songnen Plain

Soil carbon pools include both organic and inorganic carbon pools.The role of organic carbon pools in soil properties and carbon sequestration is well known,while the role of inorganic carbon pools has not received the attention it deserves.There is growing evidence that the role of soil inorganic carbon in soil fertility,carbon sequestration and CO₂ emissions should not be underestimated.While the acidification of agricultural soils due to the continuous application of large amounts of nitrogen fertilisers in agricultural production has attracted much attention,the effects of continuous nitrogen fertilisation on inorganic carbon in agricultural soils have been less studied.The black land in Northeast China is the main commercial grain producing area in China,and is known as the"panda of arable land"in China.The problem of soil acidification in black land is also one of the factors of its degradation,but there are few studies on the changes and drivers of inorganic carbon in Northeast China’s black land,especially in the transition zone between phaeozem and chernozem.Therefore,in this paper,the factors affecting inorganic carbon in the soil of the Songnun Plain were investigated by using data collection and resampling methods.The main conclusions were obtained as follows.（1）The literature was collected to establish a database of inorganic carbon in farmland soils in the 1980s and 2010s in the Songnun Plain,and to analyse the relationship between inorganic carbon density and various influencing factors.The results showed that the inorganic carbon density of farmland soils in the Songnen Plain was negatively correlated with rainfall and altitude（p<0.01）,and positively correlated with annual mean temperature（p<0.01）.The inorganic carbon density of farmland soils in the Songneng Plain was highly significantly negatively correlated with the organic carbon content and the percentage of sand particles in the soil（p<0.01）.This indicates that the inorganic carbon density of farmland soils is mainly influenced by natural factors such as rainfall,temperature,altitude,soil organic carbon and soil texture.（2）Paired points were formed by both matching the created database and by retrieving and resampling.Inorganic carbon and p H changes in agricultural soils in the study area were compared on a whole-database comparison and paired points comparisons based on a two-decade（1980s and 2010s）database.The whole-database comparison showed that soil inorganic carbon density in the Songneng Plain decreased by 0.04 kg C/m² and 0.33 kg C/m²in the 0-20 cm soil layer and 20-40 cm soil layer,respectively;the inorganic carbon density in the 0-20 cm soil layer in the paired sites decreased significantly by 0.26 kg C/m²（p<0.01）,or17%,over the past 30 years;the 20-40 cm soil layer decreased by 0.29 kg C/m² and 0.29kg C/m²,respectively.It can be seen that both soil inorganic carbon and p H have decreased over the last 30 years in the Sonnen Plain,mainly in the top 0-20 cm soils.Therefore,it is necessary to pay attention to the changes of inorganic carbon in farmland soils in this area and prevent the loss of inorganic carbon and soil acidification in the surface layer in time.（3）The meteorological data and cumulative nitrogen fertilizer input data of the study area were collected and collated to investigate the driving factors of inorganic carbon changes in farmland soils of the Songnen Plain.The results showed that the change in soil inorganic carbon density in farmland was significantly negatively correlated with rainfall evapotranspiration ratio and cumulative N input（p<0.05）,and the change in soil p H was significantly positively correlated with the change in soil inorganic carbon density（p<0.05）.The changes in soil inorganic carbon surface density over the 30-year period were mainly driven by the ratio（P/PET）of rainfall（P）and evapotranspiration（PET）and cumulative N fertilizer inputs.（4）The effects of continuous N application on soils with different inorganic carbon contents were investigated using incubation-leaching indoor simulations.The results showed that nitrogen application decreased soil calcium carbonate by an average of 31.31%and significantly reduced the abundance of calcite and sodium feldspar in soil minerals compared to the no-nitrogen treatment.The total hydrogen production was greater than the carbonate buffered hydrogen production on soils with low carbonate content（0.27-8.29 g/kg）and the p H was significantly reduced.)by 2.29 units.Nitrogen application significantly increased the EC of the soil leachate,especially the calcium and magnesium ions,and significantly reduced the bicarbonate depletion of the carbonate buffer system in the soil leachate.It can be seen that continuous N application can result in significant depletion of soil inorganic carbon and severe soil acidification,more so on low inorganic carbon soils（Ca CO₃ content 0.27-8.29 g/kg）.Therefore,more attention should be paid to soil inorganic carbon depletion on soils with low inorganic carbon content.In summary,this study clarifies the main environmental conditions and drivers affecting the inorganic carbon content of agricultural soils in the study area and the effect of continuous nitrogen application on soils with different inorganic carbon contents.It shows that the loss of soil inorganic carbon caused by agricultural production,especially long-term nitrogen fertilizer application,is an important factor in the acidification of black soils in Northeast China,and the results of the study have some theoretical and practical significance for the conservation of black land use and the reduction of agricultural carbon emissions.