Interactive comment on “ Nitrous oxide emissions from crop rotations including wheat , rapeseed and dry pea ”

The question addressed concerns the effects of different cropping systems (rotations) on nitrous oxide emissions from soils, ultimately driven by the processes causing fluxes in reactive nitrogen used by the plants, either provided by fertiliser or by biological nitrogen fixation. The paper rightly points to the scarcity of research results on effects of rotations and the uncertainty surrounding emissions driven by reactive nitrogen from biological nitrogen fixation in particular.


Introduction
N 2 O is a potent greenhouse gas (GHG), which accounts for 6 % of the total anthropogenic radiative forcing (IPCC, 2007).It is also one of the main compounds involved in the ozone layer degradation (Crutzen and Ehhalt, 1977).Its concentration increased regularly since the end of the 19th century with more rapid increase in the second half of the 20th century (see e.g.IPCC, 2007;Davidson, 2009).It is widely admitted that microbial production in soils by both nitrification and denitrification is the dominant nitrous oxide source (Firestone and Davidson, 1989), with contribution of these two processes to N 2 O emissions varying with climate, soil conditions and soil management (H énault et al., 1998;Skiba and Smith, 2000).Since the end of the 19th century, the increased use of nitrogen for food and feed production, as both synthetic fertilizer and organic manure, increased the N 2 O emission.At present, approximately 65 % of anthropogenic emissions of N 2 O originate from soils at global scale (Smith, 2004) or Europe (Leip et al., 2011).But these estimates are still very uncertain and the N 2 O emissions are considered as the most uncertain estimate within the main direct GHG.This is mainly due to large uncertainties in emission factors and this is still under debate (see e.g.Crutzen et al., 2008;Davidson, 2009).Since the agricultural sector contributes for more than 20 % of French GHG emissions in 2008 (13.5 % of the global GHG emissions, of which 75 % in the developed countries) and since N 2 O represents more than 50 % of the GHG emitted by the agriculture (and 12 % of the French part of the GWP, global warming potential), the alternative practices to decrease N 2 O emissions represent key levers to mitigate climatic change.As N 2 O emissions generally increase with N-fertilisation, as proposed in the IPCC method, crops that do not require N fertilisation appear as a possible solution to limit N 2 O emissions.Legumes, thanks to their capacity to fix atmospheric N 2 through biological nitrogen fixation (BNF), allow a reduction of N fertilizer use, both on the legume crop and on the following crop as soil mineral N availability is higher during the following year (Jensen and Hauggaard-Nielsen, 2003).Using legume crops as a source of nitrogen has thus been envisaged Introduction

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Full as a solution for decreasing N 2 O emissions, but it is still under debate.As a matter of fact, legume crops could produce N 2 O by different pathways: during the biological N 2 fixation itself, by decomposition of crop organic matter (esp.fine roots or root exudates), or when legume residues are returned to the soil (Zhong et al., 2009).While the two latter might be a N 2 O source, as legume residues have larger N contents than cereal residues (but in lower amounts), the BNF pathway seems less certain.Whereas denitrification is known to occur in legume root nodules, the magnitude of this process and its contribution to soil N 2 O emissions could be low compared to the N 2 O production by the soil microbial biomass.H énault and Revellin (2011) showed that N 2 O could even be consumed in legume nodules.Under field conditions large emissions were observed over some legumes crops: Duxbury et al. (1982) had reported relatively high cumulative fluxes of 2.3 and 4.2 kg N ha −1 yr −1 for an alfalfa field.In contrast, Velthof and Oenema (1997) estimated N 2 O-N emissions from grass-clover canopy to vary between 0 and 1 % of biologically-fixed N 2 , probably lower than from an equivalent amount of N fertilizer because the biologically-fixed N is released slowly into the soil.More recently, Rochette and Janzen (2005) made a synthesis of published studies on N 2 O emissions from legume crops, and concluded that "much of the increase in soil N 2 O emissions in legume crops may be attributable to the N release from root exudates during the growing season and from decomposition of crop residues after harvest, rather than from Biological Nitrogen Fixation per se".This led them to propose not to account for N 2 O emission from N 2 fixation in legume crops, but only from N residue decomposition after the crop.However, this was based mostly on results from soybean, grass-legume/clover and alfalfa (Stehfest and Bouwman, 2006) and there are very few references on European annual arable grain legumes cultivated in the Europe climatic conditions and harvested at maturity (most of the nitrogen being allocated to seeds and not to straws).For example, values for annual legume crops such as pea resulted from one single reference (Lemke et al., 2007).Moreover, these values were never compared to emissions from other non-leguminous crops, grown in the same conditions, while it is known that N 2 O fluxes are highly sensitive to weather and Introduction

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Full soil conditions (Philibert et al., 2012).Thus, it is difficult to compare the values of N 2 O emissions from legumes, synthesized by Rochette and Janzen (2005), with values from fertilized crops, from other experiments, and to conclude to the increase or decrease in N 2 O emissions when legumes are grown instead of fertilized crops.Consequently, more references are needed on emissions from annual legume crops as a component of crop rotations in cropping systems.
The objective of this study was to quantify N 2 O emissions from a pea crop and from the subsequent crops in comparison with N 2 O emissions from wheat and oilseed-rape crops, fertilized or not, in various rotations.Our approach was to analyse the different crop stages where N 2 O emissions may arise from N 2 fixation, i.e. during and after the legume crop in a crop rotation.We assessed whether the N 2 O emissions during a dry pea crop were similar to a non-fertilized crop, and whether the N 2 O emissions in autumn following a dry pea crop were higher than following a wheat crop or an oilseed rape crop.Finally, we estimated to what extent it was interesting to include a legume crop in a crop rotation.

Experimental site
A field trial was conducted at INRA Grignon experimental unit (Paris Basin, 48.9 • N, 1.9 • E) from 2007 to 2010, combining the comparison of several crops during each year, and the comparison of the effect of several preceding crops on the current crop.
The focus was on the pea crop in order to estimate N 2 O emissions during the crop cycle and during the subsequent period (residue decomposition before sowing of the next crop and during the subsequent crop cycle).The trial was set up over a 0.19 ha experimental field divided in three blocks, with each treatment having one replicate in each block.The soil was a clay loam with 25.7 % clay, 66.6 % loam and 7.7 % sand, Introduction

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Full 1.29 g kg −1 total N and 18 g kg −1 organic C. Weather data were available at a meteorological station nearby the field.

Treatments
Eight different rotations were set up in the field to obtain different combinations of three crops (pea, wheat and oilseed rape) with their preceding crops (Table 1) in order to analyse the different effects of the pea crop during its cycle or during the next crop.
A winter wheat crop (cv.Isengrain, 250 seeds m −2 ) was grown after wheat, pea or oilseed rape; a winter oilseed rape (cv.Mendel, 52 seeds m −2 ) crop was grown after wheat or pea; a winter pea crop (cv.Cartouche, 92 seeds m −2 ) was always grown after wheat.Each rotation was repeated three times and randomized in a complete 3-block design.Each plot area was 60 m 2 .The rape and the wheat crops were either fertilised at the optimum (using recommended decision support tools) or not fertilized.The pea crop was not fertilised.The sowing dates as well as the dates and amounts of fertiliser application are given in Table 2.The non-fertilized areas were always at the same place from year to year.Crops were fully protected against weeds and pests by chemical treatments, to prevent growth limitation due to these factors.Each year, crop residues were incorporated and a 20 cm deep ploughing was done before each sowing.Due to practical limitations, it was not possible to measure N 2 O emissions on all plots every year.Consequently, measurements were made on fertilized wheat (W), fertilized oilseed rape (R), non-fertilized wheat (W0N) and pea (P) on selected rotations in order to take samples over the different crops with the preceding crop including pea or not.The plots on which N 2 O measurements were made are indicated in Table 1.

Measurements
N 2 O fluxes were measured using the static closed chamber technique (Hutchinson and Livingston, 1993).Two chambers were set up on each of the three replicates per treatment, i.e. six chambers per treatment in total.The bases of the chamber were installed Introduction

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Full at the beginning of the crop cycle, over several seeded rows, and were inserted about 5 cm into the soil.They remained in place during the whole crop cycle.Care was taken to disturb the soil as little as possible during the installation process.Their internal area was 0.185 m 2 .Their internal volume was determined by measuring the height above ground of the chambers.It was in average 40 dm 3 at the beginning of the cycle.When the crop grew, a 60 cm high extension was added to avoid any damage of the plants.
The volume of the chambers increased to approximately 170 dm 3 afterwards.Measurements were performed approximately twice a month, and the frequency was increased to twice a week during the two weeks following fertilizer application.At each date of measurement, all the treatments were measured, whether they received N fertilizer or not.For each chamber, the air was sampled in a pre-evacuated 10 ml vial at the closure of the chamber, and 45 min, 90 min and 135 min afterwards.After sampling, the tubes were analysed in the lab by using a gas chromatograph with an electron-capture detector (Model 3400 Cx, Varian, Walnut Creek, USA).On the same days, the soil was sampled in the upper soil layer by taking three soil cores in the 0-30 cm layer using an auger in each treatment.The soil samples were mixed together and analysed in order to determine the mineral nitrogen content and the soil water content.Soil inorganic N was determined in a KCl extract with a Skalar Autonalyser, using copper reduction and the Griess-Ilosvay reaction for nitrate and the indophenol method for ammonium.Soil water content was determined by weighing before and after oven drying during 48 h the combustion of dehydrated and ground plant tissue at about 1800 • C, reduction of nitrogen oxides by reduced Cu at 600 • C and analysis of N 2 by catharometry (NA 1500 analyser, Fisons Instruments).After harvest, soil mineral N content was measured.Three soil cores of each of the three layers (0-30 cm, 30-60 cm, 60-90 cm) per block and per treatment were mixed in order to get one sample per layer.Soil inorganic N content was determined in a KCl extract with an autoanalyser (Skalar).

Statistical analysis
Mixed models were fitted to the data in order to analyse the effects of the crop, and the preceding crops (considered as fixed effects), and taking into account the effects of block and date of measurement (considered as random effects).As nitrogen fertilizer application may interfere with N 2 O emission, the crop cycle was divided into 3 periods: (i) before the first N application of the season (before Napp) including the autumn dates of measurement, (ii) more than 14 days after the last N fertilizer application (after Napp), and (iii) between these two dates (following Napp).The same period distinction was applied for all crops during each cropping season.The period was included in the mixed model as a fixed effect, as well as the two-way interactions between period and respectively crop and preceding crop.Each effect was tested by comparing the same models with and without the considered effect with a likelihood ratio test.In the case of significant interactions, a new factor was created by taking the combination of the two interacting factors (e.g.crop period).When the effect was not significant, it was suppressed from the model.In the case of significant effects, multiple comparisons with Tukey contrasts were performed in order to compare the treatments.
In order to study the effect of crop residues in more detail, a further analysis was conducted on the mean N 2 O emissions during autumn (from the beginning of September until the end of December) for each year, because we assumed that during this period, the effect of the current crop had not appeared yet, and that the N 2 O emissions Introduction

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Full linked with the crop mainly resulted from residues decomposition.The preceding crop was considered as a fixed effect and the year and block as random effects.
We also analysed the effect of the crop, and the preceding crop on the soil mineral nitrogen content measured after harvest, and on the amount of N present in the crop residues at harvest, as both variables represent sources of N 2 O emission which must be considered according to the IPCC guidelines (IPCC, 2006).The same approach was taken, starting from a model including crop and preceding crop as fixed effects, and year and block as random effects, on the amount of nitrogen contained in the crop residues remaining on the ground after harvest, or on the amount of soil mineral nitrogen content measured at harvest.All analyses were performed using R statistical software, with packages lme4 (Bates and Maechler, 2009) for the mixed models and multcomp (Hothorn et al., 2008) for the multiple comparison tests.The assumptions of the linear mixed model were checked by visual examination of the plot of residuals against predicted values and qqplots for the residuals and the random effects.after.N 2 O emission during autumn was even lower, around ±1 g ha −1 day −1 .The same trends were observed during the three years.

Effect of crops and preceding crops on N 2 O emissions
The comparison of the mixed models for N 2 O emissions showed significant date and block random effects (p < 0.001 and p = 0.008 respectively).The interaction between preceding crop and period of measurement was not significant (p = 0.8) and was thus suppressed from the model.The interaction crop x period was highly significant (p < 0.001), indicating that N 2 O emissions varied along the crop cycle differently according to the crop.Consequently the analysis was carried out on the combinations of crop x period (i.e. on a new variable crop period).The results then showed that there was no effect of the preceding crop (p = 0.48) but a highly significant crop period effect (p < 0.001).The multiple comparison tests showed that for the fertilized crops, emissions were higher after N fertilisation than before, with intermediate values during the period of N application (Fig. 2).For non-fertilized crops (W0N and P), emissions were not significantly different among the three periods.

Effects of crop residues on N 2 O emissions during autumn
When the mean N 2 O emissions during autumn of each year were analysed separately, there was a significant (random) year effect (p < 0.001) but no significant effect of block (p = 0.38).There was also no significant effect of the type of residues (i.e.preceding Introduction

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Full crop) (p = 0.49).Figure 3 shows the mean emissions during autumn for the different preceding crops and years.

Soil mineral N content after harvest
Figure 4 shows the mineral N content in soil after harvest of each crop during each year.
Values highly varied according to the crop and, to a lesser extent, according to the year.
The comparison of the mixed models showed a significant effect of the year (p = 0.035) but no block effect (p = 1).There was no effect of the preceding crop (p = 0.17) but a highly significant effect of the crop (p < 0.0001).The multiple comparisons showed that soil mineral content after harvest was lower for oilseed rape and non-fertilized wheat than for fertilized wheat and pea (Fig. 4), both being non significantly different.

N content in crop residues at harvest
The N contents in crop residues at harvest highly varied according to the year and the crop.The comparison of the mixed models showed significant year and block effects (p < 0.0001 and p = 0.0023, respectively).There was a significant effect of the preceding crop (p = 0.003) as well as a significant effect of the crop (p < 0.0001; Fig. 5).
The multiple comparisons showed that the crop effect was only due to the difference between the residues of unfertilized wheat and oilseed rape vs. pea, fertilized wheat and oilseed rape, and the effect of the preceding crop is only visible through the significant difference between unfertilized wheat after unfertilized wheat and unfertilized wheat after pea (Fig. 5).Introduction

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N 2 O emissions for the different crops in the crop rotations
As a whole, N 2 O emissions were low for all crops on the Grignon site, with background emission lower than 1 g ha −1 day −1 and averages after fertilization between 2 and 3 g ha −1 day −1 .Most values were below 10 g ha −1 day −1 .Consequently, the total emissions over the crop cycle were relatively low, with low variability between years (Fig. 1).Loubet et al. (2011) and Laville et al. (2011) observed similar fluxes over a nearby site with similar soil conditions and agricultural practices, using automated chambers with continuous measurements (16 values per day).However, they also observed much larger fluxes under three circumstances: in summer 2007 after barley harvest and residues incorporation, in December 2007 after a low temperature period, and in May 2008 after N application over maize.In the first case, the summer was cooler than the average but much wetter, which certainly favoured N 2 O emission from residues decomposition.Large emission in May 2008 was linked to the conjunction of large N inputs for maize fertilization (130 kg N ha −1 as ammonium nitrate following 107 kg N ha −1 as slurry), high temperature (monthly average almost 2 • C above the average) and larger rainfall than the average.This was not observed on our plots because of limited N availability due to low inputs at this period (Table 2).But the main reason for the low emissions observed during the 3 yr could be the low rainfall which occurred all along the year ( 458 to underestimate cumulated fluxes by approx.30 %, while Smith and Dobbie ( 2001) and Laville et al. (2011) estimated that the bias was less than 15 %.As expected, a clear effect of fertilization was observed, with emissions five to tenfold larger than background emissions (Fig. 2).However a surprising feature was that emissions were larger after two weeks after N application.Once again, this might be attributed to the relatively dry conditions.Le Cadre (2004) observed that pellet dissolution, which is necessary to make N available to the soil, and thus to nitrification or denitrification, may take more than 10 days under dry soil conditions.Moreover soil microorganisms might have a low activity in the top soil layers under such situations.
Despite such low fluxes, significant differences were observed both between crops and between periods, as related to fertilization events (Fig. 2) for the three years of the experiments (data not shown).The robustness of this observation was strengthened by the experimental protocol organized in blocks and the methods used for statistical analysis.It can then be considered that, even if the cumulated fluxes are biased, due to low observation frequency, the relative differences and hence the effects, can be considered with confidence.On the opposite, no effect of the preceding crop was observed on N 2 O fluxes, either on the whole crop cycle, or during the shorter period of autumn.This result was observed each of the three years.It brings new elements against the hypothesis that N 2 O emissions in legumes should mainly be linked with the decomposition of their residues after harvest (Rochette and Janzen, 2005).It is consistent with previous results from Lemke et al. (2002), who did not find significantly higher N 2 O emissions after a pea crop than after a wheat crop.Our result is also consistent with the observations on the possible sources of nitrogen for N 2 O emissions: soil mineral nitrogen after harvest was not higher after pea than after wheat, and N in crop residues was not significantly different between pea and fertilized rape and wheat.In contrast, Hauggaard-Nielsen et al. (2003) found a significantly higher N amount in pea straw than in barley straw.More data should be gathered to confirm this result.

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Full Concerning the amount of N in pea residues, it has to be reminded that this experiment concerns a dry pea crop.Vining pea crop (or garden pea) is the same botanical species but it is harvested at an earlier stage, before the translocation of N from the leaves to the seeds.Consequently, the total N amount of vining pea residues and their N / C ratio are significantly higher than for dry pea or wheat residues (Carrou ée et al., 2006): different results concerning N mineralization and possibly N 2 O emissions could then be observed.The references published in the literature dealing with "pea crop" do not always precise clearly which type of pea is concerned: they have to be gathered with caution.

N 2 O emission from the pea crop
Our results clearly evidenced the lower emissions over the pea crop, compared with the fertilized wheat or rapeseed crops, with an average ratio 1 : 4-1 : 6 respectively over the growing season for the three years, and a range of 1 : 3-1 : 12 respectively according to the considered year.They were not significantly different from those of the unfertilized wheat crop.Moreover, these results were observed systematically over the three years.The cumulated fluxes were not larger than emissions under natural vegetation as estimated by Stehfest and Bouwmann (2006).They are in the low range of N 2 O emissions from legumes, as published in the synthesis by Rochette and Janzen (2005).They are even lower than the values given by Lemke et al. (2007), which is the only reference over a pea crop in Rochette and Janzen (2005) the pea residues are not a larger N 2 O source than wheat/rapeseed residues.This also means that the other possible source of N 2 O in a pea crop, N-rich root exudates, did not add significant emissions while the contribution of roots to the N pool of the soil has been shown to reach 22-25 % relative to total plant-N at maturity (Mahieu et al., 2007).We could then consider that there were no subsequent effects of the pea crop that would imply to consider this crop as a larger or a lower N 2 O emitter than a cereal or an oilseed crop.These data should be confirmed in other conditions of higher emissions, particularly in another soil type.Moreover, in order to avoid underestimate due to unfrequent sampling and catch up emission pulses, it should be more precise to measure N 2 O fluxes by using automated chambers or to model daily fluxes.Models simulating N 2 O fluxes are currently being enhanced and more precise and their quality of prediction can be improved by real data assimilations (Lehuger et al., 2009).

GHG budget of the pea crop compared to other crops
Besides reducing GHG emissions during the crop cycle, the reduction of GHG budget of a pea crop is also linked with the reduced consumption of GHG by production and transport due to the avoided nitrogen fertilizer which is not applied on the pea crop, and which rate is reduced on the following crop: mean values reach 0.7 to 1.4 t eq CO 2 ha −1 less than a crop receiving 180 kg N ha −1 of mineral fertilizer mineral (ADEME-DIREN, 1997;Nemecek et al., 2000).Moreover, on this crop, reduction of energy use and GHG emissions are also linked with the suppression of the machinery cost of the fertilizer application.Using the emission factors of IPCC, a crop fertilized with 180 kg N ha  Full Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | at 105 • C. Measurements occurred during the crop cycle in spring (from the end of winter in February until harvest) in 2008 (14 dates of measurement), 2009 (20 dates) and 2010 (18 dates), and during the autumn (from September until December) in 2008 (8 dates of measurements), 2009 (8 dates) and 2010 (5 dates).At harvest, the amount of N in the crop residues was measured.Three micro-plots (0.35 m 2 each for wheat, 0.875 m 2 for pea, and 1 m 2 for oilseed rape) per block and per treatment were sampled.Vegetative parts and grains were separated just after sampling and subsequently weighted after oven-drying (48 h, 80 • C).Vegetative organs were then ground and N content was determined with the Dumas method.This involves Introduction Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | N 2 O fluxes N 2 O emissions measured during the four cropping seasons on the four crops are presented in Fig. 1 for both the autumn and spring periods.The measured fluxes were relatively low, ranging between −4 and 10 g ha −1 day −1 with negative fluxes at several periods.They show a high variability of N 2 O emissions on fertilized wheat and rape along the crop cycle.Despite low values, significant differences can be observed between crops, with fertilised wheat and rapeseed having larger fluxes than unfertilized wheat and the pea crop, especially after fertilizer application.The maximum fluxes were not always observed just after fertilizer application, but sometimes 2-3 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , 388 and 406 mm during the cropping period, from October untilJune, respectively in 2008June, respectively in  , 2009June, respectively in   and 2010) )  and especially during the period of fertilizer application in 2009 and 2010, where rainfall was much lower (71 and 48 mm respectively) than the average over this site (89 mm), with only 4 days with rainfall larger than 2 mm day −1 in 2009 (max = 6.2 mm day −1 ) and 2010 (max = 3.5 mm day −1 ).The last reason might be that, due to their low measurement frequency (two measurements per week or per month), manual chamber did not succeeded in catching up the N 2 O emission pulses which contributes significantly to the annual emissions.For instance,Parkin (2008) estimated that sampling at time intervals larger than one week could lead 9300 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | . While they measured cumulated fluxes of 0.38-0.74kg N-N 2 O ha −1 yr −1 , our estimates ranged from 0.03 to 0.1 kg N-N 2 O ha −1 yr −1 over the growing season.This confirms the option of Rochette and Janzen (2005), agreed by IPCC (2006), that BNF should not be considered as a N 2 O source in N 2 O emission inventory.The total N in crop residues and in soil after harvest was similar or larger for the pea crop compared to the other fertilized crops.However, the N 2 O emissions resulting from residue decomposition, as estimated from measurements between harvest and the next crop were not significantly different between the three crops.This shows that 9302 Discussion Paper | Discussion Paper | Discussion Paper | emission of 2.8 g N 2 O ha −1 , i.e. 0.8 t eq CO 2 ha −1 (IPCC, 2006).With our results, showing a reduction of the N 2 O emissions during the pea crop of 75-80 % compared with a fertilized crop, the overall GHG emission economy can reach up to 2.5 t eq CO 2 ha −1 .Through LCA modeling, the comparison of N 2 O emission linked with several crops underlined similar trends with 70 % of reduction in favour to pea crop compared to fertilized arable crops (T.Nemecek, personal communication, 2012).Discussion Paper | Discussion Paper | Discussion Paper | 4.4 Assessment of N 2 O emissions of a segment of three years of a crop succession Our punctual measurements could be extrapolated at the level of the year scale to calculate the N 2 O emissions of cropping systems over several years.If our data are confirmed, when we extrapolate them over a three year segment of crop rotation, including one pea crop, compared with 3 fertilized crops segment, leads to a significant 20-25 % reduction of GHG emissions in the fields at the rotation scale.This value is consistent with the 14 % reduction of GHG emissions for a 20 % introduction of pea in rotations calculated by Nemecek et al. (2008) through Life Cycle Assessments.Discussion Paper | Discussion Paper | Discussion Paper | Skiba, U. and Smith, K. A.: The control of nitrous oxide emissions from agricultural and natural soils, Chemosphere -Global Change Science, 2, 379-386, 2000.Smith, K. A. and Dobbie, K. E.: The impact of sampling frequency and sampling times on chamber-based measurements of N 2 O emissions from fertilized soils, Glob.Change Biol., 7, 933-945, 2001.Stehfest, E. and Bouwmann, A.: N 2 O and NO emission from agricultural fields and soils under natural vegetation: summarizing available measurement data and modeling of global annual emissions, Nutr.Cyc.Agroecosys., 74, 207-228, doi:10.1007/s10705-006-9000-7,2006 Velthof, G. L. and Oenema, O.: Nitrous oxide emission from dairy farming systems in the Netherlands, Neth.J. Agr.Sci., 45, 347-360, 1997Discussion Paper | Discussion Paper | Discussion Paper |

Figure 2
Figure 2 Mean N2O emissions as a function of the crop (wheat, winter oilseed rape or pea), fertilized or not (0N) and period of observation (before the first N fertilisation, more than 14 days after the last N fertiliser or during the period of N fertilization).Bars with the same letter are not significantly different.Error bars= standard error of the mean.

Fig. 2 .Figure 5 :
Fig. 2. Mean N 2 O emissions as a function of the crop (wheat, winter oilseed rape or pea), fertilized or not (0N) and period of observation (before the first N fertilisation, more than 14 days after the last N fertiliser or during the period of N fertilization).Bars with the same letter are not significantly different.Error bars = standard error of the mean.

Fig. 5 .
Fig. 5. N in crop residues (kg ha −1 ) at harvest according to the crop and the previous crop.Bars with the same letter are not significantly different.Error bars = standard error of the mean.

Table 2 .
Sowing dates of each crop, total rate, dates and amounts of nitrogen fertilizer applied on each crop according to each rotation.