The Studys On Mechanism Of N₂O Emissions And Isotopic Signature Under Different Soil Water Content And PH

A microcosm experiment was performed to investigate the effects of different water-filled pore space and pH on N₂O emissions and isotopic signatures(?¹⁵N^bulk,?¹⁸O and nitrogen isotopomer site preference of N₂O)of soil at Shunyi district,Beijing.The objective of this research is to understand the contribution of microbial processes to N₂O production and its mechanisms under different water contents to provide a theoretical basis for reducing agricultural N₂O emissions.?1?The study combined stable isotope technique and gas inhibitor method to analyze N₂O flux and its isotope signatures that emitted from soil.The experiment was set up three different water content levels,including 67%,80%and 95%WFPS,and with three inhibitor levels,?without C₂H₂,with 0.1%?V/V?C₂H₂ and with 10%?V/V?C₂H₂?.The two end-members mixing model was applied to quantify the respective contributions of microbial processes to N₂O production.To further validate whether there are fungal denitrification in soil,we conducted other treatments that adding streptomycin and cycloheximide.According to the incubation of the soil,compared with 67%WFPS,the cumulative?N₂O+N₂?emissions of 95%and 80%WFPS increased 13.53 and 3.24 times,respectively.The cumulative emissions of?N₂O+N₂?in 95%,80%and 67%WFPS were 18.05%,5.27%,and 1.24%of initial mineral nitrogen,respectively,over the entire incubation period.The weighted average?¹⁵N^bulk values varied from-42.93‰to-4.07‰,and the higher level of soil water content showed significantly higher N₂O emissions.10%?V/V?C₂H₂ would inhibit the reduction of N₂O to N₂.The?¹⁸O values with 10%?V/V?C₂H₂ were significantly smaller than that of with 0.1%?V/V?C₂H₂ in three water content levels.And the ratio of N₂O/?N₂O+N₂?reduced with the increase of soil moisture.There were larger SP values in95%WFPS treatment with 10%?V/V?C₂H₂ in the first seven incubation days,ranged from 7.61‰to21.11‰.Compared with 0.1%?V/V?C₂H₂,the weighted average SP values of N₂O under 95%,80%and 67%WFPS treatments with 10%?V/V?C₂H₂ produced from soil reduced by 0.10,0.33 and 0.06times respectively.In 67%WFPS treatment,denitrification was the dominant microbial process,then the contribution of nitrification to N₂O production became greater.The weighted average SP value was10.26‰in 80%WFPS treatment,indicating denitrification?40.90%-74.04%?was the major N₂O production process.Adding fungi and bacteria inhibitors to the soil in 95%and 67%WFPS treatments,the preliminary studies revealed the contribution of bacterial and fungi to soil N₂O emissions were 61%and 46%in 67%WFPS treatment,respectively.And the contribution of bacterial and fungi to soil N₂O emissions were 24%and 81%in 95%WFPS treatment,respectively.?2?The experiment was set up three different pH levels,including pH 6.2,pH 7.1 and pH 8.7,and two inhibitor levels?without C₂H₂ and with 10%?V/V?C₂H₂?.The study combined stable isotope technique and gas inhibitor method to analyze N₂O flux and its isotope signatures that emitted from soil after adding acids,and to estimate the production mechanism according to these.The results showed that N₂O emissions deceased with incubation time in all treatments;N₂O emissions increased with decreasing pH.N₂O emissions were significantly increased after carbon source added.There was no significant change of soil N₂O emission in the whole incubation time.Compared with no carbon treatment,the weighted average N₂O emissions of pH 6.2,pH 7.1 and pH 8.7 treatments increased 2.59,1.86 and 2.63 times with carbon source addition,respectively.?N₂O+N₂?emissions increased significantly after adding acid;In pH 6.2,pH 7.1 and pH 8.7 treatments,the weighted average N₂ emissions were 19.8%,12.1%and 14.6%of total emissions of?N₂O+N₂?,respectively.After adding glucose,the total?N₂O+N₂?emissions increased significantly.In pH 6.2,pH 7.1 and pH 8.7 treatments,the weighted average N₂ emissions were 33.4%,39.5%and 52.8%of total emissions of?N₂O+N₂?,respectively.When it comes to no glucose treatment,the ratios of N₂/?N₂O+N₂?have increased obviously.All treatments without glucose,the contents of NO₂^-were small;after adding glucose,the contents of NO₂^-increased significantly.?¹⁵N^bulk values increased with decreasing soil pH.Compared with no glucose treatments,?¹⁵N^bulk values in all treatments increased significantly with glucose addition.With no C₂H₂ treatments,?¹⁸O values increased with increasing soil pH,as well had the negative correlation with N₂O?r=-0.78,P<0.0001?.Compared with no glucose treatment,?¹⁸O values increased significantly in pH 6.2,pH 7.1 and pH 8.7 treatments with glucose.All treatments without glucose had higher SP values in the first three days of incubation time.