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%.

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.

SUMMARYFollowing the surface application of granulated urea to grassland, high ammonia (NH 3 ) losses of up to 30% have been reported. The addition of a urease inhibitor (UI) to urea granules could be a way to abate these losses. Field experiments were conducted at two intensive grassland sites in 2007 and 2008 to evaluate the potential of the new UI N-(2-nitrophenyl) phosphoric triamide (2-NPT; concentrations of 0·75, 1·0 and 1·5 g N/kg) to reduce NH 3 emissions resulting from the application of granulated urea. Ammonia losses were continuously measured on plots fertilized with urea, urea + 2-NPT, calcium ammonium nitrate and a control (0N). The measurements were made with a dynamic chamber system. All measurement periods were started after a period of precipitation with a following rainless period being forecasted. Results over measurement periods of 10 days following fertilization are presented. Ammonia losses following the application of granulated urea varied between 4·6 and 11·8 kg N/ha, corresponding to 4·2 up to 14·0% of the applied nitrogen. The addition of 2-NPT to urea granules at three concentrations significantly reduced NH 3 losses by 69-100%. Comparable losses of NH 3 were observed for urea containing the UI 2-NPT as well as calcium ammonium nitrate, and were not significantly different from the control treatment. No relationships between losses, meteorological factors and soil moisture were observed. The addition of the UI 2-NPT to urea granules applied on grassland effectively reduced NH 3 losses.

Urea (U) is the most important nitrogen (N) fertilizer in agriculture worldwide, and as N fertilizer can result in large gaseous losses of NH3 and N2O. Thus, urease inhibitors (UIs) and nitrification inhibitors (NIs) have been coupled with U fertilizers to mitigate NH3 and N2O emissions. However, it is still unclear whether adding NIs and/or UIs to U stimulates other pollutants, while reducing one pollutant. Furthermore, part of the NH3 deposition to earth is converted to N2O, leading to indirect N2O emission. To estimate direct and indirect effect of UIs and NIs on the N2O-N and NH3-N losses from U; therefore, we analyzed multi-year field experiments from the same site during 2004 to 2005 and 2011 to 2013. The field experiments with U fertilization with or without UI (IPAT, N-isopropoxycarbonyl phosphoric acid triamide) and NI (DCD/TZ, Dicyandiamide/1H-1, 2, 4-Triazol) in winter wheat and with calcium ammonium nitrate (CAN) were conducted in southern Germany. Fluxes of NH3 or N2O emissions were determined following each split N fertilization in separate experiments on the same site. Our results showed that U with NIs considerably reduced N2O emissions, and adding UIs decreased NH3 emissions. However, the effect on N2O emissions exerted by (U + UIs) or (U + UIs + NIs) was inconsistent. In contrast to the treatment of (U + UIs + NIs), the addition of NIs alone to U stimulated NH3 emission compared to treatment with U. When 1% indirect N2O emission from NH3 (IPCC emission factor (EF4)) was considered to estimate the indirect N2O emission, total N2O emissions from (U + NIs) were approximately 29% compared to that from U alone and 36% compared to that from (U + UI), indicating that indirect N2O emission from NH3 induced by NIs may be negligible.

Urea is not only the most important mineral nitrogen (N) fertilizer worldwide, but it is also the mineral N fertilizer with one of the highest potential for ammonia (NH3) losses. The European emission inventory guidebook estimates an average loss of 16% of the applied urea as NH3. For mitigating NH3 losses from urea, the best option would be to immediately incorporate the fertilizer. However, the addition of a urease inhibitor (UI) represents a potent alternative. In a multi‐year experiment from 2002 to 2005, 17 measurement periods were conducted in winter wheat (Triticum aestivum L.) and on bare soil in Southern Germany to evaluate the extent of NH3 losses following the application of urea and to determine the mitigating effect of urease inhibitors. In all measurement periods, 80 kg N ha−1 as urea without or with the UI N‐(n‐butyl) thiophosphoric triamide (nBTPT) 0.3% w/w or N‐(isopropoxycarbonyl) phosphoric acid triamide (IPAT) 0.4% w/w were applied. For 6–24 d following fertilizer application, NH3 emissions were continuously measured using a dynamic chamber system. Generally, low NH3 emissions were detected from urea, varying between 0.1 and 2.7% of the N applied to winter wheat and between 2.6 and 16.3% of the N applied on bare soil. Addition of the UIs IPAT and nBTPT had a significant effect on the course of NH3 emissions and reduced losses on average by 32% and 24%, respectively.