Effects Of Conservation Tillage Practices On Soil Nitrification And Rice N Uptake Of Paddy Field

Unreasonable agricultural practices,such as intensive conventional tillage and straw burning have resulted in many serious problems,including the degradation of soil quality,environmental pollution,damage to the agricultural sustainability and low crop nitrogen use efficiency,where have prompted a search for alternative management practices.No tillage and straw returning,which represent two major solutions of conservation agriculture or tillage,are now widely adopted for their potential to improve soil quality,reduce nitrogen fertilizer input and improve nitrogen use efficiency（NUE）.However,it remains many uncertainties in which way the conservation tillages affect soil nitrification and crop nitrogen use efficiency.Therefore,more efforts should be made to understand the relationship between soil nitrification and nitrogen use efficiency under different conservation practices.This research is based on a field experiment estabilished in 2011,four treatments in split plot design were set,including conventional tillage without straw returning（CTNS）,conventional tillage with straw returning（CTS）,no tillage without straw returning（NTNS）and no tillage with straw returning（NTS）.The responses of soil chemical and physical properties,microbial biomass,nitrification microbia,N₂O emission,soil nitrification potential,root nitrogen metabolism enzymes（NR and GS）activity,crop production and nitrogen accumulation to different conservation tillages during rice growth stage were analyzed.This research is based on the results from 2016 to 2017.The main results are as follows:（1）Tillage and straw returning did no affect the paddy soil p H while significant effect on the soil ammonium and nitrate concentration during rice growth stage were found.Compared with conventional tillage,NH₄⁺-N and NO₃^--N content under no tillage were increased by 25.2%³7.4%,13.0%²2.0%,12.2%²7.1% and 14.5%³0.6%,15.8%²9.8%,9.9%⁵3.9% in rice booting stage,full heading stage and maturation stage respectively.Straw returning significantly enhanced the soil NH₄⁺-N content by 10.1%²6.7%,17.0%³1.3%,18.7%³3.8%,and NO₃^--N content 15.5%³2.5%,25.8%³9.7%,35.4%⁴8.2% in rice booting stage,full heading stage and maturation stage respectively.There has no tillage effect on the soil microbial biomass carbon while straw returning treatments strongly stimulated soil microbial biomass carbon.Both tillage and straw returning did not affect soil microbial biomass nitrogen during rice growth.（2）In this study,AOA and AOB abundance were significantly influenced by tillage and straw returning.No tillage treatments have more AOA and AOB than those in conventional tillage by 4%¹4% and 9%¹1%,respectively.Compared with no straw returning treatments,straw returning have strongly increased AOA and AOB by 14%⁴5% and 21%⁹1%.Significant higher soil nitrification potential were found in no tillage and straw returning treatments.The soil nitrification potential in conventional tillage treatments were improved by 41%⁷8% and 31%⁵6% higher in straw returning treatments.（3）Paddy soil N₂O cumulative emission was strongly affected by tillage practices and straw returning.The soil N₂O cumulative emission under no tillage treatments were much higher than those in conventional tillage（12.5%¹8.2% increase for 2016 and 21.1%³8.6% increase for 2017）.Straw-retained plots produced much higher N₂O than without straw treatment（38.5%⁴5.5% for 2016 and 13.1%²9.5% for 2017）.Pearson correlation analysis and multiple regression analysis indicated that soil N₂O emission was regulated by soil microbial biomass carbon,NH₄⁺ content AOA and AOB abundance.（4）Root nitrate reductase activity response to tillage was much stronger in conventional tillage that in no tillage.Straw management significantly affected the NR activity in tillering stage and maturation stage.No obvious effect was found in GS activity during rice growth between tillage treatments.Straw returning treatments had much high GS activity.Tillage treatments did not influence the nitrogen accumulation and grain yield while greater nitrogen accumulation and grain yield were seen in straw returning treatments,which respectively increased by 14.9%³8.3% and 5.9%¹9.1%.There is a significant interaction between tillage methods and straw returning on nitrogen accumulation.Pearson correlation analysis and multiple regression analysis showed that the NR activity and GS activity of different straw treatment were significantly correlated with gain yield and total nitrogen accumulation;soil inorganic nitrogen content,AOA abundance,soil MBC content,NR and GS enzyme activity were closely related to the accumulation of nitrogen in rice.It is suggested that soil nitrification has a close relationship with rice nitrogen accumulation.Therefore,our research shows that the soil nitrification microbe AOA under the treatment of straw returning mainly regulates soil nitrification and affects the supply of soil inorganic nitrogen and further shift the root growth.At the same time,higher NR and GS enzyme activity in the root system are favorable for the uptake the soil inorganic nitrogen into root and then the transport to the aboveground of rice,eventually improving rice N use efficiency.