Carbon flow and management in regional rice production in Thailand

Abstract

This research aims to study the Carbon (C) flow, stock and management in rice production system, including rice cultivation, rice processing and management of rice by-products. The study boundaries are the rice production systems of four districts in Phayao province and the assessment technique is Material Flow Analysis (MFA). The results show that there was a positive C stock each year from large input of biomass residue. However, C loss from soil respiration still was very high because of the long fallow period. Straw was plowed back to the soil and tilled just a few days before flooding time causing high methane emissions at the beginning of cultivation. The recommendations for technology and management practices include harvesting straw for composting and returning the compost to soil, plowing the rest of the straw back to soil more than 30 days before the flooding period, accepting crop rotation practice instead of leaving the land fallow and finally, setting up a gasification or pyrolysis plant using straw to produce energy and by-product of bio-char for farmer to return it back to the soil. Applying feed-in-tariff or incentives like C-credit for reducing methane and C-credit for increasing C stock in the soil are also recommended.

Keywords:

• Material flow analysis
• biomass residue
• C sequestration
• Bio-char

Introduction

Biomass residues can be considered as valuable resources because they contain essential substances such as nutrients and carbon as well as energy. Thus, manure and other biomass residues can potentially be used as a renewable energy source and can be returned to the soil to improve the nutrient and drainage structure of agricultural soils [1, 2]. As a result, sustainable utilization and management of biomass residues can have a significant impact on the reduction of greenhouse gas emissions [3]. Recycling manure and biomass residues into soil can also increase soil organic carbon (SOC). Increased levels of SOC can both improve soil quality and reduce atmospheric levels of greenhouse gases. Processes that can transform organic carbon in biomass into stable SOC therefore represent a potentially very significant approach to mitigating climate change. These issues have often been considered previously in separate analyses and managed by different agencies or organizations.

Tracking the flow of key substances associated with biomass residues can help assess the sustainability of current agricultural and biomass management practices regarding essential substances, which represent both resources and pollution. This approach can indicate the main sources of substance emissions, efficiency of use and eventual fate of the key substances. Different choices in biomass management can have a marked effect on where and in what form key substances travel through the system. As such, Material Flow Analysis (MFA) represents a potentially valuable tool for assessing the substance flow implications of different alternative options with the aim of improving the metabolism of the region (i.e. to enhance energy recovery, carbon and nutrient cycling).

The challenge remains, of course, as to how these various implications should be assessed against each other. Valuing different resource management options with respect to substance flows across other potential factors including the value of energy production and potential soil carbon sequestration should be assessed. Significant tradeoffs might be expected as well as potential synergies for particular management choices. Some previous interesting studies have explored eco-efficiency of paddy rice production in Northeastern Thailand, looking at the tradeoff of several environmental and economic factors and impacts such as water, greenhouse gases, land, Labor and income [4, 5]. However, C balance, stock and C sequestration in the soil have not been analyzed.

The aim of this project is to apply the tool that can be used for evaluating alternative options for management and resource recovery from biomass for reducing greenhouse gas emissions, enhancing recovery of energy, and returning carbon (C) from biomass back to soil in an agricultural region. This research employed MFA and a case study of rice production system of four districts in Phayao lake watershed in Phayao province was used to demonstrate the application of the tool.

Materials and methods

Material Flow Analysis (MFA) is applied to estimate C and energy associated with key substance inputs and outputs, to and from alternative biomass processing plants. MFA is an environmental accounting tool that traces and provides an account of valuable resources or toxic substance flows through a system based on mass balance and mass conservation principles [6, 7]. MFA is employed here to approximate the biomass and gases output and C flow associated with products and by-products for the current situation and proposed alternative biomass processing plants.

Firstly, the quantities of manure and biomass residues to be processed through different biomass processing plants are determined. The substance and energy flows associated with different biomass residues for a given study area are calculated first using the Equation 1(1) $${\mathrm{X}}_{\text{ij}}={ \mathrm{M}}_{\mathrm{i}}\times { \mathrm{C}}_{\text{ij}}$$(1) shown below. (1) $${\mathrm{X}}_{\text{ij}}={ \mathrm{M}}_{\mathrm{i}}\times { \mathrm{C}}_{\text{ij}}$$(1)

where

X = substance flow (tonne/year or kg/year)

M = mass flows of biomass (tonne/year)

C = substance concentrations in the biomass (tonne/tonne or kg/tonnes)

i = 1,….,k indicates the biomass type

j = 1,….,n indicates the substance type

Secondly, several alternative biomass processing plants or other options are defined and incorporated into the analysis. Different products and by-products from these plants and processes are also defined and evaluated.

Thirdly, the outputs from these plants – energy and substance flows associated with products or by-products are determined by multiplying substance and energy flow inputs with transfer coefficients as shown in Equations 2(2) $${\mathrm{X}}_{\text{output}}={ \mathrm{X}}_{\text{input}}\times \text{TC}$$(2) and 3 [6, 7]. These coefficients are obtained from the literature and technical assessments of the performance of existing biomass processing plants in other regions.

Substance flow outputs associated with products or by-products from a particular biomass processing plant can be calculated by the following equation: (2) $${\mathrm{X}}_{\text{output}}={ \mathrm{X}}_{\text{input}}\times \text{TC}$$(2)

where

Xoutput = substance flow output (tonne/year)

Xinput = substance flow input (tonne/year)

TC = transfer coefficient of product and by-product output

Transfer coefficient is the partition or ratio of mass of input substance that is transferred into a specific mass of output. TC is specified for each output of substance.

The energy flow output from a biomass processing plant is calculated as follows: (3) $${\mathrm{E}}_{\text{output}}={ \mathrm{E}}_{\text{input}}\times \text{efficiency}$$(3)

where

E_output = energy flow output (TJ/year)

E_input = substance flow input (TJ/year)

efficiency = energy efficiency of power plant (electricity)

System definition

The spatial boundary includes the paddy fields in the Phayao lake watershed in 4 district areas in Phayao province including Muang, Mae-jai, Phukamyao and Dokkhamtai as shown in Figure 1. The main land use in the area is 64,394 ha of paddy rice. The soil type is silty loam. Average yearly rainfall in the study area is 1,100 mm and average temperature is 25 °C.

Figure 1. Spatial boundary and scope of the study.

The system boundary comprises of rice production and soil (all paddy fields in the area) and related processes including straw management, composting, using rice straw as a bedding in the animal farm, rice milling, burning of rice husk in the boiler for heat, using of rice husk in the brick factory.

In the rice production and soil process, the inputs include net primary productivity (NPP) (photosynthesis – respiration), seed, water, rice husk as soil conditioner and fertilizer. Output from the process includes harvested paddy rice, straw and off-gas from straw burning, C from soil respiration and methane from anaerobic decomposition in the paddy field, C in the drainage water.

As part of the straw management process, straw is harvested in the study area both by hand and by harvesting machine. Farmers who are harvesting by hand cut down the straw, bundle it together and use it as mulching material to grow other crops such as onion, garlic, or used to produced animal feed, animal bedding and compost. When harvesting by machine, the straw is thrown at the end of the machine and scattered throughout the field. Most of the straw and stubble is plowed back to the soil but some is burnt in the field.

Rice milling process produces rice, rice bran and rice husk. Rice and rice bran are sold to commercial shops and animal farms in the region. Rice husk (residue), 21% of the paddy rice, is at present used depending on the type of rice mill. The large rice mills mostly use rice husk as a fuel in the rice mill itself by using burner and dryer to dry paddy rice. There are 15 rice mills that have fuel burners except one that sells rice husk to a cement kiln as fuel. Cooperative rice mills sell rice husk to longan factories in Chiang Rai province to use as fuel to dry longan. The middle sized rice mills in the area sell rice husk to brick factories to use as an ingredient and for the charcoal burners in the area. The small rice mills in the area apply rice husk to soil as a soil conditioner and mix with animal feed.

Animal farms in the study area raise animals mostly for local consumption and to use for Labor. Small animal farms are spread in all areas. The most popular animals are cattle, buffalo, swine, chicken, and duck. Animal feed is the main incoming substance in the system. Most of it comes from the biomass left over from the rice production chain, i.e. rice straw as hay for cattle and buffaloes. Bran and broken rice is a food for pigs. Rice husk is a feed for ducks and chickens. In addition, rice husk is also used as animal bedding for animals in the area.

A composting plant with a capacity of 500 tonnes per year is the only large one in the area. The input materials are rice straw and cow manure. Most of the compost output is sold to other provinces.

Data sources and quantification methods

To determine the mass flow of goods and the concentration of substances in rice paddy and soil in rice fields, secondary data were collected from various academic papers and government reports. Primary data collection from interviewing with farmers, rice mill owners and animal farms was also conducted. Some technical data were sampled from the field to use as representative of the area including; CH₄ emission rate (Empa) and CO₂ emission rate (Ecpa_paddy, Ecpa_fallow) and Net Primary Productivity (NPP).

Sampling site description

This research selected representative rice growing areas from those that have common rice cultivation practice and method of soil preparation, fertilization and drainage. The study area is located in Muang district, Phayao province (19°08'25.7"N 99°53'25.9"E). Paddy rice in the area is rain-fed type and the rice variety is Khao Dokmali 105. Paddy rice cropping season is during July to November. December to June is the fallow period.

The soil in the area is classified as Hangdong soil series that is the main soil series of the study areas (classification by Office of Soil Resources Survey and Research, Land Development Department). The soil series characteristic is silty loam; 2.3% sand, 43.7% silt, 54.0% clay, pH 5.9, bulk density 1.41 g cm⁻³, 2.58% organic matter and 1.5% organic carbon. The CH₄ emission rate (Empa) and CO₂ emission rate (Ecpa_paddy, Ecpa_fallow) were collected from the sampling site to use as representative to determine total CH₄ and CO₂ from all the area in the system boundary.

CH₄ and CO₂ sampling and analysis

The CH₄ and CO₂ samples were collected using the closed-chamber method. Gas samples were taken from the top of the chamber once a week, the samples being collected at 0, 5, 10 and 15 min. The gas was analyzed using Gas Chromatography (Shimadzu GC 2010 plus, Japan) with a Flame Ionization Detector (FID) detector and Unibead C Packed column.

Net Primary Productivity (NPP) determination

The primary producers (green plants) can fix energy from light by photosynthesis and convert light energy into biochemical energy. Energy is ultimately released from the metabolism by the respiratory process. NPP is the residual part from respiration and is used to produce leaves, flowers and other parts of plants. Therefore, NPP in this study was determined from sampling and analyzing C mass in the crop biomass. Crop biomass is the total mass of rice plant including rice grain, rice straw, root and stubble. Samples were collected from the rice fields. The mass of each part and total mass per area of 1 square meter (dry weight) was measured. The samples were dried in the shade for 7 days and then crushed for carbon content analysis.

Determination of C flow

Input and output of C flow related to biomass flow including paddy rice product, rice product after milling, rice straw, rice husk, fertilizer, seed, and compost and manure can be calculated using Equation 1(1) $${\mathrm{X}}_{\text{ij}}={ \mathrm{M}}_{\mathrm{i}}\times { \mathrm{C}}_{\text{ij}}$$(1) . However, some of the output flows can be calculated using equations below.

• Output flow of C from burning of rice straw and stubble

FC₃ = Flow of CO₂-C from burning of rice straw and stubble (tonne C/y)

(4) $${\text{FC}}_3= \text{Straw} \times { \mathrm{C}}_{\text{straw}}\times { \mathrm{R}}_{\text{stubble}}\times { \text{TC}}_{\text{op}}\times { \mathrm{P}}_{\text{sb}}$$(4)

Strawis total mass of straw = FC₁ × Rs/s

FC₁ is total mass of rice product

R_s/s is ratio of rice straw to rice product

C_straw is carbon content in straw

R_stubble is ratio of stubble to rice product

TC_op is ratio of C emission gas to the air

P_sb is percent of rice straw and stubble that was burnt

• Output of CH₄-C from the paddy field

FC₂ = Flow of methane-C from the paddy field (tonne C/year)

(5) $${\text{FC}}_2={ \mathrm{E}}_{\text{mpa}}\times { \mathrm{A}}_{\text{paddy}}\times 12/16 \times \text{adjust} \text{factor}\times \text{scaling} \text{factor}$$(5)

Empa is Emission of methane per area (g CH₄ m⁻² crop⁻¹)

A_paddy is total area of paddy field in the system boundary

12/16 is the ratio of C per CH₄

Adjust factor is the ratio of the yield of the sampling plot per average yield of the overall study area = 0.62

Scaling factor include:

SFw = scaling factor to account for the differences in water regime during the cultivation period

SFp = scaling factor to account for the differences in water regime in the pre-season before the cultivation period

SFo = scaling factor for type and amount of organic amendment applied

Output flow of CO₂-C from soil respiration from the paddy field (tonne C/year)

FC₄ = Flow of CO₂-C from soil respiration from the paddy field (tonne C/year) (6) $${\text{FC}}_4={ \text{FC}}_4\_\text{paddy} +{ \text{FC}}_4\_\text{fallow}$$(6)

FC₄_paddy is Flow of CO₂-C emission from soil respiration during rice cultivation period

FC₄_fallow is Flow of CO₂-C emission from soil respiration during fallow period

(7) $${\text{FC}}_4={ \text{FC}}_4\_\text{paddy} +{ \text{FC}}_4\_\text{fallow}$$(7)

Ecpa_paddy is emission of carbon dioxide per area during rice cultivation (g CO₂ m⁻² crop⁻¹)

A_paddy is total area of paddy field in the system boundary

12/44 is the ratio of C per CO₂

Adjust factor is the ratio of the yield of the sampling plot per average yield of the overall study area = 0.62

(8) $${\text{FC}}_4{\_}_{\text{fallow}}={ \text{Ecpt}}_{\text{fallow}}\times { \mathrm{A}}_{\text{paddy}}\times 12/44 \times \text{Adjust} \text{Factor}$$(8)

Ecpt_fallow is emission of carbon dioxide per area during fallow period (g CO₂ m⁻² crop⁻¹)

Apaddy is total area of paddy field in the system boundary

12/44 is the ratio of C per CO₂

Adjust factor is the ratio of the yield of the sampling plot per average yield of the overall study area = 0.62

• Rate of C Stock in Paddy Soil

Δ C_Soil = Rate of C Stock in Paddy Soil

(9) $$\Delta C_{\text{Soil}} = \sum C_{\text{input}} - \sum C_{\text{output}}$$(9)

Output flow from biomass residue processing plant

C in products and by-product outputs from biomass processing plants (and returned to the soils) are calculated using Equations 1(1) $${\mathrm{X}}_{\text{ij}}={ \mathrm{M}}_{\mathrm{i}}\times { \mathrm{C}}_{\text{ij}}$$(1) and 2(2) $${\mathrm{X}}_{\text{output}}={ \mathrm{X}}_{\text{input}}\times \text{TC}$$(2) . Data and transfer coefficients are based on field interviews, a number of previous study reports and the literature [8–16]. Transfer coefficients used in this study are shown in Table 1.

Table 1. Transfer coefficients of biomass processing plant.

Process	Output TC description	TC value	References
Composting	Compost product Off-gas Leachate	TCcompost = 0.61 ± 0.09 TCoff-gas = 0.36 ± 0.05 TCleachate = 0.03 ± 0.004	[10]
Pyrolysis and Gas Engine	Bio-char Off-gas	TCChar_pyro = 0.15 ± 0.02 TCoff-gas Pyrolysis = 0.85 ± 0.13 Electrical efficiency 14 ± 5%	[11, 13, 15]
Gasification and Gas Engine	Bio-char Off-gas	TCChar_Gasi = 0.05 ± 0.007 TCoff-gas_Gasify = 0.95 ± 0.14 Electrical efficiency 24 ± 5%	[11, 12, 14, 16]

Products from biomass processing plants from all options including compost and bio-char were assumed to be applied to the soil as soil conditioner and also for C sequestration into the paddy soil in the area.

Scenario analysis of alternative biomass residue management options

There are 3 scenarios of alternative management of rice straw and application of by-product to soil as shown in Table 2.

Table 2. Improvement scenarios of alternative management of rice straw and application of by-product to soil.

Scenario	Description of the scenario
1 Compost	Collect 75% of all rice straw to co-compost with animal manure and apply all compost to soil in the area Return 25% of rice straw plowed back to soil farmers apply all compost product to the soil in the area
2 Pyrolysis and Gas Engine	Collect 75% of all rice straw to pyrolysis plant to produce electricity and bio-char Return 25% of rice straw plowed back to soil Sell electricity to regional grid Sell bio-char back to farmers who apply it to the soil in the area
3 Gasification and Gas Engine	Collect 75% of all rice straw to gasification plant to produce electricity and bio-char Return 25% of rice straw plowed back to soil Sell electricity to regional grid Sell bio-char back to farmers who apply it to the soil in the area

Determination of C sequestration to soil

As noted previously, not all of the C in biomass applied to the soil will remain over time as some portion will return to the atmosphere through decomposition [17]. The mechanisms of soil carbon sequestration are very complex and subject to significant uncertainties associated with local conditions (soil type, temperature, moisture, climate condition) and the time period of interest. Nevertheless, this study has attempted to estimate conservative C sequestration levels for different biomass products and by-products applied to soil as shown in Table 3. The multiplication factors here represent the assumed fraction of C in products and by-products applied to soil that is expected to remain as sequestered soil C after a period of a hundred years or more. The assumptions are conservatively adapted from a range of published estimates.

Table 3. The multiplication factors for C sequestration in different products and by-products.

Type	Potential sequestration (proportion of applied C sequestered for 100 years)	References
Compost	0.15 ± 0.03	[18–20]
Bio-Char	0.8 ± 0.24	[21,22]

Results

Current biomass residues management (Base case)

Rice production in the region is mostly rain-fed rice with only some areas being irrigated. Most of the biomass by-products are utilized in some manner. Rice straw is collected to use for vegetable farming, fodder or animal bedding. About 5% of rice straw is baled and sold; the rest is left on the soil for decomposing and some is burnt. Some part of stubble that is not decomposed yet is submerged under water at the very beginning of farming of the next crop inducing anaerobic fermentation which produces methane. Cattle manure is composted and used as soil conditioner while chicken litter (manure and bedding) sold to farmers to apply on land as a soil conditioner without pretreatment.

Biomass residues from rice mills include rice bran and rice husk, and rice bran was sold as animal feed. Rice husk in the large rice mill was used as fuel for heat in the rice mill itself or sold as fuel to cement plants. Some rice mills sold rice husk as fuel to industrial plants and longan factories and brick factory in the province. There is one composting plant in the region using rice straw as feedstock, however, the scale of the plant is small and the product is used in the nearby area.

Carbon balance in the current biomass residues management (Base case)

Carbon flow diagram in the study area is shown in Figure 2. In rice production and soil process, the main input was Net Primary Productivity (NPP), C that plants absorb during photosynthesis minus the C released during plant respiration. The largest C output from the system was the C released to the atmosphere from the rice fields and soil accounting for 40% of the total C input. Most of the C released to the atmosphere was C output in the form of carbon dioxide from the system through soil respiration, followed by methane emission from rice field. While C output from burning stubble was small, the C output from the rice product was the second largest accounting for 32% of the total C input. Paddy rice product as output from the system was 124,846 tonne C/y and 221,222 tonne C/y of rice straw was generated. Rice straw was used for fertilizer at 165 tonne/y, used for animal feed at 12,925 tonne C/y, and the rest was plowed back to the soil. C recycled back to the soil in the system in terms of recycled organics including straw and compost was 38% of the total C input. In turn, there was a net C stock of about 18% of total input into the system each year. Carbon release into the water source was only 1% of total C input. Rice husk and manure that was sold to other areas outside of the system accounted for 9% of total C input.

Figure 2. C Flow in the system in the base case (tonne/y).

In order to reduce C emissions, the emissions to the atmosphere should be focused as they are the largest component and cause greenhouse effect. In this area, methane emissions were high compared to other studies because of high organic characteristic of soil and the rice straw management practice. Rice straw was left in the field and incorporated for less than 30 days before cultivation causing anaerobic fermentation during the preparation of soil before planting. Therefore to reduce methane emission, rice straw should be incorporated more than 30 days before cultivation to allow it to decompose aerobically. In some areas where second rice was cultivated, the rice straw was often burnt before the next (second) rice cropping causing C emission to the atmosphere quickly along with other air pollution. Therefore, there should be a campaign to stop burning stubble to reduce emissions and retain carbon in soil.

Most of the area is cultivated once in July to December, after that, the land was left fallow. Carbon dioxide emissions from soil respiration are very high. Therefore, applying crop rotation to the land should be encouraged to reduce fallow land period. To increase C-cycle back to soil, all rice husk should be redirected to make compost as well as manure handled incorrectly should be redirected to production of compost and then back to the soil in the paddy field in the area.

Scenario 1 Collect 75% of all rice straw to co-compost with animal manure and apply all compost to soil in the area

The research found that Scenario 1 can reduce CH₄ from paddy field by 34,825 tonne C/y from the emission in the base case because in the Scenarios 1–3, 75% of all rice straw was harvested from soil to produce compost instead of leaving in the field and decomposing anaerobically under water after incorporating and during the planting of the new crop. Most of the rice straw was harvested from the field and all animal manure was collected and used to produce compost which was subsequently cycled C back to land. Therefore, carbon dioxide from soil respiration was reduced and C stock in the soil increased from 99,082 tonne C/y in the base case to 195,120 tonne C/y.

Scenario 2 Collect 75% of all rice straw to pyrolysis plant to produce electricity and bio-char to apply to soil

Methane-C emission was reduced from the base case of 54,249 tonne C/y to 18,703 tonne C/y because most of the rice straw was harvested from the soil to pyrolysis plant instead of leaving in the field to decompose anaerobically. This can lead to reduction of carbon dioxide from soil respiration from 143,648 tonne C/y in the base case to 30,971 tonne C/y in this scenario. C stock in the soil increased from 99,082 tonne C/y in the base case to 108,002 tonne C/y because more stable form of biomass C was returned to the soil.

Scenario 3 Collect 75% of all rice straw to gasification plant to produce electricity and bio-char to apply to soil

The research found that applying Scenario 3, methane-C emission was reduced from the base case of 54,249 tonne C/y to 18,605 tonne C/y because most of the rice straw was harvested from the soil to gasification plant instead of leaving in the field to decompose anaerobically. This can lead to reduction of carbon dioxide from soil respiration from 143,648 tonne C/y in the base case to 30,971 tonne C/y in this scenario. However, this gasification produced less bio-char than pyrolysis in Scenario 2. C stock in the soil increased from 99,082 tonne C/y in the base case to 91,712 tonne C/y because a more stable form of biomass C was returned to the soil.

The estimated GHG emission, C stock in the soil, energy production and GHGs offsets from renewable energy produced, resulting from manure and straw processing and recovery options in the scenario are shown in Table 4.

Table 4. Energy and environmental benefit from the scenarios.

Item	Base case	Scenario 1	Scenario 2	Scenario 3
CH4 emission (tonne CH4/y)	54,249	19,424	18,703	18,605
CH4 reduction from base case (tonne CH4/y)	–	34,825	35,545	35,644
CH4 reduction from base case (tonne CO2 eq/y)	–	870,628	888,636	891,094
Annual C budget (stock) in the soil (tonne C /y)	99,082	195,120	108,002	91,712
Type of by-product returned to soil		Compost	Bio-char	Bio-char
C in by-product returned to soil (tonne /y)	–	117,099	24,583	8,194
Potential C sequestration (proportion of applied C sequestered for 100 years( (tonne)	–	17,565	19,666	6,555
Electricity produced (MWh/y)	NA	NA	50,400,000	216,000,000
CO2 offset (from electricity production from biomass) (tonne CO2 eq/y)	NA	NA	29,298*	125,561*

* Calculated from emission factor of electricity production from Thai grid mix in 2010 (0.5813 kgCO₂ eq/kWh).

As shown in Table 4, Scenarios 2 and 3 can reduce CH₄ more than Scenario 1 because they transform rice straw into bio-char that is in a more stable C form that will not lead to further decomposition. However, the compost produce that was returned back to the soil can lead to some CH₄ emission after that. As a result, Scenarios 2 and 3 can reduce greenhouse gas from methane emission the most.

On the other hand, Scenario 1 cycled the most C back into the soil, because apart from rice straw, all animal manure that was sold to other regions in the base case was collected and used to produce compost product that was cycled back to land. Moreover, the composting process transforms most of C into compost while pyrolysis and gasification transform most of C into off-gas which is released to the atmosphere more quickly than compost, leaving only a small part of C in the bio-char.

In the base case, even though most of the rice straw was incorporated into the soil which in turn leads to large amount of C back to soil, C in the form of biomass residue will be decomposed and released to the atmosphere more quickly than other by-products such as compost, ash and char after application to soil. This issue is incorporated into the analysis using the multiplication factors for C sequestration as described in the methods section. The result shows that when C balance in the soil and rice production was calculated, Scenario 1 yielded the highest C stock in the soil followed by Scenarios 2 and 3. Even though C in the compost in Scenario 1 was returned to soil the most, potential C sequestration (proportion of applied C sequestered for 100 years) of compost was less than bio-char. Therefore, potential C sequestration for 100 years from bio-char in Scenario 2 showed the best performance.

In terms of energy production and biomass processing, in the Scenario 2, pyrolysis technology was applied and the major product was bio-char and the minor product electricity. On the other hand, Scenario 3 applied gasification technology and the major product was electricity and the minor product bio-char. Therefore, Scenario 3, gasification option produced the most electricity and hence offset the most GHGs emissions from electricity production.

Uncertainty analysis

C stock and flows results in Table 4 are associated with a number of uncertainties regarding to parameters that is used to calculate the results. Methane emission (CH₄) involves two sources of uncertainties including emission of methane per area (g CH₄ m⁻² crop⁻¹) that is obtained from representative field that have uncertainty up to ±30% when applied to the whole region. Scaling factors also have a default level of uncertainty as follows; SF_w ±25%, SFp ±15%, SFo ±2% [23]. Therefore, those uncertainties incur uncertainty in methane emissions of (CH₄) up to ±50%.

Yearly total stock change rate of C in soil has a broad source of uncertainties. Soil microbial respiration is one of the large uncertainties (±30%) that have significant impact to the stock change rate of C in soil. The uncertainty in recycled organic back to soil is ±20% uncertainty and methane emission (CH₄) has up to ±50% uncertainty that can also affect total stock change rate of C in soil.

Uncertainty of C in by-product returned to soil and Potential C sequestration (for 100 years) are associated with three main kinds of uncertainty in primary factors including; 1. uncertainty in mass of recycled organic by-product (±20%), 2. uncertainty in TC of process (±20%) and 3. uncertainty in potential sequestration (proportion of applied C sequestered for 100 years) (±30%).

Discussion

Rain-fed farming practice in the region after harvest always leave the stubble in the field until the next cropping period because farmers will not afford more labor or money or equipment to incorporate of stubble after harvesting. Also, the fallow season corresponds to massive outmigration of farm labor. As a result, the common practice is pre-season flooding more than 30 days before the next cropping period to facilitate the stubble plowing and incorporation causing large amount of GHG emission. The good practice should be to incorporate straw and stubble into the soil right after harvesting to allow straw and stubble to decompose aerobically and reduce methane emission that will occur at the next cropping period. This practice also has other co-benefits of soil conservation and soil fertility improvement thus reducing chemical fertilizer.Stubble incorporation and applying compost and growing rotational legume crop, even though, incurring more labor or operating cost, can compensate with reduction of chemical fertilizer cost. Collection of rice straw to produce compost to use in the paddy field can also accelerate soil fertility and adding more stable C stock into the soil. The composting plant does not have to be a large centralized plant; instead it can be small composting piles for each farm which the farmers can operate by themselves after proper training from the Land Development Department. The associated cost is labor cost for harvesting and cost of manure that can be initially supported by the government. Enabling environment in terms of socio-economics is the hardest part to overcome. The challenge remains as to how to gain the trust and acceptance of farmers regarding the benefits of these options.

Collaborative farming can enable feasibility of those good practices mentioned above in terms of technical and economic aspects. Collaborative farming is the new policy for Thailand promoting the gathering together of small farmers and being associated in order to get the advantage of sharing infrastructure and resources such as water, and plowing and harvesting machine, getting benefit of economies of scale, and strengthening bargaining power for buying resources and selling products. Dealing with the collaborative farm also enables the simplification of support by the government in terms of infrastructure, machine and fuel.

In order to get farmers to change their straw management practices, substantial coaching and learning processes have to be initiated by the government agencies. Demonstration of collaborative farming applying the good agricultural practices mentioned above and showing the proof to farmers of the co-benefits i.e. soil quality and fertility improvement nutrient benefit and cost reduction in reducing chemical fertilizer which can also lead to productivity improvement, is necessary. Promoting and training of Sustainable Rice Platform (SRP) (supported from UN environment) to promote resource efficiency and sustainability in rice supply chain in Thailand and to reduce GHG the main rice growing province can also assist in the learning process. Moreover, it is necessary to give the farmers incentives for the C-credit from the CH₄ reduction and the C-credit from the C stock increase in the soil. Moreover, to encourage such a good practice to occur, several mechanisms or incentives should be put in place. Related organization should work together holistically on this issue.

The organizations that should work together to support and overcome the barrier mention above include Provincial Agricultural Extension Office, Land Development Department (LDD), Agricultural cooperative, Bank for Agriculture and Agricultural Cooperatives, Rice department, Ministry of Agriculture and Cooperatives. These organizations have full responsibility and direct mission to improve agricultural productivity, soil fertility, cost reduction and conserve environment and farmers’ quality of life. Federation of Thai Farmers Association and the Thai Chamber of Commerce collaboration should work together setting Thai rice strategies and supporting farmer to follow the road map through Public-Private collaboration projects to get the right direction and also government support policy. The Thailand Greenhouse Gas Management Organization (TGO) should also play an important role for creating C-credit scheme and C market to have competitive price to support good practices that reduce GHG.

Scenario 1 that has the several practices as mentioned above has high environmental benefits directly for agricultural areas in terms of increased carbon deposition in soil, increasing organic matter to the soil and reducing the cost of chemical fertilizer use. Moreover, with the lowest investment cost and simplest technology, Scenario 1 should be the first priority to implement. Therefore, to encourage such a good practice to occur, several mechanisms or incentives should be put in place. Related organizations mentioned earlier should work together holistically on this issue.

The next priority after good agricultural and straw management practice is the option of bioenergy and bio-char production like Scenarios 2 and 3 because they are more complicated. The key factors for success of bioenergy plants include feedstock cost and security of feedstock price, electricity price, market price of by-product like bio-char, C-credit from GHG reduction. Feedstock (straw) price should be not only attractive for farmers to collect and sell but also should be reasonable price for bioenergy plant investor to buy (1,200–1,500 baht). The most challenges are in the logistics of collection, storage and delivery of rice straw to the plant. The farmers should set up a cooperative to collect and sell biomass to the plant. Electricity price increase and concerns on renewable energy options to reduce GHG can help boost the economic viability of the bioenergy option. Pyrolysis or gasification plant can be feasible with some initial subsidy or incentive or soft loan from the government, incentive from government like feed-in-tariff or renewable energy credit from the Ministry of Energy [24].

The main mechanism to enhance the viability of the plant can be C-credit from substituting electricity from the grid by electricity produced from renewable energy (biomass power plant) from Thai Voluntary Emission Reduction (T-VER) (mechanism from Thailand that is being operated by the Thailand Greenhouse Gas Management Organization or TGO).

Conclusion

This approach can be used for evaluating alternative options for management and resource recovery from biomass residues with regard to all the potential benefits identified above. Assessment of these options took into account technical feasibility. The tool appears to be appropriate for application to a range of agricultural regions given appropriate modifications to meet the particular sustainability issues facing these regions, and availability of data.

Recommendations for C management improvement of the region in this research was prioritized as follows. The first priority should be to remove straw and plowing soil after the harvest to reduce methane emissions from anaerobic decomposition. Moreover, the reduction of carbon dioxide from soil respiration by encouraging crop rotation in order to avoid fallow period should be recommended. The second priority is to encourage farmers to bring straw to compost and return to paddy field to add organic matter and increase carbon deposition to the soil.

The last priority is that the government should support the setting up of biomass gasification or pyrolysis plant and facilitate farmers to bring rice straw to sell as the fuel. Moreover, by-product from the plant, bio-char, should be sold to farmers to apply to soil because bio-char is the most stable and with long-term retention with the ability to improve soil quality and yield.

The model developed here is a static model. There is an inherent limitation in such a static model, as it does not show the variability and general trend over time. Therefore, extending the static model to a dynamic model by utilizing the anthropogenic and geogenic process equations with time as a variable is recommended.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the Thailand Research Fund [MRG5380201], University of Phayao and Office of the Higher Education Commission.