Drought-exposed leaves adversely affect soil nutrients, study shows

April 5, 2011

Jeff Dukes is studying the effects of a changing climate on plants and soils. (Purdue Agricultural Communication photo/Tom Campbell)

Download image

WEST LAFAYETTE, Ind. - Chemical changes in tree leaves subjected to warmer, drier conditions that could result from climate change may reduce the availability of soil nutrients, according to a Purdue University study.

Jeff Dukes, an associate professor of forestry and natural resources, found that red maple leaves accumulate about twice as much tannin when exposed to hot, droughtlike conditions. Those tannins, which defend leaves from herbivores and pathogens, were shown to interfere with the function of common enzymes in soil.

"When the leaves are particularly water-stressed by drought or drought with higher temperatures, we see more protective compounds, more tannins and a change in the chemistry of the tannins," said Dukes, whose findings were published in the early online version of the journal New Phytologist. "This suggests that when these leaves fall, they may slow down soil processes such as decomposition and nutrient cycling. This could, in turn, affect plant growth and nutrient uptake."

The findings are the first for the Boston Area Climate Experiment, a National Science Foundation-funded project that Dukes directs. Plants on several field plots are exposed to various future climate scenarios using heaters and other means to control conditions.

"We've basically built a big time machine that moves different plots of land into different possible futures by changing temperatures and precipitation levels," Dukes said.

The increase in leaf tannins observed in this experiment could cause leaves to decompose more slowly and also interfere with critical soil enzymes, leaving fewer nutrients for plants. The tannins in the red maple leaves also were chemically different, making them interact more strongly with the soil enzymes.

Dukes said the tannin issue could effect a sort of tug-of-war in the carbon cycle. With fewer nutrients, plants would take carbon dioxide out of the air more slowly. But if fallen leaves are decomposing slower, then the carbon would be released back to the atmosphere more slowly.

"This is an issue that could affect many natural processes," Dukes said. "We just don't know what the net result will be."

In this experiment, leaves were removed from the experiment plots and tested in laboratories. Dukes said he would next test other plants' leaves exposed to similar conditions to see how their tannins are affected. He also will test his findings in the field to see how an increase in tannins affects soil in a natural setting.

The work was carried out in collaboration Nishanth Tharayil at Clemson University, as well as researchers at Purdue, Clemson University, the University of Massachusetts Boston and Natural Resources Canada.

Writer: Brian Wallheimer, 765-496-2050, bwallhei@purdue.edu

Source: Jeff Dukes, 765-494-1446, jsdukes@purdue.edu

Ag Communications: (765) 494-2722;
Keith Robinson, robins89@purdue.edu
Agriculture News Page


Changes in the Structural Composition and Reactivity of Acer Rubrum Leaf Litter Tannins Exposed to Warming and Altered Precipitation: Climatic Stress-induced Tannins are More Reactive

Nishanth Tharayil, Vidya Suseela, Daniella J. Triebwasser, Caroline M. Preston, Patrick D. Gerard, Jeffrey S. Dukes

Climate change could increase the frequency with which plants experience abiotic stresses, leading to changes in their metabolic pathways. These stresses may induce the production of compounds that are structurally and biologically different from constitutive compounds. We studied how warming and altered precipitation affected the composition, structure and biological reactivity of leaf litter tannins in Acer rubrum at the Boston-Area Climate Experiment, in Massachusetts, USA. Warmer and drier climatic conditions led to higher concentrations of protective compounds, including flavonoids and cutin. The abundance and structure of leaf tannins also responded consistently to climatic treatments. Drought and warming in combination doubled the concentration of total tannins, which reached 30% of leaf-litter DW. This treatment also produced condensed tannins with lower polymerization and a greater proportion of procyanidin units, which in turn reduced sequestration of tannins by litter fiber. Furthermore, because of the structural flexibility of these tannins, litter from this treatment exhibited five times more enzyme (β-glucosidase) complexation capacity on a per-weight basis. Warmer and wetter conditions decreased the amount of foliar condensed tannins.

Our finding that warming and drought result in the production of highly reactive tannins is novel and highly relevant to climate change research as these tannins, by immobilizing microbial enzymes, could slow litter decomposition and thus carbon and nutrient cycling in a warmer, drier world.