Over time when metals were slowly introduced by atmospheric deposition, versus their response to sudden increases in metal concentrations when heavy metals were spiked into soils at high concentrations. Another challenge in determining the effects of metals on nitrogen transformations had been the difficulty in relating Life Science Reagents changes in soil enzyme activities to changes in the abundance of molecular markers for specific microbial taxa and genes that could be involved in nitrogen transformations. This is especially true for rice paddy soils where wet/dry cycles drive significant changes in redox that simultaneously affected metal bioavailability, microbial community structures, and rates for biological transformations of nitrogen. Nitrous oxide had been widely accepted as the most radiative greenhouse gas increasing at a year rate of approximate 0.26% per year such that this greenhouse gas had reached with a concentration of 319 1029 mol mol21 in global air by IPCC. However, it had been known also as a product resulted from uncompleted denitrification, in which reduction of nitrite was not completed to form N2 in soils. Agriculture accounted for about 60% of the global total anthropogenic N2O emission, of which rice paddies had been considered a major contributor. Total N2O emission from China’s rice paddies was estimated at 29.0 Gg N2O per year, accounting for 7–11% of annual overall greenhouse gas emission from mainland China croplands. Ammonia oxidization to nitrite had been well known as the initial and rate limiting step in nitrification, being mediated by microorganisms which carry genes encoding for the enzymes AOB and/or AOA. In contrast to the nitrifier which could be comprised by a few functional taxa, denitrifying bacteria responsible for denitrification could be broadly distributed among many different taxa using nitrate as an alternate electron acceptor for respiration. China had been the largest rice producing country in the world with approximately 20% of global rice production. In the last decade, metal pollution had been widely reported to occur in extensive rice production areas that included the lower Yangtze River delta, the Pearl River delta and river valleys in the Jiangxi and Guangdong provinces. Much attention had given to the potential health risk through food chain transfer of heavy metals and adverse effects on ecosystem health. Recently, there had been observed in metal polluted rice paddies a decline in microbial biomass and fungal to bacterial ratio with an increase in the metabolic quotient that could lead to changes in C cycling. Many laboratory studies had shown that heavy metal contamination in soil could affect the rates of microbial-mediated biogeochemical processes. Soil nitrification rates had been known to be suppressed under metal pollution both in spiked soil samples in short term studies, and in polluted fields where metals typically accumulated at a slower rate. In a study on surface wetland sediments, total denitrification activity was significantly decreased in multiple metal spiked wetland samples.