Study On The Direct N₂ Method And Its Application In Nitrification And Denitrification From Rice Fields

Nitrification and denitrification are important pathways of nitrogen loss and nitrous oxide emission from paddy field;Dry/wet cycle is a key process which affects nitrification and denitrification in paddy soil;During the wet/dry cycle,the redox potential of paddy field fluctuated dramatically,which leads to the enrichment of iron oxides in the rice roots?i.e.,Fe-plaque?.Amorphous iron in Fe-plaque can act as electron donor or acceptor to participate in the nitrification and denitrification processes in paddy soils.The aims of this work were to illustrate the effects of dry/wet cycle on gaseous nitrogen loss and products composition during nitrification and denitrification in a typical paddy soil,using the direct N₂ method.The dynamics of key parameters,e.g.nitrate,ammonium and dissolved organic carbon,were monitored to investigate the involved environmental and microbial driving mechanisms during the dry/wet cycle.In addition,the regulatory mechanisms on nitrification and denitrification by Fe plaque are also investigated.The results show:?1?The dinitrogen flux from soils can be directly and accurately determined by the cylinder-in-cylinder setup,which prevented the N₂leakage from atmosphere.In this study,the N₂ concentration in the headspace of the outer cylinder increased as high as 1400 ppmv with the increasing incubation time.While the N₂ concentration in the headspace of the inner cylinder containing autoclaved soil cores stayed consistent and did not exceed 400 ppmv.The cylinder-in-cylinder setup can minimize the leakage rate atmospheric N₂ into the incubation vessels and increase the tightness of the whole system.The detection limit of the new method in this study was 5.7?g m^-2 h^-1,which was low enough to detect the N₂ flux released from soil.Moreover,it took 2.5 hours to replace the gas in the soil via new cylinder-in-cylinder setup.It is practicable,time-saving,labor-saving and low cost.Therefore,its application is beneficial.?2?N₂ was the dominant product of nitrification and denitrification in paddy soil.The N₂ emission rates ranged 0.32⁵.13?mol L^-1 h^-1 and0.15¹9.90?mol L^-1 h^-1 in paddy soil with rice present and absent,respectively.The N₂O emission rates ranged 0.13⁰.25?mol L^-1 h^-1 and0.02¹.68?mol L^-11 h^-1 in paddy soil with rice present and absent,respectively.The N₂O/?N₂O+N₂?ratio ranged 0.12%¹3%and 0.03%³7%in paddy soil with rice present and absent,respectively.These results indicated that N₂ was the main product and the typical denitrifying bacteria,Pseudomonas,Sulfuritalea and Thiobacillus,was enriched in the flooding period during the dry/wet cycle.These results showed that dry/wet cycle was the hot-moment for nitrogen loss,and the N₂O/?N₂O+N₂?emission ratio from paddy soil increased from wetting to drying,leading to more nitrogen loss by nitrous oxide emission in the soil.?3?Fe-plaque enhanced the nitrogen loss during the dry/wet cycle via Fe???oxidation-coupled denitrification.With Fe plaque present,the N₂O production doubled and the abundances of Fe-redox bacteria and denitrifying functional genes were enriched on the root-soil interface,like Thermomonas?Pseudomonas and Thiobacillus.The promoting effect of Fe plaque on N₂O emission reappeared under hydroponic conditions when NO₃^-was the sole N source,but not when NH₄⁺was the sole N source.In addition,scavenging the Fe???on the Fe plaque eliminated the promoting effects of Fe plaque on N₂O emission while Fe???addition with the Fe plaque-free roots recovered the N₂O emission.These results demonstrate that Fe plaque promotes soil N₂O emission and N loss predominately via Fe???oxidation-coupled denitrification.