<p>Nitrogen (N) fertilization is an essential agronomic practice, which increases crop yields and improves soil fertility. Globally, more than 110 x 10<sup>9</sup> kg of chemical N fertilizers are applied each year with urea-N being one of the most affordable options. Upon urea hydrolysis, any portion not assimilated by crops is either volatilized as NH<sub>3</sub> or microbially nitrified (i.e., NH<sub>4</sub><sup>+</sup> oxidized) to leachable NO<sub>3</sub><sup>-</sup> and NO<sub>2</sub><sup>-</sup>. Nitrification inhibitors (NI) are increasingly co-applied as a sustainable agricultural practice and block the process of nitrification, resulting in a temporal increase of NH<sub>4</sub><sup>+</sup> in the soils. Several studies have documented the effectiveness of NIs in retaining soil NH<sub>4</sub><sup>+</sup> and increasing crop yields, but less is known about the effects of NIs on the fate of urea&#8211;N and the overall impact of NIs on the soil microbial community.</p><p>In a 60 day soil mesocosm experiment, we investigated the effects of Nitrapyrin (NI; 2-chloro-6-(trichloromethyl)pyridine) co-applied with a selection of urea-based fertilizers: urea (U); U with urease inhibitors (U+UI); methylene-urea (MU); and zeolite-coated urea (ZU), on a typical Mediterranean soil under ambient summer conditions. We showed that NI applied with urea fertilizers resulted in a slower decay of extractable NH<sub>4</sub><sup>+</sup> with a concurrent increase in NH<sub>3</sub> volatilization. Integrated measures of soil NH<sub>4</sub><sup>+</sup> were 1.5 to 3.3-fold greater when NI was applied. At the same time, there was a 10 to 60% reduction in integrated measures of NO<sub>3</sub><sup>-</sup> and NO<sub>2</sub><sup>-</sup> when NI was applied with the tested fertilizer types, except MU fertilizer where the integrated measures of NO<sub>3</sub><sup>-</sup> and NO<sub>2</sub><sup>-</sup> doubled. Upon urea hydrolysis, the released NH<sub>4</sub><sup>+</sup> was transformed to NO<sub>3</sub><sup>-</sup> and NO<sub>2</sub><sup>-</sup>, which subsequently decreased in concentration following a typical nitrification - denitrification pathway in the absence of plants. Soil N<sub>2</sub>O emissions from urea fertilizers were reduced by 40% with UI, 50% with NI, and 66% with NI + UI.</p><p>Interestingly, 15 days after the application of NI, there was a decrease in bacterial abundance (eub genes; qPCR) in all fertilized treatments. NI dramatically reduced the abundance of ammonia-oxidizing microbes (amoA genes) and there were fewer bacteria associated with denitrification genes (nirK, nirS, nosZ) when NI was applied.&#160;</p><p>At the end of the experiment, there was no significant difference in total N among all fertilized soils. Total N was in excess when compared to the control, and it was a considerable N pool potentially immobilized in microbial biomass in the absence of crops.</p><p>In conclusion, the use of NI doubled NH<sub>4</sub><sup>+</sup> retention in the soil and decreased soil N<sub>2</sub>O emission by 50%, through negatively affecting ammonia oxidizing and denitrifying microbes and subsequently reducing soil available NO<sub>3</sub><sup>-</sup> and NO<sub>2</sub><sup>-</sup>. The application of NIs should be carefully planned and synchronized (timing) with crop growth to reduce subsequent N transformations and N loss to the environment.</p><p><strong>Keywords</strong><strong>:</strong> urea, zeolite, methylene-urea, nitrification inhibitor, nitrapyrin, calcareous soil, soil nitrogen</p>
Use of nitrification inhibitor DMPP to improve nitrogen recovery in irrigated wheat on a calcareous soil
