Effects Of Corn Straw And Biochar Application On Organic Matter And ¹⁵N Fertilizer Utilization In Black Soil With Different Structures

The continuous planting of maize resulted in the decrease of soil organic matter and the deterioration of tillage structure in the black soil area of northeast China,resulting in the imbalance of nutrient supply and the decrease of nitrogen use efficiency.Application of corn straw and biochar can effectively improve soil structure,increase organic matter content and crop yield,and there are many related studies in recent years.However,it is not clear how the differences in physical and chemical properties,organic matter content,humus composition and HA structure of soils affect the absorption and utilization of ¹⁵N urea.Therefore,it is of great theoretical and practical significance to clarify the internal relationship between the differences in physical and chemical properties of different soil structures and ¹⁵N fertilizer.In this paper,the mechanism of improving the utilization rate of ¹⁵N urea was discussed by studying the effects of straw and biochar returning to the field on the physical and chemical properties,humus composition,humic acid structure,¹⁵N utilization and crop growth and yield of soils with different structures.Two groups of pot experiments and one group of field experiments were set up in this study.The first group of pot experiments consisted of six treatments:GS("good"structure soil with continuous year straw returning selected from the field+¹⁵N-labeled urea);BS("bad"structural soil for continuous maize planting without straw returning selected from the field+¹⁵N-labeled urea);GSC("good"structured soil+crushed corn straw+¹⁵N-labeled urea);BSC("bad"structure soil+crushed corn straw+¹⁵N-labeled urea);GSB("good"structure soil+biochar+¹⁵N-labeled urea);BSB("bad"structure soil+biochar+¹⁵N-labeled urea).In the second group,three treatments were set up:CK(control soil+¹⁵N-labeled urea);CKU(the soil on the 2mm sieve+¹⁵N labeled urea);CKW(flooded+¹⁵N-labeled urea).The field experiment consisted of 5 treatments:CK（control）;CKS1（0-20 cm soil layer crushed corn straw mixed return）;CKS2（20-40 cm soil layer crushed corn straw mixed return）;CKB1（0-20 cm soil layer biochar mixed return）;CKB2（20-40 cm soil layer biochar mixed return）.The effects of straw and biochar on improving the physicochemical properties,humic acid structure and ¹⁵N utilization of black soil with different structures were studied.Nitrogen utilization was studied by using ¹⁵N tracer technique,and the structural changes of humic acid were studied by elemental,Infrared spectral,differential thermal,three-dimensional fluorescence and nuclear magnetic analysis.The results are as follows:（1）The physical and chemical properties of the"good"structure soil were better than that of the"bad"structure soil.The water content,dominant grain components of aggregates,available nutrients and ammonium nitrogen and nitrate nitrogen of soil were higher;The carbon content of organic matter and its humus components was high,and the humification degree was high.The average organic carbon content of GS and BS were 13.79 and 9.81 g/kg after two plantations in 2018,and 13.33 and9.27 g/kg after two plantations in 2019,respectively;HA structure is relatively simple;Crops are growing well;¹⁵N has a high utilization rate,and the nitrogen use efficiency of GS and BS after two planting in 2018 was 10.35%and 8.16%,respectively.（2）The addition of straw and biochar could improve the physical and chemical properties of soils with different structures,which increased soil moisture content,content of dominant grain fractions in aggregates,content of available nutrients,content of ammonium nitrogen and nitrate nitrogen.After planting in 2019,the organic carbon content of GSB and GSC increased by 18.07%and 3.99%compared with GS,and the organic carbon content of BSB and BSC increased by 20.18%and8.70%compared with that of BS;The HA structure becomes simpler;crops grow better,yields increase;After planting in 2018,corn yield of CKS1,CKS2,CKB1 and CKB2 treatments increased by 5.88%,18.25%,12.54%and 25.38%,respectively,compared with CK.¹⁵N utilization improved.（3）In terms of the above changes,compared with the improvement effect of straw addition on the physical and chemical properties,organic matter and humus composition contents,HA structure and ¹⁵N utilization rate of soil with different structures,the improvement effect of biochar addition was greater.After the second planting in 2018,the organic carbon content of GSC and GSB increased by 18.82%and 9.90%,respectively,compared with GS.Compared with BS,BSC and BSB increased by 23.80%and 17.35%respectively;Compared with the improvement of"good"soil structure,the change of"bad"soil structure was greater.After planting in2019,compared with GS,the ¹⁵N utilization rate of GSC and GSB increased by 2.38and 10.21,respectively,and the result of BS was 4.93 and 11.12,respectively;The field experiment also showed that subsurface returning had better improvement effect than surface returning.（4）The physical and chemical properties of soil deteriorated after soil structure was destroyed.Soil water content,dominant grain component content,available nutrient content,ammonium nitrogen and nitrate nitrogen content decreased.After planting in 2019,the PQ value of CKU and CKW decreased by 2.63%and 5.56%compared with CK,respectively;The degree of condensation and oxidation of HA structure increase,and the structure becomes complicated;Plant growth deteriorated;¹⁵N utilization rate gets smaller,after two plantings in 2018,the ¹⁵N utilization rate of CK,CKU and CKW was 23.09%,20.63%and 13.77%,respectively.