Evolution Characteristics And Driving Mechanism Of Black Soil Organic Carbon And Nitrogen During The Post 113 Years Of Reclamation In Northeastern China

As one of the most important grain production bases in China,the black soil region in Northeast China has been seriously threatened by the significant reduction of soil organic carbon（SOC）and total nitrogen（TN）due to excessive reclamation and soil erosion in the past 100 years.Previous studies have focused on the changes of SOC and TN content of a single soil type within 50 years of reclamation.However,there are few studies on the evolution of SOC and TN,as well as the relationship among soil erosion rate,carbon,nitrogen and microbial community,especially over long periods of reclamation.To assess this,the black soil（uncultivated and reclaimed in 1906,1956,1965,1986,1996）and meadow soil（uncultivated and reclaimed in1952,1986,1997,2001）with different reclamation time of Heshan Farm in Nenjiang County,Heilongjiang Province were taken as the research object,the soil samplings of each site were sampled in 2006 and 2019,and the soil physical and chemical properties were determined,and the soil erosion rate was estimated using ¹³⁷Cs and²¹⁰Pb_ex methods,as well as soil microbial community structure and diversity were analyzed by high-throughput sequencing.Also,the data on soil organic carbon and total nitrogen content of about 18 reclamation time for black soil and 13 reclamation time for meadow soil were compiled from the literature.On this basis,the correlation between soil carbon,nitrogen content and reclamation years of the two types of soil was carried out,and the driving mechanism of the changes in carbon and nitrogen content was studied.The main findings are as follows:（1）The evolution of soil organic carbon and total nitrogen content in black soil and meadow soil with different reclamation years was clarifiedThe uncultivated surface SOC contents of black soil and meadow soil were between 40.05-87.34 g kg^-1 and 80.28-129.74 g kg^-1,respectively.With the increase of reclamation years,both decrease exponentially,and the functions were y=22.99+34.64*exp（-0.04*x）and y=30.44+73.08*exp（-0.21*x）,respectively,as well as the decline rate of SOC content decreased with the increase of reclamation years in an exponential function of y=1.49*exp（-0.04*x）and y=15.49*exp（-0.21*x）,respectively.With the SOC content of black soil and meadow soil tended to be stable after 110 and 25 years of reclamation,respectively,the SOC content of black soil was lower than that of meadow soil.In the past 13 years,the average decline rate of SOC content in black soil of cultivated land showed a decreasing trend with the increase of reclamation years;the SOC content of cultivated meadow soil increased in 67 years of reclamation,which may be related to sedimentation.The uncultivated surface TN contents of black soil and meadow soil were between 2.47-7.32 g kg^-1 and 5.26-10.88 g kg^-1,respectively.With the increase of reclamation years,both decrease exponentially,and the functions were y=1.88+2.81*exp（-0.04*x）and y=2.49+5.80*exp（-0.22*x）,respectively,as well as the decline rate of TN content decreased with the increase of reclamation years in an exponential function of y=0.10*exp（-0.04*x）and y=1.29*exp（-0.22*x）,respectively.With the TN content of black soil and meadow soil tended to be stable after 130 and23 years of reclamation,respectively,the TN content of black soil was lower than that of meadow soil.In the past 13 years,the average change rate of TN content in cultivated black soil and meadow soil had no obvious regularity with the increase of reclamation years.（2）The influence mechanism of soil erosion on SOC and TN content in black soil and meadow soil was elucidatedWith the increase of reclamation years,the average annual soil erosion thickness,average annual soil erosion rate and total soil erosion amount of black soil increased,except for 113 years of black soil reclamation;the average annual soil erosion thickness and total soil erosion amount of meadow soil increased with the increase of reclamation years,but the average annual soil erosion rate of cultivated land was similar.The soil erosion intensity of meadow soil was lower than that of black soil.The SOC and TN content of black soil and meadow soil were negatively correlated with soil erosion rate and soil erosion amount.The decline rate of SOC and TN content in surface layer（0-15 cm）of black soil and meadow soil with the increase of soil erosion intensity was greater than that in 0-30 cm soil,and soil erosion had a greater impact on surface layer SOC and TN content than subsurface layer（15-30 cm）.The decline rate of black soil carbon and nitrogen content with the increase of soil erosion intensity was less than that of meadow soil.However,since the erosion rate of black soil was higher than that of meadow soil,the quality of black soil became worse after erosion.（3）The relationships between soil microbial community structure and diversity and the SOC,TN content of black soil and meadow soil were establishedProteobacteria,Acidobacteria,Verrucomicrobia and Actinobacteria were the main dominant phyla of black soil and meadow soil with different reclamation years,which together accounted for 68.66%-75.85%and 71.47%-81.00%of the total bacterial abundance of the two soils,respectively.The changes of relative abundance of each dominant bacteria phylum were different after reclamation.For black soil,the correlation degree between bacterial abundance and soil erosion amount was higher than its correlation degree with soil erosion rate,and for meadow soil,the correlation degree between the bacterial abundance and the soil erosion rate was higher than the correlation degree with erosion amount.The species richness and diversity of black soil first increased and then decreased with the increase of reclamation years,while that of meadow soil shown the opposite trend.Uncultivated plots and reclaimed plots were clustered into two categories.Tillage and water erosion lead to the reduction of SOC and TN,and the nitrogen fixation function of Proteobacteria was conducive to the increase of nitrate nitrogen in black soil,and then soil available nutrients increased,which restricted the growth of Verrucomicrobia.In conclusion,tillage erosion caused by reclamation,together with natural erosion,reduced the contents of SOC and TN in black soil and meadow soil,resulting in an exponential decrease of surface SOC and TN content with the increase of reclamation years.For black soil with greater erosion intensity,SOC and TN content were more difficult to stabilize.Soil microbial community composition was changed under erosion,which affected soil carbon and nitrogen content.Therefore,the protection of black soil needs to start from reducing soil erosion.