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Popular Science: Focus on litter decomposition

Sulfur and nitrogen deposition has significantly affected the ecosystems of the entire northern hemisphere. The impact of acid rain has had a significant effect in terms of lowering water pH and leaching of important ions from the soil into surface waters, as well as in terms of the decomposition rate of plant litter and thus on the global carbon cycle. The decomposition rate is influenced by several factors. How does the type of substrate and nitrogen addition contribute to the decomposition process? This is the question that Michal Růžek from the Department of Physical Geography and Geoecology, Faculty of Science, Charles University asked, and together with his team he made an experiment that yielded interesting results.

The carbon cycle has become a hot topic with the improving understanding of the impact of climate change. The various forms of carbon and their representation in the individual parts of nature have an impact on ecosystem vitality, biochemical processes and, last but not least, the climate. Carbon is exchanged mainly between the biosphere and the atmosphere. That is why the study of the decomposition of organic matter is so important, because it will suggest to us what processes will happen in the future. In addition to climate and soil conditions, the composition and character of plant litter play a fundamental role in the process of organic matter decomposition. Due to the slowdown of decomposition, carbon and nutrients can be sequestrate in the soil, which causes a well-known negative effect on the vitality of forest ecosystems.

In 2014, scientists started a long-term experiment on the border between the Czech Republic and Germany in the Ore Mountains, which were affected by strong acid deposition in the past. Under the crowns of 140-year-old beeches and 80-year-old spruces, an experiment has begun, which has now been in progress for more than 7 years. In each stand, they simulated an increased supply of sulfur and nitrogen into the soil. To do this, they set 16 plots with a size of 3x3 meters, which they divided into 4 types of experiments - with the addition of sulfur (S), nitrogen (N), a combination of both, and a control plot without treatment. During the snow-free period, they added the required amounts of compounds (50 kg S/ha/year and/or 50 kg N/ha/year) to the experimental areas. Gradually, soil in both types of spots responded similarly to the acid input by a reduction of soil water pH and DOC concentrations (by 35%–48%) and the amount of CO2 released from the soil by the soil respiration process.

Between 2017 and 2019, the scientists expanded the experiment by adding another part, focusing directly on the decomposition of organic matter under the influence of experimentally increased S and N input. They selected four types of material for the experiment: naturally shed beech leaves (Fagus sylvatica (L.)), spruce needles (Picea abies (L.)) and tea bags filled with either green tea or rooibos tea. These teas differ fundamentally in their composition, because green tea is rich in nitrogen and poor in poorly decomposing lignin, while the composition of rooibos tea has the opposite character. Spruce needles also differ from beech leaves by their higher lignin content. A total of 1,536 samples were allowed to rest and decompose in the ground for up to two years. The samples were continuously excavated to measure mass loss, material C/N ratio and lignin concentration. The data were further supplemented by monitoring soil water pH and soil respiration.

In both beech and spruce stands, the addition of acid treatment (S) decreased the pH of soil water, reduced soil respiration and suppressed the decomposition of green tea. With the exception of green tea, the decomposition was suppressed after the acid addition only in the spruce forest that had already been more acidic.

Needles or leaves? The type of substrate plays a fundamental role in the decomposition rate of plant litter under acidic conditions. Photo: K. Fraindová.

An interesting finding was that, unlike in other studies, the addition of nitrogen as an essential nutrient for plants did not have a significant effect on decomposition processes and mass loss in most cases. Only the rate of decomposition was reduced by lowering the pH of soil water. The researchers explain this pattern by the previous adaptation of forest soil to historical nitrogen loading in these areas.

The decomposition rate of organic matter has a major impact on the carbon cycle, the subsequent vitality of ecosystems, and climate change. The research results showed that these changes are significantly influenced not only by acid rain, but also by the historical loads and by the type of vegetation and litter composition. The scientists found that under ambient conditions, the beech stand contained more C and N in the entire soil profile compared to the spruce stand, with periodical soil N leaching. While the decomposition rate and thus the availability of organic carbon decreased with increasing soil acidity in both stands, decomposition suppression was stronger in the spruce stand as opposed to the beech stand.

Růžek, M., Tahovská, K., Guggenberger, G., Oulehle, F., 2021. Litter decomposition in European coniferous and broadleaf forests under experimentally elevated acidity and nitrogen addition. Plant Soil, pp. 15

Kateřina Fraindová

Published: Jul 12, 2021 10:05 AM

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