Effect of urea fertilizer and nitrification inhibitor on soil nitrogen and microbes
Nitrification inhibitors (NI) are increasingly co-applied with nitrogen fertilizers as part of sustainable agricultural practice. Several studies in temperate regions have documented the effectiveness of NI in retaining soil NH4+, minimizing N loss, and increasing crop yields. However, less is known about the effects of NI in Mediterranean regions, where agricultural production is challenging and requires intensive irrigation and fertilization. We investigated the short-term impact of Nitrapyrin (NI; 2-chloro-6-(trichloromethyl)pyridine) in a two-factor mesocosm experiment,using a typical Mediterranean soil, where NI was 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). NI co-applied with urea fertilizers resulted in higher availability of soil NH4+ and a concurrent increase in NH3 volatilization. Net cumulative soil NH4+ was 1.5 to 3.3-fold greater when NI was applied. Concurrently, net cumulative NO3- and NO2- were reduced by 10-60 %; this was found for all the tested fertilizer types except MU fertilizer, where the net cumulative soil NO3- and NO2- doubled. N2O emissions  were reduced by 40 % with UI, 50 % with NI, and 66 % with NI+UI. Interestingly, 28 d after the application of NI, there was a decrease in bacterial abundance in all fertilized treatments. NI application dramatically reduced the abundance of ammonia-oxidizing bacteria and there were fewer bacteria associated with denitrification genes. The application of NI in the fertilized treatments resulted in distinctly different soil microbial communities compared to those not receiving NI . At the end of the incubation, total N did not differ among all treatments. NI were effective in reducing N2O emissions in this calcareous soil, however, NH3 emissions were remarkably enhanced. These findings have important implications for the large-scale adoption of inhibitor technologies in Mediterranean agroecosystems and the reduction of greenhouse gas emissions.