Influence Of Nitrate Concentrations On The Production Of N₂, N₂O, NO, And CO₂ By Soil Cores During Anaerobic Incubation

Since Industry Revolution, more and more reactive N has been introduced into the biosphere by humans, due primarily to N fertilizer production and fossil fuel combustion used to support the food and energy demands of a rapidly expanding human population. This increased reactive N bringed a series of environmental problems. However, the last step of denitrification (the microbial production of N2) which is the key process of nitrogen transformation to close the global N cycle can convert reactive forms of N back into N2. Apart from reduction of reactive N in the atmosphere, denitrification also results in less plant-available soil N and is a major source of N2O. Consquently, the research on denitrification is very essential.In this study, a system of gas-flow-soil-core technique with helium-environment incubation was introduced to simultaneously and directly measure dynamic emission rates of dinitrogen (N2), nitrous oxide (N2O), nitric oxide (NO) and carbon dioxide (CO2), which are the gaseous products of microbial denitrification. As a new technique determining microbial denitrification rate, the gas-flow-soil-core system worked under the theory that intact or mixed soil cores are enclosed in a gas tight vessel which is then flushed with an artificial gas mixture (He or He-02) to remove N2 stored in aggregates and pores of the soil before measurements begin. Our experiment included four levels of initial soil nitrate (NO3-) content, which were around 10,50,100 and 250 mg N kg"'d.s., respectively (hereinafter referred to as ION,50N,100N and 250N). For either nitrate level, sufficient dissolved organic carbon (DOC) were initially supplied (glucose was added to establish an initial DOC content of around 300 mg C kg-1 d.s.). The incubated fresh soil was a silty clay loam of the northern China. It was sampled from a typical cropland rotationally cultivated with the double cropping system of summer maize and winter wheat and the single cropping system of cotton. Soil parameters (including NH4+, NO3-, soil microbial bacteria carbon (SMBC) and soil microbial bacteria nitrogen (SMBN)) were determined by routine method. The objective of this study was to test the reliability of the measurements by this system and to explore the effect of soil NO3-content on microbial denitrification.The major conclusions of this study are as follows: 1. Soil NO3- content had a pronounced effect on denitrification rate, dynamics and cumulative emissions of N2, N2O, NO and CO2, and ratios of denitrification products. At low rate of NO3- input, denitrification rate increased with increasing soil NO3-content, while at high rate of NO3- input, denitrification rate decreased with increasing soil NO3- content; The timings of N2O and NO peaks were the same in all treatments. The N2 peak was delayed and followed N2O and NO peaks, and the delay of N2 peak increased with increasing soil NO3- content; The dynamic of CO2 emission during the whole incubation period presented " two peaks" style in 10N and 100N treatments, and "one peak" style in 50N and 250N treatments, respectively; The cumulative emissions of N2O, N2 and CO2 increased with increasing soil NO3- content; NO/N2O molar ratios in 10N,50N and 100N treatments were greater than 1 (1.09,1.58 and 1.36, respectively), while NO/N2O molar ratio in 250N treatment was smaller than 1 (0.26) because of the inhibition of the reduction of N2O by strong concentration of NO3-. N2O/N2 molar ratio ranged from 0.20 to 0.65, which was weakly positively correlated with soil NO3- content;2. The ranking of amounts of individual denitrification gases emitted during the whole period in all treatments is N2>N2O>NO, with mass fractions varied between 51-77%, 15～37%, and 5～22%, respectively;3. The measurements of individual denitrification gases recovered 77～89% of disappeared nitrate during incubation. The direct dynamic detection of individual denitrification gases and the measurements of ammonium, nitrate and microbial biomass nitrogen at the beginning and end of incubation gave nitrogen mass balance rates of 92～103%, which indicates that nitrogen balance before and after the incubation was realized;4. A significant linear relationship between the total amount of all nitrogen gases (N2+N2O+NO) and CO2 emitted during denitrification period was observed (R2=0.995, p<0.001);5. The good nitrogen recovery rate, the realization of nitrogen balance and the logical dynamics of N2, N2O, NO and CO2 emissions during the whole incubation period suggested that the tested system could precisely determine the dynamical rate of microbial denitrfication and its ratios of products.