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Ammonia (NH 3) volatilization is the major source of nitrogen (N) loss resulting from the application of synthetic and organic N fertilizers to croplands. It is well known that in Mediterranean cropping systems, there is a relationship between the intrinsic characteristics of the climate and nitrous oxide (N 2 O) emissions, but whether the same relation exists for NH 3 emissions remains uncertain. Here, we estimated the impact of edaphoclimatic conditions (including meteorological conditions after N fertilization), crop management factors, and the measurement technique on both the cumulative emissions and the NH 3 emission factor (EF) in Mediterranean climate zones, drawing on a database of 234 field treatments. We used a machine learning method, random forest (RF), to predict volatilization and ranked variables based on their importance in the prediction. Random forest had a good predictive power for the NH 3 EF and cumulative emissions, with an R2 of 0.69 and 0.76, respectively. Nitrogen fertilization rate (N rate) was the top-ranked predictor variable, increasing NH 3 emissions substantially when N rate was higher than 170 kg N ha−1. Soil pH was the most important edaphoclimatic variable, showing greater emissions (36.7 kg NH 3 ha−1, EF = 19.3%) when pH was above 8.2. Crop type, fertilizer type, and N application method also affected NH 3 emission patterns, while water management, mean precipitation, and soil texture were ranked low by the model. Our results show that intrinsic Mediterranean characteristics had only an indirect effect on NH 3 emissions. For instance, relatively low N fertilization rates result in small NH 3 emissions in rainfed areas, which occupy a very significant surface of Mediterranean agricultural land. Overall, N fertilization management is a key driver in reducing NH 3 emissions, but additional field factors should be studied in future research to establish more robust abatement strategies. [Display omitted] • NH 3 emissions and EF are assessed in Mediterranean climate zones after N fertilization. • Random Forest predict accurately NH 3 volatilization and EF. • Nitrogen rate and soil pH are the main factors that influence NH 3 emissions and EF. • N fertilization management is a key driver in reducing NH 3 emissions. • Mediterranean conditions as indirect NH 3 emissions drivers through the lower N rates. [ABSTRACT FROM AUTHOR]

Urea fertilizer applications to calcareous soils can result in significant nitrous oxide (N 2 O) and nitric oxide (NO) emissions, predominantly via nitrification rather than denitrification. To address this, we explored several mitigation strategies based on improved urea management in a rainfed winter wheat ( Triticum aestivum L.) crop during two consecutive cropping seasons with contrasting rainfall quantities and distribution. The strategies we investigated included the split application of urea at top dressing, the use of nitrification inhibitors (e.g. 2-(3,4-dimethyl-1H-pyrazol-1-yl) succinic acid isomeric mixture, DMPSA, and nitrapyrin), the urease inhibitor N-butyl thiophosphorictriamide (NBPT), or the double inhibitor DMPSA + NBPT. Emissions of N 2 O, NO, methane (CH 4 ), as well as measurements of grain and straw yield and bread-making quality (protein content, reserve protein composition: glutenins and gliadins) were measured. Nitrogen (N) use efficiency (NUE) and N surplus were also calculated. Results were affected by rainfall, since the first cropping season experienced typical rainfall quantity and distribution, whilst the second cropping season was very dry, thus increasing significantly the yield-scaled emissions and N surplus, and markedly decreasing the NUE. In comparison to the single application of urea without inhibitors, all treatments generally decreased surface-scaled and yield-scaled emissions, with urea+DMPSA being the most effective and consistent mitigation option. Split urea and NBPT did not mitigate surface-scaled emissions in the dry cropping season, because of the marked peaks in N oxides after flowering, caused by inefficient crop N uptake. In the typical rainfall cropping season, the use of the double NBPT+DMPSA inhibitor led to the best balance between mitigation of yield-scaled N oxides emissions, N efficiency, crop yield and bread-making quality (i.e. increments in total protein, gliadins and glutenins). We did not observe any effect of nitrification inhibitors on grain yield (except in the dry cropping season) or the composition of gluten proteins. Our results suggest that the use of DMPSA with or without NBPT should be recommended to mitigate yield-scaled emissions of N oxides in rainfed semi-arid crops. [ABSTRACT FROM AUTHOR]

[Display omitted] • Tillage versus no tillage and the use of DMPSA tested in rainfed semiarid barley. • DMPSA decreased 15N 2 O emissions during the preharvest period. • Remarkable postharvest N 2 O peak after rewetting derived from endogenous N. • No tillage and DMPSA increased crop recovery of exogenous and endogenous N, respectively. • No tillage reduced soil residual N, while DMPSA increased topsoil N retention. The sustainability of rainfed crops under semiarid conditions is threatened by low plant nitrogen (N) recovery as well as the potential loss of reactive N to the environment. A field 15N tracing experiment on barley (Hordeum vulgare L.) under rainfed conditions was carried out to study how different tillage management practices and the use of the nitrification inhibitor DMPSA affected the fate of N. The experiment consisted of a factorial combination of tillage (i.e., no tillage, NT, and conventional tillage, T) and fertilizer treatments (unfertilized control and ammonium nitrate, AN, with or without DMPSA). Single-labelled ammonium nitrate (15NH 4 NO 3 , 15AN, or NH 4 15NO 3 , A15N) was applied at top-dressing to microplots at a rate of 80 kg N ha−1. Our results show out that DMPSA modulates the nitrification process from both fertilizer-N and endogenous soil-N (which was the main contributor to plant N uptake and N 2 O emissions), affecting soil residual N at the end of the cropping period (i.e., higher topsoil retention of 15AN in DMPSA-amended plots). Generally, cumulative N 2 O emissions from fertilizer were derived from 15AN rather than from A15N, thus confirming the site-specific choice of the source of synthetic N as an effective N 2 O mitigation strategy. Two months after harvest, a rewetting event produced a remarkable N 2 O emission peak that drove total cumulative N 2 O emissions and was also mainly derived from endogenous N. These results suggest that dry seasons could decrease N 2 O losses after fertilization while causing critical peaks after rewetting, thus potentially limiting the effectiveness of mitigation strategies. The average plant N recovery from the synthetic fertilizer was 22.6%, while the use of DMPSA combined with NT enhanced plant N uptake from endogenous soil-N. This could be a result of the improved crop development and plant N acquisition under NT, consistent with the decrease of soil N retention for A15N in the deeper layer at the end of the experiment in the nontilled plots. This study contributes to the mechanistic understanding of the effect of nitrification inhibitors and tillage on N 2 O emissions, soil N dynamics and N plant recovery, revealing relevant effects of both management strategies and a critical role of endogenous soil-N under dry rainfed conditions. It can be concluded that, under the conditions of our study, combining DMPSA with NT could help to improve plant N recovery, thus resulting in positive impacts on reactive N loss and climate change mitigation and adaptation. [ABSTRACT FROM AUTHOR]

Increasing drought severity and evaporative demand in Mediterranean areas makes it necessary to implement irrigation systems with high water and nutrient supply efficiency. The combined management of drip irrigation burial depth and different nitrogen (N) sources, thus far unexplored, predicting these effects using proximal and spectral vegetation indices. A 2-year field experiment was conducted comparing maize yield and N uptake from four N fertilization treatments: ammonium sulfate (AS), AS with the nitrification inhibitor DMPP (AS+INH), calcium nitrate (CN) and a control without N fertilization combined with surface or subsurface (30 cm depth) drip fertigation. Multispectral data were collected to calculate various vegetation indices, while the chlorophyll content was measured with a soil plant analysis development (SPAD) sensor in the second year. Subsurface drip and AS+INH increased maize grain yields compared to surface drip and AS-only (by 12 % and 18 %, respectively, P < 0.05). However, this was observed only in the second season, as were increases in grain N content. The results show that the use of CN performed better in surface drip, while the use of NH 4 +-N-based fertilizers were recommended for subsurface irrigation. Regarding the spectral data, at the flowering-milky kernel and dent kernel phenological stages Normalized Difference Red Edge (NDRE) and the canopy chlorophyll content index (CCCI) were the two vegetation indices that best estimated agronomical parameters and were able to discriminate the phenological differences between irrigation systems. This study highlights the potential for (i) predicting yield and N uptake using proximal and multispectral sensors in drip-fertigated maize and (ii) optimizing crop performance by combining drip burial depth and N source (DMPP combined with subsurface irrigation), with relevant implications for climate change adaptation (i.e., potential improvements in crop phenology and water saving). • Surface and subsurface drip fertigation were combined with Ca(NO 3) 2 , (NH 4) 2 SO 4 or (NH 4) 2 SO 4 + DMPP in a maize crop. • Ca(NO 3) 2 and (NH 4) 2 SO 4 maximized yield and N acquisition in surface and subsurface drip, respectively. • The nitrification inhibitor DMPP increased grain yield and protein in the second year. • Spectral indices CCCI and NDRE were the best predictors of maize development. • Subsurface drip showed positive effects on grain yield and phenology (i.e., maize development acceleration) in the second year. [ABSTRACT FROM AUTHOR]

The transition year from tillage to no tillage in semiarid areas and its effects on nitrous oxide (N 2 O) emissions and related microbial communities, as well as the potential interaction with N management, including enhanced-efficiency fertilisers, are not well studied despite their economic and environmental implications. In tilled and nontilled plots, the effectiveness of the double DMPSA + NBPT inhibitor (applied with urea at basal fertilisation) and that of DMPSA (applied with calcium ammonium nitrate at top-dressing) in the mitigation of N 2 O emissions were evaluated in a rainfed barley (Hordeum vulgare L.) crop in central Spain. Crop yield, nitrogen (N) uptake, the abundances of key genes involved in nitrification and denitrification processes and meteorological conditions and soil ancillary properties were monitored. In addition, the composition of bacterial communities was determined by sequencing the 16S rRNA gene. Fertilisers with inhibitors decreased cumulative N 2 O emissions and yield-scaled N 2 O emissions by 53% and 56%, respectively, with respect to those without inhibitors, which coincided with a trend of increasing grain and biomass yield and aboveground N uptake (by 11.3%, 9.2% and 7.2%, respectively). The highest N 2 O emissions were measured 49 days after harvest (immediately after a rainfall event that like reactivated soil microorganisms), reaching 15 mg N m−2 d−1 for the treatment with fertiliser without inhibitor combined with tillage. This peak was linked to a remarkable increase in the abundance of denitrifiers. The abundance of nitrifiers and denitrifiers successfully explained the N 2 O dynamics observed after basal fertilisation (i.e. an increase in the amoA / nosZ ratio in fertilised plots with inhibitors, where the highest emissions were observed). Our results also showed a reduction in the abundance of the phylum Nitrospirae throughout the cropping period in the plots that received inhibitors. No tillage led to a higher abundance of Cyanobacteria , Verrucomicrobia and Bacteroidetes and resulted in better implantation of the crop and higher plant density compared with tillage, thus increasing yields and N use efficiency and decreasing N 2 O emissions. Under the conditions of our study, shifting from conventional tillage to no tillage enhanced the balance between N use efficiency and yield-scaled N 2 O emissions in the first year of conversion, particularity with the use of the double inhibitor with urea at basal fertilisation and DMPSA with CAN at dressing. • We addressed the transition from tillage to no tillage (NT) under semiarid conditions. • The highest N 2 O pulse occurred after a rewetting episode after harvest. • Changes in the N-cycling microbiota were observed even under short-term NT management. • Inhibitor-based treatments caused a decrease in the abundance of Nitrospirae. • Double inhibitor was effective in reducing N 2 O through stimulation of nosZ. [ABSTRACT FROM AUTHOR]

Highlights: [•] Meta-analysis of nitrification and urease inhibitors effect on crop yield and NUE. [•] DCD, DMPP and NBPT were selected for analysis. [•] Grand mean effect is 7.5% and 12.9% increase in crop yield and NUE respectively. [•] Their effectiveness was dependent on environmental and management factors. [•] Informational gaps are identified and recommendations for further research provided. [ABSTRACT FROM AUTHOR]