- Original article
- Open Access
Legume-based mixed intercropping systems may lower agricultural born N2O emissions
© Senbayram et al. 2015
Received: 11 August 2015
Accepted: 8 December 2015
Published: 16 January 2016
The area used for bioenergy crops (annual row crops (e.g., wheat, maize), herbaceous perennial grasses, and short-rotation woody crops (e.g., poplar)) is increasing because the substitution of fossil fuels by bioenergy is promoted as an option to reduce greenhouse gas (GHG) emissions. However, biomass used for bioenergy production is not per se environmentally benign, since bioenergy crop production is associated with negative side effects such as GHG emissions from soil (dominated by N2O). N2O emissions vary greatly in space and time; thus, direct comparison of soil N2O fluxes from various agro-ecosystems is certainly crucial for the assessment of the GHG reduction potential from energy crops.
Therefore, our study aimed to evaluate the two different agro-ecosystems (cropland and agro-forestry) cultivated in central Germany for their environmental impact. In a 1-year field experiment, we compared N2O fluxes from cropland (non-fertilized wheat, N-fertilized wheat, non-fertilized faba bean, and wheat mixed intercropping with faba bean) and agro-forestry (non-fertilized poplar, N-fertilized poplar, non-fertilized Robinia, and poplar mixed intercropping with Robinia) as a randomized split-block design.
Rainfall at the field site was slightly over average during the period from 1 April to 1 July in 2014 (201 mm rain) and considerably below average during the same period in 2015 (100 mm rain). Cumulative mean N2O fluxes were up to five fold higher in agro-forestry than in arable crop treatments during 2014 growing period. We hypothesized that the difference in N2O emissions when comparing arable land and agro-forestry was mainly due to the limited water and nutrient uptake of plantations during the first year. Among the arable crops (wheat, N-fertilized wheat, wheat mixed intercropped with bean, and bean), seasonal and annual N2O emissions were highest in soils when faba bean was grown as a mono-crop. On the other hand, cumulative mean N2O fluxes were 31 % lower (p < 0.05) when faba bean mixed with wheat than in soils planted with N-fertilized wheat.
The latter clearly suggests that using legume crops as intercrop or mixed crop in wheat may significantly mitigate fertilizer-derived N2O fluxes and may be an effective proxy for increasing GHG emission savings for energy crops.
The observed increase in global average temperatures over the last decades is very likely due to the observed increase in anthropogenic greenhouse gas concentrations in the atmosphere. Nitrous oxide (N2O) is a potent greenhouse gas as it absorbs long-wave radiation and contributes to the reduction of the ozone layer in the stratosphere . Data from the ice-core analysis show that for thousands of years, mean atmospheric N2O concentrations were close to 270 ppbv; however, the latter increased about 20 % in recent years [2, 3]. The Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report identified microbial production of N2O in expanding and fertilized agricultural lands as a primary driver of this increase . However, large uncertainties remain on the estimates of N2O fluxes from the biosphere [2, 5] due to complex interactions between the related processes and controls on production, consumption, and transport through the soil and on the release into the atmosphere [6, 7].
High-yielding agricultural systems have specifically high nitrogen (N) demand which cannot be supplied by soil N reserves. Therefore, additional N input is needed; however, in most agricultural land-use systems, the application of organic and inorganic nitrogen fertilizers triggers the emissions of anthropogenic N2O emissions. The present IPCC default factor for direct N2O emissions arising from the nitrogenous fertilizer application to managed soils is 1 %. However, soil N2O emissions vary significantly depending on soil type, plant species, climate, crop rotation, tillage method, and fertilizer application rates . At high N2O-emitting sites, most N2O release is characterized by short peak emissions (up to 90 % of the annual emissions) connected mainly to (i) precipitation events and change in soil moisture [8–10], (ii) N fertilization [11–13], (ii) freeze-thaw cycle , and (iv) soil tillage . Such peak N2O emissions from soils are highly variable in space and time; thus, measuring and quantifying variance in N2O emissions are rather difficult, and there are only a few field experiments available allowing long-term comparison of various crops and other factors . It is absolutely crucial to simulate such N2O peak events and to identify the driving factors in order to be able to develop mitigation options.
The main microbial reactions involved in the production of N2O are nitrification (oxidation of NH4 + to NO3 −) and denitrification (reduction of NO3 −, via NO, N2O, to N2). There is now growing evidence that denitrification (bacterial or fungal) is the dominating process responsible for N2O losses from agricultural soils [8, 10, 17]. Therefore, in many agricultural field studies, N2O emission events are found in periods when high mineral nitrogen concentrations in soils coincide with high soil moisture (increasing the rate of denitrification) [7, 18]. The relevance of soil moisture can also be seen in field studies with similar climatic, soil, and substrate conditions, where N2O emissions nevertheless show significant inter-annual variability caused by the differences in soil water content . These differences can usually be deduced from the weather records as they are related to the differences in the input of water through rainfall . But the soil-water balance is also strongly influenced by the offtake of water through evapotranspiration. Crop plants play an important role, as they may transpire 500 to 600 mm of water per growth cycle which is 30 to 90 % of the precipitation input in that period. In this respect, crops differ substantially, not only in the total amount of water that is transpired but also in the growth and transpiration pattern in the course of the year [19, 20].
Biofuels are often called “CO2 neutral” in the sense that CO2 which is emitted in the course of their combustion has previously been fixed from the atmosphere via photosynthesis during plant growth. Many industrialized countries have established ambitious policy targets and often offer financial incentives to stimulate the production or use of bioenergy. The main reasons for the promotion of biofuel production are that it is made from renewable resources (organic manures, plant materials, food waste), that it is expected to have no or even positive effect on the atmospheric greenhouse gas balance, and that it may reduce the dependency on fossil fuel . However, biomass used for bioenergy generation is not per se environmentally benign, since its production is inevitably associated with negative side effects such as GHG emissions or N leaching. Soil N2O emissions are likely to be the dominating greenhouse gas emissions associated with bioenergy crop production . It has been reported that the production and use of biofuels compared with the use of conventional fossil fuels may lead to a reduction or even increase in the total greenhouse gas emissions (72 to 107 %), depending on the type of bioenergy crop used and combustion technology chosen . Here, the authors showed that N2O would typically make up 10 to 80 % of the total greenhouse gas emissions in the biofuel production chain.
Intercropping, defined as any system of multiple cropping within the same space can be used as an alternate bioenergy cropping system . The intercropping of cereals with legumes is particularly common, and introducing N2-fixing legumes into cereal-based crop rotations may reduce synthetic mineral N-fertilizer use and thought to mitigate N2O fluxes. A reduction of N2O in tree-based intercropping systems has been reported . In contrast, in a review study, Rochette and Janzen  concluded that legumes can produce substantial N2O emissions. The cultivation of N2-fixing legume species (e.g., faba bean (Vicia faba L.) as arable crop or Robinia (Robinia pseudoacacia) as woody plant) could stimulate N2O emissions simply by increasing N input to soils, thus providing additional substrate for nitrification and denitrification . Authors reported that faba bean could release 13 % of their fixed N as rhizodeposition . Furthermore, denitrification by N-fixing bacteria can be another source of N2O in legume domains .
As discussed above, N2O emissions vary greatly in space and time; thus, the direct comparison of soil N2O fluxes from various agro-ecosystems is certainly necessary and only very few studies are available on this subject. Therefore, our study aimed to evaluate two different agro-ecosystems (cropland and agro-forestry) cultivated in central Germany for their environmental impact. In a 1-year field experiment, we compared N2O fluxes from cropland (non-fertilized wheat, N-fertilized wheat, non-fertilized faba bean, and wheat mixed intercropping with faba bean) and agro-forestry (non-fertilized poplar, N-fertilized poplar, non-fertilized Robinia, and poplar mixed intercropping with Robinia) as a randomized split-block design. Therefore, the objectives of the present study were (i) to obtain year-round N2O emission data in various agro-ecosystems (cropland and agro-forestry) in central Germany and (ii) to provide more knowledge on how a legume-based crop production system may affect N2O budget.
Sowing dates and fertilizer application rate of each treatment
Fertilizer (kg CAN–N ha−1)
Date of sowing spring and winter crops
Spring wheat/winter wheat
25 March 2014/28 October 2014
Spring wheat + N/winter wheat + N
25 March 2014/28 October 2014
Mixed intercropping (wheat and faba bean)a
25 March 2014/28 October 2014
25 March 2014/28 October 2014
30 April 2014
30 April 2014
30 April 2014
30 April 2014
Soil mineral N
For the analysis of soil mineral N, soil from 0–15 cm depth was sampled extracted with a 0.0125 M CaCl2 solution (1:5 w/v) and shaked for 1 h. The extracts were then filtered with Whatman 602 filter paper and stored at −20 °C until analysis. The extracts were analyzed colorimetrically for the concentrations of NO3 − and NH4 + using the San++ continuous flow analyzer (Skalar Analytical B.V., Breda, The Netherlands).
Trace gas flux measurement
After the N application, gas samples were taken daily for a period of 1 week, followed by intervals of 2–3 days until the end of the vegetation period and once per week during the winter period using the closed chamber method described by Hutchinson and Mosier . On each of 24 plots, basal rings made of polyvinyl chloride (PVC) (height 10 cm, diameter 60 cm) were pressed 5 cm into the soil. For the measurements, PVC chambers with an inner diameter of 60 cm and a height of 30 cm were put onto the basal rings, and tightened by a butyl rubber band all around the junction. The chambers were closed for 40 min within the period of 10.00–14.00 h at each sampling day. Gas samples were taken from the chamber atmosphere at 0–20–40 min after closing the chamber using pre-evacuated 12-ml glass vials (Labco, Crewe, UK). The top of the PVC chambers were covered with white polystyrene to reflect solar radiation and have temperature stability within the chamber.
Concentrations of N2O were analyzed by a Bruker gas chromatography system (456-GC, Bruker, Billerica, USA) by deploying an electron capture detector (ECD) for N2O. Operating conditions for the GC were as follows: injector temperature 95 °C, column temperature 85 °C, and detector temperature 320 °C. Samples were introduced using a Gilson auto-sampler (GX-281) (Gilson Inc., Middleton, WI, USA). Data processing was performed using the CompassCDS (vers. 3.0) software.
Daily N2O flux rates for dates between sampling dates were calculated using linear interpolation, and annual cumulative N2O emissions were calculated as the sum of all daily flux rates for the vegetation period of each crop and the entire investigation periods during March 2014–August 2015. We used general linear model and Tukey’s test for pairwise comparison of cumulative N2O flux between the treatments within each year. Statistical analyses were done using SPSS version 13.0.
Results and discussion
Rainfall and crop production
Rainfall at the site Reinshof was slightly over average during the period from 1 April to 1 July in 2014 (201 mm) and considerably below average during the period from 1 April to 1 July in 2015 (100 mm). Therefore, the 2014 spring period was much wetter than spring 2015. Overall, 2014–2015 winter period was quite mild with no significant freeze-thaw event.
Soil mineral N
Overall treatments, soil NO3 − concentrations in 0–15 cm soil segment varied between 10 and 85 kg NO3 −–N ha−1. Here, NO3 − was generally the dominant soil N form and highly variable when sampled soon after additions of fertilizer N. In all soils, concentrations of NO3 − in the 0–15-cm layer decreased over time with the largest decrease found in the arable crops specifically in WT and NWT treatments. During the vegetation period in 2014, soil NO3 − content was generally higher in agro-forestry soils than in arable soils. Plant nutrient and water uptake was expected to be higher in cropland compared to the young agro-forestry treatments in 2014 due to small size and low growth rate of young trees. Thus, more rapid depletion of soil mineral N in arable crops than agro-forestry treatments can mainly be attributed to the differences in plant N uptake.
In arable land stand, soil NO3 − concentrations were clearly higher (significant in 2014, p < 0.01) in FB than in other non-fertilized treatments during the vegetation period (Fig. 2a, b). For legume crops, inputs of biologically fixed N largely supplement to the uptake of soil mineral N to meet crop N demand. Thus, the legume species also take up mineral N from soils for growth before fixing additional N. The preferential use of soil mineral N helps explain why there is also significant depletion of soil NO3 − in FB treatment . In a review study, authors reported that average 41 % (for chickpea), 65 % (for faba bean), and 66 % (for field peas) of N that were present in legumes were derived from soil N . However, slightly higher soil NO3 − concentration in FB treatments than non-legume soils suggests that there were still reasonably more NO3 − available for potential denitrification losses during the legume-growing season.
Seasonal N2O emissions
In 2014, N2O emissions gradually decreased to the background levels (below 10 g N2O–N ha−1 day−1) from the months of May to July and remained in background levels until March 2015 (Fig. 3). Interestingly, the latter was less than 0.01 kg N2O–N ha−1 day−1 during the early summer period in 2015 for all treatments (including N-fertilized treatments). As seen in Fig. 1, early summer period in 2014 was relatively wet (April–June, 201 mm rainfall) compared to the same period in 2015 (April–June, only 100 mm rainfall). Therefore, we may attribute higher daily N2O fluxes in 2014 than in 2015 (in early summer period) to the differences in mineral N and moisture content of the soil. Mineral N content of all soils in June 2014 was almost similar as compared to the same period in 2015, whereas N2O fluxes were still about 10-fold higher in June 2014 than in June 2015. In this context, we may conclude that soil moisture seems to be the major driving factor of higher N2O emissions in the 2014 summer period than in 2015.
All the abovementioned factors (e.g., high moisture, high soil temperature in early summer, and moderate or high NO3 − content of the soil) are known to trigger specifically the denitrification rate in soils. Thus, we may speculate that denitrification (fungal or bacterial) was the potential key source of measured large N2O fluxes in the 2014 early summer period. Our earlier report supports this hypothesis in which sandy loam soil was incubated under laboratory conditions, and similar to the field experiment, large N2O peak events were observed immediately after rewetting of the soil. Here, a stable isotope-labeling study clearly showed that denitrification was the major source (over 90 % of emitted N2O) of large N2O peaks that occurred in wet seasons [8, 19]. Furthermore, there is now a growing evidence that fungal denitrification may be the key process producing N2O in such situations rather than bacterial denitrification . Surely, more research is needed to reveal (i) the dominant processes and (ii) key microbial or fungal strains producing N2O under these specific conditions (especially during early summer period).
Effect of plant species on N2O emissions
In agro-forestry treatments, both daily and cumulative N2O emissions did not differ among each other in the 2014 growing period. Here, mean seasonal N2O emissions (during the growing season of arable crops) were 1121 ± 161, 1102 ± 159, and 1052 ± 266 g N2O–N ha−1 in PL, PRB, and RB treatments (no significant difference), respectively. The cumulative mean N2O fluxes during the 2014 growth period were considerably higher in agro-forestry than in arable crop treatments. During the first year, young plantations in agro-forestry domains generally have limited N and water uptake, while wheat and faba bean as arable crops are at their most productive growth stage specifically during May and June (growth rates are almost at their maximum during this period). Here, soil conditions seem to be more favorable specifically for denitrification in agro-forestry treatments than in soils planted with arable crops that may explain large N2O emissions . The cumulative mean N2O emissions in agro-forestry treatments were about fivefold higher than in both WT and WFB treatments, and the latter was still more than twofold higher when compared to FB plots. In line with the present data set, authors reported large N2O fluxes after conversion of pastures lands  or grasslands . Here, authors attributed large N2O fluxes to the soil disturbance associated with tillage and cultivation that accelerate soil organic matter decomposition and microbial activity (nitrification and denitrification) leading to N2O emissions. In the present experiment, soil tillage has been done almost at the same time for all treatments. Thus, we may speculate that the difference in N2O emissions when comparing arable land to agro-forestry was mainly due to differences in water and nutrient uptake of plant species. Water and nutrient demand of young plantations during the first year are generally low which in return may cause more favorable conditions specifically for denitrification and N2O losses from denitrification. Overall, we can therefore summarize that direct plant effect seems to be one of the key variables that regulates N2O losses from soils. Zona et al. (2013) concluded that vegetation uptake of NO3 − together with water ultimately may reduce the anaerobic volume of soils and may lower both denitrification rate and product stoichiometry of denitrification (lower N2O/N2O + N2 ratio; meaning reduced N2O and enhanced N2 production) in agricultural soils [8, 17].
Effect of mineral N supply on N2O emissions
Although it was not the main goal of the present experiment to study the effect of mineral-N addition on N2O fluxes, we added fertilizer N to mono-crop wheat (80 kg N ha−1; calcium-ammonium-nitrate) in parallel plots to be able to compare N2O fluxes from soils planted with N2-fixing plants (faba bean mono-culture or faba bean intercropped with wheat) with fertilized and non-fertilized wheat soils. N fertilization during the first year of new agro-forestry plantations is also not a common practice. However, N doses similar to the arable treatments were applied at the same date in order to be able to gain better scientific knowledge about the dominant factors regulating N2O fluxes in agro-forest ecosystems. Expectedly, in all N-fertilizer treatments, N2O fluxes increased immediately after fertilizer application, however, only in 2014 (wet early summer) but no response observed in 2015 (dry early summer). The latter clearly suggests that environmental factors specifically soil moisture was the most dominant factor in 2014 that leads to relatively high N2O fluxes and without sufficient moisture or rainfall, fertilizer application alone does not affect N2O fluxes significantly, e.g., in 2015. In line with the present study, authors also reported that the application of organic or inorganic fertilizers affects N2O fluxes only in wet seasons but not in dry years [10, 17, 19].
Overall, the application of nitrogenous fertilizer affected N2O fluxes predominantly in cropland soils and had limited impact in agro-forestry soils. Here, cumulative N2O fluxes (from May to September 2014) were 46 and 121 % higher in fertilized than in non-fertilized agro-forestry (non-fertilized poplar vs. fertilized poplar) or in cropland treatments (non-fertilized wheat vs. fertilized wheat), respectively. The latter clearly suggests that N2O fluxes were more dependent on soil mineral-N content in arable crops than in agro-forestry most likely due to greater competition between plants and N2O-producing soil microorganisms in cropland than in agro-forestry soils. Fertilizer-derived cumulative N2O emissions (emission factor) during the period from April to December 2014 were 0.21 and 0.45 % of applied N in wheat and poplar soils, respectively. Measured emission factors were significantly lower than what IPCC predicts (1 %; ). However, the latter was similar to what we reported in our previous study for central and northern Germany ([12, 19]). For operational reasons in the present IPCC protocol, the N2O emission factor was set to 1 % for all fertilizer N regardless of crop or soil type. Large variations in N2O emissions from different agricultural systems due to differences in management, climate, and soil type are very well known. Low N2O emission factors in the present experiment and in our previous reports suggest that in the future, different emission factors should be considered at least for different crops or regions that account for their different risks of N2O emissions.
Cumulative mean N2O emission during the growing season was still 31 % higher (p < 0.05) in FB than in NWT treatment. In a review study, Rochette and Jansen  summarized that legumes can increase N2O emissions during growth compared to evenly fertilized arable crops most likely due to the N release from the root exudates and decomposition of crop residues. Our study clearly agrees with Rochette and Jansen  and many others (e.g., [7, 27]) that growing legumes as mono-crop can increase N2O fluxes compared to N-fertilized arable crops. On the other hand, seasonal N2O fluxes were 35 % lower in WFB (wheat mixed intercropped with faba bean) than in NWT (wheat as mono-crop) treatment. The latter suggests that using legume crops as intercrop or mixed crop in wheat may significantly mitigate fertilizer-derived N2O fluxes. However, surely more research is needed to upscale current findings due to the complexity and variability of N2O fluxes in complex agricultural systems, e.g., mixed cropping systems.
Currently, biogas production from energy crops is mainly based on anaerobic fermentation of mono-crops; however, high-yielding mono-crops require high N-fertilizer input that increases the risk of N2O losses. Present study clearly showed that mixed intercropping agricultural systems (legume and non-legume plant species) may significantly lower (about 35 %) N2O losses compared to the N-fertilized mono-crops.
Cumulative N2O emissions in agro-forestry soils were about 2–5-fold higher than in cropland soils. Soil conditions in agro-forestry treatments seem to be more favorable specifically for denitrification (due to limited water and nutrient uptake of young plantations during their initial growth stage) than soils planted with arable crops that may be responsible for large N2O emissions.
Cumulative mean N2O emissions during the growth period of annual crops were 31 % higher (p < 0.05) in soils planted with faba bean than in N-fertilized wheat. We can conclude that legumes (when grown alone) can produce substantial N2O emissions most likely due to enhanced denitrification activity in their rhizosphere due to N-rich root exudates/dead organic matter.
This study was funded by the Federal Ministry of Education and Research (FKZ 031A351A). IMPAC3 is a project of the Centre of Biodiversity and sustainable Land Use at the University of Gottingen.
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