Effects Of Long-term Fertilization On Nitric Oxide Emissions And Gross Nitrification Rates In Agricultural Soils In Guanzhong Plain

Nitric oxide?NO?plays a potent role in regional balance of atmospheric oxidants.Its re-deposition contributes to eutrophication and acidification of ecosystems.Agricultural soils are significant NO source,releasing approximately 4²1 Tg N per year.Microbial nitrification and denitrification are the predominant source of soil NO,in which nitrification is the key step of NO production during the nitrogen?N?biogeochemical cycle.Soil temperature,moisture and N substrates are among those environmental factors that strongly affect the NO production and emission.Organic amendment is one of the important measures of conservation agriculture.A clear understanding of the effect of organic amendments on NO emissions is still lacking,mostly due to the complex interaction between climate,soil and field management.The primary objective of this study was to quantify the effects of long-term organic amendments on NO emissions and gross nitrification rates in a summer maize-winter wheat cropping system in Guanzhong Plain.Field experiments were based on the“Chinese National Loess Fertility and Fertilizer Effects Long-term Monitoring Experiment”.The control(CK,0 kg N hm^-2)treatment was unfertilized throughout the years.The fertilized treatments were synthetic fertilizer(NPK,165kg N hm^-2),synthetic fertilizer plus maize stalk[NPKS,?165+40?kg N hm^-2]and synthetic fertilizer plus dairy manure[NPKM,?50+115?kg N hm^-2]during the winter wheat season;they were fertilized with synthetic fertilizer(188 kg N hm^-2)during the summer maize season.We measured NO fluxes by static chambers and related environmental factors for one year?June 2016 to June 2017?.An incubation experiment was conducted with the acetylene inhibition method under laboratory condition to detect soil gross nitrification rates.The results showed that:?1?The NO fluxes ranged from-1.7 to 112.0 g N?hm²·d?^-1 across all treatments within the experimental period.Emission peaks[up to 112.0 g N?hm²·d?^-1 in NPK treatment]were captured following the sowing and/or fertilization in all fertilized treatments.Soil temperature and NO₃^--N content were the significant factors influencing NO fluxes?P<0.01?,with the optional soil temperature within 20³0?.Soil uptake of NO occurred during winter time due to low temperature.Large NO fluxes were always captured between WFPS 0.4 to 0.8.?2?Annual NO emissions and direct emission factors ranged from 0.13 to 0.57 kg N hm^-2and from 0.04%to 0.12%,respectively,across the treatments.The emission factors were lower than the global mean?0.7%?.?3?Annual NO emissions from the NPKS and NPKM treatments were 17.6%lower and68.0%?P<0.05?larger than that from NPK treatment,respectively.Seasonal NO emissions from the NPKS and NPKM treatments were 41.1⁶0.0%?P<0.05?lower than that from NPK treatment during the winter wheat season.Seasonal NO emissions from the NPKS and NPKM treatments were 25.2²92.1%?P<0.05?larger than that from NPK treatment during the summer maize season.?4?The gross nitrification rate[2.2⁷.9 mg N?kg·d?^-1]and NO production rate[0.1⁸5.6?g N?kg·d?^-1]were small from the CK treatment.They ranged from 0.2 to 36.4 mg N?kg·d?^-1and from 0.1 to 198.0?g N?kg·d?^-1,respectively,across those fertilized treatments.The largest gross nitrification rate and NO production rate always occured on the first day of incubation.?5?Long-term fertilization significantly increased the gross nitrification rates and NO productions?P<0.05?.The largest accumulative gross nitrification rate(37.9 to 93.5 mg N kg^-1)and NO production(14.8 to 214.3?g N kg^-1)occurred in NPKS treatment.?6?The NO production rates via nitrification under laboratory condition related significantly to the NO emission peaks during winter wheat season at WFPS 0.4 and 0.6?P<0.05?.The NO production rates via denitrification under laboratory condition related significantly to the NO peaks during summer maize season?P<0.05?.In summary,the following conclusions are drawn:?1?Organic amendments reduced NO emissions during the winter wheat season because decomposition of organic materials restricted the supply of N substrates.Large NO emissions from organic amended treatments during the summer maize season were mostly due to a positive effect of soil organic matter content.?2?Annual NO emissions and direct emission factors were small in the studied agroecosystems,mostly due to high pH and low soil organic matter content.?3?Nitrification was the main NO source during winter wheat season.Denitrification was the NO main source during summer maize season.