―Crop yield improvement, climate mitigation, and nitrogen input saving―
Increasing the level of soil organic matter (mainly soil carbon) by managing agricultural land such as increasing organic matter input is known to result in crop yield increase. The increase in soil organic matter also reduces atmospheric concentration of carbon dioxide(CO2), thereby mitigating climate change. National Agriculture and Food Research Organization(NARO) has quantitatively estimated the environmental conservation effects of increasing soil carbon in the global cropland area on the growth of six major crops (wheat, maize, rice, soybean, sorghum, and millet). The amount of soil carbon in the world's agricultural land can be increased by 12.78 billion tons at the maximum within the range where the crop yield increase can be expected. By increasing this amount of soil carbon, we can expect an increase in crop production by 38.25 million tons, avoiding a global decadal average temperature rise of 0.03°C, and saving input of inorganic nitrogen fertilizer by 5.82 million tons. This result is considered to be a useful input to national governments and international organizations to establish policies and institutions promoting agricultural land management that increases soil carbon.
Increasing the amount of soil organic matter through land management is known to improve the fertility and water retention capacity of the soil, increase crop yield and reduce drought damage. Moreover, an increase in soil carbon content helps mitigate global warming. However, there has been no quantitative, global study evaluating multiple environmental conservation effects such as climate mitigation and yield increase.
NARO combined the yield of major crops in the world with climate, soil, and cultivation management data, and analyzed the relationship between the amount of carbon contained in the surface layer of agricultural land soil up to 30 cm and the yield by machine learning. Computer simulations were performed using the obtained relationships to estimate the expected increase in crop production due to the increase in soil carbon content. In addition, a computer simulation using the relationship between nitrogen input and yield was performed. Inorganic nitrogen input equates to the increased yield associated with soil carbon increase was also estimated. This corresponds to the amount of inorganic nitrogen that can be saved by increasing the soil carbon.
From the relationship between soil carbon content and yield, although the yield increases as soil carbon increases, it was estimated that yield increase would eventually reach a plateau. Based on this estimation, if the maximum amount of soil carbon is increased within the range where the yield increase can be expected, assuming the world's cultivated area at present (2010), the increase in soil carbon is estimated to be 12.78 billion tons worldwide. At the same time, the increase in global crop production was estimated to be 38.25 million tons in total for the six crops, and the avoided global decadal temperature rise was estimated to be 0.03°C. In addition, it was estimated that this yield increase due to the increase in soil carbon content corresponds to the yield increase obtained by inputting 5.82 million tons of inorganic nitrogen fertilizer. This is equivalent to 7.2% of the world's nitrogen input (relative to 2000). It has been pointed out that excessive input of inorganic nitrogen fertilizer is a cause of water quality deterioration. Increasing soil carbon content thus contribute to water quality conservation by reducing the use of inorganic nitrogen fertilizers.
This research result is expected to be useful when establishing policy and institutions to promote agricultural land management that increases soil carbon content. In addition, improving food productivity and mitigating climate change are respectively listed in Goal 2 to achieve "Zero Hunger" and Goal 13 "To take urgent action to combat climate change and its impacts" of the Sustainable Development Goals (SDGs). Soil conservation is also listed in Goal 15 "To Protect, restore and promote sustainable use of terrestrial ecosystems". This research result also indicates that farmland management that increases soil carbon contributes to the achievement of these three SDGs goals.
「世界の乾燥地域では、農地土壌の炭素量増加により穀物生産の干ばつ被害が軽減」(Japanese only) https://www.naro.go.jp/publicity_report/press/laboratory/niaes/133701.html
2. Toshichika Iizumi, Nanae Hosokawa, Rota Wagai (2021) Soil carbon-food synergy: sizable contributions of small-scale farmers. CABI Agriculture and Bioscience, 2, 43. https://doi.org/10.1186/s43170-021-00063-6