ASSESSING THE GREENHOUSE GAS EMISSION CONCENTRATION LEVELS FROM A RICE FIELD UNDER TWO WATER MANAGEMENT PRACTICES

Abstract

An attempt was made to evaluate the impact of Greenhouse Gas (GHG) emissions from paddy rice fields in Akure, Nigeria. This was to determine the effect of these emissions on the growth development and yield of rice under normal farming conditions and also to ascertain concentration levels under different water management practices. A New Rice for Africa (NERICA) 2 variety was planted in two water management scenarios which were intermittent irrigation (II) and continuous flooding (CF) scenarios. Two closed gas chambers (GCs) were designed, constructed and used for gas accumulation from the paddy rice field in the two water management cases while sampling bags were used for collection. The collection was done at 2 hours intervals from 6am through 12 noon weekly throughout the four growing stages of rice viz: sowing, emergence, tillering and harvesting. The sampled gases were analyzed in the laboratory for GHGs such as Methane (CH4), Nitrous oxide (N2O), Hydrogen Sulphide (H2S) and Oxygen (O2) using gas chromatograph (GC) equipment GH200-9. Soil analyses were carried out to determine the presence of the following parameters viz: nitrogen (N), phosphorus (P), potassium (K), magnesium (Mg), Manganese (Mn) and calcium (Ca). Others are Organic Carbon (OC), Moisture Content (MC), Iron (Fe), Chloride (Cl) and Electrical conductivity (EC) using standard laboratory procedures in other to ascertain effect of their availability on the GHGs concentration levels. From the study, CH4 emission was statistically not significant (p<0.05) as it was not detected during the four growing stages and under the two sets of water management strategies. This might be as a result of some limiting factors that affect CH4 production such as soil type, its disturbance and redox reactions in the rice irrigation water. CO2emission was statistically not significant (p<0.05) as the quantities ranged between 0.01 and 1.2% in three of the four growing stages in both water management practices. It was only during vegetative stage that its emission was not detected. Constant ebullition and diffusion through rice stalk which is known as aerenchyma, photosynthetic reaction and root exudation may be responsible for the absence in this stage. High O2 values between 17 and 20% were detected in all the growing stages during CF scenarios while slightly lower values were recorded in II when compared with CF at vegetative and harvest stages. This could be due to a reaction between O2 in the oxidized soil during shortage. The presence of other gases such as H2S which ranged between 5 and 14 ppm and inert gases ranging between 79 and 81% were detected in the gas samples. The presence of N in the soil which ranged from 0.75±0.18 mg/kgto 0.80±0.04 mg/kg and OC which was from 3.30±0.17 % to 3.32±0.01 %in all the four growing stages of paddy rice did not have significant (p<0.01) effect on the overall GHGs concentration levels since their presence was relatively insignificant in the soil under consideration. The presence of GHGs in various degrees of concentration levels at different growing stages of rice and under two different water management practices considered clearly underscores the contributions of these gases from agricultural fields to global warming which ultimately is responsible for changes experienced in climate.