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Agricultural soils are the main anthropogenic source of nitrous oxide (N 2 O), largely because of nitrogen (N) fertilizer use. Commonly, N 2 O emissions are expressed as a function of N application rate. This suggests that smaller fertilizer applications always lead to smaller N 2 O emissions. Here we argue that, because of global demand for agricultural products, agronomic conditions should be included when assessing N 2 O emissions. Expressing N 2 O emissions in relation to crop productivity (expressed as above-ground N uptake: 'yieldscaled N 2 O emissions') can express the N 2 O efficiency of a cropping system. We show how conventional relationships between N application rate, N uptake and N 2 O emissions can result in minimal yield-scaled N 2 O emissions at intermediate fertilizer-N rates. Key findings of a meta-analysis on yield-scaled N 2 O emissions by non-leguminous annual crops (19 independent studies and 147 data points) revealed that yield-scaled N 2 O emissions were smallest (8.4 g N 2 O-N kg −1 N uptake) at application rates of approximately 180-190 kg N ha −1 and increased sharply after that (26.8 g N 2 O-N kg −1 N uptake at 301 kg N ha −1 ). If the above-ground N surplus was equal to or smaller than zero, yield-scaled N 2 O emissions remained stable and relatively small. At an N surplus of 90 kg N ha −1 yield-scaled emissions increased threefold. Furthermore, a negative relation between N use efficiency and yield-scaled N 2 O emissions was found. Therefore, we argue that agricultural management practices to reduce N 2 O emissions should focus on optimizing fertilizer-N use efficiency under median rates of N input, rather than on minimizing N application rates.

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The high N inputs to agricultural systems in many regions in 27 member states of the European Union (EU-27) result in N leaching to groundwater and surface water and emissions of ammonia (NH(3)), nitrous oxide (N(2)O), nitric oxide (NO), and dinitrogen (N(2)) to the atmosphere. Measures taken to decreasing these emissions often focus at one specific pollutant, but may have both antagonistic and synergistic effects on other N emissions. The model MITERRA-EUROPE was developed to assess the effects and interactions of policies and measures in agriculture on N losses and P balances at a regional level in EU-27. MITERRA-EUROPE is partly based on the existing models CAPRI and GAINS, supplemented with a N leaching module and a module with sets of measures. Calculations for the year 2000 show that denitrification is the largest N loss pathway in European agriculture (on average 44 kg N ha(-1) agricultural land), followed by NH(3) volatilization (17 kg N ha(-1)), N leaching (16 kg N ha(-1)) and emissions of N(2)O (2 kg N ha(-1)) and NO(X) (2 kg N ha(-1)). However, losses between regions in the EU-27 vary strongly. Some of the measures implemented to abate NH(3) emission may increase N(2)O emissions and N leaching. Balanced N fertilization has the potential of creating synergistic effects by simultaneously decreasing N leaching and NH(3) and N(2)O emissions. MITERRA-EUROPE is the first model that quantitatively assesses the possible synergistic and antagonistic effects of N emission abatement measures in a uniform way in EU-27.

Ruminant production contributes to emissions of nitrogen (N) to the environment, principally ammonia (NH 3 ), nitrous oxide (N 2 O) and di-nitrogen (N 2 ) to air, nitrate (NO 3 2 ) to groundwater and particulate N to surface waters. Variation in dietary N intake will particularly affect excretion of urinary N, which is much more vulnerable to losses than is faecal N. Our objective is to review dietary effects on the level and form of N excreted in cattle urine, as well as its consequences for emissions of N 2 O. The quantity of N excreted in urine varies widely. Urinary N excretion, in particular that of urea N, is decreased upon reduction of dietary N intake or an increase in the supply of energy to the rumen microorganisms and to the host animal itself. Most of the N in urine (from 50% to well over 90%) is present in the form of urea. Other nitrogenous components include purine derivatives (PD), hippuric acid, creatine and creatinine. Excretion of PD is related to rumen microbial protein synthesis, and that of hippuric acid to dietary concentration of degradable phenolic acids. The N concentration of cattle urine ranges from 3 to 20 g/l. High-dietary mineral levels increase urine volume and lead to reduced urinary N concentration as well as reduced urea concentration in plasma and milk. In lactating dairy cattle, variation in urine volume affects the relationship between milk urea and urinary N excretion, which hampers the use of milk urea as an accurate indicator of urinary N excretion. Following its deposition in pastures or in animal houses, ubiquitous microorganisms in soil and waters transform urinary N components into ammonium (NH 4 1 ), and thereafter into NO 3 2 and ultimately in N 2 accompanied with the release of N 2 O. Urinary hippuric acid, creatine and creatinine decompose more slowly than urea. Hippuric acid may act as a natural inhibitor of N 2 O emissions, but inhibition conditions have not been defined properly yet. Environmental and soil conditions at the site of urine deposition or manure application strongly influence N 2 O release. Major dietary strategies to mitigating N 2 O emission from cattle operations include reducing dietary N content or increasing energy content, and increasing dietary mineral content to increase urine volume. For further reduction of N 2 O emission, an integrated animal nutrition and excreta management approach is required.Keywords: nitrogen, urine, cattle, nitrous oxide, mitigation ImplicationsCattle contribute to global warming through emission of nitrous oxide (N 2 O) from urine and faeces. Urinary nitrogen (N) is much more susceptible to gaseous losses than faecal N. To reduce urinary N excretion and N 2 O emission and improve N efficiency of cattle, dietary levels of N should be decreased and an optimal balance between N and energy substrates in the diet should be aimed at. Increasing urine volume by increased dietary mineral contents appears a promising N 2 O mitigation strategy, particularly in pasture. Further reduction of effective mitigation strategies...

Increasing nitrogen (N) and phosphorus (P) inputs have greatly contributed to the increasing food production in China during the last decades, but have also increased N and P losses to the environment. The pathways and magnitude of these losses are not well quantified. Here, we report on N and P use efficiencies and losses at a national scale in 2005, using the model NUFER (NUtrient flows in Food chains, Environment and Resources use). Total amount of "new" N imported to the food chain was 48.8 Tg in 2005. Only 4.4.Tg reached households as food. Average N use efficiencies in crop production, animal production, and the whole food chain were 26, 11, and 9%, respectively. Most of the imported N was lost to the environment, that is, 23 Tg N to atmosphere, as ammonia (57%), nitrous oxide (2%), dinitrogen (33%), and nitrogen oxides (8%), and 20 Tg to waters. The total P input into the food chain was 7.8 Tg. The average P use efficiencies in crop production, animal production, and the whole food chain were 36, 5, and 7%, respectively. This is the first comprehensive overview of N and P balances, losses, and use efficiencies of the food chain in China. It shows that the N and P costs of food are high (for N 11 kg kg(-1), for P 13 kg kg(-1)). Key measures for lowering the N and P costs of food production are (i) increasing crop and animal production, (ii) balanced fertilization, and (iii) improved manure management.