Impact of stubble burning on air quality and soil nutrient status in rice-wheat growing regions

Introduction

Stubble burning refers to the open-field combustion of crop residues left after mechanical harvesting, and it is practiced across millions of hectares of rice-wheat growing areas worldwide [1] . Farmers burn residues primarily to clear fields quickly between rice harvest and wheat sowing, as the narrow turnaround window of 10 to 20 days makes mechanical incorporation impractical without specialized equipment [2] . The environmental costs are steep. Burning releases large quantities of particulate matter (PM2.5 and PM10), carbon monoxide, sulfur dioxide, and nitrogen oxides, degrading regional air quality for weeks during peak burning season [3] . On the soil side, combustion destroys organic matter, volatilizes nitrogen and sulfur, kills surface-dwelling microorganisms, and damages soil structure [4,5] . Yet field-level data linking burning intensity to measured changes in both air quality and soil nutrient pools remain limited for East African rice-wheat systems. This research measured the paired effects of rice stubble burning on ambient air pollutant concentrations and topsoil nutrient status in rice-wheat areas near Nairobi, Kenya, over two consecutive post-harvest seasons. The objectives were to quantify pollutant spikes during burning events and to assess the immediate nutrient losses from burned versus unburned plots.

Research area description

The research was conducted at the University of Nairobi's Field Research Station in Kabete, Nairobi County, Kenya (1.25°S, 36.74°E, elevation 1,820 m). The soil was a humic Nitisol with baseline pH 5.9, organic carbon 2.14 percent, and clay content 62 percent. The region practices irrigated rice during the long rains (March-August) followed by wheat in the short rains (October-February), with stubble burning typically occurring in October.

Soil Quality baseline

Pre-burn soil analysis (0-15 cm depth) showed the following baseline values across the experimental area: available nitrogen 218 kg/ha, available phosphorus 19.7 kg/ha, exchangeable potassium 174 kg/ha, organic carbon 2.14 percent, and microbial biomass carbon 347 mg/kg.

The soil had a bulk density of 1.18 g/cm³ and water-holding capacity of 42.3 percent. These values were consistent across all 16 experimental plots prior to treatment application, with coefficients of variation below 8 percent for all parameters.

Material and Methods Material

Rice stubble (cv. Basmati 370) remaining after combine harvesting was used as the burning material. Residue load averaged 5.8 t/ha dry weight. Air quality monitoring equipment included two DustTrak DRX aerosol monitors (TSI Model 8534) for PM2.5 and PM10, and portable electrochemical gas analyzers (Aeroqual Series 500) for CO, SO2, and NO2. Soil sampling was done using a Dutch auger to 15 cm depth. All analytical reagents were sourced from Merck (Darmstadt, Germany).

Methods

Eight plots (20 × 15 m each) were assigned to burning and eight to residue retention in a randomized complete block design with four replications per treatment. Burning was conducted under controlled conditions on calm-wind days in October 2021 and 2022. Air quality monitors were placed at 1.5 m height, 50 m downwind from the burn center, recording at 5-minute intervals from 2 hours before through 6 hours after ignition [6] . Soil samples were collected 1 day before and 3 days after burning from each plot at 0-15 cm depth. Organic carbon was determined by the Walkley-Black method, available N by alkaline permanganate, available P by Olsen's method, exchangeable K by ammonium acetate, and microbial biomass carbon by the chloroform fumigation-extraction method [7,8] . Data were analyzed using paired t-tests and two-way ANOVA in R 4.3.1. All five air pollutants rose sharply during burning events (Table 1). PM2.5 increased 3.8-fold from 48.3 to 184.2 μg/m³, far exceeding WHO's 24-hour guideline of 25 μg/m³. PM10 peaked at 312.6 μg/m³. Carbon monoxide levels nearly quadrupled. Pollutant concentrations at the non-burning control site remained stable throughout the monitoring period. Burning caused a 28.5 percent decline in soil organic carbon and a 41.7 percent drop in available nitrogen (Table 2). Microbial biomass carbon fell by 53.6 percent, reflecting the thermal destruction of surface-dwelling microorganisms. Interestingly, available P and exchangeable K increased slightly after burning, likely due to ash deposition releasing these nutrients from organic matter.

Results

. The box plots (Figure 1) highlight the dramatic spread in pollutant levels after burning, with PM2.5 and PM10 showing the widest variability. The cycle diagram (Figure 2) illustrates how burning triggers a self-reinforcing degradation loop-organic carbon loss reduces microbial activity, which slows nutrient cycling and further depresses soil fertility over successive burning seasons.

Discussion

The 3.8-fold spike in PM2.5 during burning events is consistent with satellite-based and ground-level observations from South Asian rice-wheat belts, where seasonal burning creates regional air quality crises [9,10] . The health implications are serious, as PM2.5 at 184 μg/m³ exceeds safe thresholds for respiratory and cardiovascular function, even during short-term exposure [11] .

On the soil side, the 41.7 percent nitrogen loss through volatilization represents a direct economic cost to farmers who must replace it with purchased fertilizer. At current urea prices, this equates to roughly 91 kg N/ha lost per burning eventenough to meet half the wheat crop's N requirement [12] . The 53.6 percent decline in microbial biomass carbon is equally concerning, as soil microbes drive organic matter decomposition and nutrient mineralization [13] . The short-term gains in available P and K from ash are misleading, since these are offset by the long-term loss of the organic matter that sustains nutrient cycling [14] .

Conclusion

Rice stubble burning caused severe spikes in all measured air pollutants, with PM2.5 reaching 3.8 times the pre-burn baseline. Soil organic carbon declined by 28.5 percent, available nitrogen by 41.7 percent, and microbial biomass carbon by 53.6 percent. The small increases in available P and K from ash deposition do not compensate for these losses.

Transitioning from burning to residue incorporation or mulching would preserve soil organic matter, retain nitrogen, and avoid the air quality degradation documented here. Policy incentives for affordable residue management machinery and farmer awareness programs are needed to make this transition practical at scale.

References

1. Jain N, Bhatia A, Pathak H. Emission of air pollutants from crop residue burning in India. Aerosol Air Qual Res. 2014;14(1):422-430.
2. Gupta PK, Sahai S, Singh N, Dixit CK, Singh DP, Sharma C, et al. Residue burning in rice-wheat cropping system: causes and implications. Curr Sci. 2004;87(12):1713-1717.
3. Gadde B, Bonnet S, Menke C, Garivait S. Air pollutant emissions from rice straw open field burning in India, Thailand and the Philippines. Environ Pollut. 2009;157(5):1554-1558.
4. Mandal KG, Misra AK, Hati KM, Bandyopadhyay KK, Ghosh PK, Mohanty M. Rice residue management options and effects on soil properties and crop productivity. Food Agric Environ. 2004;2(1):224-231.
5. Yadvinder-Singh, Bijay-Singh, Timsina J. Crop residue management for nutrient cycling and improving soil productivity in rice-based cropping systems in the tropics. Adv Agron. 2005;85:269-407.
6. Cheng Z, Luo L, Wang S, Wang Y, Sharma S, Shimadera H, et al. Status and characteristics of ambient PM2.5 pollution in global megacities. Environ Int. 2016;89-90:212221.
7. Vance ED, Brookes PC, Jenkinson DS. An extraction method for measuring soil microbial biomass C. Soil Biol Biochem. 1987;19(6):703-707.
8. Subbiah BV, Asija GL. A rapid procedure for estimation of available nitrogen in soils. Curr Sci. 1956;25:259-260.
9. Kaskaoutis DG, Kumar S, Sharma D, Singh RP, Kharol SK, Sharma M, et al. Effects of crop residue burning on aerosol properties, plume characteristics, and long-range transport over northern India. J Geophys Res Atmos. 2014;119(9):5424-5444.
10. Cusworth DH, Mickley LJ, Sulprizio MP, Liu T, Marlier ME, DeFries RS, et al. Quantifying the influence of agricultural fires in northwest India on urban air pollution in Delhi. Environ Res Lett. 2018;13(4):044018.
11. Pope CA, Dockery DW. Health effects of fine particulate air pollution: lines that connect. J Air Waste Manag Assoc. 2006;56(6):709-742.
12. Dhillon SS, et al. The Happy Seeder enables direct drilling of wheat into rice stubble. Aust J Exp Agric. 2007;47(7):844-854.
13. Jenkinson DS, Ladd JN. Microbial biomass in soil: measurement and turnover. In: Paul EA, Ladd JN, editors. Soil Biochemistry. New York: Marcel Dekker; 1981. p.415471.
14. Heard J, Cavers C, Adrian G. Up in smoke: nutrient loss with straw burning. Better Crops. 2006;90(3):10-11.
15. Lohan SK, Jat HS, Yadav AK, Sidhu HS, Jat ML, Choudhary M, et al. Burning issues of paddy residue management in north-west states of India. Renew Sustain Energy Rev. 2018;81:693-706.