Assessment of nitrous oxide emissions from legume-based cropping systems under varying moisture conditions

Introduction

Nitrous oxide is the third most important long-lived greenhouse gas after carbon dioxide and methane, with a 100-year global warming potential 265 times that of CO₂ and a stratospheric ozone depletion potential that makes it the single largest contributor to ozone layer destruction in the 21st century [1,2] . Agriculture accounts for approximately 60% of global anthropogenic N₂O emissions, primarily through soil microbial processes of nitrification and denitrification that convert reactive nitrogen from fertilizers, manures, and Biological Nitrogen Fixation (BNF) into N₂O [3] . Legume crops occupy a unique position in agricultural N₂O budgets. Through symbiotic BNF, legumes introduce 40-200 kg N ha⁻¹ per season into soil, and decomposition of nitrogen-rich nodules creates substrate for N₂O-producing microorganisms [4,5] . The IPCC assigns a default emission factor of 1% to all nitrogen inputs including BNF, but field measurements range from 0.1% to over 3% depending on soil type, climate, and moisture status [6,7] . Soil WFPS controls the balance between aerobic nitrification and anaerobic denitrification, with peak emissions at 60-80% WFPS [8,9] . Yet most field measurements treat moisture as uncontrolled [10] . This research quantified N₂O emissions from soybean, groundnut, and chickpea under experimentally controlled moisture regimes.

Research area description

The field experiment was established at the Shangzhuang Experimental Station of China Agricultural University, 40 km northwest of Beijing (40°8′N, 116°10′E, and elevation 48 m). The experimental area comprised a fluvo-aquic soil (Calcaric Cambisol) with pH 7.8, organic carbon 11.4 g kg⁻¹, total nitrogen 1.08 g kg⁻¹, bulk density 1.38 g cm⁻³, sand 42%, silt 38%, and clay 20%. The site has been under continuous arable cropping for over 30 years [11] .

Soil and water quality analysis

Baseline soil samples (0-20 cm, five cores per plot) were collected in March 2022 and analyzed for mineral nitrogen (NH₄⁺-N and NO₃⁻-N in 2M KCl, continuous-flow colorimetry), dissolved organic carbon (0.5M K₂SO₄, TOC analyzer), and microbial biomass nitrogen (chloroform fumigation-extraction). Soil moisture was monitored continuously with TDR probes (CS616, Campbell Scientific) at 10 and 20 cm depths, logged at 30-minute intervals. Irrigation water was sourced from a shallow aquifer (NO₃⁻-N < 2 mg L⁻¹, pH 7.3).

Climatic and environmental conditions

Beijing experiences a warm-temperate semi-humid monsoon climate (Köppen Dwa). During the 2022-2023 growing seasons (June-October), mean daily temperatures averaged 24.7 and 25.1 °C, with total precipitation of 487 and 412 mm respectively (∼75% during July-August). Rain-exclusion shelters covered all plots during precipitation to maintain experimental moisture control and were removed within 2 hours after cessation.

Materials and Methods

Experimental design

The experiment followed a split-plot randomized complete block design with four replications. Main plots were the three legume species (soybean cv. Zhonghuang 13, groundnut cv. Huayu 22, chickpea cv. Desi ICC 4958). Subplots were three moisture regimes: low (35-55% WFPS), moderate (55-75% WFPS), and high (75-95% WFPS), maintained by automated drip irrigation triggered by TDR readings. Individual subplots measured 4 m × 5 m with 1m buffer strips between plots. No synthetic nitrogen fertilizer was applied; basal phosphorus (60 kg P₂O₅ ha⁻¹) and potassium (40 kg K₂O ha⁻¹) were broadcast before sowing. Seeds were inoculated with species-specific Bradyrhizobium or Mesorhizobium strains [12] .

N₂O flux measurement

Nitrous oxide fluxes were measured using static vented chambers (40 cm diameter, 25 cm height above soil, stainless steel) deployed on permanently installed PVC collars. Gas samples were collected three times weekly at 0, 15, 30, and 45 minutes after chamber closure between 09:00 and 11:00 local time. Samples were analyzed within 48 hours on a gas chromatograph (Agilent 7890B) equipped with an electron capture detector (detection limit 5 ppb N₂O). Flux rates were calculated from the linear increase in headspace concentration over time; chambers with R² < 0.85 were excluded. Cumulative seasonal emissions were computed by linear interpolation between sampling dates [13,14] .

Isotopomer and source partitioning

Selected peak-emission samples were analyzed for N₂O isotopomers using IRMS (Thermo Delta V Advantage). Source partitioning between nitrification and denitrification used the site preference two end-member mixing model (33‰ for nitrification, ∑5‰ for denitrification) [15] . Cumulative N₂O emissions increased significantly with soil moisture across all three legumes (Table 1). Soybean at high WFPS produced the highest emissions (3.87 kg N ha⁻¹), approximately 4.4 times the soybean low-WFPS treatment and 9.4 times chickpea at low WFPS. The species × moisture interaction was significant (p<0.001), with soybean showing a steeper moisture response than the other two legumes. The scatter plot (Figure 1) shows the exponential acceleration of N₂O fluxes above 65% WFPS, particularly under soybean. The polynomial trend lines diverge sharply above this threshold, reflecting soybean's greater substrate availability for denitrification due to higher BNF rates and root exudation of labile carbon. Under high WFPS, ammonium accumulated toward maturity while nitrate declined sharply (Table 3)-consistent with active denitrification. Dissolved organic carbon rose to 94.7 mg C kg⁻¹ at maturity under wet conditions, likely reflecting anaerobic suppression of carbon mineralization coupled with root senescence.

Results

Discussion

The exponential increase in N₂O emissions with WFPS observed here is consistent with the conceptual model proposed by Davidson et al. [8] , who identified 60-80% WFPS as the zone where nitrification and denitrification co-occur and incomplete denitrification maximizes N₂O production. Our isotopomer data add mechanistic resolution to this pattern, showing that the nitrification-to-denitrification transition occurs near 55-65% WFPS in these fluvo-aquic soils-somewhat lower than the 70% threshold reported for heavier clay soils in tropical environments [16] . Soybean's consistently higher emissions relative to groundnut and chickpea likely reflect its substantially greater BNF input (168 vs. 127 and 84 kg N ha⁻¹). Greater BNF produces more nitrogen-rich nodule and root tissue that, upon senescence, provides readily available substrate for nitrifying and denitrifying organisms [17] . The higher dissolved organic carbon under soybean (data shown for high WFPS, Table 3) further supports this interpretation, as denitrification rates are co-limited by NO₃⁻ availability and labile carbon supply [18] . The emission factors measured here (0.18-1.34%) bracket the IPCC default of 1% but show that a single default greatly overestimates emissions from low-BNF legumes under dry conditions and slightly underestimates those from high-BNF soybean under wet conditions [6] . Our findings support the case for disaggregated emission factors that account for both legume species and moisture regime, an approach that could improve national inventory accuracy in monsoon-influenced regions where within-season moisture variation is extreme [19,20] . From a mitigation perspective, maintaining WFPS below 60% through drainage management or scheduling irrigation after the denitrification-sensitive pod-filling stage could reduce emissions by 47-63% without compromising yields in these rainfedequivalent legume systems [21] .

Conclusion

Nitrous oxide emissions from legume-based cropping systems are strongly governed by soil moisture status, with cumulative emissions increasing 4-to 6-fold between low (35-55%) and high (75-95%) water-filled pore space. Soybean produced the highest emissions across all moisture regimes (mean 2.29 kg N ha⁻¹ season⁻¹), reflecting its greater biological nitrogen fixation inputs and associated substrate availability for denitrification. Isotopomer analysis confirmed that denitrification dominated N₂O production above 65% WFPS, while nitrification was the primary pathway under drier conditions. Measured emission factors ranged from 0.18% to 1.34%, demonstrating that the IPCC Tier 1 default of 1% overestimates emissions from chickpea and groundnut under dry conditions by 2-to 5-fold. These results provide a basis for developing species-and moisture-specific emission factors that would improve greenhouse gas inventory accuracy. Practical mitigation through soil moisture management-particularly maintaining WFPS below 60% during the critical pod-filling stage-could reduce legume N₂O emissions by nearly half without yield penalties.

References

1. Ravishankara AR, Daniel JS, Portmann RW. Nitrous oxide (N₂O): the dominant ozone-depleting substance emitted in the 21st century. Science. 2009;326(5949):123-125.
2. IPCC. Climate change 2014: synthesis report. Contribution of Working Groups I, II and III. Geneva: IPCC; 2014.
3. Reay DS, Davidson EA, Smith KA, Smith P, Melillo JM, Dentener F, Crutzen PJ. Global agriculture and nitrous oxide emissions. Nat Clim Chang. 2012;2(6):410-416.
4. Jensen ES, Peoples MB, Boddey RM, Gresshoff PM, Hauggaard-Nielsen H, Alves BJ, Morrison MJ. Legumes for mitigation of climate change and the provision of feedstock for biofuels and biorefineries. Agron Sustain Dev. 2012;32(2):329-364.
5. Unkovich M, Herridge D, Peoples M, Cadisch G, Boddey B, Giller K, Alves B, Chalk P. Measuring plant-associated nitrogen fixation in agricultural systems. ACIAR Monograph No. 136. Canberra: ACIAR; 2008.
6. Rochette P, Janzen HH. Towards a revised coefficient for estimating N₂O emissions from legumes. Nutr Cycl Agroecosyst. 2005;73(2-3):171-179.
7. Zhong Z, Lemke RL, Nelson LM. Nitrous oxide emissions associated with nitrogen fixation by grain legumes. Soil Biol Biochem. 2009;41(11):2283-2291.
8. Davidson EA, Keller M, Erickson HE, Verchot LV, Veldkamp E. Testing a conceptual model of soil emissions of nitrous and nitric oxides. BioScience. 2000;50(8):667680.
9. Butterbach-Bahl K, Baggs EM, Dannenmann M, Kiese R, Zechmeister-Boltenstern S. Nitrous oxide emissions from soils: how well do we understand the processes and their controls? Philos Trans R Soc B Biol Sci. 2013;368(1621):20130122.
10. Giles M, Morley N, Baggs EM, Daniell TJ. Soil nitrate reducing processes-drivers, mechanisms for spatial variation, and significance for nitrous oxide production. Front Microbiol. 2012;3:407.
11. Zhang Y, Liu J, Mu Y, Pei S, Lun X, Chai F. Nitrous oxide emissions from a maize field during two consecutive growing seasons in the North China Plain. J Environ Sci. 2011;23(7):1070-1078.
12. Peoples MB, Brockwell J, Herridge DF, Rochester IJ, Alves BJ, Urquiaga S, Boddey RM, Dakora FD, Bhattarai S, Maskey SL. The contributions of nitrogen-fixing crop legumes to the productivity of agricultural systems. Symbiosis. 2009;48(1):1-17.
13. Hutchinson GL, Mosier AR. Improved soil cover method for field measurement of nitrous oxide fluxes. Soil Sci Soc Am J. 1981;45(2):311-316.
14. Parkin TB, Venterea RT. Chamber-based trace gas flux measurements. In: Follett RF, editor. Sampling protocols. Washington (DC): USDA-ARS; 2010. p. 3.1-3.39.
15. Toyoda S, Yoshida N, Koba K. Isotopocule analysis of biologically produced nitrous oxide in various environments. Mass Spectrom Rev. 2017;36(2):135-160.
16. Bateman EJ, Baggs EM. Contributions of nitrification and denitrification to N₂O emissions from soils at different water-filled pore space. Biol Fertil Soils. 2005;41(6):379388.
17. Uchida Y, Clough TJ, Kelliher FM, Sherlock RR. Effects of aggregate size, soil compaction, and bovine urine on N₂O emissions from a pasture soil. Soil Biol Biochem. 2008;40(4):924-931.
18. Weier KL, Doran JW, Power JF, Walters DT. Denitrification and the dinitrogen/nitrous oxide ratio as affected by soil water, available carbon, and nitrate. Soil Sci Soc Am J. 1993;57(1):66-72.
19. Hergoualc'h K, Akiyama H, Bernoux M, Chirinda N, del Prado A, Kasimir Å, MacDonald JD, Ogle SM, Regina K, van der Weerden TJ. N₂O emissions from managed soils, and CO₂ emissions from lime and urea application. In: 2019 refinement to the 2006 IPCC guidelines for national greenhouse gas inventories. Vol 4; 2019.
20. Stehfest E, Bouwman L. N₂O and NO emission from agricultural fields and soils under natural vegetation: summarizing available measurement data and modeling of global annual emissions. Nutr Cycl Agroecosyst. 2006;74(3):207-228.
21. Pappa VA, Rees RM, Walker RL, Baddeley JA, Watson CA. Nitrous oxide emissions and nitrate leaching in an arable rotation resulting from the presence of an intercrop. Agric Ecosyst Environ. 2011;141(1-2):153-161.