Isotopic assessment of soil N2O emission from a sub-tropical agricultural soil under varying N-inputs

Nitrogen fertilizers result in high crop productivity but also enhance the emission of N2O, 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, N2O and N-2 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 N2O production. Here, we report the results of N2O 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 N2O emissions and identify the microbial processes involved in fertilizer N-transformation that give rise to N2O. We monitored the isotopic changes in both substrate (ammonium and nitrate) and the product N2O during the entire course of the incubation experiments. Peak N2O emissions of 122 +/- 98 mu g N2O-N m(-2) h(-1), 338 +/- 49 mu g N2O-N m(-2 )h(-1) and 739 +/- 296 mu g N2O-N m(-2) h(-1) were observed for urea application rate of 40, 80, and 120 mu g N g(-1). 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 delta N-15, delta O-15, and N-15 site preference, we inferred co-occurrence of several microbial N2O production pathways with nitrification and/or fungal denitrification as the dominant processes responsible for N2O emissions. Besides this, dominant signatures of bacterial denitrification were observed in a second N2O emission pulse in intermediate urea-N levels. Signature of N2O consumption by reduction could he traced during declining emissions in treatment with high urea level.

Automated summary

Nitrogen fertilizers increase crop productivity but also contribute to the emission of the harmful greenhouse gas N2O. Improving nitrogen use efficiency in cropping systems is a global concern.