Soil carbon sequestration in Switzerland : analysis of potentials and measures

The part of ZHAW was the settlements soils.To reach net zero greenhouse gas emissions by 2050, Switzerland will depend on domestic negative emissions of about 7 million tons of CO2-equivalents per year (Mt CO2-eq yr-1). Soil carbon sequestration is one of the cheapest and technically least demanding technologies. It is defined as a net uptake of atmospheric carbon dioxide (CO2) that leads to an increase in soil organic carbon storage on the same unit, where CO2 was taken up by photosynthesis. Compared to other technologies it has the great advantage that it rarely competes with food production and is often associated with environmental benefits. Furthermore, higher soil organic carbon stocks increase soil fertility and improve the resilience of the soil system to climate change. The main disadvantage however, is that soil carbon sequestration does not lead to a permanent storage of carbon and most measures are only effective for a few decades. This report addresses questions 1 and 2 of the postulate Nr. 19.3639 ‘Kohlenstoffsequestrierung in Böden’ by national council Jacques Bourgeois. In part 1, which addresses question 1, we assess the potential to store additional carbon in Swiss soils and show what would be necessary to improve our understanding of the actual soil carbon sequestration potentials. In part 2, which addresses question 2, we discuss advantages and disadvantages of specific measures to enhance soil organic carbon stocks. Because there are great differences between different land use types or soil categories, we discuss the measures separately for organic and mineral soils and distinguish unmanaged, agricultural, forest and settlement soils. In Switzerland soil carbon sequestration potentials are largest on agricultural mineral soils. As a result of historic land use conversions (mainly deforestation and drainage) and an intensification of agricultural use, these soils have lost significant amounts of carbon and current soil organic carbon stocks are rather low, especially on cropland. Part of the lost carbon could be regained by measures that increase soil organic carbon stocks. Permanent grassland and forest soils have higher soil organic carbon stocks and the potential for sequestration is therefore small. However, these high stocks might be at risk under climate change and efforts should focus on maintaining soil organic carbon stocks. Due the small area, settlement soils offer a limited potential for carbon sequestration. Organic soils store significant amounts of carbon but drainage-induced loss rates are high. Efforts should focus on reducing these emissions before their potential to store additional carbon can be considered. Generally, the potential for additional carbon storage is site specific and depends on current soil organic carbon stocks and management. National-scale estimates of soil carbon sequestration potentials are still highly uncertain. To improve estimates, we rely on soil organic carbon maps and spatially explicit management information. On agricultural mineral soils the measure with the highest potential is conservation agriculture (0.52–1.05 Mt CO2-eq yr-1) as it could be applied on a large area. Agroforestry and cropland to grassland conversions lead to a reduction of cropping areas and their application is only recommended for selected areas. The estimated potentials are 0–0.12 Mt CO2-eq yr-1 for agroforestry and 0.05 Mt CO2-eq yr-1 for cropland to grassland conversions. In the case of agroforestry, a significant carbon sink is expected in wood, which is not included here. Which measures would be most effective on grassland soils is not clear yet. Generally, it is important to note that additions of organic fertilizer, which can be an integral part of several measures, only count as a true sequestration measure if the biomass was produced on-farm (also excluding feed imports). Furthermore, it is important to add that the sequestration potentials presented only refer to topsoils due to a lack of data. For a full carbon accounting, effects on subsoils (below 30 cm depth) would need to be included. Biochar as another option for agricultural soils is not considered in this report, but in an accompanying study. Measures to enhance soil organic carbon stocks on forest soils include the selection of tree species, liming or wood ash application. However, they are all expected to have small effects on total soil organic carbon stocks. Afforestation on former cropland is the only measure that could lead to significantly higher soil organic carbon stocks, but would conflict with food production. However, afforestation generally leads to additional carbon storage in woody biomass. In settlements, creating new areas for carbon accumulation such as green roofs offers a potential for soil carbon sequestration of 0.07 Mt CO2-eq yr-1. This measure can have positive effects on urban climate and local biodiversity. The inclusion of biochar underneath newly built roads, could sequester 0.37 Mt CO2-eq yr-1. Biochar could also be used in tree substrates and would have positive side-effects on water uptake and retention. Drained organic soils emit 0.51–0.69 Mt CO2-eq yr-1. Measures should focus mainly on reducing these losses as soil carbon sequestration is difficult to achieve on degraded peatlands. The most promising measure to reduce emissions is rewetting, but the consequence is a severe impairment of the production function. Most likely soil covering and soil mixing cannot reduce CO2 losses. Overall, most measures to sequester carbon in mineral soils and reduce carbon losses from organic soils are relatively well known and several measures are ready to be implemented. However, careful selection of sites and measures is highly recommended as the potential to sequester carbon is strongly-site specific and any potential side-effects such as yield reductions need to be factored in. In summary, soil carbon sequestration in Switzerland could offset an upper maximum of 24% of the domestic negative emissions based on the presented measures

Canopy CO2 enrichment permits tracing the fate of recently assimilated carbon in a mature deciduous forest

How rapidly newly assimilated carbon (C) is invested into recalcitrant structures of forests, and how closely C pools and fluxes are tied to photosynthesis, is largely unknown. A crane and a purpose-built free-air CO2 enrichment (FACE) system permitted us to label the canopy of a mature deciduous forest with 13C-depleted CO2 for 4 yr and continuously trace the flow of recent C through the forest without disturbance. Potted C4 grasses in the canopy ('isometers') served as a reference for the C-isotope input signal. After four growing seasons, leaves were completely labelled, while newly formed wood (tree rings) still contained 9% old C. Distinct labels were found in fine roots (38%) and sporocarps of mycorrhizal fungi (62%). Soil particles attached to fine roots contained 9% new C, whereas no measurable signal was detected in bulk soil. Soil-air CO2 consisted of 35% new C, indicating that considerable amounts of assimilates were rapidly returned back to the atmosphere. These data illustrate a relatively slow dilution of old mobile C pools in trees, but a pronounced allocation of very recent assimilates to C pools of short residence times

Rapid mixing between old and new C pools in the canopy of mature forest trees

Stable C isotope signals in plant tissues became a key tool in explaining growth responses to the environment. The technique is based on the fundamental assumption that the isotopic composition of a given unit of tissue (e.g. a tree ring) reflects the specific C uptake conditions in the leaf at a given time. Beyond the methodological implications of any deviation from this assumption, it is of physiological interest whether new C is transferred directly from sources (a photosynthesizing leaf) to structural sinks (e.g. adjacent stem tissue), or inherently passes through existing (mobile) C pools, which may be of variable (older) age. Here, we explore the fate of (13)C-labelled photosynthates in the crowns of a 30-35 m tall, mixed forest using a canopy crane. In all nine study species labelled C reached woody tissue within 2-9 h after labelling. Four months later, very small signals were left in branch wood of Tilia suggesting that low mixing of new, labelled C with old C had taken place. In contrast, signals in Fagus and Quercus had increased, indicating more intense mixing. This species-specific mixing of new with old C pools is likely to mask year- or season-specific linkages between tree ring formation and climate and has considerable implications for climate reconstruction using stable isotopes as proxies for past climatic conditions

Soil carbon sequestration potential bounded by population growth, land availability, food production, and climate change

Improving soil management to enhance soil carbon sequestration (SCS)—a cost-efficient carbon dioxide (CO2) removal approach—can result in co-benefits or trade-offs. Here we address this issue by setting up a modeling framework for Switzerland that combines soil carbon (C) storage, food production and agricultural greenhouse gas (GHG) emissions. The link to food production is crucial because crop types and livestock numbers influence soil organic C (SOC) stocks, through soil C inputs from plants and manure. We estimated SCS rates for the years 2020–2050 for three scenarios, each with two variants for biochar: cover cropping (0.30 t CO2 equivalents [CO2-eq] ha−1 yr−1), biochar addition (0.36–1.8 t CO2-eq ha−1 yr−1) and agroforestry-biochar addition (2.2–2.3 t CO2-eq ha−1 yr−1). Different limiting factors (land and biomass availability, population growth) affected SCS rates and indicated that they cannot be sustained until 2100 under all scenarios (cover cropping: 0.10 t CO2-eq ha−1 yr−1 [2051–2100]; biochar addition: 0.35–1.8 t CO2-eq ha−1 yr−1; agroforestry-biochar addition: 1.0–1.7 t CO2-eq ha−1 yr−1). This information together with the associated GHG emissions is critical for planning net zero strategies and highlights the importance of integrated assessments that capture links between SCS and the food system

The importance of biochar quality and pyrolysis yield for soil carbon sequestration in practice

© 2023 The Authors. European Journal of Soil Science published by John Wiley & Sons Ltd on behalf of British Society of Soil Science. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, providedthe original work is properly cited.Biochar is a carbon (C)-rich material produced from biomass by anoxic or oxygen-limited thermal treatment known as pyrolysis. Despite substantial gaseous losses of C during pyrolysis, incorporating biochar in soil has been suggested as an effective long-term option to sequester CO2 for climate change mitigation, due to the intrinsic stability of biochar C. However, no universally applicable approach that combines biochar quality and pyrolysis yield into an overall metric of C sequestration efficiency has been suggested yet. To ensure safe environmental use of biochar in agricultural soils, the International Biochar Initiative and the European Biochar Certificate have developed guidelines on biochar quality. In both guidelines, the hydrogen-to-organic C (H/Corg) ratio is an important quality criterion widely used as a proxy of biochar stability, which has been recognized also in the new EU regulation 2021/2088. Here, we evaluate the biochar C sequestration efficiency from published data that comply with the biochar quality criteria in the above guidelines, which may regulate future large-scale field application in practice. The sequestration efficiency is calculated from the fraction of biochar C remaining in soil after 100 years (Fperm) and the C-yield of various feedstocks pyrolyzed at different temperatures. Both parameters are expressed as a function of H/Corg. Combining these two metrics is relevant for assessing the mitigation potential of the biochar economy. We find that the C sequestration efficiency for stable biochar is in the range of 25%–50% of feedstock C. It depends on the type of feedstock and is in general a non-linear function of H/Corg. We suggest that for plant-based feedstock, biochar production that achieves H/Corg of 0.38–0.44, corresponding to pyrolysis temperatures of 500–550°C, is the most efficient in terms of soil carbon sequestration. Such biochars reveal an average sequestration efficiency of 41.4% (±4.5%) over 100 years.This work was financially supported by the EJP Soil project CarboSeq, which has received funding from the European Union's Horizon 2020 research and innovation programme (grant agreement No. 862695). Open access funding provided by Agroscope.Peer reviewe