Ecosystems Growth & Development

Very fine roots act as renewed carbon sink in rewetted peatland forest

New study suggests that rewetting peatland forests could increase their climate change mitigation function.

Peatlands are characterised by high water tables, cover around 3% of the Earth’s land surface and contain nearly a third of global soil carbon. These unique ecosystems have been drained to convert land for agriculture or forestry. One way to restore these drained sites is to rewet these areas but little is known how carbon (C) sequestration cycle is affected. 

Sarah Schwieger and three colleagues from the Greifswald University compared the roots of Black alders (Alnus glutinosa) at drained and rewetted areas of a peatland forest in Germany. The biomass of the roots were almost double and roots were finer at the rewetted site compared to the drained area. The higher root biomass suggests that belowground C stocks are larger at rewetted sites, so this restoration method could increase the climate change mitigation function of forest peatlands. 

Schwieger and colleagues surveyed the soil and roots around black alder trees at rewetted and drained sites that are part of the WETSCAPES project around Wöpkendorf (Germany). Five, 50-cm long soil cores were collected around black alder trees to characterise root diameter, dry weight, age and specific root area. The root diameter classes were very fine (<1 mm), fine (1-2 mm) and coarse roots (2-5 mm) and the root specific area was measured with IJ_Rhizo in ImageJ software. The weather, groundwater table, soil water content, soil temperature and plant available soil nutrients were measured at the sites.

Alder tree. Source: Canva.

Researchers found that the root biomass was more than double in the rewetted site compared to the drained site, especially for the very fine and fine root classes in the upper soil layers. Overall, very fine, 2-3 year old (i.e. young) roots made up 51% of the total root biomass sampled but decreased along the soil profile whilst coarse roots increased with depth. There were large differences in specific root areas between diameter classes within the drained site, but not in the rewetted site. Plant available nitrate was 50 times higher at drained site whilst ammonium and phosphorus were 2.5 and 7.5 times higher at rewetted site.

Cross-sections of a 2-year old root from a rewetted alder stand (left), and a 4-year old root from a drained stand. Yellow lines indicate the annual growth ring boundaries. Source: Schwieger et al., 2020.

This study offers many answers about belowground processes of a rewetted, historically drained peatland forest. The trees at rewetted sites invested proportionally more in biomass where plant available nitrogen (N) was lower than at drained sites. Whilst black alder trees can fix soil N through symbiosis, “the increased root biomass in the rewetted site might be related to the lower oxygen concentrations under waterlogging, which might reduce nodule activity, forcing the tree to rely more on nutrient acquisition through roots,” Schwieger and colleagues wrote. 

“Since the rewetted site has lower N concentrations, more roots would be needed to fulfil the nutrient requirements.”

There were contrasting patterns in biomass vs. functional trait (e.g. specific root area) changes between root diameter classes. For very fine roots (<1 mm), the functional trait did not differ between sites or along soil depths but for finer (1-2 mm) and coarse (2-5 mm) roots, the functional trait was higher and differed along soil depths at the drained site. The different functional traits of thicker roots might be indicators of greater water table fluctuations within the drained site. Additionally, the root turnover at rewetted sites is likely to be higher as there are more young, fine roots. 

“[O]ur findings indicate that rewetting not only supports the peatland function as C sink by enabling the system to sequester more C in form of root biomass, but also might lead to lower respiration rates of roots with a lower SRA [specific root area] in the rewetted state,” the authors conclude.

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