G.L. Velthof

Role of nitrifier denitrification in the production of nitrous oxide

Abstract Nitrifier denitrification is the pathway of nitrification in which ammonia (NH 3 ) is oxidized to nitrite (NO 2 − ) followed by the reduction of NO 2 − to nitric oxide (NO), nitrous oxide (N 2 O) and molecular nitrogen (N 2 ). The transformations are carried out by autotrophic nitrifiers. Thus, nitrifier denitrification differs from coupled nitrification–denitrification, where denitrifiers reduce NO 2 − or nitrate (NO 3 − ) that was produced by nitrifiers. Nitrifier denitrification contributes to the development of the greenhouse gas N 2 O and also causes losses of fertilizer nitrogen in agricultural soils. In this review article, present knowledge about nitrifier denitrification is summarized in order to give an exact definition, to spread awareness of its pathway and controlling factors and to identify areas of research needed to improve global N 2 O budgets. Due to experimental difficulties and a lack of awareness of nitrifier denitrification, not much is known about this mechanism of N 2 O production. The few measurements carried out so far attribute up to 30% of the total N 2 O production to nitrifier denitrification. Low oxygen conditions coupled with low organic carbon contents of soils favour this pathway as might low pH. As nitrifier denitrification can lead to substantial N 2 O emissions, there is a need to quantify this pathway in different soils under different conditions. New insights attained through quantification experiments should be used in the improvement of computer models to define sets of conditions that show where and when nitrifier denitrification is a significant source of N 2 O. This may subsequently render the development of guidelines for low-emission farming practices necessary.

Integrated Assessment of Nitrogen Losses from Agriculture in EU-27 using MITERRA-EUROPE

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.

Nitrous oxide emission from soils amended with crop residues

Crop residues incorporated in soil are a potentially important source of nitrous oxide (N2O), though poorly quantified. Here, we report on the N2O emission from 10 crop residues added to a sandy and a clay soil, both with and without additional nitrate (NO3). In the sandy soil, total N2O emission from wheat, maize, and barley residues was not significantly different from the control. The total N2O emission from white cabbage, Brussels sprouts, mustard, sugar beet residues and broccoli ranged from 0.13 to 14.6 % of the amount of N added as residue and were higher with additional NO3 than without additional NO3. In the clay soil, similar effects of crop residues were found, but the magnitude of the N2O emission was much smaller than that in the sandy soil: less than 1 % of the residue N evolved as N2O. The C-to-N ratio of the residue accounted for only 22–34% and the mineralizable N content of the residue for 18–74% of the variance in N2O emission. We suggest that the current IPCC methodology for estimating N2O emission from crop residues may be considerably improved by defining crop specific emission factors instead of one emission factor for all crop residues.

Spatial variability of nitrous oxide fluxes in mown and grazed grasslands on a poorly drained clay soil

Fluxes of nitrous oxide (N2O) were measured in mown and intensively-grazed plots on a slightly-sloping, poorly-drained clay soil, using 144 flux chambers on four consecutive days. We tested the hypotheses that (i) spatial variability of N2O fluxes is larger in grazed than in mown grassland and (ii) spatial dependency is larger in mown than in grazed grassland. Distributions were approximately log-normal. Fluxes from grazed grassland were larger than those from mown grassland. Multiple linear regression analyses showed weak relationships between N2O flux and moisture, NH4+, NO3− and C contents, with less than 15% of the variance in N2O flux accounted for. Spatial variability was large both on a relatively small scale (less than 6 m) and on a larger scale (10–100 m) and was larger on mown grassland than on grazed grassland. Geostatistics showed that N2O fluxes were spatially dependent for a lag distance of less than 6 m on mown grassland. On grazed grassland fluxes were spatially independent on a scale of < 6 m. The large spatial variability of N2O fluxes suggests that even measurement techniques that integrate N2O fluxes over a large area may be hampered by the large spatial variability of N2O fluxes.

Effects of type and amount of applied nitrogen fertilizer on nitrous oxide fluxes from intensively managed grassland

Five field experiments and one greenhouse experiment were carried out to assess the effects of nitrogen (N) fertilizer type and the amount of applied N fertilizer on nitrous oxide (N2O) emission from grassland. During cold and dry conditions in early spring, emission of N2O from both ammonium (NH4+) and nitrate (NO3−) containing fertilizers applied to a clay soil were relatively small, i.e. less than 0.1% of the N applied. Emission of N2O and total denitrification losses from NO3− containing fertilizers were large after application to a poorly drained sand soil during a wet spring. A total of 5–12% and 8–14% of the applied N was lost as N2O and via denitrification, respectively. Emissions of N2O and total denitrification losses from NH4+ fertilizers and cattle slurry were less than 2% of the N applied. Addition of the nitrification inhibitor dicyandiamide (DCD) reduced N2O fluxes from ammonium sulphate (AS). However, the effect of DCD to reduce total N2O emission from AS was much smaller than the effect of using NH4+ fertilizer instead of NO3− fertilizer, during wet conditions. The greenhouse study showed that a high groundwater level favors production of N2O from NO3− fertilizers but not from NH4+ fertilizers. Inereasing calcium ammonium nitrate (CAN) application increased the emitted N2O on grassland from 0.6% of the fertilizer application rate for a dressing of 50 kg N ha−1 to 3.1% for a dressing of 300 kg N ha−1. In another experiment, N2O emission increased proportionally with increasing N rate. The results indicate that there is scope for reducing N2O emission from grasslands by choosing the N fertilizer type depending on the soil moisture status. Avoiding excessive N application rates may also minimize N2O emission from intensively managed grasslands.

Seasonal variation in N2O emissions from urine patches: effects of urine concentration, soil compaction and dung

Urine patches in pastures rank among the highest sources of the greenhouse gas nitrous oxide (N2O) from animal production systems. Previous laboratory studies indicate that N2O emissions for urine-N in pastures may increase with a factor five or eight in combination with soil compaction and dung, respectively. These combinations of urine, compaction and dung occur regularly in pastures, especially in so-called camping areas. The aims of this study were (i) to experimentally quantify the effect of compaction and dung on emission factors of N2O from urine patches under field conditions; (ii) to detect any seasonal changes in emission from urine patches; and (iii) to quantify possible effects of urine concentration and -volume. A series of experiments on the effects of compaction, dung, urine-N concentration and urine volume was set up at a pasture on a sandy soil (typic Endoaquoll) in Wageningen, the Netherlands. Artificial urine was applied 8 times in the period August 2000–November 2001, and N2O emissions were monitored for a minimum of 1 month after each application. The average emission factor for urine-only treatments was 1.55%. Over the whole period, only soil compaction had a clear significant effect, raising the average N2O emissions from urine patches from 1.30% to 2.92% of the applied N. Dung had no consistent effect; although it increased the average emissions from 1.60% to 2.82%, this was clearly significant (P< 0.01) for only one application date and marginally significant (P=0.054) for the whole experiment. Both compaction and dung increased water-filled pore space (WFPS) of the topsoil for a more prolonged time than high urine volumes. No effect of amount of urine-N or urine volume on N2O emissions relative to added N was detected for the whole experiment. There were clear differences between application dates, with highest emissions for urine-only treatments of 4.25% in October, 2000, and lowest of −0.11% in June, 2001. Emissions peaked at 60–70% WFPS, and decreased rapidly with both higher and lower WFPS. We conclude that compaction leads to a considerable increase in the N2O emissions under field conditions, mainly through higher WFPS. Dung addition may have the same effect, although this was not consistent throughout our experiment. Seasonal variations seemed mainly driven by differences in WFPS. Based on this study, mitigation strategies should focus on minimizing the grazing period with wet conditions leading to WFPS > 50%, avoiding camping areas in pastures, and on avoiding grazing under moist soil conditions. Greenhouse gas budgets for grazing conditions should include the effects of soil compaction and dung to represent actual emissions.

Differentiation of nitrous oxide emission factors for agricultural soils.

Nitrous oxide (N(2)O) direct soil emissions from agriculture are often estimated using the default IPCC emission factor (EF) of 1%. However, a large variation in EFs exists due to differences in environment, crops and management. We developed an approach to determine N(2)O EFs that depend on N-input sources and environmental factors. The starting point of the method was a monitoring study in which an EF of 1% was found. The conditions of this experiment were set as the reference from which the effects of 16 sources of N input, three soil types, two land-use types and annual precipitation on the N(2)O EF were estimated. The derived EF inference scheme performed on average better than the default IPCC EF. The use of differentiated EFs, including different regional conditions, allows accounting for the effects of more mitigation measures and offers European countries a possibility to use a Tier 2 approach.

Nitrogen and phosphorus use efficiencies and losses in the food chain in China at regional scales in 1980 and 2005.

Crop and animal production in China has increased significantly during the last decades, but at the cost of large increases in nitrogen (N) and phosphorus (P) losses, which contribute to ecosystem degradation and human health effects. This information is largely based on scattered field experiments, surveys and national statistics. As a consequence, there is as yet no comprehensive understanding of the changes in N and P cycling and losses at regional and national scales. Here, we present the results of an integrated assessment of the N and P use efficiencies (NUE and PUE) and N and P losses in the chain of crop and animal production, food processing and retail, and food consumption at regional scale in 1980 and 2005, using a uniform approach and databases. Our results show that the N and P costs of food production-consumption almost doubled between 1980 and 2005, but with large regional variation. The NUE and PUE of crop production decreased dramatically, while NUE and PUE in animal production increased. Interestingly, NUE and PUE of the food processing sector decreased from about 75% to 50%. Intake of N and P per capita increased, but again with large regional variation. Losses of N and P from agriculture to atmosphere and water bodies increased in most regions, especially in the east and south of the country. Highest losses were estimated for the Beijing and Tianjin metropolitan regions (North China), Pearl River Delta (South China) and Yangzi River Delta (East China). In conclusion, the changes and regional variations in NUE and PUE in the food chain of China are large and complex. Changes occurred in the whole crop and animal production, food processing and consumption chain, and were largest in the most populous areas between 1980 and 2005.

The impact of the Nitrates Directive on nitrogen emissions from agriculture in the EU-27 during 2000–2008

A series of environmental policies have been implemented in the European Union (EU) to decrease nitrogen (N) emissions from agriculture. The Nitrates Directive (ND) is one of the main policies; it aims to reduce nitrate leaching from agriculture through a number of measures. A study was carried out to quantify the effects of the ND in the EU-27 on the leaching and runoff of nitrate (NO3(-)) to groundwater and surface waters, and on the emissions of ammonia (NH3), nitrous oxide (N2O), nitrogen oxides (NO(x)) and dinitrogen (N2) to the atmosphere. We formulated a scenario with and a scenario without implementation of the ND. The model MITERRA-Europe was used to calculate N emissions on a regional level in the EU-27 for the period 2000-2008. The calculated total N loss from agriculture in the EU-27 was 13 Mton N in 2008, with 53% as N2, 22% as NO3, 21% as NH3, 3% as N2O, and 1% as NO(x). The N emissions and leaching in the EU-27 slightly decreased in the period 2000-2008. Total emissions in the EU in 2008 were smaller with implementation of the ND than without the ND, by 3% for NH3, 6% for N2O, 9% for NO(x), and 16% for N leaching and runoff in 2008. However, regional differences were large. The lower emissions with ND were mainly due to the lower N inputs by fertilizers and manures. In conclusion, implementation of the ND decreased both N leaching losses to ground and surface waters, and gaseous emissions to the atmosphere. It is expected that the ND will result in a further decrease in N emissions in EU-27 in the near future, because the implementation of the measures for the ND is expected to become more strict.

Environmental Assessment of Management Options for Nutrient Flows in the Food Chain in China

The nitrogen (N) and phosphorus (P) costs of food production have increased greatly in China during the last 30 years, leading to eutrophication of surface waters, nitrate leaching to groundwater, and greenhouse gas emissions. Here, we present the results of scenario analyses in which possible changes in food production-consumption in China for the year 2030 were explored. Changes in food chain structure, improvements in technology and management, and combinations of these on food supply and environmental quality were analyzed with the NUFER model. In the business as usual scenario, N and P fertilizer consumption in 2030 will be driven by population growth and diet changes and will both increase by 25%. N and P losses will increase by 44 and 73%, respectively, relative to the reference year 2005. Scenarios with increased imports of animal products and feed instead of domestic production, and with changes in the human diet, indicate reductions in fertilizer consumption and N and P losses relative to the business as usual scenario. Implementation of a package of integrated nutrient management measures may roughly nullify the increases in losses in the business as usual scenario and may greatly increase the efficiency of N and P throughout the whole food chain.