Isotopic assessment of soil NO emission from a sub-tropical agricultural soil under varying N-inputs.

Nitrogen fertilizers result in high crop productivity but also enhance the emission of NO, an environmentally harmful greenhouse gas. Only approximately a half of the applied nitrogen is utilized by crops and the rest is either vaporized, leached, or lost as NO, NO and N via soil microbial activity. Thus, improving the nitrogen use efficiency of cropping systems has become a global concern. Factors such as types and rates of fertilizer application, soil texture, moisture level, pH, and microbial activity/diversity play important roles in NO production. Here, we report the results of NO production from a set of chamber experiments on an acidic sandy-loam agricultural soil under varying levels of an inorganic N-fertilizer, urea. Stable isotope technique was employed to determine the effect of increasing N-fertilizer levels on NO emissions and identify the microbial processes involved in fertilizer N-transformation that give rise to NO. We monitored the isotopic changes in both substrate (ammonium and nitrate) and the product NO during the entire course of the incubation experiments. Peak NO emissions of 122 ± 98 μg NO-N m h, 338 ± 49 μg NO-N m h and 739 ± 296 μg NO-N m h were observed for urea application rate of 40, 80, and 120 μg N g. The duration of emissions also increased with urea levels. The concentration and isotopic compositions of the substrates and product showed time-bound variation. Combining the observations of isotopic effects in δN, δO, and N site preference, we inferred co-occurrence of several microbial NO production pathways with nitrification and/or fungal denitrification as the dominant processes responsible for NO emissions. Besides this, dominant signatures of bacterial denitrification were observed in a second NO emission pulse in intermediate urea-N levels. Signature of NO consumption by reduction could be traced during declining emissions in treatment with high urea level.