Abstract
Quantification of activity data and emission factors for carbon (C) in inland wetland mineral soils (IWMS) lack suitable low cost indicators for key soil C processes in temperate forests. In a beech (Fagus sylvatica L.) forest near Sorø, Denmark, SOC stocks and the risk of losing pre-drainage legacy SOC were studied using a digital elevation model (0.4 m resolution), redox potential and soil respiration measurements. The results were compared with a digitized legacy soil map used in the national GHG reporting to UNFCCC. In upland, flat and sloping terrain, an aerobic soil environment (Eh > 400 mV) prevailed throughout most of the year, but in a peat-filled topographic depression (TD) anaerobic conditions (Eh < 400 mV) fully or sporadically occurred in the growing season, controlled by the ditching-affected water table. The relief included SOC rich TDs making up 18.9% of the area based on the “Filled sink” algorithm (Saga GIS). In contrast, the peat cover on the legacy soil map was 8.2%. Furthermore, the mapped peat polygons were offset from the TDs defined by the DEM. The SOC stocks at 0–40 cm depth outside TDs (least squares mean 8.4 ± sem 0.3 kg C m−2) were significantly lower than within TDs (11.9 ± sem 0.5 kg C m−2). Average annual soil respiration increased linearly with the SOC stock by 0.06 kg C per kg SOC up to a SOC stock of 11 kg C m−2 to 20 cm depth, and a SOC loss of 0.23 ± se 0.10 kg C m−2 yr−1 was indicated inside the TD areas, close to the IPCC estimate of 0.26 kg C m−2 yr−1 for drained organic soils under forest. Our results show that continuous sensor-based monitoring of redox potential and shallow water tables linked with high-resolution DEMs offer the possibility to estimate the spatial extent of inland wetland mineral soils and their status as aerobic or anaerobic as indicated by iron rods with higher accuracy than previously. This underpins the potential use of such data for activity data mapping in Tier 3 greenhouse gas reporting.
Highlights
Reporting of carbon stocks in biomass, soil, and harvested wood products is a part of the Land Use, Land-Use Change and Forestry (LULUCF) reporting to United Nations Framework Convention on Climate Change (UNFCCC) under the Kyoto Protocol (IPCC, 2003, 2006, 2014)
We could for the first time explore the suitability of a high-resolution digital elevation models (DEM) together with continuous measurements of Eh for locating inland wetland mineral soils, their SOC stocks and soil respiration
Soil organic carbon accumulation and soil respiration varied across the forest site due to local and seasonal soil water saturation caused by a combination of topographical depressions and lateral water flow due to low porosity of the underlying Bt horizons and compact calcareous till in the C horizon
Summary
Reporting of carbon stocks in biomass, soil, and harvested wood products is a part of the Land Use, Land-Use Change and Forestry (LULUCF) reporting to United Nations Framework Convention on Climate Change (UNFCCC) under the Kyoto Protocol (IPCC, 2003, 2006, 2014). The new category “inland wetland mineral soils” (IWMS) in the “Wetlands Supplement” (IPCC, 2014) offers a better understanding of CO2 and non-CO2 balances in soils that are water saturated within the root zone from time to time, but no emission factors for CO2 have been set for inland wetland mineral soils. These categories are relevant for SOC stock estimation, as SOC content is strongly linked with drainage class (Strand et al, 2016)
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