Assessment of Greenhouse Gas Reduction Technologies in Farmland

Background and Objectives

The rapid expansion of human activities is increasing the atmospheric concentrations of carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O: dinitrogen monoxide) and other greenhouse gases. While CO₂ contributes the largest share (about 64%) to global warming since the Industrial Revolution, CH₄ and N₂O account for about 20% and 6% of the total, respectively. Farmlands and agricultural activity, in many cases, are sources of such GHG emission, and with particular reference to CH₄ and N₂O, rice paddies, digestive processes of ruminants, animal waste and application of nitrogen fertilizers are believed to be the major sources. Thus, the worldwide expansion of agricultural activity and land use alteration has undoubtedly increased the GHG emission from those agriculture-related generation sources, contributing in no small degree to global warming.

In rice paddies, uplands and other farmlands, CH₄ and N₂O are both generated through microbial activity in the soil, and emitted into the atmosphere. In an anaerobic environment as in rice paddy soil, methanogenic bacteria produce CH₄ from decomposed organic substances. In the soil of uplands or grasslands, N₂O and nitric oxide (NO) are generated while applied nitrogen fertilizers undergo transformation by the nitrification bacteria and denitrifying bacteria. Even though NO is not a GHG, it is an air pollutant that causes ozone depletion and acid rain.

NIAES is conducting experimental station tests to measure actual emissions of GHG and other trace gases from farmland and agricultural activity, and to verify the effectiveness of various GHG reduction technologies. At the same time, we are carrying out studies to predict GHG emissions from a wide area by the use of simulation models. In this way, we are working to refine the GHG emission assessment techniques for farmland and to develop emission assessment methodologies for wider area. Furthermore, we seek to be able to quantitatively assess GHG reduction methods by way of improved cultivation and soil management techniques in farmland, and to propose efficient load reduction technology systems for farm communities in Japan and in Asia.

Expected Outcomes

The research outcomes obtained by NIAES are contributing to the work related to emission assessment and to discussions on the causes of change, which are taking place at the Intergovernmental Panel on Climate Change (IPCC), International Geosphere-Biosphere Programme (IGBP) and in other international forums that address the issue of changes in the global climate. Such outcomes are also used to prepare the reports required under the United Nations Framework Convention on Climate Change (UNFCCC) and the Kyoto Protocol, and to develop various emission reduction policy measures.

Relevant Outcomes to Date

Differences in the generation of nitrous oxide and nitric oxide depending on the kind of organic materials applied

The application of organics promotes generation of N₂O and NO as does the use of chemical fertilizers. However, it has been clarified that the amount of N₂O generation varies significantly depending on the kind of organics, and that applied organics with a smaller C/N ratio generate larger amount of N₂O. Based on these findings, we estimated the total N₂O generation from Andosol uplands throughout Japan and found that N₂O generations from applied organics and from chemical fertilizers are at the same level. It was also found that the amount of NO generated by applied organics is less than that generated by chemical fertilizers.

Effect of paddy-upland rotation on GHG emission

If a rice paddy is drained out and converted for cultivation of upland crops, CH₄ generation is averted. It was shown however that N₂O generation from such upland field rises to 3 to 5 times as much as that from a rice paddy. But, when the upland was reconverted into a rice paddy, the generation of CH₄ was found to be less than half in comparison to successive use as rice paddy. The paddy-upland rotation cycle as a whole, generate less gas than the successive use as rice paddy.

Refinement of GHG emission inventory

We have collected actual data on CH₄ and N₂O emissions from farmlands in Japan and Asia, constructed a database and analyzed it. As a result, we quantified the respective GHG emission factors (emission per unit) and the contributions of generation reduction factors. These outcomes were adopted in the IPCC guideline which is used in the calculation of GHG emissions of each country as well as in Japan's GHG inventory report, making a great contribution to the refinement of global GHG emission inventory.