Effect of legume intercropping on N2O emission and CH4 uptake during maize production in the Ethiopian Rift Valley

Abstract. Intercropping with legumes is an important component of climate smart agriculture (CSA) in sub Saharan Africa, but little is known about its effect on soil greenhouse gas (GHG) exchange. A field experiment was established at Hawassa in the Ethiopian rift valley, comparing nitrous oxide (N2O) and methane (CH4) fluxes in minerally fertilized maize (64 kg N ha−1) with and without crotalaria (C. juncea) or lablab (L. purpureus) as intercrops over two growing seasons. To study the effect of intercropping time, intercrops were sown either three or six weeks after maize. The legumes were harvested at flowering and half of the above-ground biomass was mulched. In the first season, cumulative N2O emissions were largest in 3-week lablab, with all other treatments being equal or lower than the fertilized maize monocrop. After reducing mineral N input to intercropped systems by 50 % in the second season, N2O emissions were at par with the fully fertilized control. Maize yield-scaled N2O emissions in the first season increased linearly with above-ground legume N-yield (p = 0.01), but not in the second season when early rains resulted in less legume biomass because of shading by maize. Growing season N2O-N emission factors varied from 0.02 to 0.25 and 0.11 to 0.20 % of the estimated total N input in 2015 and 2016, respectively. Growing season CH4 uptake ranged from 1.0 to 1.5 kg CH4-C ha−1 with no significant differences between treatments or years, but setting off the N2O-associated global warming potential by up to 69 %. Our results suggest that high yielding leguminous intercrops entail some risk for increased N2O emissions when used together with recommended fertilization rates, but can replace part of the fertilizer N without compromising maize yields in the following year and thus support CSA goals while intensifying crop production in the region.



Introduction 35
With a rapidly increasing population and declining agricultural land in Sub-Saharan Africa (SSA), increasing productivity per area (intensification) is the only viable alternative for producing sufficient food and feed (Hickman et al., 2014a).Intensification entails increased use of inorganic fertilizers, which may cause N2O emissions and reduce the soil CH4 sink (Castro et al., 1994, Xie et al., 2010).Climate smart agriculture (CSA), by contrast, has been proposed as a way forward to 40 simultaneously increase agricultural productivity and profits, while increasing climate resilience and reducing greenhouse gas (GHG) emissions (Neufeldt et al., 2013).However, understanding of greenhouse gas emissions from crop production in SSA in general and CSA in particular is limited and the potential of crop production in SSA as a source or sink of the greenhouse gases CO2, N2O, and CH4 is understudied (Kim et al., 2016, Hickman et al., 2014b).Moreover, modelling studies 45 predict significant negative impacts of climate change on crop productivity in Africa (Blanc and Strobl, 2013) and it is largely unknown how these and the countermeasures taken to maintain agricultural productivity will affect GHG emissions.
Crop production is a major source of nitrous oxide (N2O), the third-most important anthropogenic GHG after CH4 and CO2 (IPCC, 2014).Inorganic and organic N added to soil provide ammonium 50 (NH4 + ) and nitrate (NO3 -) for nitrification and denitrification, respectively, which are the two main processes of microbial N2O production in soil (Khalil et al., 2004).The rate of N2O formation in upland soils depends greatly on the extent and distribution of anoxic microsites, which is controlled by soil moisture, texture and the distribution of decomposable organic matter and NH4 + fueling heterotrophic and autotrophic respiration, respectively (Schlüter et al., 2019, Wrage-Mönnig et al., 55 2018).The magnitude of soil N2O emissions depends on O2 availability as controlled by soil moisture and respiration, availability of mineral N and readily decomposable C (Harrison-Kirk et al., 2013) and soil pH (Russenes et al., 2016), all of which depend on soil management practices.
The N2O yield of nitrification (Nadeem et al., 2019) and the production and reduction of N2O during denitrification (Bakken et al., 2012) are further controlled by soil pH and by the balance 60 between oxidizable carbon and available NO3 - (Wu et al., 2018).Mulching and incorporation of crop residues leads to increased N mineralization and respiratory O2 consumption, thus potentially enhancing N2O emissions both from nitrification and denitrification (Drury et al., 1991), if soil moisture is sufficient to support microbial activity and restrict O2 diffusion into the soil.
Crop diversification by combining legumes with cereals, both in rotation and intercropping, enhances overall productivity and resource use efficiency (Ehrmann and Ritz, 2014).Intercropping of maize with grain legumes is common in the rift valley of Ethiopia and central in CSA (Arslan et al., 2015).In low input systems common to the Rift Valley, integration of legumes with cereals 70 diversifies the produce and improves the nitrogen nutrition of the cereal.Moreover, by partially replacing energy-intensive synthetic N, intercropping with legumes may increase the sustainability of the agroecosystem as a whole (Carranca et al., 2015).However, to make best use of the resource use complementarity of inter and main crop, the planting time of the intercrop has to be optimized so that the maximum nutrient demand of the two components occurs at different times (Carruthers 75 et al., 2000).The timing of intercrops could also affect N2O emissions if N mineralization from legume residues is poorly synchronized with the N requirement of the cereal crop.This can be counteracted by reducing mineral N additions to intercropping systems, but the timing of the intercrop (sowing date relative to the cereal crop) remains an issue that has, to the best of our knowledge, not been studied with regard to N2O emissions.

80
Intercropping and mulching may also affect the soil's capacity to oxidize atmospheric CH4 as abundant NH4 + inhibits methanotrophs (Laanbroek and Bodelier, 2004).However, field studies with incorporation of leguminous or non-leguminous catch crops have been inconclusive (e.g.Sanz-Cobena et al., 2014).In a meta-study on CH4 fluxes in non-wetland soils, Aronson and Helliker (2010) concluded that N inhibition of CH4 uptake is unlikely at fertilization rates below 85 100 kg N ha -1 y -1 and that much to the contrary, N addition may stimulate CH4 uptake in N-limited soils.Ho et al. (2015) found that incorporation of organic residues stimulated CH4 uptake even in In a review on N2O fluxes in agricultural legume crops, Rochette and Janzen (2005) concluded that the effect of legumes on N2O emission is to be attributed to release of extra N by root exudation and decomposition of nodules rather than to the process of nitrogen fixation itself.Intercropped legumes may thus affect N2O emissions in two ways: by directly providing organic N or by 95 modulating the competition between plants and microbes for soil N. Compared to mineral fertilizers, N supply from biological fixation is considered environmentally friendly as it can replace industrially fixed N (Jensen and Hauggaard-Nielsen, 2003), provided that crop yields remain the same.However, combining easily degradable crop residues with synthetic N can lead to elevated N2O emissions (Baggs et al., 2000), potentially compromising the environmental 100 friendliness of intercropping in CSA.It is well known that the effect of crop residues on N2O emission depends on a variety of factors such as residue amount and quality (C:N ratio, lignin and cellulose content), soil properties (e.g.texture), placement mode (mulching, incorporation) and soil moisture and temperature regimes (Sanz-Cobena et al., 2014, Li et al., 2016).So far, there is only a limited number of studies addressing the effect of legume intercropping on N2O emissions 105 and CH4 uptake in SSA crop production (Baggs et al., 2000;Millar et al., 2004;Dick et al., 2008).
The main objective of the present study was to evaluate the effects of forage legume intercropping of maize on N2O and CH4 emissions during maize production in the Ethiopian Rift Valley.We hypothesized that forage legumes increase N2O emissions and decrease CH4 uptake depending on above-ground biomass, legume species and sowing date; legumes intercropped three weeks after 110 sowing of maize would result in higher yields than those intercropped six weeks after maize and lead to increased N2O emissions if used with full-dose mineral fertilization.With late intercropping, legumes yields would be suppressed having no or little effect on N2O emission.
Choosing legume species and sowing date and accounting for N inputs from legume intercrops, thus could allow to manage legume intercropping in SSA with reduced GHG emissions.

Study area
The field experiment was conducted at the Hawassa University Research Farm, 07°3'3.4"Nand 120 38°30"20.4'Eat an altitude of 1660 m a.s.l.The mean annual rainfall is 961 mm, with a bimodal pattern.The rainy season between June and October accounts for close to 80% of the annual rainfall.Average maximum and minimum monthly temperatures are 27.4 and 12.9 o C, respectively.
The soil is a clay loam (46% sand, 26% silt, 28% clay), with a bulk density of 1.25 g cm -3 , a total N content of 0.12%, an organic C content of 1.64 %, an available Olsen P content of 175 mg kg -1 125 and a pHH2O of 6.14.

Experimental design and treatments
Experimental plots (20 m 2 ) with six treatments were laid out in a complete randomized block design (RCBD) with four replicates (Tab.1).Seed bed was prepared in both years by mold board plow to a depth of 0.25 m followed by harrowing by tractor.A hybrid maize variety, BH-540 130 (released in 1995) was sown on May 30 and May 7 in 2015 and 2016, respectively.Maize was planted at a density of 53,333 plants ha -1 .Following national fertilization recommendations, diammonium phosphate (18 kg N, 20 kg P) was applied manually at planting and urea (46 kg N) four weeks after sowing maize, except for the unfertilized control.The N fertilization rate was halved for the intercropping treatments in the 2016 season to account for carry-over of N from 135 forage legumes grown in the previous year.The forage legumes crotalaria (C.juncea) and lablab (L.purpureus) were planted between maize rows at a density of 500,000 and 250,000 plants ha -1 , respectively.
The above-ground forage legume biomass was harvested at flowering and half of it was removed.
The remaining half was spread manually between the maize rows after cutting the fresh biomass 140 into ~10 cm pieces.As the mulching was done plot wise, plots within the same treatment received different amounts of mulch depending on the legume yield of each plot.In the 2016 growing season, all treatments were kept on the same plots as in 2015, capitalizing on plot-specific N and C input from previous mulch.Aboveground dry matter yield was determined by drying a subsample at 72 o C for 48 hours and C and N contents were measured by an element analyser.

N2O and CH4 fluxes and ancillary data
GHG exchange was monitored between the maize rows by static chambers (Rochette et al., 2008), using custom-made aluminum chambers with an internal volume of 0.144 m 3 and a cross-sectional  Sampling was carried out weekly during the period June to September, in 2015 and May to September, in 2016 on 15 and 17 sampling dates, respectively.Gas samples were collected between 9:00 AM and 2:00 PM.For each flux estimate, four gas samples were drawn from the chamber headspace at 15 min intervals, using a 20 ml polypropylene syringe equipped with a 3way valve.Before transferring the sample to a pre-evacuated 10 cc serum vial crimp-sealed with 155 butyl septa, the sample was pumped 5 times in and out of the chamber to obtain a representative sample.Overpressure was maintained to protect the sample from atmospheric contamination during storage and shipment to the Norwegian University of Life Sciences, where the samples were analyzed by gas chromatography.He-filled blank vials were included to evaluate contamination, which was found to be less than 3% of ambient.
160 All samples were analyzed on a GC (Model 7890A, Agilent Santa Clara, CA, USA) connected to an auto-sampler (GC-Pal, CTC, Switzerland).Upon piercing the septum with a hypodermic needle, ca. 1 ml sample is transported via a peristaltic pump (Gilson minipuls 3, Middleton, W1, USA) to the GC's injection system, before reverting the pump to backflush the injection system.
The GC is configured with two back-flushed pre-columns and a Poraplot U wide-bore capillary 165 column connected to a thermal conductivity, a flame ionization and an electron capture detector to analyze CO2, CH4 and N2O, respectively.Helium 5.0 was used as carrier and Ar/CH4 (90:10 vol/vol) as makeup gas for the ECD.For calibration, two certified gas mixtures of CO2, N2O and CH4 in He 5.0 (Linde-AGA, Oslo, Norway), one at ambient concentrations and one ca.3 times above ambient were used.A running standard (every tenth sample) was used to evaluate drift of the molecular volume of gas at chamber temperature (m 3 mol -1 ).A quadratic fit was only used in cases where N2O accumulation in the chamber showed a convex downwards and CH4 uptake a convex upwards trend (i.e.decreasing emission or uptake rates with time) to estimate time-zero 180 rates.Fluxes were cumulated plot-wise by linear interpolation for each growing season.
In 2016, soil moisture and temperature at 5 cm depth were monitored hourly using data loggers Intact soil bulk density and an assumed particle density of 2.65g cm -3 were used to calculate daily water filled pore space values for the 2016 growing season: Eq. ( 2) where WFPS is the water filled pore space, VSWC the volumetric soil water content, BD the bulk density and PD the particle density which was set to 2.65 g cm -3 .Daily rainfall data were collected using an on-site rain gauge monitored daily during the growing season.

Estimating N inputs and N2O emission factors
195 N input from forage legume crop residues was estimated from measured above-ground dry matter yield, its N content and the amount of mulch applied.To account for belowground inputs a shoot to root ratio of two was assumed for both crotalaria and lablab (Fageria et al., 2014).Dry matter yields of forage legumes differed greatly depending on sowing time, with generally larger yields in 3-week than 6-week intercropping.Also, forage legumes sown three weeks after maize grew 200 faster and were harvested and mulched earlier than those sown six weeks after maize.We assumed that 50% of the legume N (mulched and belowground) was released during the growing season but reduced this amount to 30% for the aboveground component (mulch) of the 6-week treatments to account for the later mulching date.The proportions becoming available during the growing seasons are conservative estimates based on Odhiambo (2010), who reported that about 50% of N 205 contained in crotalaria, lablab and mucuna was released during a 16-week incubation experiment at optimal temperature and moisture conditions.Placing litter bags into dry surface soil, Abera et For the second year, 50% of the N left after the growing season (below and aboveground) was 210 assumed to become available, on top of the N-input from the newly sown forage legumes.Dry matter yields of forage legumes and estimated N input for the two years are presented in table 1.
Treatment-specific, growing-season N2O emission factors were calculated as: * 100 Eq. ( 3) where N2O EF is the N2O emission factor (% of N input lost as N2O-N), N2Otreatment the cumulative 215 N2O-N emission (from sowing to harvest) in the fertilized and intercropped treatments, N2Ocontrol the emission from the 0N0P treatment (background emission) and Ninput the estimated total input of N.

Grain yields and yield-scaled N2O emissions
Maize grain yield was determined by manually harvesting the three middle rows (to avoid border 220 effects) of each plot, and was standardized to 12.5% moisture content.All values were extrapolated from the plot to the hectare.To estimate yield-scaled N2O emissions (g N2O-N ton -1 grain yield), cumulative emissions were divided by grain yield.

Statistical analysis
Differences in cumulative CH4 and N2O emissions between treatments in each cropping season

Weather conditions 230
The year 2015 was one of the most severe drought years in decades and, as a result, sowing in 2015 was delayed by 3 weeks as compared to 2016.Rain fell late during the growing season and the cumulative rainfall for April to October was about 100 mm lower in 2015 than in 2016 (Fig. 1d and 1g).
During the 2016 season, N2O emission rates in the 0N-control varied between 2.5 and 22.8 µg N m -2 h -1 , peaking at the beginning of the season when WFPS was >50%.There were no significant differences in WFPS values between treatments (data not shown).Fertilized maize had similar 250 rates (3.1 -24.2 µg N m -2 h -1 ) peaking at around four weeks after planting.Maize-forage legume treatments had larger emission rates, ranging from 1.8 to 40.2 and 3.2 to 58.6 for crotalaria planted 3 and 6 weeks after maize, respectively and 3.9 to 38.0 and 1.9 to 45.2 µg N m -2 h -1 for lablab planted 3 and 6 weeks after maize, respectively.In general, emission rates were higher in the beginning than in the end of the cropping season (Fig. 1d-f).Despite higher fluxes for 255 intercropping treatments than in the unfertilized control in week 1 (P=0.162)and 4 (P=0.061),there were no statistically significant differences in flux rates between the treatments.

Cumulative N2O emissions
During the 2015 growing season, all treatments had equal or higher cumulative N2O emissions than the unfertilized control, with the 3-week lablab intercropping system emitting significantly 260 more N2O than the unfertilized control (p=0.006) and the 6-week lablab intercrop (Fig. 2a).
Comparing intercropping treatments with the fertilized control, lablab sown three weeks after maize clearly increased N2O emissions but not significantly (P=0.35),whereas all other intercropping treatments had cumulative N2O emissions comparable with fertilized maize control.
Regarding sowing date, 3-week lablab had significantly higher N2O emissions (P<0.01)than its 6-265 week counterpart, whereas no such effect was seen for crotalaria.
During the 2016 growing season, lablab intercropping 3-weeks after maize showed significantly higher (P<0.01)cumulative N2O emissions than the unfertilized control, but there was no difference between fully fertilized maize monocrop and intercropped maize treatments fertilized with 50% of the mineral N applied in 2015, nor was there any effect of intercropping date (3 vs. 6 270 weeks; Fig. 2b).

Legume and maize yields
Aboveground yields of lablab were generally higher than those of crotalaria (Table 1).
Intercropping three weeks after maize resulted in higher biomass yields compared to six weeks for both legume species.Both legumes grew poorly during the second growing season, particularly 275 crotalaria.Maize grain yields differed greatly between the years and were roughly 20% higher in the wetter year of 2016 (Table 2).Better growth conditions for maize in the second year resulted in smaller yields of intercrop legumes.

N2O emission factor and intensity
Growing-season emission factors (EF) varied from 0.02 to 0.25 and 0.11 to 0.20% in 2015 and 280 2016, respectively (Table 2).Of the intercropped treatments, lablab intercropped three weeks after maize resulted in a significantly larger emission factor than fertilized maize and other intercropping treatments, whereas there was no significant difference in 2016.Overall, growingseason N2O emission factors were ~ 40% higher in 2016 than in 2015, which is mainly due to the smaller N input in 2016 which was 25 to 45% lower than in 2015, except for the 3-week lablab 285 system which had an estimated 18% higher N input in 2016 than 2015 (Table 1).The latter was due to the extraordinary high lablab yield in the previous year and its stipulated carryover (Table 1).
Mean yield-scaled N2O emissions in 2015 varied between 25 to 55 g N2O ton -1 grain yield.In 2015, 3-week lablab had a higher N2O intensity than 6-week lablab, whereas all other differences 290 were insignificant.In 2016, with mineral N fertilization reduced to 50%, N2O emission intensities varied from 26 to 37 g N2O ton -1 grain, with no significant effect of legume species, sowing date or N fertilization (Table 2).
To further explore the variability of N2O emissions, we plotted cumulative N2O emissions plotwise against legume N yield, but found no relationship (not shown).However, when plotting yield-295 scaled N2O emission over legume N yield, a significant positive relationship (P=0.01)emerged for 2015, but not 2016 (Fig. 3a and 3b), suggesting that leguminous N input increased N2O emissions more than maize yields in the dry year of 2015.

CH4 fluxes
All treatments acted as net sink for CH4, with uptake rates ranging from 31 to 93 µg C m -2 h -1 in 300 2015 (Fig. 4a-c).Uptake rates in 2015 were rather constant in time with somewhat elevated uptake rates towards the end of the season.There were no obvious treatment effects.By contrast, in the wetter year of 2016, CH4 uptake showed a pronounced maximum in the beginning of June with uptake rates of up to 140 µg C m -1 h -1 irrespective of treatment (Fig. 4d-f), when WFPS values declined to values below 25% (Fig. 4g).Methane uptake during this period tended to be greatest 305 in the unfertilized control, while intercropping treatments had smaller uptake rates, which, however, were not significantly different from maize monocrop treatments.Differences between treatments at single sampling dates were insignificant throughout the season.Highest CH4 uptake in 2016 was recorded with lowest WFPS (~10%).

Cumulative CH4 uptake 310
Cropping season cumulative CH4 uptake exceeded 1 kg C ha -1 in both years with no significant effect of intercropping, legume species or time of intercropping (Fig. S1a and S1b).Plots intercropped with crotalaria tended to take up less CH4 but this effect was not statistically significant in neither 2015 nor 2016 (P=0.056).Plotting cumulative CH4 uptake plot-wise over legume dry matter yield did not result in a significant relationship, but highest seasonal uptake 315 rates occurred in plots with lowest legume dry matter yield (data not shown).

Non-CO2 GWP
Non-CO2 global warming potentials (GWP) were calculated as CO2 equivalents balancing cumulative seasonal N2O-N emissions with CH4 uptake on the plot level and averaging them for treatments (Table 2, Fig. 5).The relative contribution of CH4 to the non-CO2 GWP of the different 320 cropping systems varied between 22 and 69% and was highest in the non-fertilized maize monocrop.Three-week lablab intercropping resulted in significantly higher GWP compared with 6-week lablab intercropping and maize mono-cropping (Table 2).By contrast, in 2016, legume species but not intercropping time affected the GWP balance (P<0.05).Lablab intercropped 3 weeks after maize resulted in significantly higher (P<0.05)GWP than the unfertilized control but 325 was indistinctive from the fertilized maize monocrop, or other intercrop treatments (Table 2, Fig. 5a and 5b).

Maize-legume intercropping and N2O emissions
Background N2O emissions (in unfertilized maize monocrop) fluctuated between 1.1 and 23 µg N2O-N m -2 h -1 , which is in the range of previously reported emission rates for soils in SSA with low N fertilizer input (Pelster et al., 2017).Baseline emissions were somewhat higher in the wetter season of 2016, owing ~100 mm more rainfall (Fig. 1d and 1g).Elevated emission rates >30 µg N2O-N m -2 h -1 occurred in 2015 on few occasions in intercrop treatments, notably in mid-August 335 when rainfall occurred right after mulching of the three-week intercrops.Mulching of the six-week intercrops did not affect N2O emission, probably because the mulched legume biomass too small to affect the flux (Fig. 1b, 1c; Table 1).In 2016, mulching of the 3-week legumes was followed by rainfall, increasing the WFPS to 50% (Fig. 1g), however, without resulting in elevated N2O emission rates (Fig. 1e, 1f).Together, this suggests that the direct effect of mulching on N2O 340 emission depends on soil moisture and the amount of mulched biomass, and can hence not be generalized.
Legume dry matter yields varied strongly (100 to 3000 kg ha -1 ) throughout the two experimental years (Table 1, Fig. 3), depending on species, intercropping time and weather.Three-week intercrops performed generally better than six-week intercrops, which appeared to be inhibited in growth by shading through maize.This was particularly apparent for the low-growing lablab legume.In terms of legume biomass, lablab grew more vigorously and realized larger dry matter yields than crotalaria (Table 1).Moreover, lablab is known to be a better N2 fixer than crotalaria (Ojiem et al., 2007).Together, this resulted in a wide range of potential leguminous N-inputs in our experiment, which could be used to examine their overall effect on N2O emission under 350 Ethiopian rift valley conditions on a seasonal basis.Surprisingly, we did not find any significant relationship between estimated total N input or legume N yield and cumulative N2O emission.This may be due to the notoriously high spatial and temporal variability of N2O emissions rates within treatments, or reflect the fact that intercropping had no or opposing effects on N2O forming processes.Cumulative N2O emissions and legume N yields integrate over the entire season and do 355 not capture seasonal dynamics of soil N cycling and N uptake, which could obscure or cancel out transient legume effects on N2O emissions.Possibly, N released in intercropping treatments was effectively absorbed by the main crop, even though intercropping did not lead to significantly higher maize grain yields in our experiment.Alternatively, changes in physicochemical conditions brought about by intercrops, such as potentially lower soil moisture due to more 360 evapotranspiration, may have counteracted the commonly observed stimulating effect of legume N on N2O emissions (Almaraz et al., 2009, Sant'Anna et al., 2018).
To further elucidate the N2O emission response to legume intercropping, we plotted cumulative N2O emissions normalized for grain yields ("N2O intensity") plot-wise over measured legume N yields, thereby utilizing the wide range of potential leguminous N inputs provided by our 365 experiment.A significantly positive relationship between N2O intensity and legume N yields emerged for 2015, suggesting that intercropped legumes indeed increase N2O emissions relative to maize yields (Fig. 3a).It is impossible to say, however, whether this relationship was driven by the extra N entering the system through biological N fixation, or whether an increasing legume biomass affected physicochemical conditions in the rhizosphere favoring N2O formation.In 2016, 370 legume dry matter yields were much lower than in 2015, owing early rains favoring maize growth, and no significant relationship with N2O intensity was found (Fig. 3b).This illustrates that the effect of legume intercropping on N2O emissions is highly dependent on sowing date and weather, both of which control the growth of legume and main crop and ultimately the amount and fate of leguminous N in the intercropping system.Our data suggest that excessive accumulation of 375 leguminous biomass in SAA maize cropping enhances the risk for elevated N2O emissions.We expected N2O emissions to respond more strongly to intercropping in the second year ( 2016), as legume mulches were applied according to their plot-wise aboveground yields in the previous year.Indeed, N2O emission rates were clearly higher in intercropping plots on the first sampling date in 2016 (fig.1e and 1f), indicating increased N cycling in mulched plots.This difference 380 vanished quickly, however, suggesting that the effect of intercrop mulches, even at high amounts (Table 1), on N2O emissions in the subsequent year is negligible under SSA conditions.It is noteworthy that our estimates of the fraction of N carried over between the years were based on literature data (Table 1), and that a considerable part of the mulched N may have been lost during abundant rainfalls (300 mm) early in the 2016 season before crops were sown.

385
It is striking that cumulative N2O emissions were at par with the fully fertilized maize monocrop in 2016.This effect, however, was short-lived and no significant difference in average flux rates was seen during the remainder of the season resulting in statistically indistinguishable cumulative N2O emissions.This may be partly due to the 50% reduction in mineral N application to intercrop treatments, as found by others (Tang et al., 2017).Another reason may be that a considerable 390 proportion of the cumulative emission in 2016 occurred before or shortly after 3-week intercrops were sown, and was thus unaffected by growing legumes.Overall, cumulative N2O emissions were equal or higher in 2016 than in 2015, despite reduced mineral N addition to intercrops and lower legume biomass.Ultimately, the lack of a clear emission response to legume intercropping in the second year calls for studies tracing cumulative mulching effects over multiple years.In our study, 395 amount and timing of rainfall appeared to be more important for N2O emissions in the second year than amount and carryover of legume N.
Given our finding that N2O intensity responded positively to legume biomass and its N content in a drought year with poor maize growth, intercrop species and sowing and harvest date (relative to the main crop) emerge as viable management factors for controlling N2O emissions in SSA 400 intercropping systems.Legume species and cultivar in intercropping systems are known to be critical for N loss, both during the intercropping and the subsequent seasons (Pappa et al., 2011, Weiler et al., 2018).The stimulating effect of crop residues on N2O emission has been reported to depend on residue quality and soil moisture, with denitrification being the likely process (Li et al., 2016).Our study provides evidence that vigorous growth of high yielding legume intercrops can

Seasonal N2O and CH4 emission, EFN2O and GWP
Growing season N2O emissions in fertilized treatments varied from 0.17 to 0.33 and 0.23 to 0.3 kg N2O-N ha -1 in 2015 and 2016 covering 107 and 123 days, respectively (Fig. 2), and a range of total N inputs from 36.4 to 97.8 kg N ha -1 (Table 1).There are no N2O emissions studies for maizelegume intercropping in the Ethiopian Rift valley so far.Hickman et al. (2014a) reported N2O 415 emissions of 0.62 and 0.81 kg N per ha and 99 days for 100 and 200 kg N input ha -1 , respectively, for a maize field without intercropping in humid western Kenya.Baggs et al. (2006), working in the same region with maize intercropped with legumes in an agroforestry system reported N2O emissions ranging from 0.2 to 0.6 kg N ha -1 with higher emissions in tilled intercropping treatments.The largest seasonal N2O emission for intercropping reported so far from SSA is 4.1 420 kg N ha -1 (84 days) after incorporating 7.4 t ha -1 of a Sesbania-Macroptilium mixture in humid western Kenya (Millar et al., 2004).Compared to the N2O emissions reported for humid tropical maize production systems, our data suggest that maize-legume intercropping based on mulching in the sub-humid to semi-arid Rift valley appears to be a minor N2O source.Growing season N2O emission factors (EF) in our study ranged from 0.02 to 0.25 and 0.11 to 0.20% of the estimated 425 total N input in 2015 and 2016, respectively, including assumed N inputs from legume mulch as well as belowground additions and carryover between the years (Table 1).Even if the estimated EF is doubled to account for off-season emissions, it is still lower than the annual IPCC default value of 1% N2O-N per unit added N (IPCC, 2014).Our estimated EFs thus seem to be at the lower end of those reported by Kim et al. (2016) for SSA smallholder agriculture estimated from 430 literature data (0.01 to 4.1%).The reasons for the low EFs in our study are probably the high background emissions in the fertile soil of the Hawassa University research farm which supports high maize yields even in the unfertilized control (Table 1) and the low levels of N input.The soil has been used over decades for agronomic trials with various fertilization rates with and without crop residue retention and legume intercropping (Raji et al., 2019).Thus, our field trial has to be 435 considered representative for intensive management as opposed to smallholder systems with minimal or no fertilization history.Methane uptake by the soil in both seasons varied between 1.0 to 1.5 kg CH4-C ha -1 without showing any significant treatment effect, even though maize-legume intercrops tended to take up less CH4 than maize monocrops (Fig. S1).The observed trend might relate to competitive 440 inhibition of CH4 oxidation by higher NH4 + availability (Le Mer andRoger, 2001, Dunfield andKnowles, 1995) in the presence of legume intercrops, even though estimated total N inputs remained below 100 kg N ha -1 , which is considered a threshold for NH4 + inhibition (Aronson and Helliker, 2010).Alternatively, densely growing legumes may have lowered CH4 uptake through impeding CH4 and/or O2 diffusion into the soil (Ball et al., 1997).We did not observe stimulation 445 of CH4 uptake by legume intercropping, which we attribute to the absence of N and P deficiency in this fertile soil.Methane uptake rates varied from 20 to 140 µg CH4-C m -2 h -1 which is in the range of rates reported previously for SSA upland soils (Pelster et al., 2017).Seasonal CH4 uptake in our experiment offset between 22 and 69% of the N2O-GWP without revealing any significant treatment effect (Fig. S1a and S1b), but the offset was relatively largest in the unfertilized maize 450 monocrop and smallest in lablab intercropping.Hence, CH4 uptake appears to be an important component of the non-CO2 climate footprint of SSA crop production.

Legume intercropping and climate smart agriculture
Legumes are an important N source in smallholder farming systems, where mineral fertilizers are unaffordable or unavailable.Legume intercrops maximize resource use efficiency as total 455 productivity is often higher than in mono-cropping systems (Banik et al., 2006).Moreover, N fixed biologically by legume intercrops can partly replace synthetic N fertilizers, if the release is synchronized with the nutrient demand of the cereal crop.On the other hand, surplus N from legumes may result in N losses as NO3 -, NH3 and NO, N2O or N2.Mulching and incorporation of legume biomass has been found to increase N2O emissions under temperate conditions (Baggs et 460 al., 2000, Baggs et al., 2003) and under humid tropical conditions (Millar et al., 2004).Also under semi-arid, Mediterranean conditions, vetch (V.villosa) used as a winter catch crop and mulched in spring significantly increased N2O emissions during the fallow period while rape did not (Sanz-Cobena et al., 2014).This was later confirmed by a 15 N study, highlighting the role of N mineralization from legumes as a source of N2O (Guardia et al., 2016).None of the studies found 465 an overall N2O saving effect of catch crops when scaling up to the entire crop cycle, even though the latter study used reduced mineral N fertilization rates in treatments with catch crops.By contrast, reduced NO3 -leaching and N2O emission has been reported from maize intercropped with legumes in the semi-arid North China plain, which the authors attributed to enhanced N uptake by both the inter and main crop and reduced soil moisture in treatments with intercrops during the 470 rainy season (Huang et al., 2017).This shows that legume intercrops have a potential to both increase or reduce N2O emissions with consequences for the non-CO2 footprint of cereal production and hence for the viability of intercropping as a central component of CSA (Thierfelder et al., 2017).
The legume intercrops used in our study have low C:N ratios (Table S1), and can be expected to 475 release a significant part of their N through decomposition of roots and nodules or root exudation as well as during decomposition of mulches (Fustec et al., 2010).The effect of mulching on N2O emissions depends on the C:N ratio of the residues with increased emissions for low C:N ratio residues (Baggs et al., 2000, Shan andYan, 2013).In line with this, N2O emissions in intercrop treatments of our study exceeded those in fertilized maize monocrop on several sampling dates, 480 both during active growth of legumes and after mulching.Another important aspect is the amount of legume N carried over between years which depends, among others, on amount and quality of the legume and the weather between the growing seasons.Abera et al. (2014) showed that surfaceplaced residues of haricot bean and pigeon pea decompose quickly despite relatively dry conditions during offseason.Vigorous rainfalls in the beginning of the growing season like in 2016 could lead 485 to dissolved N losses, which will lead to indirect N2O emissions elsewhere or to elevated direct N2O emissions as seen on the first sampling date in 2016.

Conclusion
While legume intercrops have the potential to improve cereal yields and diversify produces for 490 smallholders in SSA, a risk of enhanced N2O emissions remains, which became apparent as increased "N2O intensity" of the main crop in a drought year (2015).For treatment names, see Fig. 2.
fairly N-rich Dutch soils.Intercrops may indirectly affect CH4 uptake by lowering soil moisture and thus increase the diffusive flux of atmospheric CH4 into the soil.Accordingly, Wanyama et https://doi.org/10.5194/bg-2019-303Preprint.Discussion started: 13 August 2019 c Author(s) 2019.CC BY 4.0 License.al. (2019) found CH4 uptake to be negatively correlated with mean annual water-filled pore space 90 in a study on different land use intensities in Kenya.

𝐹
170 the ECD signal.Emission (CO2, N2O) and uptake (CH4) rates were estimated by fitting linear (R 2 ≥ 0.85) or quadratic functions to the observed concentration change in the chamber headspace and converting them to area flux according to eq. 1 FGHG is the flux (μg N2O-N m −2 h −1 in case of N2O; µg CH4-C in the case of CH4),   the 175 rate of change in concentration over time (ppm min -1 ), Vc the volume of the chamber (m 3 ), A the area covered by the chamber (m 2 ), Mn the molar mass of the element in question (g mol -1 ) and Vn https://doi.org/10.5194/bg-2019-303Preprint.Discussion started: 13 August 2019 c Author(s) 2019.CC BY 4.0 License.
Decagon EM50, Pullman, WA, USA) together with ECH2O sensors (Decagon) for volumetric soil water content (VSWC) and temperature at five points across the experimental field.The sensors were placed in control, M+Cr3w and M+Lb3w (2).No data are available for the 2015 185 season, due to equipment failure.
405enhance N2O emissions in years unfavorable for maize growth, whereas in years with sufficient water availability early in the growing season, maize growth is favored preventing excessive https://doi.org/10.5194/bg-2019-303Preprint.Discussion started: 13 August 2019 c Author(s) 2019.CC BY 4.0 License.growth of the intercrop.Our study therefore points to sowing date as the most promising option to control growth of the intercrop relative to the main crop and hence to deal with the risk of increased N2O emissions with legume intercrops. 410 https://doi.org/10.5194/bg-2019-303Preprint.Discussion started: 13 August 2019 c Author(s) 2019.CC BY 4.0 License.

Figure 1 :
Figure 1: Mean N2O emission rates (n=4; error bars = SEM) in 2015 (left panel) and 2016 (right panel) and daily rain fall and water-filled pore space (in 2016 only).Figures a and d show emission rates in the absence of intercrops, b and e with crotalaria and c and f with lablab

Figure 3 :
Figure 3: Relationship between N2O emission intensity and intercrop legume biomass yield in intercrop treatments in 2015 (a) and 2016 (b).Shown are single-plot values for each treatment (n=4).

Figure 4 :
Figure 4: Mean CH4 flux in 2015 (left panel) and 2016 (right panel) and daily rainfall and waterfilled pore space (in 2016 only).Error bars show standard error of the mean (n=4).Figures a and d show emission rates in the absence of intercrops, b and e with crotalaria and c and f with lablab intercropping.

Figure 5 :
Figure 5: Relative contribution of CH4 uptake and N2O emission to seasonal GWP in mono-and intercropping treatments in 2015 (a) and 2016 (b).Error bars indicate standard deviation (n=4).

Table 1 :
At the same time, our study points at possibilities to manage this risk by actively controlling legume biomass development and hence potential N input through "climate-smart" choices of legume species, sowing date and mulch amounts.Our study was conducted on a nutrient-rich soil which supports high yields of both maize 495 and leguminous intercrops.Under these conditions, intercropped legumes can replace a considerable part of synthetic fertilizer, thus supporting common CSA goals.However, more https://doi.org/10.5194/bg-2019-303Preprint.Discussion started: 13 August 2019 c Author(s) 2019.CC BY 4.0 License.studies are needed to fully explore intercropping options in the framework of CSA in the East-African Rift Valley, particularly in nutrient-poor smallholder fields.Future studies on CSA approaches in SSA should address, in addition to non-CO2 greenhouse gas emissions, N-runoff 500 and soil organic matter build up, ideally in long-term field trials with and without legume intercropping.Given that seasonal N2O emission factors and intensities in our study were in the lower range of published values for SSA, intercropping appears as a promising approach to sustainable intensification in the Ethiopian Rift Valley.N inputs from forage legumes and fertilization per treatment which was estimated as 675 outlined in the Materials and Method section 3.4.Shown are mean values (n=4 ± standard error) 505 Acknowledgements.The study is part of the NORHED program "Research and capacity building in climate smart agriculture in the Horn of Africa" funded by the Norwegian Agency for Development Cooperation (Norad).We are grateful to Teshome Geletu, Teketel Chiro and Tigist Yimer for assistance during setting up and managing the field experiment, sample collection and 510 preparation.https://doi.org/10.5194/bg-2019-303Preprint.Discussion started: 13 August 2019 c Author(s) 2019.CC BY 4.0 License.https://doi.org/10.5194/bg-2019-303Preprint.Discussion started: 13 August 2019 c Author(s) 2019.CC BY 4.0 License.

Table 2 :
Grain yield, growing-season N2O emission factors and emission intensities for 107 and 123 days in 2015 and 2016, respectively and combined global warming potential (GWP) of N2O emission and CH4 uptake for fertilized treatments with and without legume intercropping.N input was estimated as outlined in Table 1.Shown are mean values (n=4 ± standard error).Different letters indicate statistical difference at https://doi.org/10.5194/bg-2019-303Preprint.Discussion started: 13 August 2019 c Author(s) 2019.CC BY 4.0 License.