Rice cultivation does not cut levels of atmospheric CO2. Yet, rice cultivation has been approved for use to compensate CO2 emissions from sources such as coal-fired power stations.
This is the finding of a recent study conducted by an international research team that included scientists from the University of Amsterdam (UvA). The results of the study were published in the April issue of the scientific journal Global Change Biology.
The fields in which rice is grown are normally managed in such a way that they are constantly underwater (known as ‘paddy fields’). These conditions and the resulting lack of oxygen curb the breakdown of soil organic matter. For this reason, it has long been generally assumed that, on balance, paddy soils capture CO2. However, a large international study led by Prof. Karsten Kalbitz of the UvA’s Institute for Biodiversity and Ecosystem Dynamics and Prof. Ingrid Kögel-Knabner of Technische Universität München has now shown that this may not be the case.
The team investigated how much carbon is stored in paddy soils in the Yangtze Delta in China, at sites where rice has been cultivated for 50, 100, 300, 700 or 2000 years, and compared the quantity of carbon there with adjoining non-paddy soils. They found that the paddy fields had indeed been storing carbon in their topmost soil layers for up to 2000 years. However, this effect was at least partially counteracted by a higher breakdown of soil organic matter from deeper soil layers. Additionally, it turns out that paddy soils are subject to an accelerated erosion of naturally occurring carbonates, thereby potentially also releasing CO2. On balance, the topmost and deeper layers added together were even shown to emit rather than store CO2 in some cases.
According to Kalbitz, these results have major ramifications. ‘We still don’t understand exactly how different types of soil influence climate change. Differences between the top and deeper soil layers have hardly been factored into existing climate models. At the moment, carbon credits are even being issued for the first 20 years of rice cultivation on paddy soils, which can then be used to compensate CO2 emissions from other sources – such as coal-fired power stations. Our research has shown that this policy can no longer be justified and that we may need to completely rethink our understanding of how human land use impacts the global carbon cycle, particularly when you consider that CO2 is not the only greenhouse gas to be affected by land use. Methane contributes substantially as well, certainly in 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’, in: Global Change Biology (2013) 19, 1107-1113, doi: 10.1111/gcb.12080.