Climatic effects on soil carbon – distinction between young and old compounds, and new future scenarios

Person in charge of the research: Jari Liski
Finnish Environment Institute

Funded by the Academy of Finland: 1.1.2005 – 31.12.2008,

Sites of research: Finnish Environment Institute, Finnish Forest Research Institute, European Forest Institute, the Dating Laboratory of the University of Helsinki and the Department of Forest Ecology at the University of Helsinki

Abstract

Changes in soil carbon, the largest terrestrial carbon pool, are critical for the global carbon cycle, atmospheric CO2 levels and climate. Soil carbon depends on productivity of vegetation and decomposition of organic material in soil, both climate dependent processes subject to change in response to climate change.

Current predictions on changes in decomposition are especially uncertain. Climatic depend-ence is only known for the youngest soil carbon that is in the minority. The current predic-tions, assuming all soil carbon to be equally sensitive to climate, may overestimate the release of carbon from soils substantially, if the older soil carbon is more tolerant, and this is what recent studies suggest. Moreover, generally in soil carbon models, climatic effects on the de-composition are based on only a few experimental data. Yet a wealth of data has been recently collected that could be used to calibrate and test the models.

This project concentrates on upland soils of temperate and boreal forests. These are important ecosystems in this respect, because their soils contain a lot of carbon and they are expected to be exposed to a large climate change.

The objectives of this project are to 1) quantify the effects of climate on decomposition of young soil carbon, 2) quantify these effects for older soil carbon, and 3) predict changes in soil carbon in temperate and boreal forests in response to climate change.

There are three work packages (WP), one to achieve each objective. In WP1, first, a European model on climatic effects on decomposition will be tested against a large data set from the USA. Second, suitability of geographical differences for predicting temporal changes in de-composition will be tested using time series of litter bag and soil respiration data. Third, to understand the relationship between the geographical differences and the temporal changes, adaptation of microbes to changes in environment will be investigated in a field experiment.

In WP2, the first approach to quantify differences in climatic effects on decomposition of young and old soil carbon will be to fractionate soil organic matter sampled from a tempera-ture gradient and study the trends of labile and stable fractions. The second approach will be to look at the ratio of young and old soil carbon in CO2 efflux from soil under different condi-tions using carbon isotopes. One of these experiments will employ 14C, which concentration is lower in older soil carbon as a result of radioactive decay. The other will employ 13C, which concentration is higher in young soil carbon in soil samples to be collected from re-cently established maize fields.

In WP3, four soil carbon models, Century, LPJ soil module, RothC and Yasso, will be tested and evaluated against the results of WP1 and 2. LPJ and Yasso will be improved accordingly and used to simulate changes in soil carbon under IPCC climate change scenarios. The simu-lations will be conducted for individual sites across the temperate and boreal forests as well as the region as a whole.

Workpackages

The research work is divided into three workpackages (WP) (Fig. 1). In WP1, the effects of climate on decomposition of young soil carbon will be quantified. In WP2, these effects will be quantified for old soil carbon. In WP3, soil carbon models will be evaluated and their va-lidity will be tested in the light of results of WP1 and 2. Moreover, selected models will be updated according to the results of WP1 and 2, and these models will be used to simulate changes in the carbon stock of forest soils across the boreal and temperate zones under changing climate in the future.

Fig. 1. The workpackages of this research project, their linkages and main results (arrows).