Rice cultivation does not lead to a reduction of the greenhouse gas CO2 in the atmosphere. Rice cultivation is nonetheless used to compensate CO2 emissions from other sources such as coal fired power plants.
These are the conclusions from a recent study by an international team of researchers with a leading role of the Institute for Biodiversity and Ecosystem Dynamics of the University of Amsterdam. The results were published in the April issue of the scientific journal Global Change Biology.
Soils used for rice production are mostly managed under submerged conditions (paddy soils). Through the lack of oxygen, such conditions reduce microbial degradation of soil organic carbon. Therefore, until now rice paddy soils were generally thought to function as net sinks of CO2. However, a large international study led by Prof. Karsten Kalbitz of the Institute for Biodiversity and Ecosystem Dynamics of the University of Amsterdam, and Prof. Ingrid Kögel-Knabner of the Technische Universität München shows that this may not be true.
In their investigation, the research team assessed carbon stocks in paddy soils used for 50, 100, 300, 700, and 2000 years as well as in adjacent soils with non-paddy management in the Yangtze delta, China. They found that all paddy rice soils have indeed been continuously accumulating soil organic carbon for up to 2000 years in the upper soil layers. However, this was offset by an increased removal of organic carbon in the deeper soil layers In addition, weathering of naturally present carbonates increased, which might contribute to the release of CO2 as well. When the shallow and deep layers of the soil were considered together, in some cases the soils turned out to be net sources instead of sinks of CO2.
The results have far reaching implications according to Kalbitz: “We are still struggling to understand the role of soils in climate change. Differences between the upper and deeper soil layers are hardly considered in our current climate models. In fact, carbon credits are given for the first 20 years of paddy rice cultivation, meaning that you can use it to compensate CO2 emissions from other sources, such a coal fired power plants. Our study shows that such schemes are no longer justified, and that we may have to completely rethink the way our land management practices affect the global carbon cycle. Especially when you realize that CO2 is not the only greenhouse gas influenced by land management, but that the release of methane gas also plays an important role, especially in rice paddy soils.
Karsten Kalbitz, Klaus Kaiser, Sabine Fiedler, Angelika Kölbl, Wulf Amelung, Tino Bräuer, Zhihong Cao, Axel Don, Piet Grootes, Reinhold Jahn, Lorenz Schwark, Vanessa Vogelsang, Livia Wissing, Ingrid Kögel-Knabner. The carbon count of 2000 years of rice cultivation. Global Change Biology (2013) 19, 1107-1113, doi: 10.1111/gcb.12080.