Drought favors plant-pathogenic fungi in pasturelands and release of carbon dioxide from the soil

Researchers at the University of São Paulo conducted field experiments to estimate the impact of drought and rising temperatures on soil quality and plant health.

Karina Ninni | Agência FAPESP – Scientists at the University of Sao Paulo’s Ribeirao Preto School of Philosophy, Sciences and Letters (FFCLRP-USP) in Brazil have estimated the impact of water stress and rising temperatures on tropical grassland soil used for pasture. The results of the research are described in two articles, the first published in Molecular Ecology in April and the second published in Soil Biology and Biochemistry in June 2020.

According to mycologist Tássio Brito de Oliveira, first author of both articles, drought affected the soil more than a temperature rise of 2 °C – in line with the global warming projected by the UN Intergovernmental Panel on Climate Change (IPCC) for the end of this century – in terms of carbon dioxide release due to soil respiration (CO₂ efflux), among other factors.

“Periods of drought can modify the diversity of fungi in the soil, increasing the number of species that cause plant disease and also changing the dynamics of soil fertility,” said Oliveira, who has a PhD in microbiology from São Paulo State University (UNESP). “We also found that soil CO₂ efflux increased in response to drought.” 

The experiments were conducted in an open field with an area of 2,500 square meters known as Trop-T-FACE. Located on the university’s campus in Ribeirão Preto, the facility was built with funding from FAPESP under the auspices of its Research Program on Global Climate Change (RPGCC). The facility combines free-air carbon dioxide enrichment (FACE), which simulates future CO₂ conditions, and temperature free-air controlled enhancement (T-FACE), which enables researchers to manipulate variables such as temperature and soil water content.

In the first article, Oliveira and colleagues show that drought favors the proliferation of plant-pathogenic fungi such as Curvularia, Thielavia and Fusarium, potentially intensifying the occurrence of plant disease. “This may represent a risk for crops, especially monocultures,” Oliveira said.

In the other study, reported in Soil Biology and Biochemistry, they measured the effect of drought and warming of 2 °C on soil respiration, and on the activity of enzymes relating to the carbon and phosphorus cycles and released by microorganisms such as fungi and bacteria. 

“In this second study we found that the impact of water stress was much more significant than warming for the activity of the enzymes involved in soil mineralization, decomposition and nutrient cycling,” Oliveira said. “We also found that soil CO₂ efflux increased as a result of water stress.”

Thus in the case of the area of pasture studied, less soil moisture meant more carbon released into the atmosphere. “Carbon dioxide isn’t produced only by soil microorganisms. Plant roots also produce and release the gas,” he explained. 

Oliveira is currently a postdoctoral fellow conducting research in microbiology at FFCLRP-USP under the supervision of professor Maria de Lourdes Polizeli.

Diversity and fertility

The area in which the experiments were conducted was divided into 12 plots. The researchers first prepared the soil and corrected its pH. They then planted a mixture of two forage crops commonly used in Brazil, with the same proportions on all plots (80% Stylosanthes capitata and 20% Stylosanthes macrocephala).

“While the forage crops were growing [about four months], all 12 plots were constantly irrigated. When the plants reached the stage we wanted, we stopped irrigating six of the 12 plots,” Oliveira said.

The experiment was designed so that the plants had grown sufficiently by the start of the dry season and the non-irrigated plots were exposed to a 40-day drought. “The facility has its own weather station. Temperature and soil humidity were measured around the clock. This data was sent to a control system. Infrared heaters kept six of the plots 2° C above the ambient temperature recorded for the other plots,” he said.

The plots received the following treatments: irrigation and ambient temperature; soil water deficit and ambient temperature; irrigation and 2° C temperature increase; soil water deficit and 2° C temperature increase.

“No treatment affected fungus species richness [number of species], which was similar for all 12 plots, but diversity [number of species times number of individuals per species] was modified on the plots where watering was cut off,” Oliveira said. “Where there was no water stress, one or two species predominated and the rest had a very small number of individuals. Where there was water stress, predominant species abundance was better distributed, and not one but several species predominated with approximately the same number of individuals.”

According to the authors of the study, the species with higher numbers of individuals were the main cause of plant disease. “This is alarming because it was a field study involving interactions with the environment, so the results were quite close to reality,” said Carlos Alberto Martinez y Huaman, a professor in the Department of Biology at FFCLRP-USP and principal investigator for a Thematic Project funded by FAPESP for the RPGCC. 

Martinez y Huaman recalled that Fusarium, one of the pathogenic species that proliferated in response to drought, is also involved in processes that emit nitrous oxide, a powerful greenhouse gas.

According to Oliveira, water stress intensified the activity of phosphatase, an enzyme that solubilizes and mineralizes soil phosphorus. “A possible explanation is that low levels of soil phosphate around the roots lead the microorganisms associated with these plants to produce more phosphatase at a certain distance, solubilize the phosphate and take it to the plants,” he said. “The plants interact symbiotically with some microorganisms, including fungi, releasing nutrients to fuel their growth, and in exchange the fungi absorb nutrients from the soil and supply them to the plants.”

Low soil water content also impacted the activity of enzymes involved in the carbon cycle. “Both beta-glucosidase and xylanase were negatively influenced by water stress. Lack of water reduces their solubility,” he said. “In addition, with less water both the enzymes and their products move about less in the soil, reducing the substrate’s ability to reach decomposable material or fungi. Essential cycles for maintaining soil nutrients are interrupted as a result, and soil fertility is affected.”

For Martinez y Huaman, research on the impact of drought and water usage provides important decision support for policymakers. “The broader context for this study was a joint call by the National Council for Scientific and Technological Development [CNPq], the National Water Agency and the Ministry for Science, Technology and Innovation, issued in 2015 when there was a water crisis in Southeast Brazil,” he said. “The National Water Agency began receiving applications for permits to use water to irrigate pasture. It wanted researchers to provide data that could help it decide whether watering pasture was a good idea, and to find out how pasturelands were affected by lack of water associated with rising temperatures.” 

Brazil has 160 million hectares of forage crops for pasture, he noted, adding that a better understanding of the impact of climate change on pasture and the impact of pasture on climate change is very important and relevant, and that drought-resistant forage crops urgently need to be selected in order to help cope with climate change.