Assessment of Greenhouse Gases Emission in Smallholder Rice Paddies Converted from Anyiko Wetland, Western Kenya

Abstract

Rice, (Oryza sativa L.) is an important food crop in Kenya and is the third most consumed cereal crop after maize (Zea mays) and wheat (Triticum aestivum). The high demand for rice has resulted in the conversion of wetlands to rice paddies, ultimately reducing the ability of wetlands to store carbon. Farmers have also increased use of fertilizer to improve productivity. Consequently, emissions from wetlands of three potent greenhouse gases (GHGs): methane (CH4), nitrous oxide (N2O), and carbon dioxide (CO2) have increased. This study assessed the influence of fertilizer application on GHGs emission, organic carbon and nutrient stocks in rice paddies in papyrus dominated wetlands in the Nzoia River basin in Kenya. Sampling was done on a weekly basis for the first two months, and thereafter twice per month in the Anyiko rice paddies, which is a smallholder system partly converted from the Anyiko wetland. Two replicates of three fertilization treatments (standard, control and under fertilization) were assigned randomly in six rice plots. The static chamber method was used to collect the GHGs, which were then analyzed using gas chromatography. Soil samples were collected and analyzed for nitrogen and organic carbon stocks. Statistical tests revealed no significant differences in organic carbon and nitrogen stocks among the three fertilization treatments. The mean CH4 fluxes did not differ significantly among the three treatments where mean flux for control plots were 8.30 ± 4.79 mg m−2 h−1; under-fertilized plots had a mean of 6.93 ± 2.42 mg m−2 h−1 and standard fertilized plots mean fluxes were 4.00 ± 6.34 mg m−2 h−1. Similarly, CO2 mean fluxes were insignificantly different among the three treatments, where control plots had mean of 174.80 ± 26.81 mg m−2 h−1, under-fertilized plots mean were 208.81 ± 36.20 mg m−2 h−1 and standard fertilized plots mean fluxes were 248.29 ± 41.22 mg m−2 h−1. However, mean N2O fluxes were significantly different among the three treatments, control plots had a mean of −3.59 ± 2.56 μg m−2 h−1, followed by under-fertilized with mean of −0.59 ± 0.45 μg m−2 h−1 and standard fertilized plots with mean of 4.37 ± 3.18 μg m−2 h−1. In this study, different fertilization scenarios had significant effects on N2O emission but no significant effect on CO2 and CH4 emission, organic carbon and nutrient stocks. From the findings of the study, fertilizer application increases emission of N2O. Therefore, there is need for sustainable use of wetlands and fertilizer in rice paddies to minimize greenhouse emissions and wetland degradation.