20 March 2023

CO2 emissions reduce soil phosphorus and put rice crops at risk


According to an international study, soil phosphorus is reduced due to prolonged exposure to high levels of CO2 in the atmosphere. The resulting decline in paddy field yields is a threat to food security

by Matteo Cavallito


Increasing CO2 levels in the atmosphere could result in reduced phosphorus in paddy fields threatening the security of food supplies in the future. This is the hypothesis advanced by a study by a group of researchers from Australia, Spain, Canada, the United States, France and China published in the journal Nature Geoscience.

“Our research suggests that a reduction in rice yields could be particularly acute in low-income countries under future CO2 scenarios without the input of additional phosphorus fertilizers to compensate, despite the potentially reduced global risk for phosphorus pollution,” said Dengjun (Kevin) Wang, an assistant professor of aquatic chemistry in the Auburn University College of Agriculture’s School of Fisheries, Aquaculture and Aquatic Sciences.

Phosphorus presence decreases by more than 20 percent

Wang and his colleagues examined several measurements collected during two long-term experiments that lasted nine and 15 years, respectively. Specifically, the authors estimated the presence of phosphorus in soil following a free-air carbon dioxide enrichment intervention.

“Although no changes were observed in the initial year of the experiments, by the end of the experiments soil available phosphorus had declined by more than 20%: 26.9% and 21.0% for 15 and 9 years, respectively,” the research states.

According to the study, the loss can be explained by the production of organic phosphorus in the soil, which is not readily available to plants, and increased removal of the element through harvest. Increased transfers of organic, biochemical and chemical phosphorus, thus linked to fertilizer use, would not be sufficient to compensate for the long-term reduction. A phenomenon that is bound to become more acute with climate change.

A concern for food safety

The results show an obvious concern, says in a statement the Spanish Higher Council of Scientific Research, a public research organization involved in the survey. CO2 in the air is the main source of carbon for crops. Its high concentration in the atmosphere, however, limits the long-term presence in the soil of phosphorus, which is an essential element for plant metabolism and growth in agricultural soils.

“When there is a high concentration of CO2 in the atmosphere, plants take advantage of its fertilizing effect in the short term,” explains Josep Peñuelas, one of the authors. “Over time, however, phosphorus is no longer available and the soil becomes depleted, compromising plant growth.” The result is less food security, especially considering factors such as the global population growth and the central role of rice in the global diet.

Consequences for poorer countries

Reduced agricultural yields in paddy fields, the researchers argue, would be most felt in low-income countries, further widening economic inequality as a result, ultimately, of rising CO2 emissions. Fifty-five percent of the large rice paddy areas in China and India will experience increased risk of yield reduction.

In relative terms, poor nations – especially those in Southeast Asia, Central and South America, Africa, and the Middle East – will experience more critical situations. In these countries, the share of paddy fields at greater risk of yield reduction could reach 70 percent, compared with 52 percent found in middle- and high-income countries.

It’s not just the CO2

The phenomenon of phosphorus reduction, in any case, is not only affected by CO2 exposure. Also crucial, in fact, is the deterioration of soil health. In 2020, for example, a survey sponsored by the University of Basel revealed that more than 50 percent of the global phosphorus loss in agriculture is caused by soil degradation.

Erosion, in particular, leaches phosphorus from farmland to wetlands, streams and watersheds thereby damaging their ecosystem. The high content of the element in water – as reported by already available global data – reflects the loss of the element from soils.