Biennial GHG budget of a riparian forest: From soil and tree-stem to ecosystem level
Abstract
Water purification efficiency of riparian ecosystems has been thoroughly studied, yet their greenhouse-gas (GHG) emissions need to be better understood. Riparian forests sequester carbon dioxide (CO2) but little is known about the methane (CH4) and nitrous oxide (N2O) balance in the forests. Recent studies demonstrate that trees themselves can emit CH4 and the vegetation can be a significant CH4 source. Few studies show that drained nitrogen-rich organic soils under deciduous forests can be significant N2O sources with significant emissions in winter. No ecosystem-level N2O budget (fluxes in the soil, tree stems and shoots, and above the canopy) is available. Grey alder (Alnus incana) is a fast-growing tree with great potential for short-rotation forestry. Grey alder stands are typically found in riparian zones. They are considered as accumulators of carbon (C). Their symbiotic dinitrogen (N2) fixation ability makes alders important regulators of the nitrogen (N) cycle.
From July 2017 to July 2019 we measured budgets of CO2, CH4 and N2O of a 40-yr old riparian grey alder forest stand on former agricultural land in Estonia using continuous measurements from the soil surface, samples from tree-stem chambers and continuous whole-ecosystem measurements (the eddy covariance (EC) technique). Simultaneously, water level and soil temperature and moisture were measured automatically, and composite soil samples were taken for physico-chemical analysis. In the wet periods, stem flux of CH4 was the main source for ecosystem exchange, whereas in the dry periods, unexpectedly, ecosystem flux was significantly higher than fluxes from soil and tree stems. Most likely, canopy was the main CH4 emitter. N2O fluxes from the soil and stems were low during the dry periods and peaked during the wet periods and the freezing-thawing. The forest was a net annual source of both CH4 and N2O (6.33±0.29 kg CH4 ha-1 and 3.42±0.12 kg N2O ha-1, mean±SE) and a net annual sink of CO2 (-21,025±158 kg CO2 ha-1). Estimated average annual losses of total organic C and total N in groundwater were 7.5 kg C ha-1 and 1.8 kg N ha-1, thus the forest was a net annual C sink (-5,224 kg C ha-1). The total GHG budget shows that grey alder forests as pioneer communities, regardless of the CH4 fluxes, sequester large amounts C and improve soil-nutrient conditions.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMGC54D..05M
- Keywords:
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- 0402 Agricultural systems;
- BIOGEOSCIENCES;
- 0470 Nutrients and nutrient cycling;
- BIOGEOSCIENCES;
- 1615 Biogeochemical cycles;
- processes;
- and modeling;
- GLOBAL CHANGE;
- 1630 Impacts of global change;
- GLOBAL CHANGE