As PM2.5 air pollution has recently exceeded the unsafe level in Bangkok and adjacent provinces, and attention is needed on factors that caused the ultra-fine dust particles including open field burning.
As a general definition, open field burning is a human-initiated activity involving the burning of living and dead vegetation in the open air. Rice residue open burning is a part of biomass burning which occurs annually and is widely used in Thailand, particularly in the central and lower northern regions to remove residue, control weeds and release land for the next crop cycle.
This process is a source of greenhouse gases, such as carbon dioxide (CO2) which is the principal product of the combustion, carbon monoxide (CO), methane (CH4), nitrous oxide (N2O), and nitrogen oxide (NOx) and comparatively less sulphur dioxide (SO2).
Furthermore, biomass open burning contributes pollutants to the atmosphere, including aerosols and hydrocarbons due to the incomplete combustion process which contributes to global warming. It is also the cause of lost nutrients and soil organic matter. In fact, the generated rice residue is composed of stubble (stiff stalks of rice attached to the soil after harvesting) and straw (seed coverings and small pieces of stem or leaves that have been separated from the seeds).
For every four tonnes of rice harvested, 6 tonnes of rice straw is left in the field. Rice straw, a by-product and considered as a waste material in rice production, contains 41% carbon, 0.5-0.8% nitrogen, 0.05-0.1% phosphorus, 0.3-2.0% potassium, 12% silica, and 10% lignin. Residue burning takes more time and labour, and is often incomplete. It has been observed that open burning of crop residues also contributes to soil pollution, which can cause severe impacts on sustainable production.
According to the 2017 Agricultural Statistics of Thailand derived from the Office of Agricultural Economics, the total rice production in Thailand in 2017 was 31.95 million tonnes from 9.73 million hectares. According to the research results conducted by Junpen about 61.87 megatonnes of rice residue were generated. Only 23.0% of the total rice residue generated were subject to open burning of which nearly 32% wase actually burned in the fields.
Rice residues are often burned even though long-term detrimental effects on soil properties may ensue. Inadequate duration of lethal temperature or a non-uniform burn pattern may allow substantial weed, disease and insect survival. Following a rice crop, you may face substantial shock because rice produces far more crop residue than other crops. However, crop residue generated in crop rotation systems can produce substantial long-term benefits which are likely to far outweigh any short-term savings accomplished by burning residue, particularly since some of Thailand's soils are naturally low in organic matter. When rice straw is burned, most of the carbon, nitrogen (99%), phosphorus (18%) and potassium (44%) in the straw is released into the atmosphere causing air pollution and affecting human health.
Furthermore, 74-100% of the carbon is lost in the form of CO2 and CO in rice straw burning. This is an important carbon source that could have been stored in the soil that we call it carbon sequestration. Shinapas Mannae, a Kanchanaburi province farmer who is a current organic rice producer says when asked about burning corn residue:
"My opinion is that the soil benefits from the return of both root tissue and above-ground crop residues. We as a community enterprise for natural agriculture never use field burning in our high-yield fields. We do everything we can to maintain or increase the amount of organic matter in our soils."
Rice residue consisted of about 60% standing stubble (350mm tall) and 40% straw deposited on the soil surface. Temperatures, below, at, and above the soil surface reach a maximum within 30 seconds after ignition to 400°C above the soil surface. Soil surface temperatures during burning were highly variable, seldom exceeding 120C when the litter-burn was incomplete, but ranging between 170-400C where stubble and straw were completely burned. Surface temperatures above 400C suggest that all soil moisture content will disappear via evaporation. Also, all microorganisms and soil fauna located on or near the surface will die.
However, we have to know that soil is a living system. A living system consists of life forms, and the food, air, water, habitat and shelter they need to thrive, grow and reproduce. In the soil, organic matter becomes food; the soil's structure and network of pores provide habitat, air and water; and living vegetation and surface residues offer shelter. Many people don't realise that soil, especially healthy soil, is full of life. Millions of species and billions of organisms make up a complex and diverse mix of microscopic and macroscopic life that represents the greatest concentration of biomass anywhere on the planet.
Bacteria, algae, microscopic insects, earthworms, beetles, ants, mites and fungi are among them. Altogether, their value has been estimated at US$1.5 trillion a year worldwide. What these low-lying creatures lack in size, they make up for in numbers. Consider bacteria, the soil microbes with the highest numbers. For example, you can fit 40 million of them on the end of one pin. In fact, there are more soil microorganisms in a teaspoonful of soil than there are people on the earth. These microbes, which make up only one-half of 1% of the total soil mass, are the yeasts, algae, protozoa, bacteria, nematodes and fungi that process soil into rich, dark, stable humus. The healthiest soils are those with a diversity and abundance of life.
Farmers who adapted to not burning their crop residues after harvesting understand that open burning is disruptive to soil microbes and destructive to the soil system and structure. Instead, they use direct seeding rice without burning residue, generally using specially designed planters that can sow seed into residues. Such farmers grow a diversity of living plants in the soil as much of the time as practical, covering the soil and offering food to soil microbes through living roots. Those soil organisms, in turn, cycle nutrients back to the plant, allowing it to grow and flourish. It's a natural, symbiotic system that leads to healthy soil and more sustainable and profitable agriculture.
Thailand's farmers can earn more money if they keep the rice straws and use them to grow oyster mushrooms. Using rice straw for mushroom production can yield about 5–10% of mushroom products (50–100kg of mushroom per 1 tonne of dried straw). Furthermore, after the mushrooms are harvested, the farmers should still keep the substrate and make even more money. The waste from mushroom production is also the main substrate for vermicomposting, one of the best organic fertilisers produced. Even if mushroom growing and vermicomposting seem to be a handful, please do keep the rice straw.
The rice straw can also be used as mulch to reduce water evaporation from the soil surface and to keep the weeds out and protect the plants' roots from heat and cold. In conclusion, burning rice straws to clear the fields after harvest is not advisable.
Mohammad Esmaeil Asadi is Associate Professor, Water Engineering and Management (WEM), School of Engineering and Technology (SET), Asian Institute of Technology (AIT).