Knowledge on short‐term and long‐term availability of nitrogen (N) after application of organic fertilizers (e.g., farmyard manure, slurry, sewage sludge, composts) provides an important basis to optimize fertilizer use with benefits for the farmer and the environment. Nitrogen from many organic fertilizers often shows little effect on crop growth in the year of application, because of the slow‐release characteristics of organically bound N. Furthermore, N immobilization after application can occur, leading to an enrichment of the soil N pool. However, this process finally increases the long‐term efficiency of organic fertilizers. Short‐term N release from organic fertilizers, measured as mineral‐fertilizer equivalents (MFE), varies greatly from 0% (some composts) to nearly 100% (urine). The most important indicators to be used for predicting the short‐term availability of N are total and NH$ _4^+ $‐N contents, C : N ratio (especially of the decomposable organic fraction), and stability of the organic substances. Processing steps before organic fertilizers are applied in the field particularly can influence N availability. Composting reduces mineral‐N content and increases the stability of the organic matter, whereas anaerobic fermentation increases NH$ _4^+ $‐N content as well as the stability of organic matter, but decreases the C : N ratio remarkably, resulting in a product with a high content of directly available N. Nevertheless, long‐term effects of organic fertilizers rather slowly releasing N have to be considered to enable optimization of fertilizer use. After long‐term application of organic fertilizers, the overall N‐use efficiency is adequate to a MFE in the range of 40%–70%.

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At a low level of N supply, the proportion of N derived from atmosphere is expected to be close to 100% if the Legume content in legume-grass mixtures is a key parameter for legumes are grown in mixtures with nonlegumes. Then, the quantification of N 2 fixation, forage, and diet quality. This study

Urea fertilizer-induced N 2 O emissions from soils might be reduced by the addition of urease and nitrification inhibitors. Here, we investigated the effect of urea granule (2-3 mm) added with a new urease inhibitor, a nitrification inhibitor, and with a combined urease inhibitor and nitrification inhibitor on N 2 O emissions. For comparison, the urea granules supplied with or without inhibitors were also used to prepare corresponding supergranules. The pot experiments without vegetation were conducted with a loess soil at (20 ± 2)°C and 67% water-filled pore space. Urea was added at a dose of 86 kg N ha -1 by surface application, by soil mixing of prills (<1 mm) and granules, and by point-placement of supergranules (10 mm) at 5 cm soil depth. A second experiment was conducted with spring wheat grown for 70 d in a greenhouse. The second experiment included the application of urea prills and granules mixed with soil, the point-placement of supergranules and the addition of the urease inhibitor, and the combined urease plus nitrification inhibitors at 88 kg N ha -1 . In both experiments, maximum emissions of N 2 O appeared within 2 weeks after fertilization. In the pot experiments, N 2 O emissions after surface application of urea were less (0.45% to 0.48% of total fertilization) than from the application followed by mixing of the soil (0.54% to 1.14%). The N 2 O emissions from the point-placed-supergranule treatment amounted to 0.64% of total fertilization. In the pot experiment, the addition of the combined urease plus nitrification inhibitors, nitrification inhibitor, and urease inhibitor reduced N 2 O emissions by 79% to 87%, 81% to 83%, and 15% to 46%, respectively, at any size of urea application. Also, the N 2 O emissions from the surface application of the urease-inhibitor treatment exceeded those of the granules mixed with soil and the point-placed-supergranule treatments receiving no inhibitors by 32% to 40%. In the wheat growth experiment, the N 2 O losses were generally smaller, ranging from 0.16% to 0.27% of the total fertilization, than in the pot experiment, and the application of the urease inhibitor and the combined urease plus nitrification inhibitors decreased N 2 O emissions by 23% to 59%. The point-placed urea supergranule without inhibitors delayed N 2 O emissions up to 7 weeks but resulted in slightly higher emissions than application of the urease inhibitor and the urease plus nitrification inhibitors under cropped conditions. Our results imply that the application of urea fertilizer added with the combined urease and nitrification inhibitors can substantially reduce N 2 O emissions.

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Globally identifying mitigation options for the emission of reactive N gases from agricultural soils is a research priority. We investigated the effect of urea size and placement depth on sources and emissions of N gases from a Cambisol cropped to spring wheat (Triticum aestivum L.). In Exp. 1, wheat received either prilled urea (PU) mixed within the soil, urea super granule (USG; diam. 10.1 mm) point‐placed at a soil‐depth of 7.5 cm, or no N fertilizer. In Exp. 2, wheat received either USG (diam. 10.2 mm) point‐placed at 2.5‐, 5.0‐, and 7.5‐cm soil depths, or no N fertilizer. In both experiments, maximum peaks for nitrous oxide (N2O) fluxes and nitrification were delayed by 2 to 3 wk in the USG compared with the PU treatment. The added 15N‐urea lost as 15N‐N2O over 116 d was only 0.01% for both PU and USG treatments in Exp. 1. This loss for USGs was higher in Exp. 2 (0.02–0.15%) measured over 70 d, mainly related to higher moisture‐induced denitrification. Temporal N2O fluxes were significantly related to changes in soil NO3−–N, water‐filled pore space and NH4+–N (R2 = 0.50, P < 0.05). However, the previous predictive model of Khalil et al. (2006) could best estimate its cumulative fluxes over time. The relative losses of ammonia (0.07–1.17%) and nitrogen oxides (0.19–1.54%) measured in Exp. 2 over 43 d decreased with increasing depths of USG placement. The USG point‐placed at the 5.0‐ and 7.5‐cm depths decreased the pooled gaseous N losses by 35 and 77%, respectively, over the shallower placement. The 15N results imply that soil N could be the major source of N2O emissions (79–97%). Field studies are suggested to validate our findings that the deeper placement of USG can decrease N emissions under arable cropping.

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