Fertilization Effect And Mechanism Of Subsoil Under High Dosage Straw Returning In Black Soil Of Northeast China

Subsoil is an important tillage layer closely located below the topsoil in the range of 20–40cm,which is very significant for regulating soil fertility and crop growth.However,long-term shallow tillage has led to a prominent phenomenon of"upper fertilizer and lower carbuncle"in the black soil layer.The low fertility in the subsoil has seriously restricted the sustainable use of the black soil and the improvement of grain production.Although straw returning is widely used in local areas,it is difficult to return maize straw because of its large volume.When the depth of straw returning is relatively shallow,it will affect crop emergence and root growth,and also aggravate the fertility difference between topsoil and subsoil.Deep-buried straw incorporation is a method to return straw to the subsoil directly,while getting rid of the limitation of the straw amount.Nevertheless,the environment of subsoil is different from that of topsoil,and the effects of high-volume straw on the fertilization of subsoil layer is still unclear.Therefore,the deep-buried straw incorporation experiment of low(15 t·hm^-2),medium(45t·hm^-2)and high(75 t·hm^-2)dosages of crushed straw and pelletized straw was set up in this study.Soil physical and chemical analysis methods,¹³C NMR and phospholipid fatty acid analysis methods were adopted in this experiment.This field positioning test of maize continuous cropping system in black soil area was carried out from 2016 to 2018.The proportional balance between soil organic carbon and nutrient elements,the composition and structure changes of the humus,the distribution characteristics of soil physical properties and aggregates,and the structure differentiation characteristics of soil microbial community were discussed.The main progress was as follows:（1）High dosage straw returning significantly increased soil organic carbon content and balanced the proportion of carbon to nitrogen,phosphorus and potassium nutrients.Straw returning significantly increased the organic carbon content in subsoil layer.With the increase of straw dosages,the organic carbon content increased between 2%–20%,5%–27%and 1%–18%from 2016 to2018.High dosage straw return was significantly higher than low dosage.The second year of straw returning was the main accumulation period of organic carbon,and the effect of straw dosages on organic carbon was more significant.The carbon-nitrogen ratio and carbon-phosphorus ratio increased by more than 10%and carbon-potassium ratio increased by more than 20%.Pelletized straw had a rapid effect in the early stage of straw returning,while crushing straw was better in the later stage.（2）Increasing the dosage of straw returning help increase the content of subsoil humus components,and was conducive to the complexity of humus structure.Among them,the high dosage of crushed straw incorporation increased the humic acid carbon by more than 30%,which was higher than its low straw dosage by 10%–20%,while the high dosage of pelletized straw incorporation had significant advantages in the early stage.The ratio of humic acid to fulvic acid in subsoil was increased by 20%–60%and PQ was increased by 10%–30%under the medium and high dosages of straw returning.The degree of humification increased with time increasing.After three years of once-time straw returning,high dosage of crushed straw could improve the aliphaticity of humus,while high dosage of pelletized straw could improve its aromaticity.（3）High dosage straw returning was beneficial to maintain good soil physical properties and promoting the formation of water-stable aggregates.Subsoil bulk density and soil compactness decreased with the increase of straw dosages,while soil water content increased with the increase of straw multiple.The content of>0.25 mm water-stable aggregates in subsoil layer was about 1.2–1.4times higher than CK,and the content of 0.25–0.053 mm and<0.053 mm micro-aggregates decreased.The increase of macroaggregates was beneficial to the improvement of soil physical properties.Soil humus is mostly distributed in>0.25 mm water-stable aggregates.The increase of soil humic acid is an important reason for the increase of macroaggregates content.（4）High dosage straw returning was beneficial to increase the number of soil microbial flora and the ratio of fungi to bacteria in the later stage of returning.Pelletized straw returning significantly increased the content of bacteria,Gram-positive bacteria and Gram-negative bacteria in the early stage of straw returning,and crushed straw significantly increased the fungi content.With the increase of returning time,the increases of fungi:bacteria ratio and Gram-positive bacteria:negative bacteria ratio of high dosage straw returning were higher than low dosage.The high dosage was more conducive to long-term maintenance of ecosystem stability.Humic acid to fulvic acid ratio and>2 mm soil aggregates were important factors affecting the distribution of microbial communities.The content of macroaggregates was positively correlated with the molar percentage of soil fungi.（5）Soil integrate fertility quality evaluation showed that high dosage once-time straw returning could keep soil fertility at a high level for three consecutive years.The subsoil integrated fertility secore of high dosage straw returning was higher than that of low dosage during the three years.Cluster analysis showed that the first and second fertility levels were separated between high dosage of crushed straw and pelletized straw after three years,which was significantly related to the improvement of soil physical,chemical and biological properties.The maize yield increased by 7%–15%with the increase of straw dosages in three consecutive years.The maize yield was positively correlated with the integrated fertility secore of sunsoil（R²=0.8713,P<0.01）.The method of principal component analysis was reliable in evaluating soil fertility quality.In summary,high amount straw incorporation significantly improved soil fertility from four aspects:soil nutrient,chemical,physical and biological factors.Pelletized straw returning showed a rapid improvement effect in the early stage of returning to the field,while crushed straw had a long-term effect.At the same time,this straw incorporation management also solved the problem of maize straw absorption in black soil area,and has a win-win effect.So that high dosage(75 t·hm^-2)of deep-buried straw incorporation can be widely used as a safe and effective fertilization method.