Methane gas emissions (CH4) in paddy fields in Minasatene District, Pangkep Regency: Microbial abundance against increased methane gas

Global warming caused by various human activities results in increased concentrations of Greenhouse Gas (GHG) emissions. This causes the problem of climate change. One of the sources of Indonesia’s greenhouse gas (GHG) emissions comes from the agricultural sector. Methane (CH4) is the second most important greenhouse gas and has a global warming potential 25 times greater than CO2. Paddy fields contribute to GHG emissions due to the condition of the rice fields which are always flooded which produce gases such as CO2, N2O and Methane (CH4). This study aims to determine the effect of microbial abundance on increasing emissions of methane gas (CH4) in paddy fields. This research was conducted by collecting samples in the field with variations at 30th, 60th, and 90th day after planting (DAP). The results of laboratory analysis showed that the results of C-Organic analysis were 1.45-2.56%, the results of soil microbial abundance analysis for the total bacterial population ranged from 1.2 x 104 - 6.3 x 105 (CFU/g) and the results of analysis flux of CH4 at 30thDAP sampling from the three sample points ranged from 2.13 - 11.29 (mg/m2/hour). For the 60thDAP sample from the three sample points it ranges from 10.01 - 22.77 (mg/m2/hour). For the 90thDAP sample from the three sample points it ranges from 21.98 - 25.54 (mg/m2/hour). The results of the analysis of methane (CH4) emissions in one growing season show different values of methane emissions at different locations and sampling times.


Introduction
Global warming is currently a world problem that causes climate change.This is due to human activities which result in increased concentrations of greenhouse gas emissions [1].Greenhouse gas emissions from the agricultural sector include emissions of methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O).
Methane is the second most important greenhouse gas after carbon dioxide (CO2) and has a global warming potential 25 times greater than CO2, therefore, small changes in CH4 in the atmosphere can contribute significantly to global warming [2].
Paddy fields are an important anthropogenic biological source of CH4 in the atmosphere, which accounts for about 10% of global CH4 [2].Paddy fields contribute 69% of GHG emissions in Indonesia [3].Methane gas comes from the results of the anaerobic decomposition of organic matter caused by the activity of methanogenic bacteria which form methane gas.The addition of organic matter to paddy field soil with stagnant conditions can increase microbial activity and cause a significant increase in methane gas emissions.
Pangkep Regency has a paddy field area of 16,732 Ha.In an effort to reduce GHG emissions, the area can be used to calculate methane gas emissions as a source of GHG inventory data.Based on this, a study was conducted on the effect of microbial abundance on increasing emissions of methane gas (CH4) in paddy fields.By conducting this research, it is hoped that it will be able to become a guideline for regions in carrying out greenhouse gas inventory activities.

Research location
Sampling of methane gas and soil samples was carried out in Minasatene District, Pangkep Regency, South Sulawesi.Laboratory analysis of methane gas was carried out at the Jakenan Agricultural Environmental Research Institute (Balingtan), Pati, Central Java and soil sample analysis was carried out at the Soil Chemistry and Fertility Laboratory, Department of Soil Science, Department of Soil Science, Faculty of Agriculture, Hasanuddin University, Makassar, South Sulawesi.

Preparatory
Stages this stage is carried out to find research ideas that will be carried out including literature studies by searching literature from various reference sources to support methods and discussions related to research including identification of paddy fields and factors that influence CH4 from paddy fields.In addition, data collection, tools and materials needed at the time of sampling, permits for research locations from the local government and landowners were carried out.

Sampling
Sampling of methane gas and soil samples was carried out during rice treatment, namely on the 30th, 60th, and 90th day after planting (DAP).Sampling points are divided into three points which are divided based on the expanse of rice fields.The soil samples taken consisted of disturbed soil samples.
Gas samples are taken using a syringe with a rubber septum in the lid, then put into a vial.The lid is set in a flat position and covers 4 rice clumps.Sampling time intervals are 5, 10, 15, 20, and 25 minutes in a series of gas sampling.Soil sampling point symbols are described in Table 1.

Parameters and data analysis
The methods used for the analysis of soil samples are described in Table 2.

C-Organic
Based on the data analysis results (Table 3.) shows the percentage of C-Organic content which is classified as low to moderate which is affected by the low addition of organic matter to the soil and affects the quality and health of paddy field soil.--------% ------- The results of C-Organic analysis in paddy soil showed an increase in C-Organic content at 30th, 60th, and 90th day after planting (DAP).This can be caused by the weathering process of organic matter originating from the remaining vegetation from the previous growing season which is still experiencing weathering/decomposition process which is influenced by anaerobic conditions in paddy fields.In Tangketasik et al. (2015), explained that a stagnant (anaerobic) atmosphere in paddy soil can inhibit weathering and mineralization of organic matter [5].

Abundance of microbes
Based on the results of the microbial population calculation (Table 4.) shows an increase in the population of bacteria.The highest population of bacteria is found in the time of taking 60thDAP at the second sample point and the lowest bacterial population was found at the time of taking 30thDAP at the second sample point.

Emissions CH4
Described in Sharkey, Holland and Mooney (2012), that methane gas is emitted from the subsoil into the atmosphere in three ways: gas bubbles are formed and released to the surface of the stagnant water through the ebulization mechanism, secondly by the process of diffusion through the stagnant water and thirdly the methane formed enters the plant root tissue.Rice and move by diffusion in the aerenchemical vessels and then released into the atmosphere [6].Methane flux calculation results (Figure 1.) show an increase at each age of the plant.At 30thDAP to 60thDAP is the vegetative phase, where in this phase rice is in the maximum tillering stage until panicle formation, so that the increase in plant height and number of tillers will make methane transport routes more and methane emissions will increase.
The results of the 90thDAP analysis are the harvest phase in rice, so that when sampling has been carried out the harvest has been carried out and leaves rice straw in paddy fields.It can be seen from the analysis results obtained, showing a significant increase in the first and second sample points.This can be caused by the addition of organic matter to the paddy soil with conditions that are still stagnant, thereby increasing microbial activity and causing an increase in methane gas emissions.This is in accordance with the opinion, which states that CH4 formed due to the decomposition of organic matter under anaerobic conditions.This is in accordance with the opinion of Agus (2004), which stated that CH4 was formed due to the decomposition of organic matter under anaerobic conditions [7].
Microbes that play an important role in the formation of methane gas are methanogenic bacteria, and bacteria that cause reduced methane gas are methanotropic bacteria.Although this study did not identify methanogenic and methanotropic bacteria, based on the analysis of microbial abundance in the bacterial population, there was an increase in the bacterial population from 30thDAP to 90th DAP.This indicates the presence of microbial activity that occurs in anaerobic conditions in paddy soil.
In accordance with the results of C-Organic analysis which also experienced an increase, the decomposition of organic matter under anaerobic conditions also continued to occur and led to the formation of methane gas (CH4).In Susilawati,et. al (2013), explained that organic matter is food (energy) for microorganisms.Therefore, land that has a high percentage of organic matter will have a greater number of soil microorganisms.So that the increase in organic matter in the land causes the bacterial population to also increase [8].

Conclusion
The results of the microbial population calculations showed that the total bacterial population ranged from 1.2 x 10 4 -6.3 x 10 5 (CFU/g).The highest bacterial population was found at the time of taking 60thDAP at the second sample point and the lowest bacterial population was found at the time of taking 30thDAP at the second sample point.There was an increase in the microbial population from 30thDAP to 90thDAP.This indicates that there is microbial activity that occurs under anaerobic conditions in paddy soil and causes an increase in methane gas.

Table 2 .
Type and method of soil sample analysis.

Table 3 .
Results of c-organic analysis of soil in paddy field.

Table 4 .
Results of analysis of soil microbial abundance in paddy fields.